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[1]. Tin oxide has long been used to produce a white, opaque & glossy glaze[2][3]. As well as an opacifying agent tin oxide also finds use as a colour stabiliser in some pigments and glazes[4]. Minor quantities are also used in the conducting phases in some electrical porcelain glazes[5][6].
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The earliest tin-glazed pottery 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.[7] 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[8][9].
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[10]. 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[8]
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[11], 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[12]. 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. [13][14]
Tin oxide has been widely used as the opacifier in sanitaryware glazes[15]. In this application additions of up to 6% is reported to be current use[16]. The cost of tin oxide rose considerably during the 1918-1918 war, and resulted in a search for cheaper alternatives[17]. The first successful replacement was zirconia and later zircon[18]. 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.[19][20] 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[21]. The Koninklijke Tichelaar Makkum factory, or Royal Tichelaar Makkum, based in Makkum, Friesland continue the production Delftware using tin-glazed earthenware[22][23].
For glaze use only one tin compound, tin (IV) oxide Tin dioxide (SnO2), and also called stannic acid[24], 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[25]. 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[26].
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 [27]. 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.[8] 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.[8]
Lead is usually brought into the glazes with tin oxide. The reaction between lead and tin oxide results in the recrystallisation of tin oxide,[27] 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 [28].
The earliest Middle Eastern 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 [29]:
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 [30]. 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 [31]. 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.[7]
A Seger analysis of a tin glaze from the early 20th century is[32]:
A more recent recipe is[7]:
And another is[33]:
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[34][35]. 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[36]. 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[37]. 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[38].
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 [39]. 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 crazing during the firing process [40] [41]. 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 [42]
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 fritted[43]. 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.[7]
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 [44].
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.