Frit
Encyclopedia
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 widely used in glass manufacture.
The origin of the word "frit" dates back to 1662 according to the OED as " a calcinated mixture of sand and fluxes ready to be melted in a crucible to make glass". Nowadays this is more commonly called "glass batch", the unheated raw materials.
In antiquity, frit could be crushed to make pigment
s 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 biomaterial
s and additives to microwave dielectric
ceramics. Frit in the form of alumino-silicate can be used in glaze free continuous casting refractories.
, 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.
, 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 and calcium 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 curprorivaite (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
fresco
es 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
.
Stocks suggests that waste powders from the drill
ing of limestone
, combined with a minor concentration of alkali, may have been used to produce blue frits. The powders owe their copper content to the erosion of the copper tubular drills used in the drilling process. However, the archaeological record has not yet confirmed such a relationship between these two technologies.
, green frit was usually employed as a green pigment. Its earliest occurrence is in tomb paintings of the 18th dynasty
, but its use extends at least to the Roman period
. 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. 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.” 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. 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.
suggests that a frit-like substance was an intermediate material in the production of raw glass. This intermediate step would have followed the grinding and mixing of the raw materials used to make glass. 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. He deduced that the frit intermediate is necessary so that gas
es will evolve during this stage and the end product will be virtually free of bubbles
. Furthermore, grinding the frit actually expedites the “second part of the process, which is to…reduce the system to a glass.”
Moorey has defined this intermediate step as “fritting,” “a process in which the soluble
salts are made insoluble by breaking down the carbonate
s, etc. and forming a complex mass of sintered silicates.” 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.” The process was known to ancient writers such as 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.
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
. But analysis of a wide array of Egyptian frits contradicts the binder composition that Kühne offers. 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.” 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. 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.
In general, frits, glasses, and faience are similar materials: they are all silica-based but have different concentrations of alkali, copper, and lime. 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.”
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. Indeed, scientists have determined that frit objects, such as amulet
s, bead
s, and vessels, have chemical compositions similar to those of powder frits designed for use as pigments. 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.
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. This type of pottery has also been referred to as “stonepaste” and “faience” among other names. A ninth century corpus of “proto-stonepaste” from Baghdad
has “relict glass fragments” in its fabric. The glass is alkali-lime-lead-silica and, when the paste was fired or cooled, wollastonite and diopside
crystals formed within the glass fragments. The lack of “inclusions of crushed pottery
” suggests these fragments did not come from a glaze. 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.” 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. It consists of a body, slip
, and glaze, where the body and glaze are “quartz-frit.” 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. Microscopic analysis reveals that the material that has been labeled “frit” is “interstitial glass” which serves to connect the quartz particles. Tite argues that this glass was added as frit and that the interstitial glass formed on firing.
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. At least one frit porcelain remains in production 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.
, and soda are used as enamels on steel
pipes
. Another type of frit can be used as a biomaterial. Molten soda-lime-silica glass can be “poured into water to obtain a frit,” which is then ground to a powder. These powders can be used as “scaffolds for bone
substitutions.” Also, frits can be added to high-tech ceramics. Scientists have made such frits by milling
ZnO and H3BO3 with zirconium
beads, then heating this mixture to 1100°C, quench
ing it, and grinding it. This frit is then added to a Li2TiO3 ceramic powder. This addition is beneficial: the ceramic can sinter at a lower temperature while still keeping its “microwave dielectric properties." In laboratory and industrial chemical process, the term is used to denote a filter made by the sintering-together of glass particles to produce a piece of known porosity; the term is also used in a more loose sense to denote a filter of any composition (for example, stainless steel, polyethylene) substituted where a frit may have been otherwise used.
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
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
Vitreous enamel
Vitreous enamel, also porcelain enamel in U.S. English, is a material made by fusing powdered glass to a substrate by firing, usually between 750 and 850 °C...
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 widely used in glass manufacture.
The origin of the word "frit" dates back to 1662 according to the OED as " a calcinated mixture of sand and fluxes ready to be melted in a crucible to make glass". Nowadays this is more commonly called "glass batch", the unheated raw materials.
In antiquity, frit could be crushed to make pigment
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...
s or shaped to create objects. It may also have served as an intermediate material in the manufacture of raw glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
. 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 biomaterial
Biomaterial
A biomaterial is any matter, surface, or construct that interacts with biological systems. The development of biomaterials, as a science, is about fifty years old. The study of biomaterials is called biomaterials science. It has experienced steady and strong growth over its history, with many...
s and additives to microwave dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing 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 EgyptEgypt
Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world...
, Mesopotamia
Mesopotamia
Mesopotamia is a toponym for the area of the Tigris–Euphrates river system, largely corresponding to modern-day Iraq, northeastern Syria, southeastern Turkey and southwestern Iran.Widely considered to be the cradle of civilization, Bronze Age Mesopotamia included Sumer and the...
, Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
, and the Mediterranean. The definition of frit as a sintered
Sintering
Sintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...
, polycrystalline, unglazed material can be applied to these archaeological contexts. It is typically colored blue or green.
Blue frit
Blue frit, also known as Egyptian blueEgyptian Blue
Egyptian blue is chemically known as calcium copper silicate . It is a pigment used by Egyptians for thousands of years. It is considered to be the first synthetic pigment. The pigment was known to the Romans by the name caeruleum...
, was made from quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
, lime
Calcium oxide
Calcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....
, a copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
compound, and an alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
flux
Ceramic flux
A ceramic flux functions by promoting glass formation in clay bodies and glazes. Fluxes are used in glazes to lower the high melting point of silica. The most commonly used fluxes in a ceramic glaze are lead, boric, soda, potassium, lithium, calcium, magnesium, barium, zinc, and strontium...
, all heated to a temperature between 850 and 1000°C. Quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
may have been used to contribute silica and calcium 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 curprorivaite (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
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
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
Tomb
A tomb is a repository for the remains of the dead. It is generally any structurally enclosed interment space or burial chamber, of varying sizes...
painting
Painting
Painting is the practice of applying paint, pigment, color or other medium to a surface . The application of the medium is commonly applied to the base with a brush but other objects can be used. In art, the term painting describes both the act and the result of the action. However, painting is...
at Saqqara
Saqqara
Saqqara is a vast, ancient burial ground in Egypt, serving as the necropolis for the Ancient Egyptian capital, Memphis. Saqqara features numerous pyramids, including the world famous Step pyramid of Djoser, sometimes referred to as the Step Tomb due to its rectangular base, as well as a number of...
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
Ur
Ur was an important city-state in ancient Sumer located at the site of modern Tell el-Muqayyar in Iraq's Dhi Qar Governorate...
from the Early Dynastic III
Sumerian king list
The Sumerian King List is an ancient manuscript originally recorded in the Sumerian language, listing kings of Sumer from Sumerian and neighboring dynasties, their supposed reign lengths, and the locations of "official" kingship...
period. Its use in the Mediterranean dates to the Thera
Santorini
Santorini , officially Thira , is an island located in the southern Aegean Sea, about southeast from Greece's mainland. It is the largest island of a small, circular archipelago which bears the same name and is the remnant of a volcanic caldera...
fresco
Fresco
Fresco is any of several related mural painting types, executed on plaster on walls or ceilings. The word fresco comes from the Greek word affresca which derives from the Latin word for "fresh". Frescoes first developed in the ancient world and continued to be popular through the Renaissance...
es from the Late Middle Bronze Age
Bronze Age
The Bronze Age is a period characterized by the use of copper and its alloy bronze as the chief hard materials in the manufacture of some implements and weapons. Chronologically, it stands between the Stone Age and Iron Age...
.
While the glass phase is present in blue frits from Egypt, scientists have not detected it in blue frits from the Near East
Near East
The Near East is a geographical term that covers different countries for geographers, archeologists, and historians, on the one hand, and for political scientists, economists, and journalists, on the other...
, Europe, and the Aegean
Aegean Sea
The Aegean Sea[p] is an elongated embayment of the Mediterranean Sea located between the southern Balkan and Anatolian peninsulas, i.e., between the mainlands of Greece and Turkey. In the north, it is connected to the Marmara Sea and Black Sea by the Dardanelles and Bosporus...
. Natural weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
, which is also responsible for the corrosion
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
of glasses and glazes
Ceramic glaze
Glaze is a layer or coating of a vitreous substance which has been fired to fuse to a ceramic object to color, decorate, strengthen or waterproof it.-Use:...
from these three regions, is the likely reason for this absence.
At Amarna
Amarna
Amarna is an extensive Egyptian archaeological site that represents the remains of the capital city newly–established and built by the Pharaoh Akhenaten of the late Eighteenth Dynasty , and abandoned shortly afterwards...
, 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
Vitruvius
Marcus Vitruvius Pollio was a Roman writer, architect and engineer, active in the 1st century BC. He is best known as the author of the multi-volume work De Architectura ....
reports the production of ‘caeruleum’ (a blue pigment) at Pozzuoli
Pozzuoli
Pozzuoli is a city and comune of the province of Naples, in the Italian region of Campania. It is the main city of the Phlegrean peninsula.-History:Pozzuoli began as the Greek colony of Dicaearchia...
, made by a method used in Alexandria
Alexandria
Alexandria is the second-largest city of Egypt, with a population of 4.1 million, extending about along the coast of the Mediterranean Sea in the north central part of the country; it is also the largest city lying directly on the Mediterranean coast. It is Egypt's largest seaport, serving...
, 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
Thutmose III
Thutmose III was the sixth Pharaoh of the Eighteenth Dynasty. During the first twenty-two years of Thutmose's reign he was co-regent with his stepmother, Hatshepsut, who was named the pharaoh...
and later show the use of bronze filings instead of copper ore
Ore
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....
.
Stocks suggests that waste powders from the drill
Drill
A drill or drill motor is a tool fitted with a cutting tool attachment or driving tool attachment, usually a drill bit or driver bit, used for drilling holes in various materials or fastening various materials together with the use of fasteners. The attachment is gripped by a chuck at one end of...
ing of limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
, combined with a minor concentration of alkali, may have been used to produce blue frits. The powders owe their copper content to the erosion of the copper tubular drills used in the drilling process. 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 malachiteMalachite
Malachite is a copper carbonate mineral, with the formula Cu2CO32. This green-colored mineral crystallizes in the monoclinic crystal system, and most often forms botryoidal, fibrous, or stalagmitic masses. Individual crystals are rare but do occur as slender to acicular prisms...
, green frit was usually employed as a green pigment. Its earliest occurrence is in tomb paintings of the 18th dynasty
Eighteenth dynasty of Egypt
The eighteenth dynasty of ancient Egypt is perhaps the best known of all the dynasties of ancient Egypt...
, but its use extends at least to the Roman period
Roman Empire
The Roman Empire was the post-Republican period of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean....
. 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. 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
Wollastonite
Wollastonite is a calcium inosilicate mineral that may contain small amounts of iron, magnesium, and manganese substituting for calcium. It is usually white. It forms when impure limestone or dolostone is subjected to high temperature and pressure sometimes in the presence of silica-bearing fluids...
([Ca,Cu]3Si3O9) crystals and a “glassy phase rich in copper, sodium, and potassium chlorides.” In certain circumstances (the use of a two-step heating process, the presence of hematite
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...
), 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. 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 among frit, glass, and faience
An Akkadian text from Assurbanipal’s library at NinevehNineveh
Nineveh was an ancient Assyrian city on the eastern bank of the Tigris River, and capital of the Neo Assyrian Empire. Its ruins are across the river from the modern-day major city of Mosul, in the Ninawa Governorate of Iraq....
suggests that a frit-like substance was an intermediate material in the production of raw glass. This intermediate step would have followed the grinding and mixing of the raw materials used to make glass. 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.”
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. He deduced that the frit intermediate is necessary so that gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
es will evolve during this stage and the end product will be virtually free of bubbles
Liquid bubble
A bubble is a globule of one substance in another, usually gas in a liquid.Due to the Marangoni effect, bubbles may remain intact when they reach the surface of the immersive substance.-Common examples:...
. Furthermore, grinding the frit actually expedites the “second part of the process, which is to…reduce the system to a glass.”
Moorey has defined this intermediate step as “fritting,” “a process in which the soluble
Solubility
Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...
salts are made insoluble by breaking down the carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
s, etc. and forming a complex mass of sintered silicates.” A frit preserved in a “fritting pan fragment” kept in the Petrie Museum
Petrie Museum of Egyptian Archaeology
The Petrie Museum of Egyptian Archaeology in London, England, which is part of University College LondonMuseums & Collections. The museum contains over 80,000 objects and ranks among some of the world's leading collections of Egyptian and Sudanese material...
“shows numerous white flecks of unreacted silica and a large number of vesicles where gases had formed.” The process was known to ancient writers such as Pliny
Pliny the Elder
Gaius Plinius Secundus , better known as Pliny the Elder, was a Roman author, naturalist, and natural philosopher, as well as naval and army commander of the early Roman Empire, and personal friend of the emperor Vespasian...
and Theophilus
Theophilus Presbyter
Theophilus Presbyter is the pseudonymous author or compiler ofa Latin text containing detailed descriptions of various medieval arts, a text commonly known as the Schedula diversarum artium or De diversis artibus , probably first compiled between 1100 and 1120...
.
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
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
and alumina than the glasses have.
Scholars have suggested several potential connections between frit and faience
Egyptian faience
Egyptian faience is a non-clay based ceramic displaying surface vitrification which creates a bright lustre of various blue-green colours. Having not been made from clay it is often not classed as pottery. It is called "Egyptian faience" to distinguish it from faience, the tin glazed pottery...
. 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
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
. But analysis of a wide array of Egyptian frits contradicts the binder composition that Kühne offers. 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
Slurry
A slurry is, in general, a thick suspension of solids in a liquid.-Examples of slurries:Examples of slurries include:* Lahars* A mixture of water and cement to form concrete* A mixture of water, gelling agent, and oxidizers used as an explosive...
in water, and finally apply the glaze “by dipping or painting.” 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. 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.
In general, frits, glasses, and faience are similar materials: they are all silica-based but have different concentrations of alkali, copper, and lime. 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.”
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. Indeed, scientists have determined that frit objects, such as amulet
Amulet
An amulet, similar to a talisman , is any object intended to bring good luck or protection to its owner.Potential amulets include gems, especially engraved gems, statues, coins, drawings, pendants, rings, plants and animals; even words said in certain occasions—for example: vade retro satana—, to...
s, bead
Bead
A bead is a small, decorative object that is usually pierced for threading or stringing. Beads range in size from under to over in diameter. A pair of beads made from Nassarius sea snail shells, approximately 100,000 years old, are thought to be the earliest known examples of jewellery. Beadwork...
s, and vessels, have chemical compositions similar to those of powder frits designed for use as pigments. 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.
Fritware
FritwareFritware
Fritware, also known as Islamic stone-paste, is a type of pottery in which frit is added to clay to reduce its fusion temperature. As a result, the mixture can be fired at a lower temperature than clay alone....
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. This type of pottery has also been referred to as “stonepaste” and “faience” among other names. A ninth century corpus of “proto-stonepaste” from Baghdad
Baghdad
Baghdad is the capital of Iraq, as well as the coterminous Baghdad Governorate. The population of Baghdad in 2011 is approximately 7,216,040...
has “relict glass fragments” in its fabric. The glass is alkali-lime-lead-silica and, when the paste was fired or cooled, wollastonite and diopside
Diopside
Diopside is a monoclinic pyroxene mineral with composition MgCaSi2O6. It forms complete solid solution series with hedenbergite and augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but typically dull green crystals in the monoclinic prismatic class...
crystals formed within the glass fragments. The lack of “inclusions of crushed pottery
Pottery
Pottery is the material from which the potteryware is made, of which major types include earthenware, stoneware and porcelain. The place where such wares are made is also called a pottery . Pottery also refers to the art or craft of the potter or the manufacture of pottery...
” suggests these fragments did not come from a glaze. 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
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
of the [ceramic] body.” Whether these “relict glass fragments” are actually “frit” in the more ancient sense remains to be seen.
Iznik pottery
Iznik pottery
İznik pottery, named after the town in western Anatolia where it was made, is highly decorated ceramics that was produced between the late 15th and 17th centuries....
was produced in Ottoman
Ottoman Empire
The Ottoman EmpireIt was usually referred to as the "Ottoman Empire", the "Turkish Empire", the "Ottoman Caliphate" or more commonly "Turkey" by its contemporaries...
Turkey
Turkey
Turkey , known officially as the Republic of Turkey , is a Eurasian country located in Western Asia and in East Thrace in Southeastern Europe...
as early as the 15th century AD. It consists of a body, slip
Slip (ceramics)
A slip is a suspension in water of clay and/or other materials used in the production of ceramic ware. Deflocculant, such as sodium silicate, can be added to the slip to disperse the raw material particles...
, and glaze, where the body and glaze are “quartz-frit.” The “frits” in both cases “are unusual in that they contain lead oxide
Lead oxide
Lead oxide may refer to:* Lead oxide, PbO, litharge, massicot* Lead oxide, Pb3O4, minium, red lead* Lead dioxide , PbO2Less common lead oxides are:* Lead oxide, Pb2O3, lead sesquioxide...
as well as soda
Sodium oxide
Sodium oxide is a chemical compound with the formula Na2O. It is used in ceramics and glasses, though not in a raw form. Treatment with water affords sodium hydroxide....
”; the lead oxide would help reduce the thermal expansion coefficient of the ceramic. Microscopic analysis reveals that the material that has been labeled “frit” is “interstitial glass” which serves to connect the quartz particles. Tite argues that this glass was added as frit and that the interstitial glass formed on firing.
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. At least one frit porcelain remains in production at Belleek, County Fermanagh
County Fermanagh
Fermanagh District Council is the only one of the 26 district councils in Northern Ireland that contains all of the county it is named after. The district council also contains a small section of County Tyrone in the Dromore and Kilskeery road areas....
, Northern Ireland
Northern Ireland
Northern Ireland is one of the four countries of the United Kingdom. Situated in the north-east of the island of Ireland, it shares a border with the Republic of Ireland to the south and west...
. 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.
Modern uses of frit
Frits are indispensable constituents of most industrial ceramic glazes which mature at temperatures below 1150°C. Frits are typically intermediates in the production of raw glass, as opposed to pigments and shaped objects. But they can be used as their own entities in a number of high-tech contexts. Frits made predominantly of silica, diboron trioxideBoron trioxide
Boron trioxide is one of the oxides of boron. It is a white, glassy solid with the formula B2O3. It is almost always found as the vitreous form; however, it can be crystallized after extensive annealing...
, and soda are used as enamels on steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
pipes
Pipe (material)
A pipe is a tubular section or hollow cylinder, usually but not necessarily of circular cross-section, used mainly to convey substances which can flow — liquids and gases , slurries, powders, masses of small solids...
. Another type of frit can be used as a biomaterial. Molten soda-lime-silica glass can be “poured into water to obtain a frit,” which is then ground to a powder. These powders can be used as “scaffolds for bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
substitutions.” Also, frits can be added to high-tech ceramics. Scientists have made such frits by milling
Mill (grinding)
A grinding mill is a unit operation designed to break a solid material into smaller pieces. There are many different types of grinding mills and many types of materials processed in them. Historically mills were powered by hand , working animal , wind or water...
ZnO and H3BO3 with zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
beads, then heating this mixture to 1100°C, quench
Quench
In materials science, quenching is the rapid cooling of a workpiece to obtain certain material properties. It prevents low-temperature processes, such as phase transformations, from occurring by only providing a narrow window of time in which the reaction is both thermodynamically favorable and...
ing it, and grinding it. This frit is then added to a Li2TiO3 ceramic powder. This addition is beneficial: the ceramic can sinter at a lower temperature while still keeping its “microwave dielectric properties." In laboratory and industrial chemical process, the term is used to denote a filter made by the sintering-together of glass particles to produce a piece of known porosity; the term is also used in a more loose sense to denote a filter of any composition (for example, stainless steel, polyethylene) substituted where a frit may have been otherwise used.