Science Lesson: How Pottery Glaze Works
Many pottery pieces have a glossy finish that looks like glass was melted to the outside of the piece. This is called a glaze and melted glass is a good description of the process. This glass seals the piece and makes it watertight and stain resistant. Depending on the glaze used, it can add shine, take away the shine, color a piece or lighten it. Glazes are a wonderful addition to the world of pottery and the world of science.
Every glaze made has a basic recipe with only 3 ingredients. These are the glass making, flux and refractory ingredients. Each one has a purpose in the glaze and affects the final product and finish of the pottery piece. Different amounts of each ingredient are mixed together with water to create a glass that can be brushed onto a piece of pottery and stay in place until it has been baked thoroughly in a kiln.
The first ingredient is the glass maker. This ingredient melts during the firing process and turns into glass. It seals the piece by melting into the surface of the clay. It is most commonly made with silica sand, a pure form of sand that is available all over the world. The next ingredient added is the flux. It acts as a type of glue to hold the other ingredients of the glaze in place during the firing process. It is commonly made of either sodium or calcium. The final part of the recipe is the refractory ingredient, generally made of alumina. This lowers the temperature at which the silica melts. Without this ingredient the temperature needed to make the glaze melt into place permanently would be too high for the pottery.
Each ingredient in this recipe plays its own part in making a glazing recipe a success. The silica sand functions as the glass making component, the flux made of sodium or calcium holds the silica in place and the alumina refractory ingredient lowers the melting temperature so the glass can form a barrier. These three ingredients are mixed in different amounts to make thick or thin glazes that melt at different temperatures.
Once a glaze has been applied, consider further decoration using ceramic decals which is fired at 1150-1220 degress Celcius.