Porcelain and Ear thenware. 251 



of the English manufacturers at the present day, in regard to glazes 

 and enamels, and their proportions of clay and flint. M. Brong- 

 niart, director of the Sevres works, with a liberality and patriotic 

 ax-ling which merit the highest praise, published accurate statements 

 of many details and processes, based upon scientific principles. 



It can scarcely be too often repeated that all earthy compounds when 

 exposed to a red heat, act chemically upon one another; and as the 

 precise nature of those actions is unknown, every manufacturer who 

 would avoid disappointment and loss, should examine his materials 

 with the severest chemical analyses.* It needs the most accurate 

 observation, with a sound judgment, so to proportion the ingredi- 

 ents for the body of the ware, that it will receive the glaze without 

 crazing; and so to adapt the heat to both, as to reach the precise point 

 of fuzing the glaze, and uniting it with the ware previously semi-vitri- 

 fied, and liable to farther changes in the furnace. f 



When the paste has gone through all the preparatory processes 

 and has become thoroughly amalgamated by lying long in a mass, a 

 result which time alone can give it, it is ready to be fashioned into 

 any form designed by the artificer. J This is performed in three 

 ways, by throwing, pressing and casting.^ The first is done by 

 that ancient machine the potter's wheel, where circular vessels of al- 

 most every form and size are made by a succession of lathes. 

 For plates, saucers, tablets, &c. the workman has a mould fixed to 

 the center of a circular board, or table, which revolves horizontally. 

 Over the mould he places a covering of the clay, which by the aid of 

 his hand, and occasionally a small instrument of metal or wood, is 

 turned in a moment to the figure of the mould — is taken off and set 

 on a shelf by an assistant workman — is replaced by another, which 

 in its turn is worked off: and thus in a few hours a vast num- 

 ber of pieces are prepared for the next step of the process. The 

 moulds are made of plaster of Paris which has the property of ab- 



• The necessity of chemical analysis will appear, from comparing three native 

 substances much in use. The porcelain earlh of Limoges, which is often used with- 

 out any admixture, is composed of sixty two parts silex, ten alumine, twelve mag- 

 nesia, seven sulphate of Barytes ; whereas the porcelain clay of Cornwall, is a com- 

 pound of twenty per cent silex, and sixty per cent alumine. 



t Crazing is a technical word, signifying the cracking of the glaze, arising from a 

 defective union of the glaze with the body of the ware. 



t Clay does not readily part with water, beyond a certain amount, therefore the 

 mass does not dry by evaporation. The surface would dry if exposed to the sun. 



§ New Edinburgh Encyclopedia. 



Vol. XXVI.— No. 2. 33 



