English porcelain and earthenware are made from the following bodies, which are prepared by soaking the clays in a large vessel of water, and, when of the consistence of slip, passing them through the finest silk lawn into another vessel in which proper gauges are fixed, so that the other materials may be afterwards added in a slop state. Clay slip should weigh 13 1/2 lb.; Cornish clay, 13 1/2 lb.; Cornish stone, 16 1/2 lb.; and flint, 16 1/2 lb. a gallon. The passing through the lawn is repeated as often as is needful, so that the mixture may be deprived of impurities. Care must be taken that the bones used for china bodies are not decayed, and for the other materials used in making porcelain, great care is necessary to see that they are of the purest kinds. These bodies fire at a higher temperature than that usually observed, and are placed and fixed in the furnace with ground flint. For the coloured bodies the marls used should be selected of the finest quality, argillaceous marl being the best; and very fine lawn will be required if it is intended that the body should be clean and free from metallic spots.
Clays in which the silicious ingredients are in proportion of three to one are the best for the use of porcelain; those in which argil is in excess are the best for coarser earthenware, because less acted upon by alkalies. The colours in clays produced by vegetables or bituminous particles are destroyed by heat in an open fire, and are by no means prejudicial; but those which arise from metallic particles are obstinate, and should be avoided as much as possible. Clays which contain argil and silex only are very refractory, but calcareous earths in the proportion of 10 to 12 per cent. will render any clay fusible. The clays for porcelain should be those which contain the most sand, and are of the greatest fineness; also such as do not retain water with too much tenacity, which is the case when argil is not combined with fixed air, therefore all clays ought to be exposed to the action of the atmosphere for a long time previous to using. Calcareous earth in its common form is limestone or spar, magnesia, etc, which in their pure state are not so easily dissolved as when combined with fixed air.
Argillaceous clay or alumina clay forms the basis of common alum; is called argil, and is never found pure; the finest part is extracted from alum, and is not fusible in the strongest heat required for china or earthenware Argil in its usual state of dryness is capable of absorbing 2 1/2 times its weight of water. Silicious earths found in a stony state abound in flint; the purest are found in crystals and quartz of a pure white; fixed alkalies, vegetables, or minerals are their true solvents. It should be understood that flint and bones, in all instances, are to undergo the process of calcination previous to using.
Articles formed of one of the bodies are first moderately burned in earthen pots, to receive a certain degree of compactness, and to be ready for glazing. The glaze consists of an easily melted mixture of some species of earths, which, when fused together, produce a crystalline or vitreous mass; this, after cooling, is very finely ground and suspended in a sufficient quantity of water. Into this fluid the rough ware is dipped, by which the glazing matter is deposited uniformly on every part of its surface. After drying, each article is thoroughly baked or fired in the violent heat of the porcelain furnace. It is usual to decorate porcelain by paintings, for which purpose enamels or pastes, coloured . by metallic oxides, are used, so easy of fusion as to run in a heat less intense than that in which the glazing of the ware melts.
235 parts blue clay, 225 calcined ochre, 45 manganese, 15 Cornish clay; the materials must be accurately examined on account of the manganese, which ought to be free from lime or other calcareous earth; the pieces of ware when manufactured are very apt to crack, because of the sudden transition from heat to cold, provided above a certain proportion of lime is contained in the manganese. This kind of earthenware requires only once burning, after which it is scoured with fine sand, and then a small quantity of oil is rubbed over it.
50 parts red clay, 7 1/2 common clay, 1 manganese, 1 flint.
32 parts yellow clay, 10 Cornish clay, 4 flint.
20 parts red or brown clay, 8 Cornish clay, 4 blue clay, 2 flint.
1 1/2 part blue clay, 1 1/2 brown clay, 1 black clay, 1 Cornish clay, 1 flint, 1/4 Cornish stone.
1 1/2 part blue clay, 1 1/2 brown clay, 1 1/2 black clay, 1 flint.
24 parts argillaceous marl, 48 Cornish stone, 24 blue clay, 10 bones, 1 calcined nickel.
40 parts marl, 4 Cornish clay, 1 flint.
40 parts argillaceous clay, 4 blue clay, 2 flint. This body makes porous wine and butter coolers, and water bottles, on the principle of absorption and evaporation. The articles are generally ornamented with various coloured clays; they should be kept in the wet clay state, at the time of being painted, otherwise the different colours laid upon them will not sufficiently adhere, but are liable to chip and peel off when burned. A moderate degree of heat must be applied, as too great a temperature will cause the body to be too dense, and prevent absorption; it will therefore be necessary to fire such articles in the easy parts of an earthenware biscuit oven.
(a) 300 parts Cornish stone, 250 Cornish clay, 200 blue or brown clay, 100 flint, 1 blue calx.
(6) 175 pts. Cornish stone, 150 Cornish * clay, 90 blue or brown clay, 35 flint, 5 body frit, 1/2 blue calx. These bodies are very ductile, and fire at the temperature of the common biscuit oven; each piece of ware should be perfectly dry when placed in the seggars. because they are made a great deal thicker than any other kind. Setters also should be used at the bottom of each piece, and ground flint applied, but not sand, for the placing or seating; the body, when burned, is quite vitrified, and the pieces of ware are strong and heavy, ringing remarkably shrill.