Tin-glazing

Tin-glazing is the process of giving ceramic items a tin-based glaze which is white, shiny and opaque, normally applied to red or buff earthenware.

The earliest tin-glazed pottery appears to have been made in Iraq/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. [Caiger-Smith, 1973, p.23] From Mesopotamia it spread to Egypt, Persia and Spain before reaching Italy in the Renaissance, Holland in the 16th century and England and France shortly after.

Tin-glazed pottery fell out of common use in the 18th century after Josiah Wedgwood formulated a very white earthenware body. There has been a revival in the twentieth century by studio potters. Some twentieth-century artists painted on tin-glazed pottery, for example, Picasso, who produced much work of this kind in the 1940s and 1950s.

Tin oxide (IV), has found widespread use in glaze, where it has been valued as an opacifier. Tin oxide can remain in suspension in the vitreous matrix, and, with its Refractive Index being sufficiently different from the matrix light is scattered, and hence increases the opacity of the glaze. Due to the high cost of tin oxide many glaze formulations now use other opacifiers, such as zircon

The earliest Middle Eastern tin glazes used calcium, lead and sodium compounds as fluxes in combination with the silica in sand. 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. [Caiger-Smith, 1973, p.207] A more contemporary tin-glaze recipe is given by Alan Caiger-Smith: [ Caiger-Smith, 1973, p224]

*Lead bisilicate frit: 74%
*China clay: 10%
*Whiting: 2%
*Flint: 4%
*Tin oxide: 10%

Tin-glazed pottery is almost all decorated, which its white surface lends itself to. The decoration is applied as metallic oxides, most commonly cobalt oxide, copper oxide, iron oxide, manganese dioxide and antimony oxide. 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. [ [http://www.pottersconnection.co.uk/shop/33/index.htm Potters Connection] ]

The pottery vessels are given a bisque or biscuit firing, usually between 900 deg C and 1000 deg C, which makes them strong but porous. 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 deg C (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.

References

*Caiger-Smith, Alan, "Tin-Glaze Pottery in Europe and the Islamic World: The Tradition of 1000 Years in Maiolica, Faience and Delftware" (Faber and Faber, 1973) ISBN 0-571-09349-3

*Caiger-Smith, Alan, "Lustre Pottery: Technique, Tradition and Innovation in Islam and the Western World" (Faber and Faber, 1985) ISBN 0-571-13507-2

*Carnegy, Daphne, "Tin-glazed Earthenware" (A&C Black/Chilton Book Company, 1993) ISBN 0-7136-3718-8

*Harris, David, "Guide To Looking At Italian Ceramics" (J. Paul Getty Museum in association with British Museum Press, 1993)

*McCully, Marylin (ed.), "Picasso: Painter and Sculptor in Clay" (Royal Academy of Arts, 1998) ISBN 0-900-94663-6

*Piccolpasso, Cipriano, "The Three Books of the Potter's Art" (trans. A.Caiger Smith and R.Lightbown) (Scolar Press, 1980) ISBN 0-859-67452-5

ee also

Tin-glazed pottery