Porcelain and Earthenware. 255 



parts to ten of lead. These ingredients are reduced to a fine pow- 

 der, and mixed with as much water as will make them into a thick 

 cream. The mixture must be well stirred to keep them equally 

 suspended in the water. The pieces are then dipped in the fluid, 

 and turned rapidly from side to side to equalize the glaze, when they 

 are set on a board for a few minutes, and then are ready for the sag- 

 gars. The gloss oven completes the series, and after being subject- 

 ed to a degree of heat sufficient to vitrify the glaze, and unite it to 

 the body of the ware, the oven and its contents are again gradually 

 cooled, the manufacture is completed, and the ware is ready for the 

 market. 



It is a desideratum with manufacturers to find some glaze in which 

 lead may be dispensed with on account of its noxious effects upon 

 the health of the workmen, and the injury produced by the decom- 

 position, of its oxides when exposed to the action of acids. The 

 glazing introduced in England by the Ellers, (of throwing salt into 

 the apertures of the kiln, when the baking was nearly completed,) 

 is still practised for some of the common kinds of ware. 



M. Brongniart, states that " real or hard porcelain, which is that 

 of Saxony, has for its base a very white clay mixed with a siliceous, 

 and calcareous flux, and for its glaze or covering, felspar fused with- 

 out an atom of lead."* 



Twenty seven parts of felspar, eighteen of borax, four of Lynn 

 sand, three of nitre, three of soda, and three of China clay melted 

 together, and when cold, with three parts more of calcined borax, 

 ground to a fine powder, make a glaze which is successfully employ- 

 ed in one of the English establishments, and has met the approbation 

 of the Society for the Encouragement of Arts. Ground flints, 

 ground flint glass and common salt form another glaze, while another 

 and better still is of ground porcelain, flint and calcined gypsum. 

 The variety of glazes, however, is almost endless, and they are adop- 

 ted as they are found advantageous in practice, though still treasured 

 up as secrets in most countries, except France. 



As alkalies are so powerful in promoting the fusion of intractable 

 bodies, it might be anticipated that they should supersede the use of 

 lead ; but it is found in practice that when they are employed beyond 

 a certain amount they do not expand in the same proportion as the 



# Brongniart's Essay on Colors obtained from Metallic oxides, Philosophical Mag- 

 azine. Vol. xiii. p. 346 et xiv. p. 17 et seq. 



