Adding a little blue stain to a medium temperature transparent glaze can give it a more pleasant tone. Some iron is present in all stoneware bodies (and even porcelains), so transparent glazes never fire pure white. At cone 10 reduction they generally exhibit a bluish color (left), whereas at cone 6 they tend toward straw yellow (right). Notice the glaze on the inside of the center mug, it has a 0.1% Mason 6336 blue stain addition; this transforms the appearance to look like a cone 10 glaze (actually, you might consider using a little less, perhaps 0.05%). Blue stain is a better choice than cobalt oxide, the latter will produce fired speckle.
Every glossy ceramic glaze is actually a base transparent with added opacifiers and colorants. So understand how to make a good transparent, then build other glazes on it.
Medium Temperature Glaze
These are stoneware glazes that fire in the range of 1200C (2200F). They often contain boron to assist with melting.
|CoO - Cobalt Oxide