612 POTTERY 



because it.is smooth enough; but is immediately transported to the place where it is 

 to be dipped in the glaze- or enamel-tub. A child makes the pieces ring, by striking 

 with the handle of the brush, as he dusts them, and then immerses them into the 

 glaze-cream ; from which tub they are taken out by the enameller, and shaken in the 

 air. The tub usually contains no more than 4 or 5 inches depth of the glaze, to 

 enable the workman to pick out the articles more readily, and to lay them upon a. 

 board, whence they are taken by a child to the glaze-kiln. 



OF PORCELAIN. 



Porcelain is a kind of pottery ware whose paste is fine grained, compact, very hard, 

 and faintly translucid ; and whose biscuit softens slightly in the kiln. Its ordinary 

 whiteness cannot form a definite character, since there are porcelain pastes variously 

 coloured. There are two species of porcelain, very different in their nature, the 

 essential properties of which it is of consequence to establish ; tbe one is called hard, 

 and the other tender or soft: important distinctions, the neglect of which has introduced 

 great confusion into many treatises on this elegant manufacture. 



Hard porcelain is essentially composed, first, of a natural clay containing some 

 silica, infusible, and preserving its whiteness in a strong heat ; this is almost always a 

 true kaolin ; secondly, of a flux, consisting of silica and alkali, composing a quartzose 

 felspar rock, called pe-tun-tse. The glaze of this porcelain, likewise earthy, admits of 

 no metallic substance. 



The biscuit of the hard porcelain made at the French national manufactory of 

 Sevres is generally composed of a kaolin-clay, and of a decomposed felspar-rock ; 

 analogous to the china-clay of Cornwall and Cornish stone. Both of the above 

 French materials come from Saint Yrieux-la-Perche, near Limoges. 



After many experiments, the following composition has been adopted for the service 

 paste of the Royal manufactory of Sevres ; that is, for all the ware which is to be 

 glazed : silica, 59 ; alumina, 35'2 ; potash, 2'2 ; lime, 3'3. The conditions of such a 

 compound are pretty nearly fulfilled by taking from 63 to 70 of the washed kaolin or 

 china-clay, 22 to 15 of the felspar, nearly 10 of flint-powder, and about 5 of chalk. The 

 glaze is composed solely of solid felspar, calcined, crushed, and then ground fine at 

 the mill. This rock pretty uniformly consists of silica 73, alumina 16'2, potash 8-4, 

 and water 0'6. 



The kaolin is washed at the pit, and sent in this state to Sevres, under the name of 

 decanted earth. At the manufactory it is washed and elutriated with care ; and its 

 slip is passed through fine sieves. This forms the plastic, infusible, and opaque 

 ingredient to which the substance must be added which gives it a certain degree of 

 fusibility and semi-transparency. The felspar-rock used for this purpose should 

 contain neither dark mica nor iron, either as an oxide or sulphide. It is calcined to 

 make it crushable under stamp-pestles driven by machinery, and then ground fine in 

 hornstone (chert) mills. This pulverulent matter being diffused through water, is 

 mixed in certain proportions regulated by its quality, with the argillaceous slip. The 

 mixture is deprived of the chief part of its water in shallow plaster pans without 

 heat ; and the resulting paste is set aside to ripen, in damp cellars, for many months. 



When wanted for use, it is placed in hemispherical pans of plaster, which absorb 

 the redundant moisture ; after which it is divided into small lumps, and completely 

 dried. It is next pulverised, moistened a little, laid on a floor, and trodden upon by 

 a workman marching over it "with bare feet in every direction; the parings and 

 fragments of soft moulded articles being intermixed, which improve the plasticity of 

 the whole. When sufficiently tramped, it is made up into masses of the size of a 

 man's head, and kept damp till required. 



The dough is now in a state fit for the potter's lathe ; but it is much less plastic 

 than earthenware paste, and is more difficult to fashion into the various articles ; and 

 hence one cause of the higher price of porcelain. 



The round plates and dishes are shaped on plaster-moulds ; but sometimes the paste 

 is laid on as a crust, and at others it is turned into shape on the lathe. When a crust 

 is to be made, a moistened sheep-skin is spread on a marble table ; and over this the 

 dough is extended with a rolling-pin, supported on two guide-rules. The crust is 

 then transferred over the plaster-mould by lifting it upon the skin ; for it wants 

 tenacity to bear raising by itself. When the piece is to be fashioned on the lathe, a 

 lump of the dough is thrown on the centre of the horizontal wooden disc, and turned 

 into form as directed in treating of earthenware, only it must be left much thicker than 

 in its finished state. After it dries to a certain degree on the plaster-mould, the work- 

 man replaces it on the lathe, by moistening it on its base with -a wet sponge, and 

 finishes its form with an iron tool. A good workman at Sevres makes no more than 

 from 15 to 20 porcelain plates in a day ; whereas an English potter, with two boys, 



