A glassy coating consisting mostly of silica sometimes with an addition of other materials, such as colorants and fluxes to lower the melting point. A glaze must be fired together with the ceramic body either at once or separately in a second firing.
While modern technology has produced new types glazes that falls into none of these cathegories, historically, there are four principal kinds of glazes:
Feldspathic, lead, and salt glazes are transparent while tin glaze is an opaque white, unique in that sense that it was used directly onto unfired earthenware to create Delft and Majolika type of wares. Of these, hard porcelain takes a feldspathic glaze.
Glazes seems to have been discovered in China along with the technological advances necessary for Bronze casting during the Shang Dynasty (16th - 11th century BC). The technique for making glazed stoneware has thus been known in China for about 3,500 years. The earliest Shang glazes found so far are hard, thin and greenish.
By the middle of the Warring States period (4th century BC) un-glazed vessels accounted for approximately half of all pottery made.