Layering glazes is a great way to explore a wide variety of fired surface possibilities. Add an American shino glaze to the combination with an important caution, however. Here’s why.
Glaze Crawling: The term that describes a fired glaze that has pulled apart, leaving islands of thickened glaze and bare clay surface beneath. Crawling occurs while the glaze is molten because the surface tension of the liquid is stronger than the bond between the glaze and the clay it has been applied to.
Specific Gravity: A unitless measure of the density of a substance divided by the density of a reference fluid. For liquids and solids the reference fluid is almost always water.
Surface Tension: The attraction force between molecules at the surface of a liquid. This tensile force works against the attraction force between the molecules of the liquid and the surface the liquid is on.
Viscosity: A measure of the thickness or the resistance of a liquid to flow at a given rate.
“Shino first or suffer the curse” is a phrase many ceramic artists have heard and followed in the studio. It means that when layering glazes, one of which is an American shino, put the shino on first. Using it over almost any other glaze produces a disaster. I’ve learned this the hard way.
Traditional Japanese shino glazes are made by mixing water with crushed feldspar. Sometimes a small amount of clay is added, up to 10% at most. The glazed ware is typically fired for many hours to about cone 8–10.
American shino glazes also use lots of feldspar-type minerals, but they contain more clay. The clay fraction varies between 15–30% kaolin and/or ball clay, except where large amounts of bentonite (2–6%) are used for glaze suspension. There’s also typically soda ash in the recipe. The amount varies widely, ranging from as little as 1% to as much as 20%. These glazes can be and usually are fired in 4–8 hours to cone 8–10.
American shino glazes are characterized by two significant features: high clay content (>10%) and high surface tension. The relatively high clay content in these glazes produces dramatic glaze shrinking as the applied glaze dries. During firing, the soluble sodium in the recipe causes these glazes to begin to melt at a relatively low temperature, about 1564°F (851°C). That’s the melting point of soda ash. The drying shrinkage causes cracks in the surface of these shino glazes if they’re applied too thickly. Those cracks are often the starting point for glaze crawling, a usually undesirable firing fault. The relatively high clay and feldspar-mineral content of these shinos produces a melted glaze chemistry that’s high in alumina. Alumina makes glazes viscous. That counteracts the fluidity produced by the high flux content of the recipe from the soda ash combined with the feldspars, so these glazes are not runny. On the other hand, the glazes are high in surface tension. That contributes to glaze crawling, compounding issues that began with application and drying of the glazes.
Studio Take Home
There are risks when applying any glaze that shrinks a lot as it dries over another glaze. High drying shrinkage creates the possibility that the strong surface tension of the outside layer will cause both glazes to crawl in the firing. The result is usually unexpected and disappointing.
Applying a thinner coat of a glaze that shrinks a lot, as American shino glazes do, helps avoid drying cracks and later crawling (1). Another way to help reduce drying crack formation in such glazes is to add a binder to the recipe. Organic gums like CMC, a carboxymethylcellulose gum, aid glaze suspension, are good glaze binders that reduce cracking, and harden the glaze surface. Hardening the glaze allows for handling work without the glaze dusting during decoration or kiln loading. However, because CMC is an organic material, its useful properties weaken or disappear entirely over time and more may need to be added to the stored glaze in the future. CMC is not recommended as a glaze binder if the glaze will be stored for several months or longer.
Alternative glaze hardeners come from a family of extremely fine-grained, volcanic-ash derived clays called smectites—bentonites, Veegum, and Macaloid (Bentone MA) are examples. These are entirely mineral. Their useful properties will not weaken in storage.
While smectite-based hardeners are also glaze suspenders, they do not have the same binding effect on the dried glaze as CMC. Determining whether gums or mineral hardeners work best requires testing. Because their effects are very powerful, it is easy to add too much of any binder or hardener to a glaze recipe. Typical additions of gums or smectites may range from 0.1% (that’s ¹/10 of a percent) to less than 1%. American shino glaze recipes often call for a lot more of one or more smectites, which explains why, when layered, they must go on first!
the author Dave Finkelnburg is a studio potter and practicing engineer. He earned his master’s degree in ceramic engineering from Alfred University.