Saturday, November 17, 2007

European Porcelain

Hard Paste porcelain: original formula, developed in China under T'ang—vitrified mixture of kaolin (low iron, high alumina clay, china stone—quarz, mica and feldspar). Glazes include lime, feldspar—fires at about 1300C.
Soft Paste porcelain: ("pate tendre") low clay content, high quartz, glass frit, lime or gypsum—fires lower (1240 C or lower)—not as strong, slumps, cracks, but beautiful effects with glazes—used in France 18th c (Sevres) and Britain (Bow, Chelsea etc.).
Hard paste porcelain (“true porcelain”) consists of a “triaxial” blend of quartz, feldspar and kaolin. Fired at 1300 C, a dense vitreous body with an iron content less than 1% results in a white, translucent and very strong porcelain. Soft-paste porcelain is produced by a very different method, which was in common use around Paris by the mid-18th century. Soda ash, sea salt, potassium nitrate, silica and lime (or gypsum) were milled together and heated for 30 or more hours to form a white opaque product. The alkali and quartz react to produce an alkali silicate glass using a sintering technique very similar to that used since the 10th c. to make maiolica and faience glazes.
After 1750, calcined alum was added to provide a further source of potassia and alumina. The mixture was washed to remove any un-reacted alkali, as soluble alkali would migrate to the surface and cause blemishes. The sinter was milled wet and combined with 16.5% white limestone and 8.3% washed, decanted calcareous marl clay from Argenteuil, which added plasticity. This mixture was dried, wedged and stored for several months to ferment organic constituents and to improve plasticity. In this state, the material could be moulded.
However, to throw the material, black soap and parchment glue were added to increase green strength. Thrown ware was trimmed in the bone-dry rather than leather-hard state. This created dust that was very dangerous to the lungs. Soft-paste wares were fired for periods of up to 90 hours at temperatures of about 1240 C or lower. Soft-paste porcelains are generally twice-fired; lead- alkai-silicate, lead-silicate or lead-borosilicate glazes are applied to biscuit wares and re-fired to a lower temperature (Ceramic Masterpieces, Kingery and Vandiver).

Bone china: developed by Josiah Spode c. 1800--up to 50% bone ash, equal parts china clay, feldspar--ideal for casting, economical. By 1825, used extensively in Britain. Bisqued at around 1250C, then glazed and refired at around 1080C. Used lead-borosilicate glaze.
Wedgwood's cream ware/pearlware: Improved existing earthenware body after 1763--refined ball clay, calcined flint (pure silica withstands higher temperature), china stone/ china clay. Glazes lead-soda. cobalt added to glaze after 1779 to produce pearlware.
The first porcelain in Europe: In Florence, Grand Duke Francesco de'Medici (died 1587) was recorded in 1575 as having found the secret of making porcelain after years of trials and experiment. This is generally considered the first European porcelain, although the body and glaze are in fact based on Near Eastern and maiolica techniques. The paste formula and the high temperatures required in the firing kilns were difficult to achieve, and the project proved extremely costly: after Francesco's death, production dwindled.

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