While this does not mean that the Advancer surface is like a non-stick frying pan, it does mean that removal will be considerably faster and easier. Glaze and heavily fluxed porcelain clay bodies can still fuse and grip onto the oxide layer present on the shelf surface. However the non-porous surface does make glaze considerably easier to remove than glaze that has infused beneath the surface as with other types of shelves.

Round, globular drips or runs are easily removed by using a paint scraper or putty knife with a rigid, stiff metal blade and normally do not require grinding of any sort. Larger raised areas can usually be removed by blade scraping if done soon after shelves are removed from the kiln.

In the event of a very fluid run or glaze volatilization that covers a larger shelf area of an inch or more, a combination of scraping and grinding will be required. Keep in mind that even in these instances, glaze is still sitting on top of the shelf surface so it is not required or recommended to grind beneath the shelf surface. Because Advancer is very hard, it is difficult to do this anyway.

Keeping in mind that it is still preferable to use the least aggressive method possible, you may wish to start with a 60 or 80 grit, alumina oxide sand paper or emery cloth. Silicon carbide rubbing stones, knife sharpening stones or even fire brick posts may also be used for this purpose. Surface scratching and dulling are inevitable once you use any of these methods. This not detrimental to long term shelf performance, but may make ongoing glaze removal more time consuming.

Obviously this is not going to be practical if glaze thickness on the shelf is over a 1/16”. In this case you will need to grind with something like a masonry or ceramic tile grinding wheel or similar flap-disc type wheel mounted on an angle grinder or Dremel® tool.

If you are spending a lot of time removing glaze from shelves after each firing, you should consider using a high alumina wash on the shelf. This will make shelf maintenance easier in the long run. Glazes that consistently volatilize or spray onto your shelves will build up on the shelf making subsequent removal more difficult if left unchecked and will also cause pots to glaze and stick onto the shelf. Ultimately these types of glazes should only be used on kiln washed shelves.

For routine maintenance and removal of occasional glaze drips and other kiln debris removal, we will soon be introducing a hand tool made of the same material as our GlazeEraser® wheel head grinding disk. Check it out soon at www.GlazeEraser.com.

At Syzygy Tileworks we are dedicated to producing a truly handmade, aesthetically pleasing product that is durable and environmentally safe, and benefits the local community artistically, economically and environmentally. Advancer shelves help us to meet these goals. Because Advancer shelves are thinner, we are able to fire more tiles in each kiln. The kilns complete their firing cycle and cool down more quickly which allows us to increase our number of firings each week while we use less electricity in each firing. Advancer shelves have allowed us to make larger tile sizes than were feasible for us before, and we have also lowered our loss rates because the tiles stay flat. They are a sound investment for so many reasons: lower cost of firing, increased capacity, lower loss rates and they are virtually indestructible. Donna and Marshall at Smith Sharpe Fire Brick Supply are a joy to work with. Their level of service is rarely seen these days. I simply cannot say enough good things about these shelves and the people that make them available.

Josh White
Production Manager
Syzygy Tileworks
Silver City, NM
www.syzygytile.com

At Blue Heron Pottery, my life as a potter has become much easier since purchasing Advancer Kiln Shelves. The shelves are so light – causing less strain on my wrists and back when loading my deep kiln. Warping is not an issue, since the shelves do not warp. When glaze occasionally runs onto the shelves, it comes off easily and does not leave an uneven surface. They are amazing...a little sanding and voila.

I have found that with my clay body I need to use a kiln wash, so Marshall from Smith-Sharpe Fire Brick Supply (SSFBS) provided a formula for me to use. I apply it with a skinny 1 inch paint roller and it works great...no more brushing! Recently, I sanded the shelves down with 80 grit paper, per Marshall's recommendation, and recoated...heaven!

Purchasing Advancer shelves has been a great investment for us. When we were considering buying the shelves, I really had a problem with the cost and couldn't imagine it being worth that expenditure. However, I can now say they were worth every cent, and I recommend them highly as a wise investment.

With the professional and complete support from SSFBS, my doubts have been erased and my questions have been answered in regards to the care and use of my shelves. I can officially say that after using the typical 3/4 or 1 inch thick shelves for years, I only wish the Advancer shelves were available sooner. My life as a potter would have been so much easier!

There's a new product on the market for potters and ceramic artists, GlazeEraser™. The GlazeEraser is a unique, slow speed grinding tool designed to work with your potters wheel to quickly remove glaze drips and other unwanted kiln debris from pot bottoms. It's ideal for quickly smoothing foot rings and rough glaze edges. Designed to be used with or without bat pins, there is no special safety protection required. You control the speed and pressure!

Watch the video below for more information and to see GlazeEraser in action!

Richard Bresnahan
Saint John’s Pottery Studio
County Road 159
Collegeville, MN 56321
www.csbsju.edu/pottery
320-363-2930

Saint John's Pottery engages students, apprentices and visiting artists in the work of artistic creation, discipline, and the research and preparation of natural materials. All of these experiences are framed by questions of what it means to envision and create a sustainable living system.

The loading technique in our wood fired kiln uses a combination of a sand, alumina hydrate, kaolinetic clay mixed with reground, fired wadding that separates the shelves from posting, and allows the kiln shelves to expand and contract with reduced friction stress. An alumina hydrate/sand kiln wash between clay wadding and the shelves also reduces friction, but is not used on the entire shelf. We often load larger works that span across shelf seams and shelf stacks that reach six feet in height, the choice to use Advancer® kiln shelves has been an advantage since they are much easier to level than thicker shelves.

Besides being easier to level, the people at Saint John’s Pottery have experienced several additional benefits from their Advancer kiln shelves. Specifically, they’ve found Advancer shelves remarkably easy to clean, which is important since in some locations of the wood fired kiln there is often beading of ash glaze present on the shelves. Because they are lightweight, Advancer kiln shelves are easy for one person to work with in loading, particularly when stacked above chest height. “Since we regularly train new people in loading and stacking techniques in our multi- chambered wood fired kiln, choosing kiln shelves is an important decision for the people at Saint John’s Pottery.”

Saint John’s Pottery primarily uses Advancer in the second chamber of their wood kiln, which is exclusively used for glaze ware. This chamber is preheated over several days by the first firing chamber, allowing for a very gradual increase in temperature, and 4-8 hours of actual stoking of the glaze chamber. They have also used Advancer shelves in gas firings with good results.

Overall, the people at Saint John’s Pottery have been very pleased with their Advancer shelves. Here’s what Richard Bresnahan, director of Saint John’s Pottery Program, had to say: “Advancer kiln shelves have been a very good investment, and haven’t shown any serious signs of wear. They have been very flexible in loading, and have saved us time and space in loading. We have been working with Smith-Sharpe for 31 years, and both customer service and technical assistance have been far above what we have experienced with other companies!”

Come to Motawi Tileworks on any given day and you can find founder Nawal Motawi and her studio team hard at work making Motawi Tile in their Ann Arbor studio. Using locally-produced clay and glazes that are mixed on-site to their own recipes, Nawal and the Motawi team are passionate about exploring new design, technical ideas and healthy business practices. The studio team has turned their emphasis toward recycling and energy efficiency as next steps in improving their own environmental responsibility, which may be one of several reasons their production staff agrees, “Advancer® kiln shelves are our #1 choice in kiln shelves.”

Aside from the fact that the thinner cross-sections and excellent thermal properties of Advancer® shelves result in faster heat transfer and greater energy efficiency, the Motawi Tileworks team has experienced several other benefits from their Advancer® kiln shelves. When asked how using Advancer® shelves has impacted their firing, their work and/or their life, they had great things to say.

Their Work

“Dependable product with no unexpected issues.”

“Durable and easy to keep clean, helping lower our kiln debris related defects.”

“Advancers® last longer than kiln shelves made with other materials.”

“Advancers® all allow us to get more square footage in each kiln load.” 

Their Lives

“Advancers® are easier on your body because they are lighter than other kiln shelves.”

“Advancers® are easier and safer to pick up, less shape shelf fragments.”

While oxidation firing tiles in their electric kilns, Nawal and her studio team separate their Advancer® shelves with standard kiln posts. When bisque firing, they load tiles vertically from front to back on the Advancer® shelves, which is typically a very heavy load. And when glaze firing, the Motawi Tileworks team loads their tiles horizontally on the back of their Advancer® shelves.

Overall, the team at Motawi Tileworks sees their Advancer® kiln shelves as a great investment. They have not had to replace them!

 
About Motawi Tileworks

Motawi Tileworks
170 Enterprise Drive
Ann Arbor, MI 48103
www.motawi.com
Find us on Facebook – Motawi Tileworks

Having studied sculpture, ceramics and glaze chemistry, Nawal Motawi graduated from the University of Michigan with a degree in Fine Art. She founded the Tileworks in a small garage in 1992, set up a table at the local farmer’s market and was commissioned to create her first local fireplace installation. Today Nawal employs over 20 artisans and staff in the 12,000 sq. ft facility in Ann Arbor, Michigan.

“We make Motawi Tile in our Ann Arbor studio using locally-produced clay and glazes that are mixed on-site to our own recipes. Our aesthetic influences include early 20th Century decorative artists such as Mary Chase Stratton, Adelaide Robineau, Louis Sullivan, William DeMorgan, Dard Hunter and Frank Lloyd Wright. Our designs have evolved from our continuous exploration of historic decorative art.” Nawal believes “in respecting the best of historical design - rendering old motifs in new ways and creating works that will be enjoyed in this lifetime and passed down for generations to come.”

Nawal is passionate about exploring new design, technical ideas and healthy business practices. The studio team has turned their emphasis toward recycling and energy efficiency, their next steps in improving their own environmental responsibility. Motawi’s arts and education “outreach” programs are a positive artistic force in the community.

Whey you buy a piece of art with your own money, it’s because it speaks to you in a way that words cannot express. It provides a daily dose of beauty. Nawal whole-heartedly believes we are all a little happier when surrounded by such accessible art. It’s a high compliment to us that so many people choose our tile as a special gift or for their own home. Motawi Tileworks’ handcrafted tile can be found in homes and businesses nationwide and is featured in over 375 gift shops, galleries, museum shops, botanic gardens and home decor galleries worldwide.

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While many individual customers report that they do not need to use a wash coat on their ADVANCER® shelves, we generally recommend using a suitable high alumina wash on ADVANCER® shelves – particularly in studio or classroom settings where multiple users are present.

Due to the extremely low porosity of ADVANCER (less than1%) vs. conventional silicon carbide (18% typical) and cordierite (30% typical), it is true that there is much less of a tendency for glaze to penetrate into or stick onto the ADVANCER surface. Never the less it is still possible for glaze to react with the protective oxide glass surface of ADVANCER, especially at higher temperatures, and bond onto the ADVANCER surface. Glaze drips are usually easily removed without grinding because extremely low porosity of ADVANCER. On the other hand, glazes that volatilize at temperature, leaving a fine mist of glaze on the shelf may require light grinding or sanding for complete removal.

Ceramics: A Historical Overview

Clay has been used for many things throughout human history: a writing surface, building, material money (e.g., In the Near East, the Babylonians issued hollow balls of clay with little stones inside. A mark impressed on the outside showed how much it was worth.), storage containers for food and drink, cooking vessels and serving plates, ballast (weight placed in the hold of a ship to enhance stability) on ships (Clay vessels filled with spices, olive oil, and wine were shipped and traded throughout the Middle East, Europe and Asia.), chamber pots, ceramic shields on space ships, in engine parts, and in communications, and a major tool for dating cultures.

1. Ceramics - Objects made of clay fired sufficiently high in temperature for a chemical change to take place in the clay body, usually over 1550 degrees F.

Firing Clay:

1. Firing - Clay is hardened by heating it to a high temperature, fusing the clay particles. Primitive pottery is usually fired on the ground or in pits with whatever flammable material is available. Kilns allow a more efficient use of materials and more control over the atmosphere during a firing. The two basic atmospheres, oxidation and reduction, affect the color of the final piece.
2. Kiln - The furnace in which ceramics are fired. Kilns can be electric, natural gas, wood, coal, fuel oil or propane. Materials used to heat the kiln can affect the work; wood ash can build up on the surfaces of a piece and form a glaze at high temperatures. Some potters introduce chemicals into the kiln to influence the effects of the firing. Famed ceramist Beatrice Wood achieved a lustre effect by throwing moth balls into the kiln.
3. Leather Hard - A damp condition of the clay when it is too firm to bend yet soft enough to be carved. Plastic stage - clay is easily manipulated and bent. Bone dry stage - No visible moisture - no dampness to touch - Clay is ready to be fired
4. Greenware - Unfired clay ready or nearly ready for firing.
5. Bisque - Clay that has been fired once, usually at a low temperature.
6. Vitrify - A glassy, non-porous state caused by heat or fusion.
7. Kiln Wash - A mixture of china clay and flint in water solution used to coat kiln shelves to protect them from dripping glaze.
8. Kiln Shelves - The shelves inside a kiln that ceramic greenware is stacked on in the kiln. The shelves must be coated with kiln wash to prevent glazed pottery from sticking to the shelf.
9. Shelf Supports - Thick posts used to hold shelves in a kiln. 30. Pyrometer - Instrument used to record the exact temperature of the kiln.
10. Pyrometric Cones - These are slender pyramids of ceramics material made in a graded series to melt and indicate when a firing is nearly completed or completed. In an automatic cutoff kiln, they trip a switch when they melt to cut the kiln off.
11. Stilt - A triangular support for clay pieces that helps prevent glaze from melting on to shelves during a firing.
12. Elements - Coils of high temperature resistance wire that convert electricity to heat.
13. Maturing Point - Time and temperature needed to completely fire a glaze or clay object to the "vitrified" state.
14. Oxidation - (Compare to Reduction) A firing atmosphere with ample oxygen. An electric kiln always gives an oxidizing fire. In a wood or gas firing, the mixture of fuel and air is perfectly adjusted to give a clean burn. Acoma whiteware is fired in oxidation.
15. Reduction - (Compare to Oxidation) A firing atmosphere with inadequate oxygen and large amounts of carbon (smoke or unburned fuel). What would have been copper oxide in an oxidation atmosphere will be pure copper in reduction. Reduction allowed the Chinese to develop the sangue de beouf red glazes and gives Raku its metallic finishes. In Indian pottery, Maria's black pieces are the result of heavy reduction; the same piece in oxidation would be a terra cotta color.
16. Raku - Pottery is fired normally but removed when it is red hot and the glaze is molten. It is then usually placed in a bed of combustible materials and covered, creating intense reduction resulting in irregular surfaces and colors.

Types of Clay:

1. Earthenware - A low-fire clay. Porous and not waterproof. To be functional, it must be glazed.
2. Terra cotta - A brownish-orange earthenware clay body commonly used for ceramic sculpture.
3. Stoneware - A high-fire clay. Stoneware is waterproof even without glaze; the resulting ware is sturdier than earthenware.
4. Porcelain - True porcelain was being made in China and Korea around 960 AD. Porcelain is a combination of kaolin (a pure, white, primary clay), silica and feldspar. A unique aspect of porcelain is that it can be worked as clay, but when fired properly reaches a state similar to glass. Primary qualities of porcelain are translucency and whiteness. In the 17th Century, English potters invented Bone China to compete with the porcelain being imported into Europe.

Coloring and Decorating Clay:

1. Underglazes - Liquid clay slip that contains coloring oxides and chemicals used to apply color and designs to a ceramic piece.
2. Oxides - Metal oxides can be mixed with water and applied to the surface of clay. By varying the amount of material applied and rubbed off, the potter can achieve effects similar to stained wood. The most common stain is iron oxide (rust).
3. Engobe - A white or colored thin layer of clay used to decorate a bisque pot. It may or may not be glazed over.
4. Slip - A fine, liquid form of clay applied to the surface of a vessel prior to firing. Slip fills in pores and gives uniform color.
5. Incised - These decorations are surface designs cut into the clay. Mishima (inlaid clay)-variation - contrasting colored slip is inlayed into incised lines. This can bedone using wax resist - incising then applying slip. OR slip may be applied toincised lines and sanded off the raised body.
6. Sgraffito - This comes the Italian word meaning "scratched through" and is done by incising or cutting a design through a colored slip coating to reveal the clay body.

Glazes:

1. Glaze - A coating of material applied to ceramics before firing that forms a glass-like surface. Glazes can be colored, opaque, translucent or matte.
2. Matte glaze - Dull-surfaced glazes, lusterless and non shiny.
3. Crackle glaze - Minute decorative cracks in the glaze that are often accentuated by rubbed-in coloring material.
4. Dry footing - Glaze is removed from the bottom of a piece before firing, making stilting unnecessary.
5. Crazing - The fine network of small cracks that occurs on glazes. The Japanese encourage crazing and will stain cracks with concentrated tea.

I own approx. 50, 12x24" Advancer® shelves. I estimate that they’ve been fired 200-250 times in the 8 years since I bought them. In that time I believe I've cracked 2, one through personal clumsiness; one hairline crack which I spotted, cause unknown. I fire to cone 9 in reduction.

My assessment of KilnShelf.com Advancer® Shelves:

Pluses….

• A considerable portion of my production is porcelain dinnerware. Loose kiln-wash and rubble from deteriorating shelves was once a major headache for me. I've eliminated the problems almost entirely with Advancer. They are very structurally sound, no noticeable erosion after 8 years of use.

• They are still perfectly flat: I checked them with a straight-edge to be sure.

• They are much lighter than the 1" thick clay shelves I once used; no small thing to an aging potter.

• They're only slightly over ¼" thick. In a stack of 15 shelves, I've saved close to a foot of stacking space over the 1" shelves I once used. I store them all on one small cart on wheels.

Minuses….

• I use them without any form of kiln wash, wanting to eliminate as much loose stuff in the firing as possible. Used this way, they are a bit "sticky" with the feet of some of my smaller porcelain ware. I deal with this by using a small amount of alumina in wax resist as a coating on the feet of the ware.

I consider them the best investment I made when moving to my current studio. I'm sure they've paid for themselves many times over in time and effort saved. It was a significant purchase which I've never regretted.

Eric Jensen
Studio Artist
Chicago, IL
4/30/09

By: Susan Sutton, Editorial and Production Manager

Two very different pottery operations are enjoying increased kiln capacity and other benefits through the use of advanced NSiC kiln shelves.

Potters often view kiln furniture simply as a means to an end. You've worked hard to create your ware, and you just need something to put it on while it's fired in the kiln. The shelves have to be able to withstand high temperatures, and they've got to stay flat without warping over time. It would be nice if the furniture were lightweight and easy to store, but sometimes you can't have everything-so the cheaper the better, right? Well, maybe not. Two potters have discovered that paying a little bit extra up front for advanced nitride-bonded silicon carbide (NSiC) kiln shelves is saving them a lot in the long run.

A Big Operation

Peter Deneen owns Deneen Pottery in St. Paul, Minn. With about 20 employees, Deneen's primary product is custom-made, hand-thrown stoneware coffee mugs, which the company markets to country inns and bed and breakfasts across the country. Deneen fires its 48-cu-ft electric kiln daily to around cone 5 (2167°F) and produces about 3000 mugs a week.

The company had been using high-alumina kiln shelves for years, but Peter was disappointed with the warping, sagging and cracking that occurred over time. "The furniture lasted only about three to four years," he says. "Eventually, the shelves would sag in the center, and then after that they'd crack."

Peter had heard good things about Advancer(r) NSiC kiln shelves.* "I was told that these shelves wouldn't warp over time and that they'd outlast the shelves that we had been buying," he says. He began testing the shelves in November 2004 and moved them to full production in January 2005.

While not enough time has passed for Peter to evaluate the shelves' resistance to warping, other benefits were obvious right away. At 5/16 in. thick, the new NSiC shelves are considerably thinner than the 5/8-in.-thick kilns shelves he had been using, which enables the company to load an additional 100 mugs in the kiln for each firing. "We used to get 600 mugs in the kiln," explains Peter. "Now we can get 700 mugs in it with the Advancer shelves." Because of the increased capacity, the company is going to be able to cut back on its firing schedule, saving both electricity and wear on the kiln.

In addition, Peter was surprised by another immediate benefit. "The new shelves cut the firing time down by about an hour and a half over the other shelves we had been using," he says. "We were really pleased with that. The shorter firing cycle I didn't expect. It should reduce our electric bill by a couple hundred dollars a month."

*Manufactured by Saint-Gobain Ceramics, Worcester, Mass. For more detailed information on these and other lightweight kiln furniture options, see “Lighten Up,” Potter Production Practices, March 2004, pp. 3-10.

A Smaller Setting

In Chicago, Ill., Eric Jensen runs what is basically a one-man operation. (He has someone come in one day a week to help out.) He produces primarily high-temperature (about cone 9, 2300°F) porcelain dinnerware, which he sells to craft, gift and art galleries all over the country. Eric fires about once a week, with a minimum of 100 pieces in each kiln load. "For one person, it's a fairly big number because I'm making a lot of smaller items," he says.

Eric creates his own glazes, hand builds each piece, and loads and unloads his natural-gas-fired kiln for each firing. Every plate, bowl and cup represents a large investment in his time and energy, and losses can be devastating.

About five years ago, he decided his cordierite kiln shelves were more bother than they were worth. As the shelves degraded over time, they spit off tiny bits onto his ware during firing. He could either spend more time to grind down the bits and try to salvage the ware, or consider the pieces a loss. Neither option was attractive. "I had a lot of ruined plates," says Eric. "As the years went by with these shelves, it was an increasing headache."

The cordierite shelves caused other problems as well. At 12 x 24 x 1 in. thick, the shelves took up a lot of storage space and were very heavy. "As you get older, that wears on you when you're trying to load that top shelf in the back," he laughs. The shelves also warped and cracked. This was especially problematic for the plates, which need to remain flat during firing.

Eric, who was in the process of building a new studio at the time, had heard about Advancer shelves on the Internet and through his contacts in the pottery industry. He had already budgeted for new kiln furniture, so he decided to try out some Advancer shelves. "I believe I bought 10 at first, which was already a sizeable investment, and I was sold on them right away," he says. "The major thing for me is that it has completely solved that spraying bits problem. For that alone, I think I'd pay 10 times what I paid for these things."

Eric now owns 50 Advancer shelves, and while they solved his main complaint, that wasn't the only benefit. "These are only 5/16 of an inch thick, so I'm firing more ware with each firing, which makes a considerable difference, and that's also a lot of shelf mass that I no longer have to heat," he says. "Plus, the weight differential is magnificent. To not have to struggle with those heavy shelves anymore is a relief. And I can store my 50 shelves on one cart now, where in the past it would have taken three times that space or even more."

And warping has never been a problem. "After five years, these shelves are still dead flat," Eric says.

To Wash or Not to Wash?

Depending on what's being fired, and how, advanced NSiC shelves may not require the application of any kiln wash. According to Marshall Browne, application specialist for Smith-Sharpe Fire Brick Supply of Minneapolis, Minn., factors such as body and glaze formulation, firing temperature, and the level of reduction in the firing could all influence the amount of sticking potters might experience with Advancer shelves. "Most people that fire porcelain at cone 6 or above will experience some sticking if they don't use any wash," he says. "On the other hand, a lot of people can fire stoneware with no wash on their shelves and the body doesn't stick. Glaze drips tend not to get a very good grip on Advancer shelves, because the product is very dense (less than 1% porosity). Additionally, glaze drips usually happen later in the firing, so the glaze doesn't have a long time to sit on there and react the way a foot ring would."

Peter and Eric have had different results with the need for kiln wash on the shelves. "Our glaze volatilizes during the firing and coats everything in the kiln," Peter says. "The shelves were starting to get a thin coat of glaze on them, which could have been problematic after many firings." Glaze dust on the mugs also fell off during stacking, so small bits (1/32 in. diameter) would coat the shelves and cause the mugs to stick. Peter decided to test the shelves with kiln wash. After some trial and error, the company is using the same kiln wash formula it had developed for its high-alumina shelves, and sticking is no longer a problem.

Eric's experience with kiln wash has been completely different. "These shelves are so hard that my glaze does not get below the surface at all. I've now dripped glaze onto these Advancer shelves a number of times," he says. "I have no kiln wash on them, no alumina dust, nothing. They're bare shelves, and it's impossible-as far as I can tell-for my glaze to adhere to these shelves so it won't come off. You can almost pick the glaze off these shelves with your fingers." In fact, on the rare occasions when Eric finds that he does need to use a grinder to remove the glaze bits, he has to be careful. "If I bear down too hard, I'll wear down the nib on my grinder, because the shelves are considerably harder than the grinding bits are," he says.

Eric's porcelain bodies don't even stick to the shelves on a regular basis, which is very unusual, though he does sometimes use a bit of alumina powder in wax resist on the feet of pieces that he thinks might cause problems. The bottoms of most of Eric's ware are typically flat, about 2 in. in diameter, and they don't incorporate foot rings. According to Marshall, spreading the weight across the larger area on the bottom, instead of concentrating it on the relatively small foot rings, could help explain why Eric's pieces don't tend to stick.
Most people, Marshall cautions, will experience a certain amount of sticking when firing porcelain at higher temperatures. "Porcelain bodies are typically very fluxed," he explains. "When the flux sits in the body on a foot ring for a long enough period of time, it will typically interact with that glass layer [on the shelves], and the foot ring will stick. The cure for that is to use a kiln wash that is high in alumina. Silica washes don't really help with that problem. If you have a wash that normally uses flint with kaolin, I recommend substituting alumina hydrate, and that usually takes care of the problem."

Firing with a smaller amount of reduction, as Eric does, might also contribute to less sticking. "It's a rule of thumb in the refractory business that refractories melt at a lower temperature in the absence of oxygen," explains Marshall. "Most people who fire porcelain are firing in reduction, and that does cause things to flux a little bit more."

As is usually the case in the ceramic industry, there is no "one size fits all" solution to the kiln wash question. The good news is that there are solutions, and your supplier should be willing to help you discover the perfect one for you and your firing situation.

You Get What You Pay For

In this increasingly cost-conscious world, it's tempting to make decisions based on price alone. As Peter and Eric have found, though, spending more in the short term can sometimes save money in the long run. "All of the benefits outweigh the costs," says Peter. "This furniture's going to last longer, we get more layers in the kiln because it's so much thinner, and it reduces our firing time. The expense is outweighed by the savings we're going to see by having the product in production."

For more information on Deneen Pottery, contact the company at 2325 Endicott St., St. Paul, MN 55114; (888) 646-0238 or (651) 646-0238; fax (651) 646-8605; e-mail peter@cloth-clay.com ; or visit http://www.cloth-clay.com .

Eric Jensen can be reached at (773) 539-8200, fax (773) 539-0819, or e-mail ejensenceramic@hotmail.com .

For more information about advanced NSiC kiln shelves, visit http://www.refractories.saint-gobain.com or contact Marshall Browne at Smith-Sharpe Fire Brick Supply, 117 27th Ave. S.E., Minneapolis, MN 55414; (612) 331-1345; fax (612) 331-2156; e-mail marshall@ssfbs.com ; http://www.kilnshelf.com .

SIDEBAR: Perspectives on Durability

Deneen: We accidentally knocked an Advancer shelf off the top of our stack. It fell onto the concrete floor probably from about 5 ft and it didn't break. I couldn't believe that.

Jensen: I consider them very sturdy, although I treat them like a pane of glass. I'm very careful with them and I have had no problems.

Browne: With older silicon carbide shelves, sometimes you'll see little cracks in the edge that just go in a little bit. You can't get that with Advancer. If you've got a crack, it's going to make its way right through because the shelves are so dense. However, the nice thing about Advancer is that all the corners are rounded, so you can bump a shelf on the edge of the kiln or something and it's not as likely to just snap off.

Click here to download the article as a PDF.

The newest generation of lightweight, low-mass kiln furniture can help pottery producers save energy, improve productivity—and increase profits.

Think about your kiln shelves for a moment. Chances are they’re large and heavy, and consume more energy during each firing cycle than your products do. Now, imagine decreasing that block of kiln furniture by 50% or even 75%. Impossible? Not with the newest generation of kiln furniture, which is produced from several types of advanced silicon carbide (SiC) refractory materials.* These advanced SiC refractories are 10 to 28 times stronger than traditional SiC and cordierite refractory materials, which allows for substantially lighter kiln furniture components, such as plates (shelves), support beams and support posts. The important properties of these advanced SiC materials are summarized in Table 1, along with the properties of traditional SiC and cordierite for comparison.

Advanced nitride-bonded SiC (NSiC) kiln furniture in particular offers several advantages over traditional SiC and cordierite kiln furniture for pottery kiln applications. Higher strength allows for smaller cross-sections and lighter components. For example, an advanced NSiC kiln shelf is thinner (5⁄16 in.) than the typical 5⁄8- to 1-in-thick traditional kiln shelf. Thinner cross-sections and excellent thermal properties result in faster heat transfer and greater energy efficiency—as a result, firing cycles can often be completed in less time using less energy. And, depending on the percentage of the total kiln load devoted to kiln furniture, thinner kiln shelves can significantly reduce the total load density while increasing ware capacity.

These and other benefits are helping an increasing number of production potters reduce production costs while increasing profitability.

Advanced NSiC Benefits

Advanced NSiC kiln furniture—especially in the form of kiln shelves—has been steadily finding its way into a number of gas-fired pottery kiln applications for the last several years due to its unique performance characteristics and lower cost when compared to other advanced SiC materials.** (Although advanced NSiC kiln furniture has a higher initial cost than traditional SiC or cordierite kiln furniture, a thorough economic analysis typically indicates a rapid return on the investment.)
The light weight of an advanced NSiC kiln shelf is one of its most heralded attributes, because less heavy lifting is required when stacking and unstacking kiln loads. For example, a 24 x 12-in. advanced NSiC shelf weighs just 8 lbs, compared to up to 21 lbs for the same size traditional SiC or cordierite shelf. This alleviates many of the ergonomic and safety concerns often found in pottery production facilities.

The reduction in energy consumption is also an attractive benefit. Calculations based on a 24 x 12 x 1-in. cordierite shelf compared to a 24 x 12 x 5⁄16-in. advanced NSiC shelf fired at 2228°F with a cost of electricity at $0.10/kWh demonstrated that it cost $0.30 (3 kWh) to fire the cordierite shelf and $0.12 (1.2 kWh) to fire the advanced NSiC shelf—a savings of over 50%.

Additionally, advanced NSiC kiln shelves are made flat and stay flat. The manufactured maximum allowable deflection for a shelf is 0.003 in. per inch measured across the diagonal, and the shelf will remain flat after many firings to cone 10 or 12 under heavy loading. At these temperatures, a 24 x 12-in. shelf will easily support a uniform load of 200 lbs (supported at three points) without warping after repeated firings. Advanced NSiC structural beams are also often used to support heavier traditional SiC and cordierite shelves to minimize warping.

Pottery producers have also discovered that advanced NSiC kiln shelves require less maintenance compared to conventional kiln furniture. Because the typical porosity of traditional SiC and cordierite shelves ranges from 15% to 30%, glaze drips fuse onto and into the surfaces of these shelves and require grinding for removal. The porosity of advanced NSiC shelves, however, is less than 1%, which makes it very difficult for glaze drips to get a firm grip on the surface. Many users report that glaze and soda drips are easily scraped off an advanced NSiC shelf without grinding and without the use of any kiln wash. While the exclusion of kiln wash is not necessarily recommended for all users, the extremely low porosity of advanced NSiC shelves does significantly reduce or eliminate other labor-intensive maintenance operations.

Firing Atmospheres

Although advanced NSiC kiln furniture is primarily used in gas-fired kilns, it can also be successfully used in some soda-, wood- and electric-fired applications.

Soda Firing. Advanced NSiC kiln shelves can be used in soda firing when soda is introduced indirectly into fireboxes using wet or dry methods. Due to the extremely low porosity of the shelves, soda drips are usually easy to remove by hand scraping after each firing. However, advanced NSiC shelves are not recommended for applications where water/soda combinations are sprayed directly onto the shelf, because they are susceptible to thermal shock failure under these conditions. In these cases, a thicker traditional SiC shelf should be used.

Wood Firing. Advanced NSiC kiln shelves have also been used successfully in wood-fired kilns. However, the shelves must be placed so they are not subjected to direct flame impingement or uneven temperatures. In wood-fired kilns with multiple chambers, the shelves have been used successfully in the second and third chambers, where temperature rises are gradual and direct flame impingement is not an issue. If these chambers are stoked with wood at later stages of the firing, it is important that temperatures are already high enough to allow for a gradual increase, and/or that the shelves are stacked or shielded in a way that prevents direct flame impingement. A combination of advanced NSiC shelves and traditional SiC shelves is often a good option in areas where temperature uniformity is a problem.

It is also worth noting that an advanced NSiC shelf is likely to exhibit more profuse glass dripping from the alkalis present in wood-fired atmospheres. A traditional SiC shelf will exhibit a certain amount of glass dripping in wood-fired atmospheres due to oxidation; however, the problem is compounded with an advanced NSiC shelf because of its higher surface area of silicon carbide (i.e., fine grain sizing and low porosity). An advanced NSiC shelf also has a protective oxide (glass) layer, which is intentionally formed during manufacturing to further protect the shelf from the destructive effects of oxidation. When excess alkalis are present, which is the case when wood is a fuel source, the alkalis will flux the glass layer and lower its viscosity, causing the glass to drip or foam more readily. The effect is less pronounced in second or third chambers where direct wood stoking is minimal.

Electric Firing. Advanced NSiC kiln shelves are also starting to be used in top-loading resistance coil element kilns, where their light weight and lower mass offer several advantages over traditional shelves. For instance, while a 1-in.-thick cordierite shelf has been the standard choice for electric kiln firings to cone 6 and above, it also consumes a lot of energy and stacking space. This is especially the case with 7- to 20-cubic-foot top-loading kilns, where space is at a premium. Not only will an advanced NSiC shelf stay flat in this application, but its 5⁄16-in. thickness also allows for more stacking space and reduced energy costs. It is also easier to lift in and out of top-loading kilns due to its lighter weight. The initial investment for the advanced NSiC shelves is significantly higher than cordierite shelves in electric firing applications; however, it is easy to realize a substantial return on investment when energy and productivity savings are considered.

It should be noted that there has been a long-standing belief in the pottery market that silicon carbide shelves cannot be used in electric kilns due to the risk of electrical shock. Silicon carbide is an electrical conductor; however, the electrical resistance of advanced NSiC shelves depends on several variables, making the likelihood of an electrical shock minimal. As with any type of powered equipment, observing common sense rules, precautions and maintenance recommendations will eliminate potential hazards. For instance, advanced NSiC shelves should not be used in kilns that are in poor repair with unpinned elements protruding out of their grooves, and an operator should never reach into an electric kiln unless the power is turned off. More detailed information can be obtained from an application specialist.

Product Suitability

Advanced NSiC shelves can be used with a number of different pottery products. One of the most logical applications for these shelves is in tile production firings, where the advanced NSiC material provides a flat surface that remains flat after repeated firings to cone 10 and beyond. Energy savings and increased stacking space are other advantages. Due to the manufactured flatness uniformity of the shelves, tile can often span the seam of two shelves butted together without noticeable deformation, provided all posting is level. However, care must be taken in placing the tile on the shelves to avoid temperature differences that could possibly lead to thermal shock failure—especially when firing 3⁄8-in and thicker tile. For example, if thick tiles are placed to cover an entire shelf except around the edge, the exposed edge can act as a cooling fin while heat is retained under the tile, and the temperature difference could possibly lead to thermal shock failure.

Many potters choose advanced NSiC shelves specifically because of their ability to resist glaze from fusing onto the shelf surface. However, unglazed porcelain foot rings and pot bottoms will fuse onto a shelf surface so tenaciously that pots cannot be removed without breaking off the fused portion. This effect is similar to that seen in a wood-burning kiln—the alkalis in the porcelain glaze migrate under the ware and flux the glass layer of the shelf, especially at higher temperatures. This lowers the viscosity of the glass layer, making it very sticky.

These potentially destructive effects can be overcome by using an appropriate high-alumina kiln wash when firing porcelain bodies. Due to the advanced NSiC material’s extremely low porosity, kiln washing a shelf is like trying to re-glaze a fired pot that has already been glazed; however, pre-warming the shelves to promote drying can facilitate kiln wash application. Additionally, several thin coats of an appropriate low-clay-content wash will have less of a tendency to lift off during drying and firing. A fired coating specifically designed for porcelain compatibility and adherence to the advanced NSiC shelves can also be used. It is best to consult an application specialist for a specific kiln wash or coating recommendation.

Application Considerations

Although advanced NSiC kiln furniture offers a number of benefits compared to traditional kiln furniture, as with any product, some limitations exist. Potters must examine their specific kiln and firing applications to determine if advanced NSiC kiln furniture is the right choice for them.

Users must understand that the advanced NSiC material is vastly different than traditional SiC and cordierite materials. While advanced NSiC kiln furniture is much stronger and can support far heavier loads, it is also more susceptible to mechanical damage or breakage if mishandled. Care must also be taken to prevent advanced NSiC kiln furniture from being exposed to prolonged moisture, such as repeated condensation or rain, because it has a tendency to dry very slowly. Even though the porosity of the advanced NSiC material is extremely low, prolonged exposure to moisture can still penetrate into the part. If an exposed part is fired under normal conditions, the low porosity does not allow moisture to readily escape, and a steam explosion can occur. It is therefore recommended that the kiln furniture be stored in a dry, enclosed area that will not be exposed to inclement weather or ground moisture. If the kiln furniture is inadvertently subjected to moisture, it must be dried in accordance with a detailed drying schedule prior to normal use. (An application specialist can provide assistance in this situation.) It should be noted, however, that the use of an appropriate kiln wash is very acceptable and does not constitute a moisture concern.

One of the most important considerations when using advanced NSiC kiln shelves is avoiding rapid or uneven heating and cooling that could possibly lead to thermal shock failure. Extreme temperature differences lead to unequal expansion within the shelf, resulting in high internal (thermal) stresses. Sometimes these stresses are high enough to trigger a crack. Think of it as pouring a liquid at room temperature on ice—the extreme temperature difference causes the ice to crack. Traditional SiC and cordierite shelves are typically coarse-grain, pressed, porous compositions; as a result, a thermal shock crack may start at one edge and stop a short distance into the shelf by finding a spot to “dead-end.” An advanced NSiC shelf, however, is a fine-grain, slip-cast composition with a much higher density, and a thermal shock crack will almost always propagate through the shelf and cause shelf failure.

For this reason, advanced NSiC shelves are not recommended where direct flame impingement occurs, such as Raku firing and some wood-firing applications (as described previously). Similarly, the shelves should not be used whenever rapid and potentially uneven temperature changes are anticipated. Forced-cooled firing profiles are not recommended unless they are absolutely controlled for temperature uniformity.

Some advanced NSiC shelf failures have been observed on the bottom layer of kiln car settings. This is most likely caused by cold air being drawn through leaky car seals when the kiln is shut off, or by excess heat retention of the car bed when compared to the rest of the kiln. For this reason, it is typically recommended that the bottom shelf layer be composed of thicker traditional SiC parts.

Additionally, excessively massive support posts should not be used with advanced NSiC shelves. Commercially extruded posts with a hole through the center are acceptable, as are dense brick soaps and advanced SiC beams and beam sections used vertically or horizontally. Full-size dense brick used as posts can retain enough heat at the support area to cause temperature differences during cooling that can lead to thermal shock failure in the kiln shelf. Such cracks usually follow a crescent shaped path around the offending post (brick). Clay wadding can also be used to help reduce the likelihood of these problems.

A Cost-Conscious Choice

Advanced NSiC kiln furniture offers many advantages over traditional kiln furniture in a range of pottery production applications. Its substantial strength allows users to significantly reduce their refractory-to-ware ratio, resulting in increased productivity, and its thinner cross-sections (i.e., less refractory mass) and excellent thermal properties provide faster heat transfer and substantial energy savings. Any increase in energy costs makes the return on investment even more attractive. Other advantages include light weight, flatness retention, less maintenance and longer life.

Given these benefits, it is not hard to understand why many potters are now using advanced NSiC kiln furniture—and why many more are evaluating it for use in their applications.

About the Authors

Michael Arbini is a senior application engineer at Saint-Gobain
Ceramics, Bedford, Texas, and can be reached at (817) 545-2867, fax (817) 545-7524 or e-mail michael.a.arbini@saint-gobain.com . Marshall Browne is an application specialist at Smith-Sharpe Fire Brick Supply, Minneapolis, Minn., and can be reached at (612) 331-1345, fax (612) 331-2156 or e-mail marshall@ssfbs.com. For more information about advanced SiC kiln furniture, visit http://www.refractories.saint-gobain.com or http://www.kilnshelf.com .

*Available commercially under the trade names Advancer® (advanced nitride-bonded SiC, U.S. patent no. 4,990,469), Crystar® 2000 (recrystallized SiC) and Silit®SK (reaction-sintered, silicon-infiltrated SiC), manufactured by Saint-Gobain Ceramics.
**Note: Certain application variables such as temperature, load and ware compatibility may require the use of other advanced SiC materials. It is always best to consult an application specialist for material recommendations.

Click here to download the article as a PDF.

by William Schran

When starting the process of designing a kiln, one should consider the kiln shelves an important factor that may influence the shape and size of the structure. Several factors will have a role in shelf selection.

Kiln shelf size will play a major role in the design of the kiln. There are several standard sizes of square and rectangular shaped shelves. The potter should choose a shelf size that is readily available. One should consider the size of ware, especially diameter, that is made and how the pots will occupy space on a shelf, including the placement of support posts. Maximum weight one can comfortably lift may be important when thinking about the type and size of the shelves.

After kiln shelf size is factored into the design of the kiln, then the type of shelf will be the next consideration. Maximum firing temperature, type of fuel and atmosphere, cost and again, shelf weight, all must be taken into account with shelf selection.

We shall use a standard of 12” X 24” shelf size as comparisons are made of composition, thickness, weight and cost.

One factor many potters often neglect to consider is the amount of energy it takes to heat the pots and the kiln furniture. Often more energy is spent heating the furniture than the pots. We must think about the thermal conductivity of the kiln shelves, also referred to as the K factor. This is explained as the coefficient of thermal conductivity, which is the amount of heat that passes through a unit cube of material in a given time when the difference in temperature difference across the cube is one degree. Simply put, different materials will conduct heat at different rates. The lower the number designation, the more insulating the material is and thus, more energy is required to heat the shelf to a given temperature. Cordierite has a K factor 7 - 10. High alumina shelves are somewhat higher, in the low 20’s, while silicon carbide shelves have a number about 100. Silicon carbide nitride bonded shelves are similar to oxide-bonded shelves. Advancer shelves are 85 – 125, depending on the temperature. What does this mean for the potter? It takes considerably more time and energy to heat a cordierite than a silicon carbide shelf. For larger ware, especially larger diameter plates, a faster firing may result in more uneven heating between areas of the pot in contact with the shelf and the upper edge of the pot leading to possible cracking on shelves composed of material with lower thermal conductivity. Once heated, shelves with lower numbers will have a tendency to hold the heat longer, which may be an advantage in slowing the cooling of the kiln.

During this research, each manufacturer or reseller was contacted and asked many questions about the refractories they provide. One question that resulted in unanimous response was: “Should your shelf be rotated (flipped)?” With the only exception being the Advancer shelf, all responded yes, shelves should be rotated. Due to the number of variable conditions shelves may be subjected to, no specific routine for rotating was suggested. Potters should pay attention and be aware that cordierite is more susceptible to warping than silicon carbide shelves.

Shelves made of Cordierite, a naturally occurring mineral, composed primarily of alumina (33% approx.) and silica (60% approx.) with minor amounts of other minerals, have traditionally been used in top loading electric kilns. The minerals cordierite, mullite and corundum are the primary constituents in the make up of the shelf. Dry pressing under high pressure usually produces these shelves. They exhibit a low coefficient of expansion and are highly resistant to thermal shock, so they are good choices if rapid heating or cooling is desired. On the other hand, they are more susceptible to warping at higher temperatures. In industry, cordierite shelves are generally used where the maximum firing temperature is cone 8. Shelves used up to cone 6 should be at least ¾” thick, and those used to a maximum of cone 10 should be at least 1” thick. A 1” thick shelf will weigh approximately 21 pounds and cost about $30. Shelves composed of cordierite generally have a high porosity rate. Porosity of 13% up to more than 20% has been noted. This means if a glaze drip or run occurs, the glaze can more easily penetrate the surface and melt into the shelf. Glaze must be chipped or ground out of the shelf as each subsequent firing will cause the glaze to penetrate further and may compromise the integrity of the shelf. Kiln wash should be used to protect the surface, using a preferred mix of alumina and kaolin. Cordierite shelves are not recommended for wood or salt firings.

High Alumina shelves are generally similar to cordierite shelves but contain a higher percentage of alumina, giving them a higher temperature rating (cone 11) and less susceptibility to warping, though rotating of these shelves is still recommended. Dry pressing is used to create these shelves. High alumina shelves are denser than cordierite and somewhat more resistant to glaze drips, but they still may have a porosity of up to

20 % or higher, making the use of kiln wash necessary. High alumina shelves are not recommended for wood or salt firings. A 1” high alumina shelf will weigh 22 pounds and cost about $45.

A recent entry into the list of shelves is the Corelite line produced by Resco Products, Inc. These shelves are extruded with openings through the interior. Resco reports these type of shelves have been used by industrial manufacturers for several years to fire sanitary ware up to cone 9 to save on energy costs. The structural shape of the shelves lightens the weight such that a 12” x 24”x 1” shelf weighs only 12.3 pounds with cost ranging from $38 - $40. The shelves are advertised to be light in weight, ground to be very flat, resistant to warping, thermal shock and less prone to cracking. The shelf composition is a mix of mullite and cordierite, including a high content of alumina (49% approx.) and silica (44% approx.) with about 5% magnesium. As with the cordierite and high alumina shelves, these shelves are not recommended for wood or salt/soda firings. The porosity is 23% to 28%, so the use of kiln wash is highly recommended. Though a company representative stated that these shelves can be fired above cone 10, the specification sheet lists the maximum temperature as 2336°F, so long term use at temperatures at and above cone 10 may be an issue. Flipping of these shelves is suggested.

Silicon Carbide kiln shelves have long been the workhorse of the studio potter firing fuel-burning kilns. Also known as oxide bonded shelves, they are composed primarily of silicon carbide and silica. Silicon carbide shelves generally have a much higher maximum temperature rating than cordierite or high alumina shelves. Dry ram pressing under high pressure is the process used to manufacture most all silicon carbide shelves. Crystolon brand shelves, manufactured by Saint-Gobain, have a composition of silicon carbide: 88% and silicon dioxide: 10% with .5% iron and other minor materials. These are rated to a maximum firing temperature of 2730°F. Ashine Industries, Inc.is one manufacturer that produces silicon carbide shelves in China. Their shelves have a composition of silicon carbide: 90% and silicon dioxide: 6% with .5% silica and other minor materials. These shelves are rated to a maximum temperature of 2370°F. Though these shelves can be fired to higher temperatures and are more resistant to warping, they are more prone to cracking from thermal shock. Some manufacturers, most notably Chinese shelf producers, have begun to introduce expansion cuts in the shelves to address this issue. Silicon carbide shelves generally have a rather high porosity, between 14 % and 18%, so kiln wash is recommended. This porosity does make them more susceptible to warping. When silicon carbide shelves are exposed to oxidation the silicon carbide structure changes and some bond is lost, causing the shelves to weaken somewhat.

Euclid’s sells an oxide bonded silicon carbide shelf made by Ashine Industries that comes with a wash of 98% alumina and 2% bentonite applied at the factory. The shelves made by Ashine also come with expansion cuts to release thermal stress at high temperatures. A 12” x 24” x ½” shelf weighs 14.7 pounds and a 5/8” shelf weighs 16 pounds. The ½” shelf sold by Euclid’s lists for $60. The Crystolon, 12” x 24" x 5/8”, weighs 17 pounds and lists for about $88 from Smith Sharpe Fire Brick Supply. Thickness of the Crystolon shelf is determined by shelf size, application and load. A larger shelf requires a greater thickness to control warpage during firing. Due to their resistance to corrosive atmospheres, silicon carbide shelves are recommended for wood fired and salt/soda kilns. Smith Sharpe recommends ¾” thick silicon carbide shelves for wood or soda firing.

Nitride Bonded Silicon Carbide shelves provide a very strong shelf that is thinner and weighs less than the oxide bonded silicon carbide. A 12” x 24” x 3/8” nitride bonded shelf weighs 11 pounds. Nitride bonded shelves are dry ram pressed, then fired in a nitrogen atmosphere. This produces a shelf that is 75% silicon carbide, 20% silicon nitride, 1% silicon dioxide with the remaining materials being less than 1% each. Ashine Industries is one company that manufactures these shelves in China. The maximum working temperature of these shelves is 2480°F (1360°C). Ashine reports their nitride bonded shelves are fired in a nitrogen atmosphere to 1400°C. As of this writing, Ashine supplies these shelves to Euclid’s and Larkin Refractory Solutions. Reports from potters about the early versions of these shelves, that had no expansion cuts, appeared to suffer from thermal cracking more frequently than other shelves. Manufacturers have responded that potters are subjecting these shelves to thermal stress from rapid heating or cooling. Like some silicon carbide shelves, the nitride bonded shelves now have expansion cuts to release thermal stress that might otherwise lead to cracking. Whether these cuts provide sufficient relief from thermal stress is still debated in the industry. Some have suggested the inconsistent quality of the materials or inconsistent firings may be a factor with the cracking issue. Nitride bonded shelves have a porosity of 16%, similar to oxide bonded shelves, so use of kiln wash may be advised.

Advancer is a patented brand of silicon carbide nitride bonded shelves manufactured by Saint-Gobain Ceramics. The Advancer product line was originally developed in the late 1980’s for the commercial porcelain industry in Europe and soon found its way into the sanitary ware market and other technical applications. All Advancers are 5/16” thick. A 12” x 24” Advancer weighs a bit over 9 pounds, providing for faster heating and cooling. Kiln wash is not necessary to protect the surface and glaze accidents can be removed by scraping with a putty knife and as needed, a light application of an angle grinder. The shelf is produced by slip casting using careful quality control of the materials. The shelves are composed of 70% silicon carbide and 30% silicon nitride bond. The maximum firing temperature is 2642°F. The shelves are fired twice, the first time in a nitrogen atmosphere to a top temperature that is proprietary to the company, though is probably in the 2700°F range. A second firing, oxidizing in air, produces a very tight oxide layer. This process produces a shelf that has a porosity of http://www.kilnshelf.com>

Some personal observations:

I have been involved with firing ceramic ware since 1972. I have fired a variety of brands and sizes of electric kilns. I have also participated in building/firing gas fired kilns and have fired a variety of commercially manufactured gas fired kilns. There are manufactured gas kilns that are very well engineered that make firing a fairly simple operation.

In addition to my kiln experiences, I have also used a variety of kiln shelves. Back in the 1970’s potters were often limited in their shelf options. Usually ¾” silicon carbide shelves were used in fuel fired kilns and cordierite shelves were the choice for electric kilns.

Potters now have several options, with each type of shelf having advantages and disadvantages.

In our school gas fired kiln I have begun a long term testing of all of the shelves I have written about.

We have been using Advancer shelves in the kiln for about 2 years. We have never applied kiln wash to the shelves. We have experienced many glaze run issues and have even had a low fire clay pot mistakenly included in a cone 10 firing. In all instances most all of the glaze pops off with a putty knife. A quick application of an angle grinder removes any remaining glaze. I have noticed the shelves will pluck pieces of the kiln post that remain attached to the shelf and require the use of the angle grinder. Coating the post ends with alumina may resolve this. I have not observed any warping of the Advancers.

The old silicon carbide shelves are warped from years of firing and not flipping them. They are very heavy and are held in reserve to finish loading a firing if needed. I will be flipping these shelves to see if they will flatten out over time. Some have small cracks, but have been that way for many years, and the cracks have not enlarged.

The oxide bonded silicon carbide shelf that came from the factory with a coat of kiln wash seems to be holding up well, though after two firings it came out of the kiln with a rather glossy surface on the unwashed areas. Whether this is due to oxidation of the silicon carbide creating a glassy surface is unknown.

We decided not to apply kiln wash to the nitride bonded silicon carbide shelf to test how resistant the shelf might be to glaze accidents. The one glaze run that occurred so far came off fairly easily with the angle grinder. Porcelain clays are subject to plucking on these shelves. After 3 firings I have noted a slight warping of the shelf. I will continue to monitor this very carefully.

The shelf manufactured by Resco, that has the openings through it, seems to be holding up well in the cone 10 firings. I did notice the shelf color has darkened and the surface seems to be more vitrified. Some glaze drips have gotten through a layer of kiln wash, into the shelf that required removal with an angle grinder. This has resulted in small areas of the shelf being ground away. I am pleased to see this product on the market as I think this may be a good alternative for folks with larger top loading kilns that need something lighter in weight than the 1” thick solid cordierite shelves.

I would like to thank all of the individuals and companies that have provided me with valuable information, product samples and full shelves for testing in cone 10 reduction firings:

Jon Walls, Euclid’s Elements/Euclid Kilns, http://www.euclids.com/ 1-800-296-5456 ex 223 jwalls@pshcanada.com oxide and nitride bonded silicon carbide shelves and more, Jon also put me in touch with his supplier:

Peter Wu, Ashine Industries Inc., http://www.ashine.com/ 416-493-5187

ashine@ashine.com manufacturer in China of oxide and nitride bonded silicon carbide shelves

Jim Wunch, Larkin Refractory Solutions, http://www.larkinfurnace.com/, 678-336-7090

lrs@larkinrefractory.com supplier of oxide and nitride bonded silicon carbide shelves and other refractories

Sue, Bailey Ceramics Supply and Bailey Pottery Equipment Corp, http://www.baileypottery.com/

800-431-6067 info@baileypottery.com high alumina, cordierite, oxide and nitride bonded shelves

Kevin Frederes and Eric Nedreberg, Resco Products Inc, http://www.rescoproducts.com/

888-283-5505, Kevin.Frederes@rescoproducts.com , eirc.nedreberg@rescoproducts.com producer of Corelite high alumina cordierite shelves

Jon Pacini, Laguna Clay Co, http://www.lagunaclay.com/ and http://www.axner.com/

800 4-LAGUNA, 800-843-7057, jpacini@lagunaclay.com

supplier of cordierite, oxide and nitride bonded shelves, Advancer shelves

Dona Turbes and Marshall Browne, Smith-Sharpe Fire Brick Supply, http://www.kilnshelf.com/

866-545-6743, dona@ssfbs.com , marshall@ssfbs.com

Primary supplier of Advancer and Crystolon shelves and other refractories

Mike Arbini, Saint-Gobain Ceramics, http://www.refractories.saint-gobain.com/

817-545-2867, Michael.a.arbini@saint-gobain.com

Producer of Advancer and Crystolon shelves and other advanced refractories

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NEWS BULLETIN

NIOSH Criteria Document on Refractory Ceramic Fibers

The National Institute for Occupational Safety and Health (NIOSH) has just completed a comprehensive scientific review of data characterizing occupational exposures to airborne refractory ceramic fiber (RCF) and information concerning potential health effects associated with workplace exposures to RCF. This document, referred to by NIOSH as, “Criteria for a Recommended Standard: Occupational Exposure to Refractory Ceramic Fiber,” DHHS (NIOSH) Publication No. 2006-123, incorporates an extensive NIOSH evaluation of published literature with over 230 cited references. Commonly referred to as a “NIOSH Criteria Document,” these highly regarded scientific reviews result in what is called a “Consensus Standard,” incorporating comments from independent scientific peer-reviewers and a diverse group of stakeholder representatives, including the RCFC.

In 2002, the Refractory Ceramic Fibers Coalition (RCFC) and the Occupational Safety and Health Administration (OSHA) introduced a voluntary worker protection program entitled PSP 2002. PSP (Product Stewardship Program) is a comprehensive, multi-faceted risk management program designed to control and reduce workplace exposure to airborne RCF. Contained within PSP 2002 is a Recommended Exposure Guideline (REG) of 0.5 fibers per cubic centimeter (f/cc) of air over an 8-hour Time-Weighted Average (TWA), which is endorsed by OSHA in PSP 2002.

The NIOSH Criteria Document for RCF supports the intent of the Product Stewardship Program for RCF and also includes a Recommended Exposure Limit (REL) of 0.5 f/cc of air per TWA for up to a 10-hour work shift during a 40-hour workweek. NIOSH is a highly respected scientific health and safety research organization that has put a great deal of work into their evaluation of RCF. The RCFC is pleased that NIOSH, along with OSHA, endorses PSP for RCF including the industry’s REG of 0.5 f/cc of air per 8-hour TWA.

The NIOSH criteria document for RCF is available online at: http://www.cdc.gov/niosh/updates/upd-06-12-06.html If you or your customers have any questions concerning this publication, please feel free to contact the Unifrax Health Hotline at 1 (800) 322-2293.

What is NIOSH?

• NIOSH is a (non regulatory) federal agency responsible for conducting research and making recommendations for the prevention of work-related injury and illness.
• The views expressed within the Criteria Document for RCF are not “regulatory requirements,” but rather represent the findings from a well-respected scientific health and safety research organization.
• NIOSH and OSHA were both created in 1970 under the Occupational Safety and Health Act.
• NIOSH often acts as the “scientific research arm” of OSHA, while OSHA is primarily responsible for developing and enforcing occupational safety and health regulations.
• NIOSH is part of the Centers for Disease Control and Prevention (CDC) within the U.S. Department of Health and Human Services.
• NIOSH is headquartered in Washington, DC, with research laboratories and offices in Cincinnati, OH; Morgantown, WV; Pittsburgh, PA; Spokane, WA; and Atlanta, GA.

Dean E. Venturin, Ph.D.
Director, Health, Safety
and Environment
http://www.unifrax.com/web/UnifraxHome2.nsf/AllDocuments/9C62F4C6BD5305E8852571B0006716A4?

By Marshall Browne and Mike Arbini

There is currently a lot of misinformation circulating throughout the pottery market regarding the use of nitride bonded silicon carbide kiln shelves in top loading electric kilns. The two most widely discussed topics are electrical conductivity and thermal shock.

The pottery market has historically avoided the use of silicon carbide kiln shelves in top loading electric kilns due to the electrical conductivity of silicon carbide. While a portion of this belief is correct, it should be noted that there are many types of silicon carbide kiln shelves. The differences are mainly due to formulation, method of manufacture and bond type. These differences can also have an affect on the degree of electrical conductivity. For example, recrystallized silicon carbide is often used as heating elements (hot rods, glow bars, etc.) in other types of electric kilns. This is obviously a very conductive type of silicon carbide material and would be considered dangerous when used as a shelf material in an electric kiln. The danger could exist when a loose element is in contact with the top kiln shelf; the operator opens the kiln while in operation and touches the kiln shelf resulting in an electrical shock (i.e., a lack of common sense and the alignment of several variables). Traditional nitride bonded kiln shelves (there are many on the market like the type being introduced from China) have more electrical resistance than a recrystallized silicon carbide due to the type of bond, but their electrical conductivity values are still considered to be potentially unsafe.

Advancer® kiln shelves were introduced to the market by Saint-Gobain Ceramics. Advancer is a patented advanced silicon nitride bonded silicon carbide. Before its introduction to the pottery market, Saint-Gobain Ceramics conducted tests comparing the electrical properties of Advancer to other types of silicon carbide kiln shelves. The uniqueness of the Advancer material comes from its formulation, type of bond and especially the glassy oxide layer that is manufactured on the surface of the product. It is this glassy oxide layer that dramatically increases the electrical resistance of Advancer kiln shelves when compared to other types of silicon carbide kiln shelves. As a result, the electrical conductivity value of an Advancer kiln shelf is low enough to be considered safe for use in top loading electric kilns.

Regarding thermal shock, different types of silicon carbide kiln shelves (including their size and geometry) are more or less susceptible to thermal shock. While it is generally true that the higher thermal expansion of silicon carbide kiln shelves make them more susceptible to thermal shock than cordierite kiln shelves, this is only one factor among many complex variables that can contribute to thermal shock. It is thermal gradient (significant temperature differences across the shelf) more often than not that contributes to thermal stress failure of silicon carbide kiln shelves. Traditional nitride bonded silicon carbide kiln shelves are typically thicker than an Advancer kiln shelf and their increased in thermal mass make them more prone to thermal gradient failure. Furthermore, traditional nitride bonded kiln shelves are not as strong as an Advancer® kiln shelf, making them more susceptible to failure at lower thermal stresses. It is also worth noting that silicon carbide kiln shelves can actually be cycled faster than cordierite kiln shelves because of their higher thermal conductivity. This is particularly true of true lo-mass shelves like Advancer.

Generally speaking, the range of temperatures and typical even heating cycle characteristic of top loading electric kiln applications are considered mild for Advancer kiln shelves. The relatively lower operating temperatures (Advancer shelves are rated for continuous use up to 2700 degrees F) in these small kilns along with their small and well insulated firing chambers are considered to be confined and uniform when compared to larger industrial kiln applications where Advancer kiln furniture is more widely used.

It should also be noted that there is no evidence that the use of negative pressure, under-kiln venting systems are detrimental to the use of Advancer shelves in electric kilns. These systems, when properly installed and maintained do not create conditions within the kiln that can lead to thermal gradients as discussed above.

THIS PAPER IS THE RESULT of a four-year investigation to try to find combinations of insulating materials which, when allied with different hot facings, might retard or even prohibit their deterioration so that potters might be encouraged to build insulated kilns for salt and soda firing. The original proposal submitted to the Arts and Humanities Research Board (AHRB) based in Bristol, England, who gave financial backing for the research was for two kilns to be built. The first kiln was to contain a wide range of brick types and tests allied with different facings. It was hoped to fire it approximately 50 times. The knowledge gained from this kiln was to be used in a second kiln, which it was hoped to fire 100 times. The number 100 was notional but it represented a base point of acceptability in comparison with the more conventional heavy brick kiln. The heavy brick kiln is rugged, takes more energy to fire and will generally have a longer firing cycle, but the basic bricks are not prohibitively expensive. The insulated kiln, in order to be considered effective should be significantly cheaper to fire and shorter in firing duration, but it should still have a reasonable life expectancy.

For my doctoral thesis which I undertook as a full time staff member of the University of Ulster, I used a 4 cu ft insulated kiln which, while it required considerable remedial attention, was fired 100 times. But the time spent plugging holes and filling cracks within a tiny kiln would not be feasible were the kiln to be scaled up. Nevertheless, I gained a confidence and some experience to hope that with a larger kiln, the problem could also be addressed.

It is the retardation of the formation of salt-glaze on or in the fabric of the kiln which is the key to a successful outcome. The more easily the salt-glaze is formed incorporating the silica present in the kiln bricks, the more the bricks themselves will be attacked and subject to deterioration. An insulating brick has considerably less mass than the conventional heavy firebrick and, also, whereas the firebrick is dense, the insulating brick is made up of a fine series of air pockets. Thus the airborne alkalis which result from the breakdown of the salt introduced into the kiln at high temperature can easily enter the brick through the pores in the surface to form a glaze not only upon the surface but also within the brick itself.

All bricks contain silica. It is the combining of silica with sodium (released from the salt or soda when subjected to high temperature) which forms the salt-glaze. This is the source of the problem regarding the deterioration of the kiln while at the same time providing the reaction required on the surface of the vessels in the kiln, which is what the potter wants. Alumina is an inhibitor of salt-glaze; an essential part of the reaction in enabling a fluid or less fluid glaze to result. The higher the alumina ratio in the reaction the slower or dryer will be the glaze formed; the lower the alumina ratio themore fluid and vigorous will be the resulting glaze.The higher the alumina levels in the bricks the more expensive and denser they are, the lower the insulating properties.

At the commencement of this project it seemed to be a feasible proposition to coat the brick surfaces with combinations of materials known to inhibit the formation of the glaze. If enough combinations were produced, it should only be a matter of time before some were seen to be better than others. These could then be further refined to make an ideal combination. From the outset I was aware of two products: American-based ITC and UK based Nonvit Furnace coat which claimed to be successful in already having addressed the problem. I decided to use these products as ‘controls’. Also, during the building of the first kiln, I became aware of a book entitled The Art of Firing by Nils Lou in which the author made certain claims for ITC. On examining the text I noted that his tests had been with low grade bricks and for only six salt firings. The successes claimed, I knew from my own previous experience were insubstantial, as at least 15 or 20 firings are necessary as a minimum
before any optimism can be expressed about a possible solution to the problem. Also many more firings would be necessary to be able to substantiate this supposition with confidence.

I am not a kiln builder bursting with theories about innovative kiln design and construction. The essence of this research is with the materials and their resistance to deterioration as a result of repeated salt (or soda) firings. The kiln should be of a certain capacity to demonstrate its usefulness to others as an example. Accordingly, I convened a meeting with 17 UK salt and soda glaze potters to take advice and also to find out whether others had experience or ideas which might be useful for me. A few suggestions of products were mentioned but the main outcome was support for the project and an agreement that a sprung arch kiln of about 1 cum should be appropriate to test the theory. Also, it was suggested that I test as ‘controls’various clay bodies including some
with a significant iron bearing quantity; and that the firings be to cone 10+ under reducing conditions. The kilns I built incorporated two courses of bricks throughout and an additional layer of fibre in the roof. They were based upon the recommendations of Fred Olsen in The Kiln Book. Olsen recommends that the most efficient form is a cube with a sprung arch. He also recommends that the sizes of kiln shelves or bats be considered from the outset and he recommends that the height and length of the firing chamber has a specific relationship with the diameter and height of the chimney. All this was undertaken according to his recommendations. I decided to do away with complicated ironwork for the door and simply build this up by placing 15 courses of bricks on their sides in the door recess into which were fitted two ‘spy’bricks. Two LPG burners, one firing from the
front into the left hand firebox and the other from the back into the right hand firebox completed the kiln. The kiln was fitted with a thermo couple and an oxygen/carbon probe. I wanted to test as wide a range of bricks as possible from several manufacturers. I appreciated that the higher brick grade/classification the higher the manufacturers recommended ceiling temperature and the higher (and presumably the better for me) would be the alumina content of the brick and the better it would be likely to perform. However, the higher
the grade, the more expensive the brick. (A 23 grade brick is approximately £1t/$1.5 US per brick, a 28 grade brick £2 at $3 US per brick and a 34 grade £10/$15US per brick) If in the final outcome the kiln is prohibitively expensive to build, few potters will do because of the cost.

I was aware that there were various principles which needed to be tested, as follows: Certain products,that is,‘bubble alumina’(95+per cent pure alumina) might require little additional treatment and yet be resistant to the formation of the salt-glaze. The lower grade brick with a lower alumina content would deteriorate more rapidly but this may be delayed by the addition of a ‘topcoat’ to prevent the attack by the airborne gases. That an important factor might be ‘fit’of the ‘topcoat’to the base and that amorewidely varying expansion/contraction
range between ‘topcoat’ and brick would sustain damage more rapidly. That the brick with a highly porous surface might be induced to absorb the resist materials into its porous structure, building up a resist not only
on the surface but also from within. That there might be an appreciable difference in effectiveness between that of a refractory ‘cement’ type coating and a ‘glass’which seals up all the pores. And finally, that there might be other materials available, that is, fibres, laminates, foams or that it might be possible to produce or obtain castables with appropriate properties.

Early on, I made a visit to ‘Ceram Research’ in Stoke-on-Trent to discuss the problem and to find out the kinds of materials which a group of ceramic chemists believed might be useful. And although much of the discussion
between them I did not understand, the following materials were recommended for consideration in different forms: alumina; silicon carbide; zirconium; magnesium; boro-carbide; mullite and sillimanite. It was stressed that the physical characteristics between the ‘brick’ and wash should be as close as possible. Proportions of mixes, bonding agents and the specific variation of materials were discussed but recommendations varied greatly. Foams, laminates and high-emissivity coatings were mentioned,as were specific manufacturers
of cements and other materials. I came away without being given guidance on specific proportions and percentages or with any clear line of particular attack but with a confidence that if a broad enough range of different tests were to be included in the first kiln, some tests would be clearly seen to be much better than others. I begin to realize the enormity of the task ahead. This was further compounded when I considered factors such as particle size, method of application and the combined forms of the prime materials.

Subsequent to this, I discussed the project with a group of German potters in Höhr-Grenzhausen. Though their firing temperatures and salt coatings are generally lower and lighter than those with which I was familiar, they were generous with both time and enthusiasm and I was made aware of various German products; notably cements and bats which might be useful to test. Also, I maintained contact with Mick Casson whose enthusiasm and assistance and generosity provided me with a ‘spur’ when during lengthy series of firings, nothing seemed to be happening.

For the building of the first kiln, I wanted to include a wide range of bricks of differing grade sizes from different manufacturers. I was aware also that this first kiln had to fire 50 – 60 times. If I introduced a 23 grade brick in the firebox area or roof and the ‘topcoat’ test was not successful, I might lose the entire kiln prematurely. Therefore I resolved to be cautious and decided upon 26 grade as the lowest for the main structure of the kiln. This was augmented by higher grade bricks – 28, 30, 32 and 34 grades. The bagwalls and door were
constructed from 23 and 25 grades as both could be replaced easily without adversely affecting the life of the kiln.

Most of the bricks used were manufactured by BNZ. These were augmented by some from ‘Wolf ’ and ‘Premier/Vesuvius’ in the right hand wall. The entire right hand face of Kiln #1 was overlayed with vertical bands of 13 separate cements on top of bricks reducing from32/34 grade at the bottom of 26 grade equivalent at the top. The left hand face bricks were all 26-60s except the bottom course of 32s. The roof was constructed from alternating layers of 30 and 32 grade bricks. Different ‘topcoat’ tests were applied to every two adjacent
bricks in the left hand and back faces of the kiln. The inside front faces were overlayed with seven more cement or preparatory manufactured products. These included ITC and Nonvit Furnacecoat. The bagwalls and door
bricks duplicated some of the tests used elsewhere to as certain the results of these tests on lower grade bricks.

Uniformity of application of topcoat was difficult because the different grades of brick had different porosities. Also, some materials were more finely ground than others. But I decided to keep the water contents relatively consistent with the weight of the dry ingredients to produce a liquid of a thin cream consistency. Generally this varied between 500/750 gm of dry material with 1.1/1.5 litres of water. Into this was placed a 20 ml measured quantity of vinegar to keep materials in suspension. All bricks were dipped to ensure not only a flat ‘topcoat’ but also some penetration into the porous structure of the brick. The dipping covered not only the top face but also 2/3 cm of the side faces so that if some opening up between the courses did occur, some protection within would be in place. I would like to state that up to perhaps a 30 grade brick there was an apparent uniformity of pick up, but on the 32 – 34 grade bricks where porosity was so slight and particularly when using such a heavy
material as silicon carbide 220, patchiness of ‘topcoat’did result. On occasions this had to be augmented by a later brush coating.

I was uncertain what material to use as a ‘binder’ to ensure that these non/lowsalt-glaze producing materials would in fact adhere to the bricks after firing. I reasoned that materials with a natural plasticity or a silica content to stick the ‘topcoat’to the brick during firing should be considered. Accordingly I selected: china clay; a high and low silica ball clay; bentonite; a fine and coarse grade kyanite (which has a low shrinkage); two cements which were known to have high alumina contents; and a low temperature frit. Later a wide range of
high and low temperature glass melts were added to the list together with other cements.

The kiln was given a first pre-heat. The following day a high temperature pre-salt firingwas undertaken. Some tests were given an additional brush coating to ensure a visual opacity to all tests. Additionally, a light in filling of a few joints within the vertical walls and sprung arch was made with cement, alumina plus silicon carbide. A second pre-salt high fire was followed by the first salt firing. Ten kg of salt were used and a few bottles of wine consumed.

An analysis was made after every firing, together with details of remedial attention given. Every firing was undertaken to reach top temperature in eight hours and incorporated the cones 06,3,6,9,10. Cone 10 was always flattened. Salting commenced at Cone 3 and was completed by Cone 9. This is my normal salting procedure. Four body tests on numbered beakers were always included as requested by the UK potters. The clay body of the beakers contained quantities of Fe203 ranging from 0.6 to 2.4 per cent. An oxygen probe and thermocouple were fitted to the roof of the kiln. Every firing was logged, and the fired tests, firing log and subsequent analysis of action taken was available for every firing. The fired tests demonstrated that a reasonable proportion of firings were undertaken under reducing conditions, sometimes in heavy reduction. Occasionally, I introduced a few of my spouted vessels which sometimes require an oxidised result.

From the initial 10 kg salt per firing, this had reduced in quantity to 3 kg by firing 15: A heavy orange peel glaze was a standard requirement for each firing on the four test bodies. After the third firing, the M23 and C23 bricks in the right hand bag wall were replaced. Also, many of the tests in the left, back and roof faces of the kiln were given repeat brush coatings of their original mixtures. After the fourth firing, several of the cements in the right face were given additional coatings as were roof bricks. After #5,a series of free standing tests on 28 grade bricks were introduced on the 2nd level back shelf. Following firings #6 and 7 a cement comprising Secar 71:10 pts, calcined alumina: 7.5 pts,silicon carbide 220:1 pt, was mixed and applied to areas of the kiln sustaining
damage, notably the left face bottom three courses, the right face ‘Wolf ’Lupudur bricks, and the bottom course of 32 grade bricks: The 23L and 23NF bricks in the right bag wall were also replaced. After #9, Lupufest 901 (a
German cement) was used as an in fill to areas of damage in the right and left hand faces. An additional 20 free standing tests again on 28 grade bricks were introduced into the second shelf level of the kiln together with 24 tests on the third shelf level of the kiln.

After firing #10 a major overhaul of the kiln was undertaken and both bag walls were replaced. This is not surprising as the original bricks of only 23 grade were situated in the hottest and most violent part of the kiln. They were replaced with two layers of BNZ 30 grade bricks, first high fired with a brush coat of Purimachos Refractory Glaze Wash (PRGW) and afterwards with a cement coating comprising Secar 80 – 8 pts/calcined alumina – 4 pts, silicon carbide 220s 1 part. On top of the two layers were placed a course of BNZ 34 grade bricks untreated. The above cement mix was also applied to damaged areas in the left, right, back and roof faces of the kiln. The kiln was now showing extensive areas of damage after only 10 firings.

After #11, additional free standing tests were introduced to the front shelf of level three. After #14, it was noted that new damage to the kiln was a result of the too ‘hard’mix of Secar cement, alumina and silicon carbide being pushed into the vertical and horizontal cracks opening up in the brick courses. This is an important point; perhaps it is common sense – if in trying to repair damage, a mix of harder consistency than the surrounding bricks is introduced, the softer bricks will take the brunt during the later expansion and contraction of the kiln. A mix of one pt cement, together with three or four pts filler (alumina calcined) would have been softer. Firings #14 – 18 recorded two significant factors: That sheets of brick and ‘topcoat’of perhaps 3 – 5 mm thickness were
beginning to separate from the main body of some of the bricks;and when this happened a brushcoat of Secar 80 cement – 4 pts,Wades calcined alumina – 8pts, silicon carbide 220– 1 pt, was painted over the exposed brick surface. Sometimes this separating layer was removed to try to protect the new hot face.This separation of the top layer of the brick was only localised.

Before salt firing #20, it had become apparent that much of the fabric of the original kiln had deteriorated to the point that most of the original tests within the kiln were defunct having necessitated subsequent coatings of additional mixtures. The free standing tests now provided the best hope for the future. It was also being speculated that perhaps any cement coating might present problems through the differing contraction/expansion rates between brick and top coat. It was recognised that the procedure and application might be as important as the materials being applied; and that several applications of resist material and infrequent light remedial attention to damaged areas might be part of the final solution.

A glass or glaze top coat seemed a sensible alternative. This, I reasoned would seal the surface pores of the insulating brick to prevent the absorption of the gases. But it might also prevent subsequent damage to the brick through contraction. A glass is fluid for a wide temperature range and the hardening of the glass occurs gradually. If the glass is in excess it should run but still provide a thinner impervious surface. It was also questioned what was the function of silicon carbide as an addition to many of the previous cement mixes. It was noted that of the original applications of materials to the brick surfaces, those containing a high per cent of silicon carbide often tended to be among the best in resisting the breakdown of the brick surface. What seemed to happen in such cases was that initially the silicon carbide mixture formed a dark glass. This, after repeated salt firings bubbled profusely, perhaps by as much as 5 cm forming a honeycomb mixture. After many more firings, this bubbling reduced in size but the brick was still afforded significant protection. If the proportion of
silicon carbide in the mix was of the order of only 10 per cent, bubbling did not occur.

After salt firing #19 and following further discussions with staff of Ceram Research, Stoke-on-Trent, an evaluation was made of the kiln fabric and the three levels of free standing tests. From this, several additional factors emerged: That alumina seemed to be the highest provider of non-glass. It was probable that fineness and purity of material would be a significant factor. Thus Wades’ calcined alumina (99 per cent pure) is probably more effective as a resist than calcined alumina (95 per cent pure) but that this in turn is probably more effective
than alumina hydrate (85 per cent pure); That Purimachos Refractory Glaze Wash (PRGW) which had been noted as providing significant resistance when applied to the surface of the brick, formed a glaze which became more fluid in subsequent firings. It was speculated by Dave Shepherd of Ceram Research that the initial success of the material might be the reasonably high sodium content in its composition and subsequent melt and that this, in turn, might prevent later penetration by the airborne (sodium based) alkalis resulting from the introduction of salt to the kiln; That a ‘glass’may be ultimately more successful than the cements. It would be necessary to test various kinds of glasses with different melting temperatures. These in turnmight benefit from a
‘top coat’of a different type; perhaps a cement or an alumina or silicon carbide based material; That a veneer of a low grade base brick given additional protection through the bonding with a thin slice of a high alumina 34 grade brick or even Saffil blanket was considered to be worth investigating in the next series of free standing tests. (Saffil blanket and 34 grade brickswere at this point showing significant resistance when subjected to repeated firings.)

The function of the kiln had changed fundamentally after 19 firings. The kiln had now become the means of conducting further tests but would not provide the solution itself, as most original tests had been debased by being overpainted with other materials when they had become damaged through the repeated exposure to the salt gases. This meant that the kiln had to keep going for at least another 30 firings or so. Ten of the original tests on the bricks of the fabric of the kiln were still not totally discarded and these were duplicated within the new series of free standing tests. When the kiln was totally emptied to introduce the free standing tests it was found that: The quarter cut 34 grade bricks used as props were, except for a slight discolouration still in excellent condition; The silicon carbide shelves had erupted on all surfaces with small rounds of green glass. This cleaned off easily with a chisel leaving the shelf intact and in good condition; and the new tests (numbering now173) included all the better previous tests, a wide range of different glasses, both high and low temperature, together with several with a high sodium content. It also included many tests on which Purimachos PRGW high temperature glaze wash was first fired to 1200°C to provide an initial melt, after which a
‘top coat’ was applied. Most of these new tests were applied on top of a BNZ 28 grade quarter-cut bricks.

Salt firing #20 was high fired without additional salt to try to protect and harden the newly applied ‘topcoats’ preparatory to the exposure of the normal heavy salt atmosphere. Salt firings #21 – 30 displayed the continued gradual deterioration of parts of the walls of the kiln. This was, where appropriate, remedied by additional brush coats of Secar cement together with an increased quantity of alumina and a little silicon carbide (to provide a softer fired surface from that previously applied). At this point it was felt that again the kiln should be emptied of all tests and a new sequence of tests introduced to test the more successful results. Now, however, instead of quarter cut 28 grade bricks as an individual test, it was hoped to simulate the nature of a kiln wall, which is made up of course upon course of bricks. Bricks were obtained from Premier Vesuvius to augment the range provided by BNZ.

The plan was to apply an individual resist to a pile of bricks of different specifications. Twenty additional tests were included as well as the 22 piles of bricks. A Saffil blanket test and a test of a laminate of a 23 grade brick bonded to a 34 grade ‘bubble alumina’brick by Secar cementwas also included. Those tests in which a cement resist was applied on top of a previously fired glass used a high vinegar/low water base to thicken the mixture and aid application. Vinegar was also usedwhere silicon carbide was a necessary ingredient to keep it in suspension.

Salt firings #31 to 63 were undertaken between September 2001 and February 2002. During these successive firings the roof and walls of the kiln deteriorated seriously. From salt firing #43 onwards regular attention to the
roof through the application of Purimachos Refractory GlazeWash. (PRGW–NT7), did make a significant difference in limiting further deterioration.

After salt firing # 63, all new tests had undertaken 33 firings and the time had come to make a thorough analysis preparatory to the dismantling of Kiln #1. The Saffil blanket and 23/34 grade laminate were recorded as being worthy of further testing. Of the new tests, as a general principle, those tests which featured a cement on top of a glass displayed a fragmented top coat after firing. In many instances the glass beneath was still good and protecting the base brick. What was also interesting, was that in some instances, the lower grade brick was as good as or even better than a particular higher grade brick. It was recorded for instance, that a BNZ 23HD brick was, at the end of 33 firings, in a better state than a BNZ 30 grade immediately below it in the pile of tests.

Generally, the following bricks, all classified as ‘bubble alumina’ grade were in a reasonably good state irrespective of the applied resist. These included HR 33/180; HR 185; HR 185 HP; BNZ 34. They had on occasion cracked because of a higher than normal thermal contraction, but the brick was not significantly damaged through attack by the sodium vapour. However, of the lower grade bricks it was specific ones which were less damaged and also a specific resist was quite often responsible for a better than normal result through
the entire pile of bricks. From analysis (and a series of photographs) various groupings of bricks were considered to beworthy of inclusion inKiln #2.The base of the kiln was completed in March 2002. This comprised: A base of breeze blocks equipped with air spaces; a base of heavy firebricks running across the line of the breeze blocks; A hearth comprising on the left BNZ 26 grade bricks and on the right BNZ 26/60 grade bricks.

The building of Kiln #2 presented a fine balancing act as the kiln was expected to fire up to100 times. If the entire construction was made from the ‘bubble alumina’ grade bricks, it appeared that these bricks would provide the numbers of firings required,yet the insulating properties of these bricks would take the kiln out of the category of one which could truthfully be called an insulating kiln, and also the cost would be high. If on the other hand the insulating 23HD brick was used throughout, it would have required a considerable act of good faith to believe that this brick, only classified at 1260ºC would be refractory enough for the numbers of firings required,let alone the continuous bombardment from the salt vapours. Yet this brick had performed well in earlier tests.

The distribution of the selected bricks was as follows: 1800ºC ‘bubble alumina’ bricks for the bottom four courses; two courses each of the high spall resistant 155/PB, 140/HSR, and 130/HSR; The top two wall courses comprised: Calor 26, 25HD, 23HD, in banks on the left, back and right faces. The roof was constructed from 155PB arched: 22 bricks making the full span and resting on the skewbacks on the left and right face walls.A layer of fibre covered the entire roof, topped with molars (diatomaceous brick). A layer of molars backed up the side, back and front faces of the kiln.As with Kiln #1, a thermocouple and oxygen probe were positioned in the roof.

The door bricks were built course on course and comprised 23NF, 23L, 23/130, B25, 25 grade, 26/60, 28 grade bricks. All were painted with NT7 as this seemed likely from the outset to be the most protective glass on these low grade bricks (these bricks had previously not performed well with the earlier cement facings in kiln1. But as door bricks can be replaced easily without jeopardising the kiln, it seemed a useful test to make. As with kiln 1, kiln 2 was built without any kind of mortar. An amendment to the top of the chimney through the inclusion of a top cowl which included an angled top vane to deflect the wind and thus prevent the fierce ‘blow backs’
experienced with kiln 1 proved a success.Also the inclusion of two fitted wires to enable the cowl to be moved easily to face the wind without having to climb onto the roof to effect the change, greatly assisted the operation of firing.

Whereas the majority of tests in the fabric of Kiln #1 were ‘blind chances’, and many proved by as early as salt 9, not to be effective, those tests included within Kiln #2 had been found to offer significant success even after up to 30 firings. Thus, after salt firings #7 – 13 I recorded the following in my accompanying logbook. “The kiln is performing well with little need for remedial attention.” That which was necessary was as follows: 23HD bricks – some peeling back of the glaze from the soft bricks underneath; These were painted over with NT7; Some brick courses in walls and roof showed signs of movement but is not considered dangerous. Again, an appropriate “touch up”with NT7 and NT8 remedied this; Some silicon carbide shelves were better than others. It was decided to make a study of this; Some damage was occurring to the various types of bricks which made up the door. This was to be expected because, after all, the test resists incorporated into Kiln #2 were all fluid and the glaze would run down the hot face of the bricks. The damage occurred when the individual door bricks were being taken out and bricks had stuck together. The running was more noticeable on the 23 grade bricks than on the 26/60 and 28 grades.But the damage to these door bricks was solely through this cause and not through the penetration of the alkalis from the salt vapours. These same bricks, which had disintegrated quickly when situated in Kiln #1 and when covered with various cement facings, were now immediately seen to be more successful when covered with a glass. After salt firing #13, it was recorded that there was slight damage to
the bricks adjoining the door in the side walls of the kiln and where localised glass was being formed. After firing # 17, the door bricks were replaced with 25 HDs overpainted with NT7. At this point, the following appeared to be the least damaged, of the tests. Up to this point, the pack system had always been made up of silicon carbide shelves. Different shelves had been tested with differing results. But always, whether after five or 20 firings, the green glass accumulating on the top and bottom course of the shelves had caused the ultimate demise of the shelves.

By firing #59 NT8 was the standard ‘touch up’ material for all parts of the kiln requiring light attention. This was particularly so for the entire arch where the peeling back of the original NT9 was exposing parts of the roof to the possibility of attack. Prior to each firing, a five minute remedial ‘touch up’ by a two-inch brush attached by tape to a two-foot long stick enables all parts of the kiln to be easily accessible. For some serious damage to some bottom course ‘bubble alumina’ bricks close to the burners and the areas where the side wall meet the back walls and where a significant hole has occurred, Lupufest 901 cement was applied and overpainted withNT8.

Before firing #59, a major crack to the left hand wall had given cause for alarm. This was the result of inferior metal work rather than attack by the sodium vapours. The top of the kiln appeared to have moved slightly forward. (because diagonal stays were not introduced fromthe outset).

In October 2003, the 80th and last salt firing was undertaken. The final 21 firings had been undertakenwith little to comment upon. Each firing was, and always had been preceded by a visual inspection and a light ‘touch up’ with NT8. A visual analysis of the kiln and a photographic session after the final firing revealed: The difference in deterioration between the 25HDand Calor 26 bricks was slight; The differences between these two types of brick treated with NT7 and NT8 was also slight. This evidence can be assessed from those tests placed in the kiln before firing #38 and which received no ‘touch up’ before any of the successive 43 firings. The other tests of the same bricks were in place from the outset of Kiln #2 before the 80 salt firings. The bricks which have undertaken 80 firings were a little more debased but I am confident that the life expectancy of both types is good.

By Marshall Browne

Advancer forms a glass layer on the surface in an oxidizing atmosphere. The actual process is oxidation of the silicon carbide grain. We double fire Advancer in an oxidizing atmosphere to intentionally form a glass layer. The resulting glass surface is actually a protective layer (when intact) that prohibits further oxidation of the Advancer surface  beneath. Advancer forms more of a glass layer than CN192 because it has a  higher surface area of silicon carbide grain (i.e., fine grain sizing and virtually no porosity).

The combustion material (wood) is a likely source of alkalis (e.g., potassium) in the atmosphere of these kilns. The alkalis are likely fluxing the glass layer and dropping its viscosity so it drips more readily. When the glass viscosity drops, two things occur that compound the problem.

(1) Oxygen can penetrate the glass layer faster by diffusion thereby increasing the oxidation rate.
(2) Oxidation off gases produce bubbles in the glass which is the "frothing" or "foaming" observed. The bubbling is more pronounced in humid environments because a surface reaction with water can give off extra CO gas. The wood is also a likely source of water that can make a bad situation worse.

Once again, the main difference between the oxidation rate of Advancer and CN192 is surface area. CN192 will likely experience an increased oxidation rate when the surface oxide (glass layer) is infected by alkalis; however, the oxidation rate will be much slower than Advancer given the low surface area. Bubbling or "frothing" would also be less pronounced with CN192 given its low surface area. Generally speaking, Advancer has much better oxidation resistance (i.e., a protective bond) than CN192 but the ultimate life of Advancer in this environment will have to come from experience. The exact properties of the altered glass layer are difficult to predict and are greatly influenced by temperature and the type of alkalis that are created as a by-product of the combustion of wood.

TECHNICAL BULLETIN

CRYSTOLON® Kiln Furniture CN-192