As for the English soft-paste porcelain of the eighteenth century, there is a remarkable dearth of information both as to its composition and as to its manufacture. We know in fact in much greater detail how the great potteries at King-te-chen were carried on at the same period, thanks to the letters of the Père D’Entrecolles, and to the information collected in Dr. Bushell’s great work, _Oriental Ceramic Art_ (New York, 1899. I shall always quote from the text edition).
The following technical notes are based chiefly on the processes in use either at Sèvres or in the great factories of the Limoges district.[7] To begin with the Kaolin, the ‘premier’ element in the composition of porcelain. The greatest care is taken to procure a pure white clay which should approach as near as possible to the more or less theoretical mineral kaolinite, _i.e._ to a hydrous silicate of alumina. With this object the rough china-clay brought from the pit is thrown into a large tank of water and broken up with wooden spades; the milky liquid is now decanted into a second tank, leaving behind most of the quartz and the other stony particles. On its way the soup-like liquid passes through the meshes of a sieve--these may be formed either of brass wire or sometimes of finely woven silk. On this sieve all but the finest particles are retained. The greater part of the kaolin is deposited in this second tank, but a certain portion still remains suspended in the liquid, which is again decanted; the remaining kaolin then settles down in the third tank, yielding the finest clay. To dry this slimy mass, it is first forced by hydraulic pumps into canvas bags, and these bags are then pressed between fluted wooden trays, strongly clamped together. We have now got a white chalky mass which may contain as much as 98 per cent. of the hydrated silicate of alumina.
The other materials, the china-stone[8] and the quartz, have first to be reduced to the finest powder. To effect this they may, to begin with, be roasted to effect disintegration, then crushed in a stone-breaking machine, and finally passed through the grinding-pan in which they are ground fine between large blocks of chert which rotate upon a pavement of the same stone. The finely ground materials have now to be mixed in suitable proportions either by the old process of ‘slop-blending,’ where the different ‘slops,’ each of known specific gravity, are run in due proportion into the big ‘blending ark,’ or, as is now usual in the case of fine wares, by weighing out the materials in a dry state. On the relative amounts of the three elements, the china-clay, the china-stone, and the quartz, the nature of the porcelain after firing will depend. M. Vogt (_La Porcelaine_, Paris, 1893) gives a useful table showing the limits within which the materials may be varied. We may note that in the case of a normal china-stone or petuntse being used instead of felspar, very little additional quartz is required. These limits are: kaolin, 35 to 65 per cent.; felspar, 20 to 40 per cent.; and quartz, 15 to 25 per cent. The larger the percentage of the first material, the harder and more refractory will be the resultant porcelain.