What are glazes?
Basically, glazes are ground up rocks containing various minerals plus clay and water. Glazes melt when fired to maturity and form a coating of glass on the surface of the pot. Glazes contain lots of silica, which is the main glass former. As the glazes slowly melt, they bubble and boil like thick syrup. As they get hotter and hotter, the glazes become more and more fluid, first to the consistency of honey and then beyond. We turn off the kiln when the glazes are mature (fully melted). If I were to accidentally over-fire the kiln, the glaze would get too fluid and run off your pots. Eventually even the pots would melt into a puddle.
Glazes are similar to clays in their chemical composition. Both contain mostly alumina and silica. But glazes have more fluxes. A flux is something that causes things to melt at a lower temperature. The fluxes in glazes are elements like calcium, sodium, and potassium and are sourced from materials like lime, feldspar and talc. They flux the alumina and silica in the glazes and cause the glaze to melt into a glass and fuse to the pot’s surface. Colors come form metallic oxides, like red iron oxide, copper carbonate, cobalt carbonate, rutile and titanium dioxide, which we add to certain glaze. The hot, melted glaze fuses to the clay. Chemical reactions take place. The heat, atmosphere in the kiln, metallic oxides and other glaze materials all affect the color and look of the fired glaze. We fire our pots in a reducing atmosphere.
Why are some glazes runny?
When fired to maturity, some glazes are inherently more fluid (runny) than others. We use many glazes. Some are runny, some are not, some are in-between.
When application of the glaze is thick, glazes are more likely to run. When glazes are applied unevenly, resulting in thick and thin spots, the thick spots will be more likely to run. When glazes are overlapped, they are more likely to run.
Some glaze combinations become extremely fluid where they overlap, even though those same glazes, when used by themselves, may not be runny. This is known as a eutectic. Example: the overlap/combination of our Chun and Titanium glazes form a eutectic and become very runny.