Most potters don’t give much thought to kiln wash and just use the recipe they used when they first learned about firing kilns or grab whatever happens to be in the kiln wash bucket. In this post, John Britt explains that giving a bit more consideration to kiln wash might help potters avoid some of the common kiln wash headaches – like scraping cracked kiln wash off shelves or lamenting an otherwise perfect piece that was ruined by a flake of kiln wash.
So read on and learn about the many layers of kiln wash! – Jennifer Poellot Harnetty, editor.
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.
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 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 everywhere 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:
Basic Kiln Wash
|EPK Kaolin||50 %|
|Silica (Flint)||50 %|
Silicon dioxide has a melting point of 3100 degrees F (1710 degrees C) and aluminum oxide has a melting point of 3722 degrees F (2050 degrees C). Since potters fire to temperatures between 1100 degrees F (593 degrees C) and 2400 degrees F (1315 degrees 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 mid-range 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:
Basic Salt Kiln Wash
|Alumina Hydrate||50 %|
|EPK Kaolin||50 %|
Kaolin has a melting point of 3218 degrees F (1770 degrees C) and alumina oxide has a melting point of 3722 degrees F (2050 degrees 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 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.
Addressing Kiln Wash Problems
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 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.
No-Crack Kiln Wash
|Calcined EPK Kaolin||25%|
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:
Super-Awesome No-Crack Kiln Wash
|Calcined EPK Kaolin||25%|
|G-200 Feldspar||1 – 2%|
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 chips from flying around the kiln and getting onto pots.
As you can see, 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 degrees F (2700 degrees C) and zirconium silicate, which goes under various names like Zircopax, Ultrox, Superpax, milled zircon, zircon flour, etc., has a melting point of 4622 degrees F (2550 degrees 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 recipes to choose from and many solutions to common problems if we just take the time to learn about the materials we use.
To learn more about John Britt, visit www.johnbrittpottery.com.