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.Dictionary of Ceramics (3rd Edition) Edited by Dodd, A. Murfin, D. Institute of Materials. 1994. 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 Oxford English Dictionary, 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 question 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.
Ancient frit Archaeologists have found evidence of frit in Egypt, Mesopotamia, Europe, and the Mediterranean.T. Pradell et al. 2006, "Physical Processes Involved in Production of the Ancient Pigment, Egyptian Blue", Journal of the American Ceramic Society 89.4: 1431. The definition of frit as a sintered, polycrystalline, unglazed material can be applied to these archaeological contexts.L. Lee and S. Quirke 2000, "Painting Materials", In: P. T. Nicholson and I. Shaw (eds.), Ancient Egyptian Materials and Technology, Cambridge: Cambridge University Press, 109;Moorey, P. R. S., 1985, Materials and Manufacture in Ancient Mesopotamia, BAR International Series 237, Oxford: Bar Publications, 134–135;Tite, M. S., et al., 1998, "The Use of Copper and Cobalt Colorants in Vitreous Materials in Ancient Egypt", In: S. Colinart and M. Menu (eds.), La Couleur Dans La Peinture et L'Émaillage de L'Égypte Ancienne, Bari: Edipuglia, 112–113. It is typically colored blue or green.
Blue frit
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.G. D. Hatton, A. J. Shortland, and M. S. Tite 2008, "The Production Technology of Egyptian Blue and Green Frits from Second Millennium BC Egypt and Mesopotamia", Journal of Archaeological Science 35: 1591. Quartz sand may have been used to contribute silica to the frit.Tite et al. 1998, 114. The copper content must be greater than the lime content in order to create a blue frit.Tite et al. 1998, 112–113. Ultimately the frit consists of cuprorivaite (CaCuSi4O10) crystals and "partially reacted quartz particles bonded together" by interstitial glass.Hatton, Shortland, and Tite 2008, 1591. 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".Pradell et al. 2006, 1430–1431. However, alkali content—and the coarseness of the cuprorivaite crystals—contribute to the shade of blue in the frit.Lee and Quirke 2000, 109. 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.Hatton, Shortland, and Tite 2008, 1591; Lee and Quirke 2000, 110. Its use in the Mediterranean dates to the Thera frescoes from the Late Middle Bronze Age.Lee and Quirke 2000, 110.
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.Pradell et al. 2006, 1431. 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.Hatton, Shortland, and Tite 2008, 1592. 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.Hatton, Shortland, and Tite 2008, 1596.
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.Hatton, Shortland, and Tite 2008, 1592; Lee and Quirke 2000, 109. Vitruvius lists the raw materials for caeruleum as sand, copper filings, and 'nitrum' (soda). In fact, analysis of some frits that date to the time of Thutmose III and later show the use of bronze filings instead of copper ore.
Stocks suggests that waste powders from the drilling of limestone, combined with a minor concentration of alkali, may have been used to produce blue frits.D. A. Stocks 1997, "Derivation of Ancient Egyptian Faience Core and Glaze Materials", Antiquity 71: 181. The powders owe their copper content to the erosion of the copper tubular drills used in the drilling process.Stocks 1997, 180–181. However, the archaeological record has not yet confirmed such a relationship between these two technologies.
Green frit
Evidence of the use of green frit is so far confined to Egypt. Alongside malachite, green frit was usually employed as a green pigment.A. Lucas and J. R. Harris 1962, Ancient Egyptian Materials and Industries, 4th ed., London: Edward Arnold Publishers Ltd., 344–345. Its earliest occurrence is in tomb paintings of the 18th dynasty, but its use extends at least to the Roman period.P. Bianchetti et al. 2000, "Production and Characterization of Egyptian Blue and Egyptian Green Frit", Journal of Cultural Heritage 1: 179; Hatton, Shortland, and Tite 2008, 1592. The manufacture of green and blue frit relies on the same raw materials, but in different proportions. To produce green frit, the lime concentration must outweigh the copper concentration.Hatton, Shortland, and Tite 2008, 1591; Lee and Quirke 2000, 112; Tite et al. 1998, 113. The firing temperature required for green frit may be slightly higher than that of blue frit, in the range of 950 to 1100 °C. The ultimate product is composed of copper-wollastonite ([Ca,Cu]3Si3O9) crystals and a "glassy phase rich in copper, sodium, and potassium chlorides".Lee and Quirke 2000, 112. In certain circumstances (the use of a two-step heating process, the presence of hematite), scientists were able to make a cuprorivaite-based blue frit that later became a copper-wollastonite-based green frit at a temperature of 1050 °C.Bianchetti et al. 2000, 181-183. On some ancient Egyptian wall paintings, pigments that were originally blue are now green: the blue frit can "devitrify" so that the "copper wollastonite predominates over the lesser component of cuprorivaite". As with blue frit, Hatton, Shortland, and Tite have analyzed evidence for green frit at Amarna in the form of cakes, powders, and one vessel fragment and inferred the sequential production of the three types of artifacts.
Relationships with glass and faience An Akkadian text from Assurbanipal's library at Nineveh suggests that a frit-like substance was an intermediate material in the production of raw glass.A. L. Oppenheim et al. (eds.) 1970, Glass and Glassmaking in Ancient Mesopotamia, Corning: The Corning Museum of Glass, 22–23, 118. This intermediate step would have followed the grinding and mixing of the raw materials used to make glass.P. T. Nicholson and J. Henderson 2000, "Glass", In: P. T. Nicholson and I. Shaw (eds.), Ancient Egyptian Materials and Technology, Cambridge: Cambridge University Press, 199. An excerpt of Oppenheim's translation of Tablet A, Section 1 of the Nineveh text reads:
The steps that follow involve reheating, regrinding and finally gathering the powder in a pan. Following the Nineveh recipe, Brill was able to produce a "high quality" glass.R.H. Brill 1970, "The Chemical Interpretation of the Texts", In: A. L. Oppenheim et al. (eds.), Glass and Glassmaking in Ancient Mesopotamia, Corning: The Corning Museum of Glass, 113. He deduced that the frit intermediate is necessary so that gases will evolve during this stage and the end product will be virtually free of bubbles.Brill 1970, 112–113. Furthermore, grinding the frit actually expedites the "second part of the process, which is to ... reduce the system to a glass".Brill 1970, 118.
Moorey has defined this intermediate step as "fritting", "a process in which the soluble salts are made insoluble by breaking down the carbonates, etc. and forming a complex mass of sintered silicates".Moorey 1985, 135. A frit preserved in a "fritting pan fragment" kept in the Petrie Museum "shows numerous white flecks of unreacted silica and a large number of vesicles where gases had formed".Nicholson and Henderson 2000, 199. The process was known to ancient writers Pliny and Theophilus.
But whether this "fritting" was done in antiquity as a deliberate step in the manufacture of raw glass remains questionable. The compositions of frits and glasses recovered from Amarna do not agree in a way that would imply frits were the immediate precursors of glasses: the frits have lower concentrations of soda and lime and higher concentrations of cobalt and alumina than the glasses have.Tite et al. 1998, 116.
Scholars have suggested several potential connections between frit and faience. Kühne proposes that frit may have acted as the "binding agent for faience" and suggests that this binder was composed predominantly of silica, alkali and copper with minor concentrations of alkali earths and tin.A. Kaczmarczyk and C. Lahanier 1985, "Ancient Egyptian Frits and Colored Faience Bodies: Problems of Classification", In: P. A. England and L. Van Zelst (eds.), Application of Science in Examination of Works of Art, Boston: Museum of Fine Arts, 97. But analysis of a wide array of Egyptian frits contradicts the binder composition that Kühne offers.Kaczmarczyk and Lahanier 1985, 97. Vandiver and Kingery argue that one method of producing a faience glaze was to "frit or melt the glaze constituents to form a glass", then grind the glass and form a slurry in water, and finally apply the glaze "by dipping or painting."P. B. Vandiver and W. D. Kingery 1992, "Manufacture of an Eighteenth Dynasty Egyptian Faience Chalice", In: M. Bimson and I.C. Freestone (eds.), British Museum Occasional Paper 56: Early Vitreous Materials, London: British Museum, 81. However, their use of "frit" as virtually synonymous with "melt" represents yet another unique take on what a "frit" would constitute. Finally, Tite et al. report that frits, unusually colored blue by cobalt, found in "fritting pans" at Amarna have compositions and microstructures similar to that of vitreous faience, a higher-temperature form of Egyptian faience that incorporated cobalt into its body.Tite et al. 1998, 116, 118. In their reconstruction of the manufacture of vitreous faience, Tite et al. propose that the initial firing of raw materials at 1100–1200 °C produces a cobalt-blue frit, which is then ground, molded, and glazed.