Tin-glazing
Encyclopedia
Tin-glazing is the process of giving ceramic
items a tin-based glaze
which is white, glossy and opaque, normally applied to red or buff earthenware
. The opacity and whiteness of tin glaze make it valued by its ability to decorate with colour.
Tin oxide
is valued in glazes as both an opacifier and as a white colorant. Tin oxide has long been used to produce a white, opaque & glossy glaze. As well as an opacifying agent tin oxide also finds use as a colour stabiliser in some pigments and glazes. Minor quantities are also used in the conducting phases in some electrical porcelain glazes.
appears to have been made in Abbasid
Iraq
(750-1258 AD)/Mesopotamia
in the 9th century, the oldest fragments having been excavated during the First World War from the palace of Samarra
about fifty miles north of Baghdad
. From Mesopotamia, tin glazes spread to the Islamic Egypt (868–905 AD) during the 10th century, and then to the Islamic Spain (711-1492 AD), leading to the maximum development of Islamic lusterware
.
The history of tin glazes in the Islamic worlds is disputed. One possible reason for the earlier production of tin-glazed wares could be attributed to the trade between the Abbasid Empire and ancient China from the 8th to 9th century onwards, resulting in imitation of white Chinese stoneware
by local Islamic potters. Another might be local glaze-making rather than foreign influence, supported by the similarility between the chemical and microstructural features of pre-Islamic white opaque glazes and that on the first tin-opacified wares
From the Middle East, tin-glaze spread through the Islamic world to Spain. In the 13th century, tin glazes reached Italy
, where the earliest recorded reference to its use is in 1330s, resulting in the emergence of Italian majolica
. Amongst others Luca della Robbia
, born in Florence circ. 1400, used tin oxide as an opacifier in glazes. Potters began to draw polychrome paintings on the white opaque surface with metallic oxides such as cobalt oxide and to produce lustreware. The off-white fired body of Delft ware and English majolica was made to appear white, and hence mimic the appearance of Chinese porcelain, by the application of a glaze opacified and coloured white by the addition of tin oxide.
. Whilst zirconium compounds are not as effective their low price led to a gradual increase in popularity with an associated reduction in use of tin oxide. Today, tin oxide usage in glazes finds limited use in conjunction with zircon compounds, although it is generally restricted to specialist low temperature applications and use by studio potters. The whiteness resulting from the use of zirconia has been described as more clinical that that from tin oxide, and is consequently preferred in some applications. The Koninklijke Tichelaar Makkum factory, or Royal Tichelaar Makkum, based in Makkum
, Friesland
continue the production Delftware
using tin-glazed earthenware
.
(SnO2), and also called stannic acid, is commercially exploited. Opacity is produced in glazes by the addition of some substance to scatter and reflect some of the incident light.
The opacity of glaze could be determined by the particles which spread through the glaze, therefore the light is absorbed by the particles, being scattered back before reaching the ceramic body, leading to the opaque glaze. As a result, the concentration of the absorbing or scattering particles in the glaze could determine the degree of opacification. Generally speaking, the more different the refractive index
between the particles and the glaze matrix, the larger the opacity. Similarly, the closer the particle size to the light wavelength (100-1000 nm for visible light) and the more irregular the surface, the larger the degree of opacification.
Tin oxide
remains in suspension in vitreous matrix of the fired glazes, and, with its high refractive index being sufficiently different from the matrix, light is scattered, and hence increases the opacity
of the glaze. The degree of dissolution increases with the firing temperature, and hence the extent of opacity diminishes. Although dependant on the other constituents the solubility of tin oxide in glaze melts is generally low. Its solubility is increased by Na2O, K2O and B2O3, and reduced by CaO, BaO, ZnO, Al2O3, and to a limited extent PbO.
Some research on medieval tin glaze has shown that the particle size of tin oxide which appears as cassiterite is around several hundred nanometer, which corresponds to the range of wavelength of visible light . In some cases, the tin oxide is presented not only as small crystals but also as aggregates of particles. These factors – the high refractive index, the low solubility in glazes and the particle size make tin oxide an excellent opacifier.
In the beginning of the use of tin oxide, it is mainly viewed as a slip
layer between the glaze and ceramic body. This could be seen from the SEM
photomicrographs of some earlier Islamic glazed ceramics, of which the particles of tin oxide are concentrated at the interface, together with the existence of wollastonite
, diopside
and air bubble as other opacifiers. The microanalysis of later tin glazes reveals the distribution of tin oxide through the glazes rather than just at the interface, which indicates that tin oxide is really acting as an opacifier
instead of only a surface coating layer.
Lead is usually brought into the glazes with tin oxide. The reaction between lead and tin oxide results in the recrystallisation
of tin oxide, and thus enhances the degree of opacification in tin-opacified glazes than in tin-opacified glass. A high PbO/SnO2 ratio is often found in ancient glazes. During the firing process, lead oxide
react with quartz at approximately 550℃ to form PbSiO3, which then reacts with tin oxide to produce lead-tin oxide (PbSnO3) at a temperature higher than 600℃. After the formation of lead-tin oxide, the melting of PbSiO3, PbO and PbSnO3 occurs at the temperature in the range of 700℃ to 750℃, resulting in the dissolution of PbSnO3 to SnO2. The degree of the crystallisation of SnO2 increases with the increasing of temperature. During either heating or cooling, the recrystallisation is taken place until the supply of tin is exhausted. In the second heating, lead in the form of lead oxide no longer reacts with tin oxide to form lead silicate, thus the recrystallised cassiterite
(SnO2) remain undissolved and precipitate in the glazes. The nucleation
and growth
rates of the precipitation depend upon temperature and time. The particle size of the casssiterite developed is also dependent on the temperature, and smaller than that used in the very beginning. It is the smaller particle size of the recrystallised SnO2 in glazes that increases the opacity in tin-opacified glazes. Besides the increasing the opacity, the high lead oxide to tin oxide ratio also reduces the melting point of glazes, lead to a lower firing temperature during production .
ern tin glazes used calcium
, lead
and sodium
compounds
as fluxes in combination with the silica in silica. A Islamic opaque white glaze has been analysed, and is quoted below as a Seger formula :
In this recipe, the addition of alkali
helps to increase the hardness of the surface and clarify the colour of the glaze as well. With the development of tin glazes, the significant amount of tin oxide indicates its deliberate addition as an opacifier. A recipe involving the use of three ingredients was given in Abu’l-Qasim’s treatise from Persia
in the 14th century: a glass-frit
of quartz
and potash
, a lead-tin calx
and a calcination
of limestone
and quartz . Afterwards, with the spread of tin glazes, lead gradually became the principal background in tin glazes, though a small proportion of alkali was still introduced in order to increase the fusibility
. No specific recipes alluding to tin glazes in Spain
have been found in ancient archives. However, recent research has shown that, at least since the 10th century AD, most Islamic white glazes in Spain were lead-silica glazes with tin oxide as an opacifier. That is, no alkaline glazes or lead-alkaline glazes have been found . Piccolpasso recorded several glazes used in Italy in the 1550s, all variations of lead, tin, lime, soda and potash glazes. It is believed early Spanish glazes were similar.
A Seger analysis of a tin glaze from the early 20th century is:
A more recent recipe is:
And another is:
during the firing process . On the other hand, the calcareous clay fired in an oxidising atmosphere results in a buff colour, thus lower the concentration of tin oxide used
The white opaque surface makes tin glaze a good base for painted decoration. The decoration is applied as metallic oxides, most commonly cobalt oxide
for blue, copper oxide
for green, iron oxide
for brown, manganese dioxide for purple-brown and antimony
for yellow. Late Italian maiolica
blended oxides to produce detailed and realistic polychrome paintings, called istoriato. To these oxides modern potters are able to add powdered ceramic colours made from combinations of oxide, sometimes frit
ted. In the sixteenth century, the use of subtle and blended colours which were not strong enough to penetrate the opaque glaze made the delicate control of ton-value possible, and the painting therefore had to be done on the glaze surface, which then becomes a common manner of painting on tin-glazed wares.
The pottery vessels are biscuit fired, usually between 900 ℃ and 1000 ℃. The fired vessel is dipped in a liquid glaze suspension
which sticks to it, leaving a smooth and absorbent surface when dry. On this surface colours are applied by brush, the colours made from powered oxides mixed with water to a consistency of water-colour paint, sometimes with the addition of a binding agent such as gum arabic
. The unfired glaze absorbs the pigment like fresco
, making errors difficult to correct but preserving the brilliant colors of the oxides when fired. The glazed and decorated vessels are returned to the kiln for a second firing, usually between 1000 and 1120 ℃ (the higher temperatures used by modern potters). Lustered wares have a third firing at a lower temperature, necessitating a delicate control of the amount of oxygen in the kiln atmosphere and therefore a flame-burning kiln.
Traditional kilns were wood firing, which required the pots to be protected in the glaze and luster firings by saggars or to be fired in a muffle kiln. Except for those making luster ware, modern tin-glaze potters use electric kilns.
The recrystallisation of tin oxide during the firing provides evidence of the slightly different methods of different production sites, as the crystal size, the distribution and the concentration may be influenced. For instance, the analysis of the 14th century Islamic tin glazes from eastern Spain indicates that theses samples may be produced by non-fritting methods, as the heterogeneous distribution of tin oxides may be the remains of original grains of tin oxides .
The interaction between glaze and body also give clues to different handling and firing processes. As mentioned above, tin glaze suspension is applied to bisque or biscuit body made of calcareous clay with high content of calcium oxide
. This could be inferred from the absence of trapped glaze bubbles. If it is applied to an unfired body, the calcium carbonate
will decompose, generating carbon dioxide
, the releasing of which from the body to the glaze results in trapped bubbles in the glaze layers.
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...
items a tin-based glaze
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:...
which is white, glossy and opaque, normally applied to red or buff earthenware
Earthenware
Earthenware is a common ceramic material, which is used extensively for pottery tableware and decorative objects.-Types of earthenware:Although body formulations vary between countries and even between individual makers, a generic composition is 25% ball clay, 28% kaolin, 32% quartz, and 15%...
. The opacity and whiteness of tin glaze make it valued by its ability to decorate with colour.
Tin oxide
Tin dioxide
Tin dioxide is the inorganic compound with the formula SnO2. The mineral form of SnO2 is called cassiterite, and this is the main ore of tin. With many other names , this oxide of tin is the most important raw material in tin chemistry...
is valued in glazes as both an opacifier and as a white colorant. Tin oxide has long been used to produce a white, opaque & glossy glaze. As well as an opacifying agent tin oxide also finds use as a colour stabiliser in some pigments and glazes. Minor quantities are also used in the conducting phases in some electrical porcelain glazes.
History
The earliest tin-glazed potteryTin-glazed pottery
Tin-glazed pottery is a majolica pottery covered in glaze containing tin oxide which is white, shiny and opaque. The pottery body is usually made of red or buff colored earthenware and the white glaze was often used to imitate Chinese porcelain...
appears to have been made in Abbasid
Abbasid
The Abbasid Caliphate or, more simply, the Abbasids , was the third of the Islamic caliphates. It was ruled by the Abbasid dynasty of caliphs, who built their capital in Baghdad after overthrowing the Umayyad caliphate from all but the al-Andalus region....
Iraq
Iraq
Iraq ; officially the Republic of Iraq is a country in Western Asia spanning most of the northwestern end of the Zagros mountain range, the eastern part of the Syrian Desert and the northern part of the Arabian Desert....
(750-1258 AD)/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...
in the 9th century, the oldest fragments having been excavated during the First World War from the palace of Samarra
Samarra
Sāmarrā is a city in Iraq. It stands on the east bank of the Tigris in the Salah ad-Din Governorate, north of Baghdad and, in 2003, had an estimated population of 348,700....
about fifty miles north of 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...
. From Mesopotamia, tin glazes spread to the Islamic Egypt (868–905 AD) during the 10th century, and then to the Islamic Spain (711-1492 AD), leading to the maximum development of Islamic lusterware
Lusterware
Lusterware or Lustreware is a type of pottery or porcelain with a metallic glaze that gives the effect of iridescence, produced by metallic oxides in an overglaze finish, which is given a second firing at a lower temperature in a "muffle kiln", reduction kiln, which excludes oxygen.The first use...
.
The history of tin glazes in the Islamic worlds is disputed. One possible reason for the earlier production of tin-glazed wares could be attributed to the trade between the Abbasid Empire and ancient China from the 8th to 9th century onwards, resulting in imitation of white Chinese stoneware
Stoneware
Stoneware is a vitreous or semi-vitreous ceramic ware with a fine texture. Stoneware is made from clay that is then fired in a kiln, whether by an artisan to make homeware, or in an industrial kiln for mass-produced or specialty products...
by local Islamic potters. Another might be local glaze-making rather than foreign influence, supported by the similarility between the chemical and microstructural features of pre-Islamic white opaque glazes and that on the first tin-opacified wares
From the Middle East, tin-glaze spread through the Islamic world to Spain. In the 13th century, tin glazes reached Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
, where the earliest recorded reference to its use is in 1330s, resulting in the emergence of Italian majolica
Maiolica
Maiolica is Italian tin-glazed pottery dating from the Renaissance. It is decorated in bright colours on a white background, frequently depicting historical and legendary scenes.-Name:...
. Amongst others Luca della Robbia
Luca della Robbia
Luca della Robbia was an Italian sculptor from Florence, noted for his terra-cotta roundels.Luca Della Robbia developed a pottery glaze that made his creations more durable in the outdoors and thus suitable for use on the exterior of buildings. His work is noted for its charm rather than the drama...
, born in Florence circ. 1400, used tin oxide as an opacifier in glazes. Potters began to draw polychrome paintings on the white opaque surface with metallic oxides such as cobalt oxide and to produce lustreware. The off-white fired body of Delft ware and English majolica was made to appear white, and hence mimic the appearance of Chinese porcelain, by the application of a glaze opacified and coloured white by the addition of tin oxide.
Current use and alternatives
Tin oxide has been widely used as the opacifier in sanitaryware glazes. In this application additions of up to 6% is reported to be current use. The cost of tin oxide rose considerably during the 1918-1918 war, and resulted in a search for cheaper alternatives. The first successful replacement was zirconia and later zirconZircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
. Whilst zirconium compounds are not as effective their low price led to a gradual increase in popularity with an associated reduction in use of tin oxide. Today, tin oxide usage in glazes finds limited use in conjunction with zircon compounds, although it is generally restricted to specialist low temperature applications and use by studio potters. The whiteness resulting from the use of zirconia has been described as more clinical that that from tin oxide, and is consequently preferred in some applications. The Koninklijke Tichelaar Makkum factory, or Royal Tichelaar Makkum, based in Makkum
Makkum
Makkum is a village in Wûnseradiel in the province Friesland of the Netherlands and has around 3500 citizens .The dutch queen Beatrix came to visit Makkum in 2008 at Queensday....
, Friesland
Friesland
Friesland is a province in the north of the Netherlands and part of the ancient region of Frisia.Until the end of 1996, the province bore Friesland as its official name. In 1997 this Dutch name lost its official status to the Frisian Fryslân...
continue the production Delftware
Delftware
Delftware, or Delft pottery, denotes blue and white pottery made in and around Delft in the Netherlands and the tin-glazed pottery made in the Netherlands from the 16th century....
using tin-glazed earthenware
Earthenware
Earthenware is a common ceramic material, which is used extensively for pottery tableware and decorative objects.-Types of earthenware:Although body formulations vary between countries and even between individual makers, a generic composition is 25% ball clay, 28% kaolin, 32% quartz, and 15%...
.
The nature of tin glaze
For glaze use only one tin compound, tin (IV) oxide Tin dioxideTin dioxide
Tin dioxide is the inorganic compound with the formula SnO2. The mineral form of SnO2 is called cassiterite, and this is the main ore of tin. With many other names , this oxide of tin is the most important raw material in tin chemistry...
(SnO2), and also called stannic acid, is commercially exploited. Opacity is produced in glazes by the addition of some substance to scatter and reflect some of the incident light.
The opacity of glaze could be determined by the particles which spread through the glaze, therefore the light is absorbed by the particles, being scattered back before reaching the ceramic body, leading to the opaque glaze. As a result, the concentration of the absorbing or scattering particles in the glaze could determine the degree of opacification. Generally speaking, the more different the refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
between the particles and the glaze matrix, the larger the opacity. Similarly, the closer the particle size to the light wavelength (100-1000 nm for visible light) and the more irregular the surface, the larger the degree of opacification.
Tin oxide
Tin dioxide
Tin dioxide is the inorganic compound with the formula SnO2. The mineral form of SnO2 is called cassiterite, and this is the main ore of tin. With many other names , this oxide of tin is the most important raw material in tin chemistry...
remains in suspension in vitreous matrix of the fired glazes, and, with its high refractive index being sufficiently different from the matrix, light is scattered, and hence increases the opacity
Opacity (optics)
Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...
of the glaze. The degree of dissolution increases with the firing temperature, and hence the extent of opacity diminishes. Although dependant on the other constituents the solubility of tin oxide in glaze melts is generally low. Its solubility is increased by Na2O, K2O and B2O3, and reduced by CaO, BaO, ZnO, Al2O3, and to a limited extent PbO.
Some research on medieval tin glaze has shown that the particle size of tin oxide which appears as cassiterite is around several hundred nanometer, which corresponds to the range of wavelength of visible light . In some cases, the tin oxide is presented not only as small crystals but also as aggregates of particles. These factors – the high refractive index, the low solubility in glazes and the particle size make tin oxide an excellent opacifier.
In the beginning of the use of tin oxide, it is mainly viewed as a 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...
layer between the glaze and ceramic body. This could be seen from the SEM
Scanning electron microscope
A scanning electron microscope is a type of electron microscope that images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern...
photomicrographs of some earlier Islamic glazed ceramics, of which the particles of tin oxide are concentrated at the interface, together with the existence of 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...
, 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...
and air bubble as other opacifiers. The microanalysis of later tin glazes reveals the distribution of tin oxide through the glazes rather than just at the interface, which indicates that tin oxide is really acting as an opacifier
Opacifier
An opacifier is a substance added to a material in order to make the ensuing system opaque. An example of a chemical opacifier is tin dioxide , which is used to opacify ceramic glazes and milk glass; bone ash is also used....
instead of only a surface coating layer.
Lead is usually brought into the glazes with tin oxide. The reaction between lead and tin oxide results in the recrystallisation
Recrystallization (metallurgy)
Recrystallization is a process by which deformed grains are replaced by a new set of undeformed grains that nucleate and grow until the original grains have been entirely consumed. Recrystallization is usually accompanied by a reduction in the strength and hardness of a material and a simultaneous...
of tin oxide, and thus enhances the degree of opacification in tin-opacified glazes than in tin-opacified glass. A high PbO/SnO2 ratio is often found in ancient glazes. During the firing process, 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...
react with quartz at approximately 550℃ to form PbSiO3, which then reacts with tin oxide to produce lead-tin oxide (PbSnO3) at a temperature higher than 600℃. After the formation of lead-tin oxide, the melting of PbSiO3, PbO and PbSnO3 occurs at the temperature in the range of 700℃ to 750℃, resulting in the dissolution of PbSnO3 to SnO2. The degree of the crystallisation of SnO2 increases with the increasing of temperature. During either heating or cooling, the recrystallisation is taken place until the supply of tin is exhausted. In the second heating, lead in the form of lead oxide no longer reacts with tin oxide to form lead silicate, thus the recrystallised cassiterite
Cassiterite
Cassiterite is a tin oxide mineral, SnO2. It is generally opaque, but it is translucent in thin crystals. Its luster and multiple crystal faces produce a desirable gem...
(SnO2) remain undissolved and precipitate in the glazes. The nucleation
Nucleation
Nucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...
and growth
Crystal growth
A crystal is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. Crystal growth is a major stage of a crystallization process, and consists in the addition of new atoms, ions, or polymer strings into...
rates of the precipitation depend upon temperature and time. The particle size of the casssiterite developed is also dependent on the temperature, and smaller than that used in the very beginning. It is the smaller particle size of the recrystallised SnO2 in glazes that increases the opacity in tin-opacified glazes. Besides the increasing the opacity, the high lead oxide to tin oxide ratio also reduces the melting point of glazes, lead to a lower firing temperature during production .
Analyses and Recipes
The earliest Middle EastMiddle East
The Middle East is a region that encompasses Western Asia and Northern Africa. It is often used as a synonym for Near East, in opposition to Far East...
ern tin glazes used calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
, lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
and sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
compounds
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
as fluxes in combination with the silica in silica. A Islamic opaque white glaze has been analysed, and is quoted below as a Seger formula :
-
-
- PbO=0.32
- CaO=0.32
- K2O=0.03
- Na2O=0.29
- MgO=0.04
- Al2O3=0.03
- SiO2=1.73
- SnO2=0.07
-
In this recipe, the addition of 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,...
helps to increase the hardness of the surface and clarify the colour of the glaze as well. With the development of tin glazes, the significant amount of tin oxide indicates its deliberate addition as an opacifier. A recipe involving the use of three ingredients was given in Abu’l-Qasim’s treatise from Persia
Iran
Iran , officially the Islamic Republic of Iran , is a country in Southern and Western Asia. The name "Iran" has been in use natively since the Sassanian era and came into use internationally in 1935, before which the country was known to the Western world as Persia...
in the 14th century: a glass-frit
Frit
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...
of 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,...
and potash
Potash
Potash is the common name for various mined and manufactured salts that contain potassium in water-soluble form. In some rare cases, potash can be formed with traces of organic materials such as plant remains, and this was the major historical source for it before the industrial era...
, a lead-tin calx
Calx
Calx is a residual substance, sometimes in the form of a fine powder, that is left when a metal or mineral combusts or is calcinated due to heat.Calx, especially of a metal, is now known as an oxide...
and a calcination
Calcination
Calcination is a thermal treatment process applied to ores and other solid materials to bring about a thermal decomposition, phase transition, or removal of a volatile fraction. The calcination process normally takes place at temperatures below the melting point of the product materials...
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....
and quartz . Afterwards, with the spread of tin glazes, lead gradually became the principal background in tin glazes, though a small proportion of alkali was still introduced in order to increase the fusibility
Fusibility
Fusibility is the ease with which a material will melt. Materials such as solder require a low melting point so that when heat is applied to a joint, the solder will melt before the materials being soldered melt, i.e. high fusibility. On the other hand, firebricks used for furnace linings only...
. No specific recipes alluding to tin glazes in Spain
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...
have been found in ancient archives. However, recent research has shown that, at least since the 10th century AD, most Islamic white glazes in Spain were lead-silica glazes with tin oxide as an opacifier. That is, no alkaline glazes or lead-alkaline glazes have been found . Piccolpasso recorded several glazes used in Italy in the 1550s, all variations of lead, tin, lime, soda and potash glazes. It is believed early Spanish glazes were similar.
A Seger analysis of a tin glaze from the early 20th century is:
-
-
- PbO=0.52
- CaO=0.16
- K2O=0.03
- Na2O=0.29
- Al2O3=0.15
- SiO2=2.77
- SnO2=0.23
-
A more recent recipe is:
-
-
- LeadLeadLead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
bisilicate fritFritFrit 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...
: 74% - China clay: 10%
- WhitingCalcium carbonateCalcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
: 2% - FlintFlintFlint is a hard, sedimentary cryptocrystalline form of the mineral quartz, categorized as a variety of chert. It occurs chiefly as nodules and masses in sedimentary rocks, such as chalks and limestones. Inside the nodule, flint is usually dark grey, black, green, white, or brown in colour, and...
: 4% - Tin oxideTin oxideTin oxide may refer to:* Tin oxide , SnO* Tin dioxide , SnO2...
: 10%
- Lead
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And another is:
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- Potassium feldsparFeldsparFeldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
: 65% - Limestone: 11%
- Silica: 11%
- Zinc oxideZinc oxideZinc oxide is an inorganic compound with the formula ZnO. It is a white powder that is insoluble in water. The powder is widely used as an additive into numerous materials and products including plastics, ceramics, glass, cement, rubber , lubricants, paints, ointments, adhesives, sealants,...
: 9% - Tin oxideTin oxideTin oxide may refer to:* Tin oxide , SnO* Tin dioxide , SnO2...
: 4%
- Potassium feldspar
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As a glaze colorant
In combination with chromium compounds addition of 0.5 - 1.5% tin oxide to a glaze result in a pink colour, with such glazes being known as Chrome-tin pinks. In conjunction with small additions of zinc oxide and titanium oxide, additions of tin oxide up to 18% to lead glazes can produce a satin or vellum surface finish. The firing temperatures of such glazes are low, in the region of 950 – 1000 ℃ because of the variable degrees if solution of the individual oxides. The amount of tin oxide used for coloured glazes depends upon the opacifying property of the chosen chromophore and the intensity of the colour desired; if a deep colour is required less opacifier will be needed than for pastel shades.Manufacture
Though the recipe of tin glazes may differ in different sites and periods, the process of the production of tin glazes is similar. Generally speaking, the first step of the production of tin glazes is to mix tin and lead in order to form oxides, which was then added to a glaze matrix (alkali-silicate glaze, for example) and heated . After the mixture cooled, the tin oxide crystallises as what has been mentioned above, therefore generates the so-called white tin-opacified glazes. Besides, the body of tin-opacified wares is generally calcareous clays containing 15-25% CaO, of which the thermal expansion coefficient is close to that of tin glazes, thus avoid crazingCrazing
Crazing is a network of fine cracks on the surface of a material, for example in a glaze layer.Crazing is a phenomenon that frequently precedes fracture in some glassy thermoplastic polymers. Crazing occurs in regions of high hydrostatic tension, or in regions of very localized yielding, which...
during the firing process . On the other hand, the calcareous clay fired in an oxidising atmosphere results in a buff colour, thus lower the concentration of tin oxide used
The white opaque surface makes tin glaze a good base for painted decoration. The decoration is applied as metallic oxides, most commonly cobalt oxide
Cobalt oxide
Cobalt oxide may refer to*Cobalt oxide - CoO*Cobalt oxide - Co2O3*Cobalt oxide - Co3O4...
for blue, copper oxide
Copper oxide
Copper oxide is a compound from the two elements copper and oxygen.Copper oxide may refer to:*Copper oxide , a red powder;*Copper oxide , a black powder...
for green, iron oxide
Iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...
for brown, manganese dioxide for purple-brown and antimony
Antimony
Antimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...
for yellow. Late Italian maiolica
Maiolica
Maiolica is Italian tin-glazed pottery dating from the Renaissance. It is decorated in bright colours on a white background, frequently depicting historical and legendary scenes.-Name:...
blended oxides to produce detailed and realistic polychrome paintings, called istoriato. To these oxides modern potters are able to add powdered ceramic colours made from combinations of oxide, sometimes frit
Frit
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...
ted. In the sixteenth century, the use of subtle and blended colours which were not strong enough to penetrate the opaque glaze made the delicate control of ton-value possible, and the painting therefore had to be done on the glaze surface, which then becomes a common manner of painting on tin-glazed wares.
The pottery vessels are biscuit fired, usually between 900 ℃ and 1000 ℃. The fired vessel is dipped in a liquid glaze suspension
Suspension (chemistry)
In chemistry, a suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometer. The internal phase is dispersed throughout the external phase through mechanical agitation, with the use of certain...
which sticks to it, leaving a smooth and absorbent surface when dry. On this surface colours are applied by brush, the colours made from powered oxides mixed with water to a consistency of water-colour paint, sometimes with the addition of a binding agent such as gum arabic
Gum arabic
220px|thumb|right|Acacia gumGum arabic, also known as acacia gum, chaar gund, char goond, or meska, is a natural gum made of hardened sap taken from two species of the acacia tree; Acacia senegal and Acacia seyal...
. The unfired glaze absorbs the pigment like 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...
, making errors difficult to correct but preserving the brilliant colors of the oxides when fired. The glazed and decorated vessels are returned to the kiln for a second firing, usually between 1000 and 1120 ℃ (the higher temperatures used by modern potters). Lustered wares have a third firing at a lower temperature, necessitating a delicate control of the amount of oxygen in the kiln atmosphere and therefore a flame-burning kiln.
Traditional kilns were wood firing, which required the pots to be protected in the glaze and luster firings by saggars or to be fired in a muffle kiln. Except for those making luster ware, modern tin-glaze potters use electric kilns.
The recrystallisation of tin oxide during the firing provides evidence of the slightly different methods of different production sites, as the crystal size, the distribution and the concentration may be influenced. For instance, the analysis of the 14th century Islamic tin glazes from eastern Spain indicates that theses samples may be produced by non-fritting methods, as the heterogeneous distribution of tin oxides may be the remains of original grains of tin oxides .
The interaction between glaze and body also give clues to different handling and firing processes. As mentioned above, tin glaze suspension is applied to bisque or biscuit body made of calcareous clay with high content of calcium oxide
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....
. This could be inferred from the absence of trapped glaze bubbles. If it is applied to an unfired body, the calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
will decompose, generating carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, the releasing of which from the body to the glaze results in trapped bubbles in the glaze layers.