S ^efeafchss refpeBtng thi 



pofe this fand by art ; and though I have not dccompofed lt> 

 I have found by fvnthefis, that three parts of filiceous fand 

 and one of talc form an excellent fand for enamel. From 

 this it may be readily fecn, that, to compofewith facility fand 

 for enamel, nothing is necelTary but to determine, by a good 

 analvfis, the quantity of talc. This fand may be prornred in 

 places where earthen-ware is made. It may be eafily known j 

 for, befides the filiceous fand, which forms the greateft part 

 of it, you may obferve in it talcky particles in great abun- 

 dance; and, to be good, it mufl: contain nearly a quarter. 

 When it does not contain a^fufficient quantity, the enamel it 

 produces fufes with more difficulty, and does not become 

 fmooth ; it remains granulated and pitted. There are cer- 

 tainly fome combinations of earth which may produce very 

 good fluxes, either for enamel or for tranfparent colours. It 

 might be attended with advantage to try fome of thefe com- 

 binations. Ponderous earth (barytes) and lime fufe very 

 well together : by adding a little filex, or a little magnefia, 

 it is probable that an excellent matter might be produced. 

 If this glafs, compofed of lime and barytes only, had fuffi- 

 cient folidity to refill the air and weak acids, there would be 

 no neceffity perhaps to add filex ; but if the marine fait, as 

 I am inclined to think, ought alfo to enter into the compo- 

 fition of this kind of glafs, filex ought likewife to form a part 

 of it. The experiments on this head, for the fake of trial, 

 may be varied different ways. When the glafs deftined to 

 ferve as flux for colours is employed, it is cullomary, in order 

 that they may be rendered more fufible, to add a little nitre 

 and borax. The common borax of the fhops contains an 

 excefs of foda, which, in my opinion, it would be of benefit 

 to faturate with the nitric acid. I think alfo that the flux 

 mio-ht be rebaked with the dofe of nitre and borax, or of 

 nitric borax, which might be added before being employed. 

 It is only to colours fuch as purple and the oxyd of cobalt 

 that nitre and borax are added. 



I have tried to find a fubftitute for marine fait in the com- 

 pofition of white enamel. Potafh produced only an ugly 

 and ill fufed gray mafs, which acquired no luftre in the fur- 

 nace j nitre produced a green mafs, but exceedingly friable 3 



fulphat 



