Crystal glazes are a relatively new phenomenon. There are not many examples before the early 20th century. The early examples were discovered by accident without an understanding of what happened. The ceramic industry in Europe and in America started experimenting with the glaze in the late 1800’s but decided that the glaze was not practical economically because of several factors which were difficult and time consuming. It was not until the 1980’s, when electronics started making fully programmable, automatic kilns possible, that crystal-glazed ware started to become more common. Even today, however, most potters do not want to deal with the difficulties involved in producing these glazes.
Crystal glazes work best on porcelain clay and require an intricate long cooling schedule. They run off the pot and need special containers to collect the running glaze so that it does not ruin the kiln shelves. It is impossible to repeat something again. Each piece is unique. I fire my crystal glazed ware to approximately 2300 degrees F. and then hold the temperature in the kiln on cooling between 2000 F. and 1830 F. for approximately 3 to 5 hours depending on the glaze. During this time, the crystals grow larger and larger.
Each glaze composition, together with the firing schedule and glaze thickness, makes different forms and colors of crystals. I use cobalt oxide, nickel oxide, iron oxide, copper oxide, and manganese oxide for different colored crystals.
All crystal-glazed ware can be washed by hand with any dish detergent. They should not be cleaned in a dishwashing machine.
Dark Star Bulb by Bill Boyd