POTTERY 615 



contain. This being continued till the oven is filled, the aperture is then bricked 

 up. The firing of earthenware bisque continues sixty hours, and of china forty-eight. 

 The quantity of coals necessary for a 'bisque' oven is from 16 to 20 tons; for a 

 'glost ' oven from 4.} to 6 tons. 



The ware is allowed to cool for two days, when it is drawn in the state technically 

 called ' biscuit ' or bisque, and is then ready for ' glazing,' except when required for 

 printing or a common style of painting, both of which processes are done on the 

 bisque prior to being ' glazed.' 



Tender porcelain, or soft china-ware, is made with a vitreous frit, rendered less 

 fusible and opaque by an addition of white marl or bone-ash. The frit is, therefore, 

 first prepared. This, at Sevres, is a composition, made with some nitre, a little sea 

 salt, Alicant barilla, alum, gypsum, and much siliceous sand or ground flints. That 

 mixture is subjected to an incipient pasty fusion in a furnace, where it is stirred about 

 to blend the materials well ; and thus a very white spongy frit is obtained. It is 

 pulverised, and to every three parts of it, one of the white marl of Argenteuil is 

 added ; and when the. whole are well ground, and intimately mixed, the paste of 

 tender porcelain is formed. 



As this paste has no tenacity, it cannot bear working till a mucilage of gum or black 

 soap be added, which gives it a kind of plasticity, though even then it will not bear the 

 lathe. Hence it must be fashioned in the press, between two moulds of plaster. The 

 pieces are left thicker than they should be ; and when dried, are finished on the lathe 

 with iron tools. 



In this state they are baked, without any glaze being applied ; but as this porcelain 

 softens far more during the baking than the hard porcelain, it needs to be supported 

 on every side. This is done by baking on earthen moulds all such pieces as can be 

 treated in this way, namely, plates, saucers, &c. The pieces are reversed on these 

 moulds, and undergo their shrinkage without losing their form. Beneath other 

 articles, supports of a like paste are laid, which suffer in baking the same contraction 

 as the articles, and of course can serve only once. In this operation saggers are 

 used, in which the pieces and their supports are fired. 



The kiln for the tender porcelain at Sevres is absolutely similar to that for the com- 

 mon earthenware ; but it has two floors ; and while the biscuit is baked in the lower 

 story, the glaze is fused in the upper one ; which causes considerable economy of 

 fuel. The glaze of soft porcelain is a species of glass or crystal prepared on purpose. 

 It is composed of flint, siliceous sand, a little potash or soda, and about two-fifth parts 

 of lead oxide. This mixture is melted in crucibles or pots beneath the kiln. The 

 resulting glass is ground fine, and diffused through water mixed with a little vinegai 

 to the consistence of cream. - All the pieces of biscuit are covered with this glazy 

 matter, by pouring this slip over them, since their substance is not absorbent enough 

 to take it on by immersion. 



The pieces are encased once more each in a separate sagger, but without any sup- 

 ports ; for the heat of the upper floor of the kiln, though adequate to melt the glaze, 

 is not strong enough to soften the biscuit. But as this first vitreous coat is not very 

 equal, a second one is applied, and the pieces are returned to the kiln for the third 

 time. See STONE, ARTIFICIAL, for a view of this kiln. 



The manufacture of soft porcelain is longer and more difficult than that of hard ; 

 its biscuit is dearer, although the raw materials may be found everywhere ; and it 

 furnishes also more refuse. Many of the pieces split asunder, receive fissures, or become 

 deformed in the biscuit-kiln, in spite of the supports ; and this vitreous porcelain, 

 moreover, is always yellower, more transparent, and incapable of bearing rapid transi- 

 tions of temperature, so that even the heat of boiling water frequently cracks it. It 

 possesses some advantages as to painting, and may be made so gaudy and brilliant in 

 its decorations, as to captivate the vulgar eye. 



The best English porcelain is made from a mixture of the Cornish and Devonshire 

 kaolin (called china-clay), ground flints, ground Cornish stone, and calcined bones 

 in powder, or bone-ash, besides some other materials, according to the fancy of the 

 manufacturers. A liquid pap is made with these materials, compounded in certain 

 proportions, and diluted with water. The fluid part is then withdrawn by the 

 absorbent action of dry stucco basins or pans. The dough, brought to a proper stiff- 

 ness, and perfectly worked and kneaded on the principles detailed above, is 

 fashioned on the lathe, by the hands of modellers, or by pressure in moulds. The 

 pieces are then baked to the state of biscuit in a kiln, being enclosed, of course, in 

 saggers. 



This biscuit has the aspect of white sugar, and being very porous, must receive a 

 vitreous coating. The glaze consists of ground felspar or Cornish stone. Into this, 

 diffused in water, along with a little fire-powder and potash, the biscuit ware is 

 dipped, as already described. The pieces are then fired in the glaze-kiln, care being 



