GLAZE RECIPES: WHY SOMETIMES THEY DON'T WORK

Here is a common scenario. A potter sees a piece with a glaze they really like. They get the recipe, mix up a batch, and are disappointed with the results. Their glaze looks nothing like the one they originally saw. If this has happened to you, you know the frustration.

This happens for a few reasons, but most of all by what has influence over the glaze itself during firing.

A Glaze Is Affected By:

• Final temperature. Even if fired to the same Cone, the temperature may vary from one part of the kiln to another, or two people might interpret the bending of the Cones differently.
• Rate of change in temperature, particularly cooling rate. This is a main reason why glazes look different when fired on a whole piece than when fired in a test kiln, because the smaller test kilns usually cool faster.
• Clay bodies. Color as well as the materials in the clay body. For example, iron in a clay will often cause spotting through the glaze, or will act as a flux causing glazes to melt earlier).
• The thickness or thinness of the application or the method of application (sprayed, dipped, or brushed).
• Variations in materials (Glaze materials are taken from the earth, and their compositions are not pure. There is always some variation from batch to batch, and sometimes this variation is enough to affect the glaze in a substantial way).
• Atmosphere: The amount of oxygen present or not present (reduction).
• The presence of other glazes nearby.
• Particle size differences. If a material is available in 200 mesh and 325 mesh, these will melt differently and give different effects.
• Mixing and screening. How well the materials are mixed and to what mesh they are screened will effect the final result.
• Venting. Whether and how much a kiln is vented can affect the final firing.

As you can see, there are many factors, so it is not surprising that it is hard to replicate a glaze.

Commercial glazes have to be formulated to work over as wide of a range as possible, or they would not be marketable. Not that they are perfect. As you can see, there is such a variety of factors that can affect them that sometimes they will vary too. But they are usually chosen for their more stable properties.

In fact, that’s why some people think commercial glazes tend to be “boring." You are not likely to get a beautiful commercial multi-colored breaking glaze, because to achieve such a glaze requires a very specific range of conditions. In fact, breaking means that it changes colors or textures around the piece, such as where there is variation in thickness or surface texture.

Commercial low fire glazes are the most stable. Because of the way they have been formulated and the lower temperatures they are fired at, they usually are very consistent. They can still be affected by the above factors, but are less susceptible to minor changes. This is why people who work with low fire glazes are usually those who are looking for consistent, repeatable colors, rather than unexpected, breaking colors.

Generally, the higher the temperature, the more interaction there is between the clay and the glaze. So the clay body will play an increasingly important role as you go up in temperature. At Cone 6, there is some interaction. At Cone 10, there is a lot of interaction.

So there you have it. Happy firing!