«| Bcfearches refpeShtg ttii 



to dircover that bell lulled to the mixture on which yotl 

 operate. 



A hundred parts of the calx above mentioned, which in the 

 Fiencii potteries is called calcine, is generally taken with lOO 

 parts of fund. From 25 to 30 pounds of fea-falt, or muriat of 

 f:>du a e a' d.d : the v\hole is well mixed together, and it is 

 fufed iu the bottom of a furnace in which potter's ware is 

 bhked. This matter is generally placed on fand, on lime 

 quenched in the open air, or on afhes. The bottom of the 

 mafs is in general badly fufed. This, however, does not 

 prevent the matter, after it has been pounded, and applied on 

 the articles, from becoming exceedingly white and hard in 

 the furnace. When taken from the furnace it is not while j 

 it is even often very black : in general it is marbled with 

 black, gray, and white. 



This procefs is that generally ufed in potteries. In the 

 •ompofitions deltined for earthen- ware, the proportion of 25 

 parts of tin to 100 of lead is never exceeded : for common 

 earthen-ware, the manufacturers are even fatisficd with 15 of 

 tin to 100 of lead. It may be eafily feen, that if you wifh 

 to obtain an enamel whiter and more fufihle, you muft di- 

 minifh the quantity of fand ; but there is no neceflity for 

 augmenting that of the fea-falt, or muriat of foda : as the 

 whitcnefs and opacity depend on the quantity of tin, you 

 may ufe calcine, which contains 25 or 30 per cent. For ex- 

 ample, 100 of fuch calcine, 60 of fand, and 25 of marine 

 fait, give a compofition exceedingly fufible. 



But it is to be obferved, that it is necefl'ary to employ fome 

 further manipulations when you wilh to have enamel proper 

 for being api)lied on metal, and are defirous to give it all the 

 pcrfeAion of which it is fufceptible. In that cafe, you do 

 not employ crude fixnd, but calcine it, in a ftrong heat, with 

 a quarter of its weight of marine fait, either in a fmall quan- 

 tity in a crucible, or on a large fcale in a potter's furnace. 

 If you wifh to have a very fufible enamel, you may even add 

 minium, or lead calcined by the former operation, and nearly 

 as much fca-fiilt, that is to fay, a fourth. You then obtain 

 a white mat's half fufed and porous, which you pulverife, and 

 >employ in the compofition of enamel inflead of fand, and in 

 7 the 



