Soluble Glaze Materials
May 21, 2009
Many potters do not fully realize the potential problems caused by the use of highly soluble glaze materials. The five most commonly used soluble materials are Gerstley borate, colemanite, borax, soda ash and pearl ash (potassium carbonate). While other materials (nepheline syenite, lithium carbonate, some frits and various sodium-based feldspars) can be partially soluble, they statistically do not produce glaze defects caused by the highly soluble materials.
Soluble glaze materials are hygroscopic, meaning they can take on moisture in storage. A potter buying a bag of Gerstley borate stored in a high moisture area might discover that it has taken on extra water, resulting in a higher material weight. If excess glaze water is poured off, the potter might find that some of the actual soluble material has been removed from the glaze formula.
When glazing a pot, soluble materials travel with the glaze water onto the bisque surface. As the glaze dries, soluble materials migrate or are wicked to the higher areas of the pot, resulting in a different glaze formula that contains a higher amount of flux. This type of defect can often be observed as glaze discoloration. Blistered glaze rough areas are frequently observed on the pottery edges and high points on the form.
Some glaze formulas require the use of soluble materials to achieve a specific color or surface variation. In such instances, a substitute material never quite fills the requirements. When the glaze requires a soluble material, it is important to to store the material in an air-tight container and only mix enough for one glazing session. While this last step is often impractical, always be aware that the glaze mixed on a Monday might not be the same glaze by Friday.
While the leaching of soluble materials into the glaze water might not occur at such a fast rate, there is no definitive line at which a glaze changes in wet storage. Once a glaze breaks down, the potter might be unaware of the change until the fired glaze defect is observed. However, potters often report thin crystals of condensed soluble material floating on the wet glaze surface. While the crystals can be removed by sieving the glaze, it does alter the glaze formula. Glaze material solubility can be increased or decreased, in part, by the percentage of soluble material in the glaze, the pH level of the glaze water and the temperature at which the wet glaze is stored.