A traditional alkaline glaze is made from limestone and or ashes (which contains alkaline salts including calcium), clay, and sand or crushed glass. Clay is an Alumina silicate, Al2O3 2SiO2 2H2O. The green color comes from iron in an oxygen starved, or reduction atmosphere during the firing process. I thought it might be interesting to look at an example of this in the context of a quote from Steiner from his Agricultural lectures:
“Lime is Generalized Outer desire. Silica is Generalized outer Perception. Clay mediates between them. Clay is closer to silica but mediates towards lime.”
In ceramics, Limestone is a flux causing silica, the glass former, to melt and flow. Clay contains silica and alumina. The alumina helps stabilize the glaze. What we are seeing in the glaze above is Limestone, silica and clay (with a small amount of iron) working together. It is the visual expression of what Steiner is saying.
Art and Anthroposophy 