Frit

A frit is a ceramic composition that has been fused, quenched, 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.You keep a good and smokeless fire burning until the ‘metal’ (molten glass) becomes fritted. You take it out and allow it to cool off. A frit is a ceramic composition that has been fused, quenched, 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. The earliest appearance of blue frit is as a pigment on a tomb painting at Saqqara dated to 2900 BC, though its use became more popular in Egypt around 2600 BC. Blue frit has also been uncovered in the royal tombs at Ur from the Early Dynastic III period. Its use in the Mediterranean dates to the Thera frescoes from the Late Middle Bronze Age. While the glass phase is present in blue frits from Egypt, scientists have not detected it in blue frits from the Near East, Europe, and the Aegean. Natural weathering, which is also responsible for the corrosion of glasses and glazes from these three regions, is the likely reason for this absence. . At Amarna, archaeologists have found blue frit in the form of circular cakes, powder residues, and vessel fragments. Analysis of the microstructures and crystal sizes of these frits has allowed Hatton, Shortland, and Tite to deduce the connection among the three materials. The cakes were produced by heating the raw materials for frit, then they were ground to make powders, and finally, the powders were molded and refired to create vessels. In On Architecture, the first century BC writer Vitruvius reports the production of ‘caeruleum’ (a blue pigment) at Pozzuoli, made by a method used in Alexandria, Egypt. Vitruvius lists the raw materials for caeruleum as sand, copper filings, and ‘nitrum’ (soda). Indeed, analysis of some frits that date to the time of Thutmose III and later show the use of bronze filings instead of copper ore.