Book: Ceramics - Art or Science? Author: Dr. Stan Jones

11. Pottery Technology 2

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More recently, using tunnel kilns, biscuit firing was reduced to 23 hours at 1235 degrees C with about 12% shrinkage, and glost firing took place at 1100 degrees C for 14 hours. The pots were then decorated with coloured enamels and gilding. The enamels with the same firing temperature can be fired together; otherwise the highest temperature enamel is fired first. There can be as many as six separate firings between 900 and 750 degrees C for 1 to 2 hours. The brilliance of enamel colours and gilding on bone china is greater than on other porcelains. In the most modern kilns bone china only requires a biscuit firing of between 16 and 9 hours at 1235 to 1254 degrees C. The biscuit items are then pre-heated, spray-glazed and the glost firing takes 8-9 hours at 1104 degrees C. Each of these improvements represents a significant reduction in energy use.

Spode bone china cups and saucers

Spode bone china cups and saucers

11.13 Soft-Paste Porcelain

There is a form of porcelain called “soft-paste porcelain” that was discovered in Europe in the late 16th century, again in the attempt to discover Chinese hard-paste porcelain. Whereas hard-paste porcelain contains a high percentage of kaolin, feldspar and silica, soft-paste porcelain contains little or no kaolin, and the principal ingredient is a glassy frit. Compared with hard-paste porcelain, fired in practice between 1350 and 1460 degrees C, soft-paste porcelain contains more flux and is usually fired between 1100 and 1350 degrees C. The body is more granular as it is not as vitrified. It was called soft-paste as it becomes much softer in the kiln than hard-paste porcelain. As with bone china it was fired in moulds dusted with quartz. The soft-paste bodies were generally brittle and not very robust as they could not withstand extremes of heat and cold, hence the habits of warming the teapot before pouring in boiling water and for the same reason putting the milk into the teacup first. The wares also tended to warp in the kilns causing high wastage. After the first biscuit or “bisque” firing, soft-paste porcelain is then glazed and glost fired.

The first French soft-paste porcelain had a complex set of constituents, reported to be 75% frit, 17% soft chalk and 8% chalky clay. The frit itself was made up of 60% silica, 22% potassium nitrate, 7.2% sodium chloride, and 3.6% each of aluminium-potassium sulphate, sodium carbonate and calcium sulphate. The “clay” body was not very plastic, so soft soap and gum tragacanth were added to make it sufficiently plastic to be worked. The vessels were then fired at 1250 degrees C.

The fired body was not absorbent, so the low temperature glaze (7-800 degrees C) had to be made viscous to fuse with it. However there were advantages with soft-paste porcelain decoration, as the overglaze enamels mingled/fused with the softened glaze providing a unique and attractive effect that hard-paste porcelain decorators tried but failed to reproduce. Also the lower temperatures permitted a wider palette of more delicate tones.

Chelsea soft paste cup and saucer

Chelsea soft paste cup and saucer

Standard palette for enamels - courtesy Worcester Porcelain Museum

Standard palette for enamels - courtesy
Worcester Porcelain Museum

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