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. [1] 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. [2] The definition of frit as a sintered, polycrystalline, unglazed material can be applied to these archaeological contexts. [3] [4] [5] 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. [6] Quartz sand may have been used to contribute silica to the frit. [7] The copper content must be greater than the lime content in order to create a blue frit. [8] Ultimately the frit consists of cuprorivaite (CaCuSi4O10) crystals and "partially reacted quartz particles bonded together" by interstitial glass. [9] 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". [10] However, alkali content—and the coarseness of the cuprorivaite crystals—contribute to the shade of blue in the frit. [11] High alkali content will yield "a large proportion of glass". thereby diluting the cuprorivaite crystals and producing lighter shades of blue. [11] Regrinding and resintering the frit will create finer cuprorivaite crystals, also producing lighter shades. [11]
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. [9] Blue frit has also been uncovered in the royal tombs at Ur from the Early Dynastic III period. [12] Its use in the Mediterranean dates to the Thera frescoes from the Late Middle Bronze Age. [13]
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. [14] Natural weathering, which is also responsible for the corrosion of glasses and glazes from these three regions, is the likely reason for this absence. [14] [15]
At Amarna, archaeologists have found blue frit in the form of circular cakes, powder residues, and vessel fragments. [16] 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. [17]
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. [18] Vitruvius lists the raw materials for caeruleum as sand, copper filings, and 'nitrum' (soda). [11] 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. [11]
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. [19] The powders owe their copper content to the erosion of the copper tubular drills used in the drilling process. [20] However, the archaeological record has not yet confirmed such a relationship between these two technologies.
Evidence of the use of green frit is so far confined to Egypt. [16] Alongside malachite, green frit was usually employed as a green pigment. [21] Its earliest occurrence is in tomb paintings of the 18th dynasty, but its use extends at least to the Roman period. [22] The manufacture of green and blue frit relies on the same raw materials, but in different proportions. [9] To produce green frit, the lime concentration must outweigh the copper concentration. [23] The firing temperature required for green frit may be slightly higher than that of blue frit, in the range of 950 to 1100 °C. [9] The ultimate product is composed of copper-wollastonite ([Ca,Cu]3Si3O9) crystals and a "glassy phase rich in copper, sodium, and potassium chlorides". [24] 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. [25] 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". [13] 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. [17]
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. [26] This intermediate step would have followed the grinding and mixing of the raw materials used to make glass. [27] An excerpt of Oppenheim's translation of Tablet A, Section 1 of the Nineveh text reads:
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. [28]
The steps that follow involve reheating, regrinding and finally gathering the powder in a pan. [28] Following the Nineveh recipe, Brill was able to produce a "high quality" glass. [29] 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. [30] Furthermore, grinding the frit actually expedites the "second part of the process, which is to ... reduce the system to a glass". [31]
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". [32] 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". [33] The process was known to ancient writers Pliny and Theophilus. [33]
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. [34]
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. [35] But analysis of a wide array of Egyptian frits contradicts the binder composition that Kühne offers. [36] 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." [37] 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. [38] 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. [34]
In general, frits, glasses and faience are similar materials: they are all silica-based but have different concentrations of alkali, copper and lime. [39] However, as Nicholson states, they are distinct materials because "it would not be possible to turn faience into frit or frit into glass simply by further, or higher temperature, heating". [40]
The use of frit as pigments and as entire objects does give credence to the idea that frit-making was, to some extent, a "specialized" industry. [41] Indeed, scientists have determined that frit objects, such as amulets, beads and vessels, have chemical compositions similar to those of powder frits designed for use as pigments. [42] Nevertheless, determining the exact technical relationships among the frit, glass and faience industries is an area of current and, likely, future scholarly interest. The excavations at Amarna offer a spatial confirmation of these potential relationships, as the frit, glass and faience industries there were located "in close proximity" to one another. [43]
Fritware refers to a type of pottery which was first developed in the Near East, where production is dated to the late first millennium AD through the second millennium AD. Frit was a significant ingredient. A recipe for "fritware" dating to c. 1300 AD written by Abu’l Qasim reports that the ratio of quartz to "frit-glass" to white clay is 10:1:1. [44] This type of pottery has also been referred to as "stonepaste" and "faience" among other names. [45] A ninth-century corpus of "proto-stonepaste" from Baghdad has "relict glass fragments" in its fabric. [46] The glass is alkali-lime-lead-silica and, when the paste was fired or cooled, wollastonite and diopside crystals formed within the glass fragments. [47] The lack of "inclusions of crushed pottery" suggests these fragments did not come from a glaze. [48] The reason for their addition would have been to release alkali into the matrix on firing, which would "accelerate vitrification at a relatively low firing temperature, and thus increase the hardness and density of the [ceramic] body". [48] Whether these "relict glass fragments" are actually "frit" in the more ancient sense remains to be seen.
Iznik pottery was produced in Ottoman Turkey as early as the 15th century AD. [49] It consists of a body, slip, and glaze, where the body and glaze are "quartz-frit". [50] The "frits" in both cases "are unusual in that they contain lead oxide as well as soda"; the lead oxide would help reduce the thermal expansion coefficient of the ceramic. [51] Microscopic analysis reveals that the material that has been labeled "frit" is "interstitial glass" which serves to connect the quartz particles. [52] Tite argues that this glass was added as frit and that the interstitial glass formed on firing. [53]
Frit was also a significant component in some early European porcelains. Famous manufacturers of the 18th century included Sèvres in France, and at Chelsea, Derby, Bow, Worcester and Longton Hall in England. [1] [54] Frit porcelain is produced at Belleek, County Fermanagh, Northern Ireland. This factory, established in 1857, produces ware that is characterised by its thinness, slightly iridescent surface and that the body is formulated with a significant proportion of frit. [1]
A small manufacturing cluster of fritware exists around Jaipur, Rajasthan in India, where it is known as 'Blue Pottery' due its most popular glaze. The technique may have arrived in India with the Mughals, [55] with production in Jaipur dating to at least as early as the 17th century. [56] [57]
Frits are indispensable constituents of most industrial ceramic glazes which mature at temperatures below 1150 °C. [58] Frits are typically intermediates in the production of raw glass, as opposed to pigments and shaped objects, [59] but they can be used as laboratory equipment in a number of high-tech contexts.
Frits made predominantly of silica, diboron trioxide (B2O3) and soda are used as enamels on steel pipes. [60] Another type of frit can be used as a biomaterial, which is a material made to become a part of, or to come into intimate contact with, one or more living organisms. Molten soda-lime-silica glass can be "poured into water to obtain a frit", which is then ground to a powder. [61] These powders can be used as "scaffolds for bone substitutions". [61] Also, certain frits can be added to high-tech ceramics: such frits are made by milling zinc oxide (ZnO) and boric acid (H3BO3) with zirconium (Zr) beads, then heating this mixture to 1100 °C, quenching it, and grinding it. [62] This frit is then added to a lithium titanate (Li2TiO3) ceramic powder, which enables the ceramic to sinter at a lower temperature while still keeping its “microwave dielectric properties." [62]
In laboratory and industrial chemical process equipment, the term frit denotes a filter made by the sintering-together of glass particles to produce a piece of known porosity referred to as fritted glass.
Automotive windshields incorporate a dark band of ceramic dots around the edges called a frit. [63]
In 2008, the Spanish ceramic frit, glaze and colours industry included 27 companies employing nearly 4,000 people with a combined annual turnover of approximately €1 billion. [64] In 2022, the global market for ceramic frits was estimated to be worth a total of US$ 1.67 billion. [65]
Pottery is the process and the products of forming vessels and other objects with clay and other raw materials, which are fired at high temperatures to give them a hard and durable form. The place where such wares are made by a potter is also called a pottery. The definition of pottery, used by the ASTM International, is "all fired ceramic wares that contain clay when formed, except technical, structural, and refractory products". End applications include tableware, decorative ware, sanitary ware, and in technology and industry such as electrical insulators and laboratory ware. In art history and archaeology, especially of ancient and prehistoric periods, pottery often means vessels only, and sculpted figurines of the same material are called terracottas.
A pigment is a powder used to add color or change visual appearance. Pigments are completely or nearly insoluble and chemically unreactive in water or another medium; in contrast, dyes are colored substances which are soluble or go into solution at some stage in their use. Dyes are often organic compounds whereas pigments are often inorganic. Pigments of prehistoric and historic value include ochre, charcoal, and lapis lazuli.
Faience or faïence is the general English language term for fine tin-glazed pottery. The invention of a white pottery glaze suitable for painted decoration, by the addition of an oxide of tin to the slip of a lead glaze, was a major advance in the history of pottery. The invention seems to have been made in Iran or the Middle East before the ninth century. A kiln capable of producing temperatures exceeding 1,000 °C (1,830 °F) was required to achieve this result, the result of millennia of refined pottery-making traditions. The term is now used for a wide variety of pottery from several parts of the world, including many types of European painted wares, often produced as cheaper versions of porcelain styles.
Lead glass, commonly called crystal, is a variety of glass in which lead replaces the calcium content of a typical potash glass. Lead glass contains typically 18–40% lead(II) oxide (PbO), while modern lead crystal, historically also known as flint glass due to the original silica source, contains a minimum of 24% PbO. Lead glass is often desirable for a variety of uses due to its clarity. In marketing terms it is often called crystal glass.
Creamware is a cream-coloured refined earthenware with a lead glaze over a pale body, known in France as faïence fine, in the Netherlands as Engels porselein, and in Italy as terraglia inglese. It was created about 1750 by the potters of Staffordshire, England, who refined the materials and techniques of salt-glazed earthenware towards a finer, thinner, whiter body with a brilliant glassy lead glaze, which proved so ideal for domestic ware that it supplanted white salt-glaze wares by about 1780. It was popular until the 1840s.
Ancient Egyptian art refers to art produced in ancient Egypt between the 6th millennium BC and the 4th century AD, spanning from Prehistoric Egypt until the Christianization of Roman Egypt. It includes paintings, sculptures, drawings on papyrus, faience, jewelry, ivories, architecture, and other art media. It was a conservative tradition whose style changed very little over time. Much of the surviving examples comes from tombs and monuments, giving insight into the ancient Egyptian afterlife beliefs.
Islamic pottery occupied a geographical position between Chinese ceramics, and the pottery of the Byzantine Empire and Europe. For most of the period, it made great aesthetic achievements and influence as well, influencing Byzantium and Europe. The use of drinking and eating vessels in gold and silver, the ideal in ancient Rome and Persia as well as medieval Christian societies, is prohibited by the Hadiths, with the result that pottery and glass were used for tableware by Muslim elites, as pottery also was in China but was much rarer in Europe and Byzantium. In the same way, Islamic restrictions greatly discouraged figurative wall painting, encouraging the architectural use of schemes of decorative and often geometrically patterned titles, which are the most distinctive and original speciality of Islamic ceramics.
Underglaze is a method of decorating pottery in which painted decoration is applied to the surface before it is covered with a transparent ceramic glaze and fired in a kiln. Because the glaze subsequently covers it, such decoration is completely durable, and it also allows the production of pottery with a surface that has a uniform sheen. Underglaze decoration uses pigments derived from oxides which fuse with the glaze when the piece is fired in a kiln. It is also a cheaper method, as only a single firing is needed, whereas overglaze decoration requires a second firing at a lower temperature.
Tin-glazing is the process of giving tin-glazed pottery items a ceramic glaze that is white, glossy and opaque, which is normally applied to red or buff earthenware. Tin-glaze is plain lead glaze with a small amount of tin oxide added. The opacity and whiteness of tin glaze encourage its frequent decoration. Historically this has mostly been done before the single firing, when the colours blend into the glaze, but since the 17th century also using overglaze enamels, with a light second firing, allowing a wider range of colours. Majolica, maiolica, delftware and faience are among the terms used for common types of tin-glazed pottery.
Han purple and Han blue are synthetic barium copper silicate pigments developed in China and used in ancient and imperial China from the Western Zhou period until the end of the Han dynasty.
Egyptian blue, also known as calcium copper silicate (CaCuSi4O10 or CaOCuO(SiO2)4 (calcium copper tetrasilicate)) or cuprorivaite, is a pigment that was used in ancient Egypt for thousands of years. It is considered to be the first synthetic pigment. It was known to the Romans by the name caeruleum. After the Roman era, Egyptian blue fell from use and, thereafter, the manner of its creation was forgotten. In modern times, scientists have been able to analyze its chemistry and reconstruct how to make it.
Egyptian faience is a sintered-quartz ceramic material from Ancient Egypt. The sintering process "covered [the material] with a true vitreous coating" as the quartz underwent vitrification, creating a bright lustre of various colours "usually in a transparent blue or green isotropic glass". Its name in the Ancient Egyptian language was tjehenet, and modern archeological terms for it include sintered quartz, glazed frit, and glazed composition. Tjehenet is distinct from the crystalline pigment Egyptian blue, for which it has sometimes incorrectly been used as a synonym.
Blue pottery is widely recognized as a traditional craft of Jaipur of Central Asian origin. The name 'blue pottery' comes from the eye-catching cobalt blue dye used to colour the pottery. It is one of many Eurasian types of blue and white pottery, and related in the shapes and decoration to Islamic pottery and, more distantly, Chinese pottery.
Ceramic glaze, or simply glaze, is a glassy coating on ceramics. It is used for decoration, to ensure the item is impermeable to liquids and to minimise the adherence of pollutants.
This is a list of pottery and ceramic terms.
Fritware, also known as stone-paste, is a type of pottery in which frit is added to clay to reduce its fusion temperature. The mixture may include quartz or other siliceous material. An organic compound such as gum or glue may be added for binding. The resulting mixture can be fired at a lower temperature than clay alone. A glaze is then applied on the surface.
Forest glass is late medieval glass produced in northwestern and central Europe from approximately 1000–1700 AD using wood ash and sand as the main raw materials and made in factories known as glasshouses in forest areas. It is characterized by a variety of greenish-yellow colors, the earlier products often being of crude design and poor quality, and was used mainly for everyday vessels and increasingly for ecclesiastical stained glass windows. Its composition and manufacture contrast sharply with Roman and pre-Roman glassmaking centered on the Mediterranean and contemporaneous Byzantine and Islamic glass making to the east.
The ways in which glass was exchanged throughout ancient times is intimately related to its production and is a stepping stone to learning about the economies and interactions of ancient societies. Because of its nature it can be shaped into a variety of forms and as such is found in different archaeological contexts, such as window panes, jewellery, or tableware. This is important because it can inform on how different industries of sections of societies related to each other – both within a cultural region or with foreign societies.
Andrew J. Shortland is an archaeologist at Cranfield University where he is director of the Cranfield Forensic Institute (CFI), a position he has held since 2016.
Blue pigments are natural or synthetic materials, usually made from minerals and insoluble with water, used to make the blue colors in painting and other arts. The raw material of the earliest blue pigment was lapis lazuli from mines in Afghanistan, that was refined into the pigment ultramarine. Since the late 18th and 19th century, blue pigments are largely synthetic, manufactured in laboratories and factories.
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