Oil Spot Tea Bowl, Jin Dynasty (Sung 12th-13th Century). Seikado Foundation Tokyo. D: 12 cm

Crystalline glazes appeared already during the Chinese Sung period of the 12th century (Japanese and Korean oilspot and kaki glazes).

In Europe, the history of the crystal glazes begins at the 19th century in Sèvres France, followed by the Royal Porcelain Manufacture of Copenhagen and the famous Meissen Manufacture in Germany. 

 In the 20th century the german potters Richard Bampi and Hubert Griemert and  in Switzerland Arnold Zahner experimented with crystal glazes. In all generations this type of glazes all have  two things in common: The hazard and a high estethic attraction.

Crystalline glazes are of two types: one has small single crystals suspended in the glaze, and the second has large crystal clusters in or on the surface. Both types of crystals tend to catch and reflect the light.

 In practice their composition has to be carefully related to the pot form, for the jewel like clusters seem to float from the surface of the more radical forms. Higher additions of the compounds zinc, rutile, and iron oxide tend to favor the development of crystals in the glaze. 

The presence of these compounds makes the glaze more opaque than usual and the absence of aluminum tends to make them quite fluid and brilliant in surface effect.

Detail of a laboratory sample (System Me2O-SrO-TiO2-SiO2 + Co)

Close-Up of vase in Gallery 1

The most critical part of producing crystalline glazes is in the firing cycle. The heating phase is carried out in the normal fashion, but the cooling must be slowed to about 100°F./38°C. above the maturing point, then held there several hours before slowly cooling in the last stages of firing. 

A pyrometer is indispensible for the proper firing and cooling of the crystal­line glaze, for without an instrument to indicate the rise and fall of the temperature inside the kiln the process cannot be properly controlled.  

Due to the fluidity of this glaze type, the pottery is usually fired sitting on a pedestal of insulating brick or an a high stilt within a shallow dish. After firing, the brick or stilt can be easily ground from the base of the pot.