History

The beginning of man's application of glaze on his terra cotta vessels are unclear, but evidence shows that this technique was known since times immemorial. The oldest example of glazing found, to the present, belonged to 15th century BC and concerns an Egyptian vessel, whose surface is decorated with glass powder.

From Achaemenian Monarch, glazed brick with alkali glaze have been found in Apadana, Susa. Examples of these are preserved in Louver, Paris. Two types of bricks were unearthed in Susa: Glazed Silicon bricks with relief patterns and flat bricks bearing monochrome patterns. These bricks are made of sand and lime, whether relief-patterned or flat, were formed in molds and fired thrice.

The first firing, nowadays called biscuit concerned the formation of the body. Then glaze was applied and a second firing took place. Finally a clear transparent silicon glaze, colored with various oxide dyes (lead-antimony for yellow, copper for green-blue, Ferro manganese for black and brown and tin for matte glaze and white color) was applied before the third firing. Examples of fired bricks bearing this type, are found in Persepolis, Fars.

Preparation of Glass-Earthenware involves a different paste. Clay, from which ceramic wares are made, is aluminum silicate with water. Kaolin is the purest kind of clay. Other elements such as sodium, potassium, calcium and magnesium are considered impurities, when present in it. GLASSWARE is made of glass paste or kaolin. This glaze ideally appears on smooth, glass-like surfaces.

Production of Glass-Earthenware became possible as soon as the technique of glazing hard ceramic wares containing kaolin was mastered. Thus, glazed vessels manufactured in 12th century AD, were perfectly suited to this type of decoration, as attested to by superb relics from Seljuk period. Also in Kashan, probably in about 1090 AD, quartz powder was mixed with kaolin to create limpid-boiled wares with sufficiently hard surfaces to allow them to be engraved.

The method currently used in production of Glass-Earthenware in Kashan, probably originated in experiments carried out by Neishabour and Samarkand potters, who tried to apply additive patterns to already fixed patterned or glazed items in a second firing, so as to obtain white patterns. A collection of bowls dated from 1160 to 1170 AD, represents the top of evolution of this method.

As these operations are very complicated and require several consecutive firings at approximately 750 degrees Celsius, this method gradually declined.

Although a wide range of colors were obtained by adding various minerals and oxides to the glaze, the physical incompatibilities between dyes and glaze were quite problematic. High temperatures needed to melt the glaze either marred the colors or affected their durability. Near the end of 12th century AD, Iranian potters clearly realized that they have only two options left, either to limit their palette to dyes that could be applied in a single firing, or to use dyes that required lower temperature in the second.