The Many Layers of Kiln Wash John Britt

Instructions

Some people might think that kiln wash is the place where you take your car kiln to get it cleaned. Well, that may be a good idea for a lot of kilns I have seen, but kiln wash is really a necessary and valuable tool for potters. It protects kiln shelves from glaze runs, drips and other accidents that occur in red hot kilns, like pots that tip over, bloating or melting clay bodies, etc. It is also used to protect shelves from volatiles in atmospheric kilns like wood ash or sodium oxide in salt and soda kilns.

Most potters don’t give it a second thought and grab any recipe or just use anything that is in the bucket labeled “kiln wash.” However, in order to make a good kiln wash you need to select materials that have very high melting points and that, when combined, do not create eutectics that cause melting. Knowing a bit about the properties of materials and the principles of kiln wash allows you to choose the ingredients that make the best kiln wash for your specific situation and avoid costly problems.

Kiln wash is used in the full range of ceramics firing from cone 022 to cone 14 and every where in between. The type of kiln wash needed varies for each specific situation because some potters work in electric kilns at low-fire temperatures, while others work with fuel-fired kilns at very high temperatures.

Understanding the structure of a glaze is helpful when selecting or creating kiln wash recipes so you can understand how not to create a glaze on your kiln shelf. Very simply, a glaze is composed of a glass-former (silica), a flux (sodium, potassium, lithium, calcium, barium, magnesium, zinc, boron or lead oxide) and a refractory (alumina, usually sourced from clay/kaolin). Historically, what potters did was to leave out the flux in their glaze recipe to make their kiln wash. That meant that only silica and alumina (kaolin/clay) were used as the kiln wash.

One of the first kiln wash recipes I used was:

This means you use 50 grams of silica and 50 grams of kaolin. In everyday practice, potters rushing to load a kiln, often just use a scoop of kaolin and a scoop of silica. This is not technically accurate because silica weighs more than kaolin, but it is close enough to work. Silicon dioxide has a melting point of 3100°F (1710°C) and aluminum oxide has a melting point of 3722°F (2050°C). Since potters fire to temperatures between 1100°F (593°C) and 2400°F (1315°C) a mixture of these two materials will not melt, will not form a eutectic, and will protect the kiln shelves. (The source of alumina in kiln wash is often kaolin, but it can also be alumina hydrate or alumina oxide. The source of silicon dioxide is usually 200 mesh silica.) This is a good kiln wash for low and midrange electric firings. The only problem is that it contains silica, which is a glass-former. So, if a lot of glaze drips onto the shelf, it can melt the silica in the kiln wash and form a glaze on the shelf. Also, when you scrape your shelves to clean them, you create a lot of silica dust, which is a known carcinogen. So using silica in your kiln wash is not always the best choice. Another drawback of this recipe is that, if it is used in salt or soda firings, it will most certainly create a glaze on the shelf. This is because silica, as noted above, is a glass-former. When sodium oxide, which is a strong flux, is introduced atmospherically, it can easily melt the silica in the kiln wash into a glass. This is why silica should not be used in a kiln wash recipe for wood, salt or soda kilns.

For these types of firings, this kiln wash is better:

Kaolin has a melting point of 3218°F (1770°C) and alumina oxide has a melting point of 3722°F (2050°C), so it will not melt, even in a cone 10–13 firing. These ingredients are called refractory because they are resistant to high temperatures. The refractory industry, which includes bricks, kiln shelves, posts, etc., relies heavily on these materials. This kiln wash recipe can be used at all temperatures and in all kiln atmospheres. It can also be used as a wadding recipe to set the pieces on in wood, salt and soda kilns. Just mix it up thicker than the kiln wash—like bread dough—and roll it into wads. The recipe can also be adjusted to 60% kaolin and 40% alumina hydrate, which produces similar results but costs less. Since alumina hydrate costs about $1.44 a pound and EPK costs $0.32 a pound, tilting the recipe toward EPK quickly reduces the price. Another high-temperature wadding recipe that is cheaper is:

This wadding is easily removed from the bottoms of pots because the sawdust burns out and the wad becomes very fragile and smashes easily with pliers or small hammer. Once you understand the principle of kiln wash you can easily substitute other refractories like zirconium oxide (or Zirconia, ZrO2), zircon (ZrSiO4 a.k.a zirconium silicate, zircon flour, or Zircopax), kyanite, sand, fireclay, ball clay or calcined clay to make variations in your wash recipe if you have problems.

For example, a common problem with kaolin-based kiln washes is that they crack off the shelf. The reason for this is that clay has the physical property of shrinkage. When you put it on the shelf, it looks really uniform and smooth, but then as it dries it cracks like Texas soil in the summer sun. After several firings, you typically just scrape off the glaze drips and the pieces that have chipped up, apply more kiln wash to hide that firing’s issues; and then that new layer cracks and the crevasses just keep getting worse. This can cause your pots to crack when they get hung up on the uneven wash during periods of expansion/contraction. Or, when using porcelain, the foot can even become warped and uneven as it fluxes and conforms to the uneven surface of the shelf. Another more insidious problem with cracked kiln wash is that the turbulence created by the burners blows some of the kiln wash chips up into the air and they inevitably land in your favorite bowl, ruining it.

The best way to avoid this is to calcine the kaolin or buy calcined kaolin called Glomax. You can calcine kaolin by putting some in a bisque bowl and firing it to red heat (or just put it in with your bisque firing.) Calcining will eliminate the physical property of shrinkage but leave the chemical refractory properties of kaolin intact. What you have made is very fine ceramic grog. So you can adjust your kiln wash recipes by substituting half the kaolin with calcined kaolin or Glomax.

You can add more calcined kaolin—like 35%—if you want. You just want to keep enough kaolin in the recipe to suspend the other materials so that it goes on smoothly and doesn’t settle out.

I discovered a small refinement of this recipe after visiting the Homer Laughlin China Company in Newell, West Virginia. There, the Chief Ceramic Engineer told me that, because they have high air turbulance in their kilns, he adds approximately 1% feldspar to help “stick” the kiln wash together. They determined how much feldspar to add by trying to rub it off with their finger after the firing. If it rubbed off, then there wasn’t enough flux. More flux was then added until it took a fingernail to scrape it off. If it took a key or screwdriver blade to scrape it off, there was too much flux. So, the recipe then becomes:

Although it seems crazy to add flux to a kiln wash, this very small amount actually is just enough for the kiln wash to stick it lightly to itself and to the shelf, preventing the kiln wash chips from flying around the kiln and getting onto pots.

As you can see in the recipes, there are many kiln wash variations. However, it is essential to know the melting properties of ingredients to make sure that they don’t melt on your shelf. For example, zirconium oxide is a refractory and melts at 4892°F (2700°C) and zirconium silicate, which goes under various names like Zircopax, Ultrox, Superpax, milled zircon, zircon flour, etc., has a melting point of 4622°F (2550°C). So these can make excellent additions to a kiln wash recipe. The only drawback is that zirconium silicates can cost from $1.33 to $3.00 a pound, depending on the amount you buy.

To illustrate the wide variety, some potters just dust alumina hydrate on their shelves to protect them, while some wood firing potters use 100% silica and wall paper paste to make a very thick (1/2-inch) coating that protects their shelves from excessive ash deposits. Still others, who have the new advanced nitride-bonded silicon carbide shelves, don’t even use kiln wash at all because the glaze drips shiver off when the shelves cool. Other potters, who are very neat and don’t share their space with others, may not even use kiln wash so that they can flip the shelves after every firing to prevent warping.

Kiln wash is such a ubiquitous material in the ceramics studio that we take it for granted. Potter’s make a significant investment in their kiln shelves but rarely take more than a few minutes to mix up two scoops of kaolin and alumina to protect them. They also spend countless hours making and perfecting their work only to suffer unnecessary breakage and loss of pots because they just don’t know that a kiln wash doesn’t have to crack or fly off into the bottoms of pots. There are many kiln wash recipes to choose from and many solutions to common kiln wash problems if we just take the time to learn about the materials we use.

Kiln Wash Application

Most potters apply kiln wash with some kind of brush. If you are coating the whole shelf, use a 4- or 5-inch house-painting brush, but if you are touching up bare spots after scraping off glaze drips, use a small 1–inch glaze brush and just dab it on in the spots that need it. If you use a brush, work very fast because the shelf will suck up the wash as soon as the brush touches it, making areas of uneven thickness.

Mix up the wash about as thick as heavy cream and paint on several thick layers to protect your shelves, allowing each to stiffen before applying the next coat. Then clean the edges with a wet sponge. Some potters leave a bare 1/2–inch or 1/4–inch band at the edge of the shelf so that chips don’t fall onto the shelf below.

If you have a lot of shelves to kiln wash all at once, one of the best and fastest ways is to use a spray gun. Lay out all of your shelves in a row and coat them all very quickly and evenly. Depending on your spray gun, you may need to adjust the nozzle spray pattern and the thickness of the wash to get it to spray properly, but once you get that figured out you will be very happy with the consistency of the results. Any overspray on the sides of the shelves can be wiped off with a damp sponge.

If you don’t have a spray gun, another excellent method of coating the whole shelf is to use a paint roller with a short nap length. Just fill the rolling pan with kiln wash and roll on the wash for a smooth, even coat. Allow it to get tacky to the touch and then apply another one or two coats, depending on the thickness desired.

At my studio, I have a lot of students working and testing glazes, so the shelves get really beat up and have a lot of glaze drips. Once or twice a year I grind my shelves clean and re-apply the wash. Since I don’t have a spray gun, I prefer to use a roller because it gives a smooth even coat very quickly.

This article was written by John Britt and excerpted from the March 2009 issue of Ceramics Monthly.