Compojilion of Enamel. j^ 



the red Into vermilion, according to the quantity added. I( 

 >ve had certain procefles for making this colourj we (hould 

 obtain all the fliades of red from pure red to oraii,ge, by 

 Uiing, in different proportions, the oxyd of copper and that of 

 iron. The calx of copper fufcs argil more callly than filcx : 

 the cafe is the fame with calx of iron. If vou fufe two or 

 three parts of argil with one of the oxyd of copper, and if the 

 heat be fufHcient, you will obtain a very opake enamel, and 

 of a vermilion red colour: the oxvd of copper. palTcs from 

 red to green through yellow ; fo that the enamel of copper, 

 which becomes red at a ftrong heat, may be yellow with a 

 weaker heat. The fame eOciSt may be produced bv deoxv- 

 dating copper in different degrees: this will be eflectcd ac- 

 cording as the heat is more or lefs violent.. The above com- 

 polition might, I think, be employed to. give a vermilion red 

 colour to porcelain. The heat of the porcelain furnace ought 

 to be of fufiicient llrength to produce the proper effcft. 



The calx of iron fufed alfo with argil in the fame propor- 

 tions as the calx of copper, gives a very beautiful black. Thefc 

 proportions may, however, be varied. 



Blue. ■ 



Blue is obtained from the oxyd of cobalt. It is the mofl 

 fixed of all colours, and becomes equally beautiful with a 

 weak as with a ftrong heat. The blue produced by cobalt 

 is more beautiful the purer it is, and the more it is oxvdatcd. 

 Arfenic does not hurt it. The faline fluxes which contain 

 nitre are th:)fe befi: fuited to it: you add a little alfo when 

 you employ that flux which contains a little calcined borax 

 or glafs of borax, though you may employ it alfo with that 

 flux alone. 



But the (Uix which, Jiccording to my experiments, gives to 

 cobalt blue the greatcll fplendour and beauty, is that com- 

 pofed of white glafs (which contains no metallic calx), of 

 borax, nitre, and diaphoretic antimonv well wafted. When 

 this glafs is made for the purpofe of being employed as a 

 flux for blue, you may add lefs of the white oJ;vd of anti- 

 fijony : a fixih of the whole will be fuffi(;:ient. 



Vklh. 



