Frit

A frit is a ceramic composition that has been fused in a special fusing oven, quenched to form a glass, and granulated. Frits form an important part of the batches used in compounding enamels and ceramic glazes; the purpose of this pre-fusion is to render any soluble and/or toxic components insoluble by causing them to combine with silica and other added oxides. However, not all glass that is fused and quenched in water is frit, as this method of cooling down very hot glass is also widely used in glass manufacture.

According to the OED, the origin of the word "frit" dates back to 1662 and is "a calcinated mixture of sand and fluxes ready to be melted in a crucible to make glass". Nowadays, the unheated raw materials of glass making are more commonly called "glass batch".

In antiquity, frit could be crushed to make pigments or shaped to create objects. It may also have served as an intermediate material in the manufacture of raw glass. The definition of frit tends to be variable and has proved a thorny issue for scholars. In recent centuries, frits have taken on a number of roles, such as biomaterials and additives to microwave dielectric ceramics. Frit in the form of alumino-silicate can be used in glaze-free continuous casting refractories.

Archaeologists have found evidence of frit in Egypt, Mesopotamia, Europe, and the Mediterranean. The definition of frit as a sintered, polycrystalline, unglazed material can be applied to these archaeological contexts. It is typically colored blue or green.

Blue frit, also known as Egyptian blue, was made from quartz, lime, a copper compound, and an alkali flux, all heated to a temperature between 850 and 1000°C.Quartz sand may have been used to contribute silica to the frit. The copper content must be greater than the lime content in order to create a blue frit. Ultimately the frit consists of cuprorivaite (CaCuSi₄O₁₀) crystals and “partially reacted quartz particles bonded together” by interstitial glass. Despite an argument to the contrary, scientists have found that, regardless of alkali content, the cuprorivaite crystals develop by “nucleation or growth within a liquid or glass phase.” However, alkali content—and the coarseness of the cuprorivaite crystals—contribute to the shade of blue in the frit. High alkali content will yield “a large proportion of glass,” thereby diluting the cuprorivaite crystals and producing lighter shades of blue.Regrinding and resintering the frit will create finer cuprorivaite crystals, also producing lighter shades.

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Wikipedia