PORCELAIN. An argillaceous substance characterized by its translucency and vitrifica tion; hence the ware made from the same is often so termed. These two qualities are not found combined in other pottery, earthenware being porous and opaque and stoneware being vitreous but not translucent. The derivation of the word is not clear, but some claim it comes from the cowry shell named porcellana (Italian) or porcelana (Spanish), part of the enamel coating of which has the appearance of porcelain.
The body or substance of which porcelain is made is termed paste or pate (French). The Chinese hard-paste porce lain (first to become known to Western civiliza tion) is made up of certain proportions of clays known as kaolin and petuntse or cornish stone. Kaolin is frequently called china clay and cor nish clay on account of its most prolific source of high quality being Cornwall (also Devon), England. Both kaolin and cornish stone are the natural products of weathered granite (pegmatite). Kaolin is a silicate or alumina and infusible, while cornish stone (petuntse), a feldspar containing silicate of alumina com bined with potash, soda and lime or barium, is fusible and acts in the furnace to produce with the kaolin a molten mass of homogeneous texture and having conchoidal fracture like glass. Porcelain is generally divided into two main classes known as hard-paste and soft paste or natural and artificial. The French know them as piite dure and pate tendre, terms very frequently used by English-speaking con noisseurs. In general it may be said that the hard-paste is not scratched with a file or steel point while soft-paste is abraded under such a test. Soft-paste is slightly porous. While hard paste porcelain is, as before stated, produced (body and glaze) from koalin and china stone, soft-paste was made (the Chinese clays being unknown at first) in imitation of the Eastern wares from substitutes after an enormous amount of research. But the old Sevres soft paste was an artificial body and glaze of as tonishing beauty and perfection, its unctuous, soft tone contrasting much in its favor on com parison with the glaring whiteness of the Chinese natural porcelain body. Brongniard,
the great ceramist and director at the Sevres works (middle 18th century), gives the follow ing formula as a favorite at the royal factory: molten nitre, 0.220; gray sea salt, 0.072; alum, 0.036; Alicant soda, 0.036; Montmartre gypsum, 0.036; Fontainebleau sand, 0.600; all made into a frit (fused into a vitrified mass, then pulver ized). This glassy substance was mixed with the following ingredients, in the following pro portions, to produce the actual pate: Frit (as above) 0.75; calcareous marl from Argenteuil, washed, 0.068. A glaze formula was: calcined Fontainebleau sand, 027; calcined flint, 0.11; litharge, 0.38; carbonate of soda, 0.09; carbonate of potash, 0.15. The physical softness of this pate and glaze allowed the colors in tne decora tion to sink into paste and glaze so as to leave a lovely mellowness like wax. In 1769 Sevres started hard-paste. One considerable advantage which soft-paste has over hard-paste from the manufacturing point of view is the fact that less heat is required in the baking; hut, hard paste firing at from 2,462° F. to 2,642° F. with safety, while soft-paste permits only a margin of heat of from 2,012° F. to 2,102° F. gives hard-paste a safety margin against crazing of 180° F. against a range of but 90° F. for soft paste firing. The firing, which is done in specially constructed ovens, takes place after the wares are stacked in safety, protected by fire-clay covers, known as saggers. The in candescent heat of the hard-paste furnace is generally called by experts by the French term grand feu, sometimes hard fire, while the lesser heat required in baking the body of other wares is termed demi grand feu. The preparation of the clays for porcelain is done in blungers, sifting machines, etc.; the forming or shaping of the mass or body on the potters' mold or in the modern turning lathes, "jiggers," gjollies,D all of which processes common to all classes of pottery, do not belong under this specialized head (see CERAMICS; CLAY; POT TERY, and related references).