248 Porcelain and Earthenware. 
then ground in water in a mill of very hard stones, and in this semi- 
fluid state, passed off in troughs, through a succession of sieves, each 
finer than the other, until divested of every coarse particle. Great 
care is to be used that the stones employed in the mills do not con- 
tain any calcareous substances.* A very hard siliceous stone call- 
ed chert has been employed in the English manufactories for this 
purpose. 
Porcelain clay with 100 or 200 per cent. of sand would make a 
perfectly opaque body, therefore it is essential to add some alkaline 
material as a flux, to give the ware its semitransparency. ‘The clays 
used for porcelain in China, probably contain this ingredient in their 
native state, which may account for the superior fineness, hardness 
and semitransparency of the Chinese wares. None of the Europe- 
an clays are identical with the Chinese. ‘The difficulty of adding 
correctly the fusible ingredient which is so critical in its adjustment ; 
too much rendering the ware vitreous and liable to crack, too little, 
forming an opaque substance, incapable of that semifusion essen- 
tial to the real porcelanous texture ; is sufficient cause why European 
porcelain is inferior to the Chinese. By some the superiority of the 
Chinese is imputed to the great degree of heat which is never reach- 
ed except in the furnaces of Persia and China; but the more prob- 
able cause is a difference in the materials. 
It is a curious fact which the manufacturer should not lose sight of, 
that alumine, silex and lime when separate cannot be melted in furna- 
ces, but when mixed in certain proportions are readily fused ; the one 
mineral acting as a flux upon the other. ‘The perfection of pdreclatt 
appears to be obtained when the proportions of the pure ingredients 
are such, as that the highest and longest heat of the furnace, reaches the 
point of fusing the silex, and thoroughly incorporating it with the alu- 
mine, without melting it, or diminishing its volume. Undecomposed 
felspar is sometimes added to the porcelain clay and flint, to produce 
the desired semi-transparency. Its fusible property is owing to the 
presence of about an eighth part of potash, which acting asa flux 
upon the silex, causes a semi-vitrification of the whole mass. Vau- 
quelin says that silex forms two thirds of most pottery—alumine from 
* Mr. Parkes states that a severe loss was sustained by some large manufactur- 
ers, in consequence of having been supplied with prepared flint, which had been 
ground on stones, containing carbonate of lime. The abrasion of the stones mixed 
an unknown quantity of lime with the flint. 
