Porcelain - (Ceramics Porcelain)

Porcelain - (Ceramics, Porcelain) (1) HARD-PASTE. Hard or `True' porcelain contains two essential ingredients known to the Chinese as kao-lin (china clay) and pai-tun-tzu (china stone), both of which are products of feldspar rock in varying stages of decay. The main characteristic of kaolin is that it will take and retain almost any shape. Pai-tun-tzu, or petuntse, which is the more usual French form, acts as the cement. Kaolin requires a higher temperature to fuse it than does the petuntse. The mixture of refractory white kaolin and fusible petuntse unite in the firing into a dense, white, translucent, resonant material, namely porcelain. The temperature required to bring this about is approximately 1,450 degrees Centigrade. The mixture before firing is usually called the `paste'.
Porcelain comprises the `body' and the `glaze'. The latter is an outer skin containing petuntse and in hard porcelain is nearly always fired at the same time as and in one operation with the body. Rarely, the glaze is applied later in a second firing which will be at a somewhat lower temperature. Hard porcelain is translucent though the degree of translucency may vary greatly. It should also `ring' when struck. And it should be so hard that it will withstand efforts to cut it with the edge of a file. Worth bearing in mind is that for the Chinese the criterion for porcelain is that it `rings' when struck, whereas for the European translucency is all important.
It is generally agreed that porcelain was first made by the Chinese in the late seventh or early eighth century A.D. A merchant writing in 851 tells of drinking vessels made of a clay as fine as glass through which the shimmer of water could be seen. The body of surviving specimens cannot be scratched with steel and is white and translucent; but it is said that kaolin was not used. Those Chinese porcelains with which we are most familiar-of the Ming and Ching dynasties-do contain kaolin and a greater proportion of petuntse than is usual in European porcelain. This means that the wares are somewhat softer and did not require such a high firing temperature as the European.
The Chinese kept their secret and their monopoly for a millenium and it was not till the early years of the eighteenth century that Johann Friedrich Bottger produced the first true porcelain to be made in Europe, which led to the founding, in 1710, of the Royal Saxon Porcelain Manufacture at Meissen, near Dresden. A factory was established at Vienna some ten years later and by the mid-eighteenth century there were a number of German factories producing hard porcelain. S6vres, which at first had made soft-paste porcelain, introduced a hard-paste body in 1770. It may be said that on the Continent soft-paste formulae were used only until such time as the hard-paste formula could be obtained.
But this was not so in England, where true porcelain has been but little made. William Cookworthy experimented for many years before founding his factory at Plymouth in 1768. He made hard-paste porcelain there till 1770 when the factory was removed to Bristol. Cookworthy soon withdrew from the enterprise which continued under Richard Champion till about 1782. Some hard-paste porcelain was made at the small New Hall (Staffordshire) factory from about 1780 to 1812. All the other English factories made soft-paste porcelains. But from the early years of the nineteenth century `bone china' was the staple English product.

Porcelain - (2) SOFT-PASTE. Soft or `artificial' porcelain differs from hard-paste porcelain (see above) in that it is a `softer' material, that it requires less heat (about 1,100 degrees Centigrade as against 1,450 degrees Centigrade for hard) to fuse it, that it can be scratched or cut with metal (the edge of a file) that the glaze was always added afterwards. It was inevitable the European potters desirous of producing a ware that would partake of the translucency of Chinese porcelain should introduce glass into the mix. This was first done successfully at Florence about 1560, the product being known as Medici porcelain. Surviving examples are very rare indeed. It was not till the beginning of the eighteenth century that any great quantity of porcelain was produced. St Cloud was probably the first successful French factory (from before 1700), followed by Chantilly, Mennecy, Vincennes, S6vres; in Italy, Nove (before 1730), Doccia, Capo-di-Monte ...
Whereas in Europe the basic ingredients were clay and ground glass, in England bone-ash and, in a few cases, soaprock, were preferred to glass. Bone-ash makes for easier working and seems to have been first used at Bow about 1750, then later at Chelsea, Derby and other centres. Soaprock was first used at Bristol in 1748, other factories to use it being Worcester, Caughley, Liverpool.


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