COLOURS OBTAINED FROM METALLIC OXIDES. 107 



all thefe colours except one are unchangeable in the fire ; The flux is glafc 

 whereas it has been often afferted in books, that paintings in° a3 J a 

 enamel are fubjecl to confiderable change. 



When the porcelain is put in the fire to bake the colours, the The hard glare 

 feld fpar glaze dilates and opens its pores, but does not become dilute" th^co- 02 * 

 i'oft. As tiie colours do not penetrate it, they are not fubj eft lours, 

 to the changes they undergo on tender porcelain. It muft 

 however be obferved, that they lofe a little of their intenfity, 

 by acquiring the tranfparence given them by the fufion. 



When works of little importance are made they need nolfo that only fin» 

 be re-touched ; but this is neceflary when a painting is to be ^uGhing! S "" 

 highly finimed. This re-touching is not diftinguifhable in the 

 paintings on porcelain from that of any other fpecies of 

 painting. 



One of the great inconveniences of thefe colours, is the faci- The colours are 

 lity with which they fcale or fly off when the fire is often ap- apt t0 ac * 

 plied. 



This has been particularly remarked at SeVres, on account Hard porcelain I* 

 of the folidity and infufibility with which porcelain is there™* |£™ tifuI » 

 manufactured. JSut thefe qualities caufe it to refill the alter- changes of heat 



nations of heat and cold for a longer time, and gives its ground and coId bettei ' 



, •„• i -v i £ , , , , . than the foft. 



a more brilliant white colour. On the other hand, the porce- 

 lains of Paris being more vitreous, tranfparent, and of a blue- 

 ifti call, generally crack if boiling w T ater is frequently poured 

 info them. 



In order to remedy this evil without altering the qualify of The fcaiing is 

 the body, I foftened the glaze a little, by introducing moreJjJ t ^ d ^ 

 •filiceous or calcareous flux according to the nature of the feld glaze, 

 fpar. This method fucceeded, and for this twelvemonth pall 

 the colours have pafled two and three times through the fire 

 without cracking, provided there be not too much flux, and 

 they be not laid on too thick. 



It has been remarked, that when foda and potafli were in- Alkalies are too 

 troduced the colours fcaled ; fo that they cannot be ufed a« vo . ,at,le t , a . be 



a <t«i r ll t t • i m- i i » U * ea " ln tftlS art » 



fluxes, lhele alkalies being volatilized abandon the colours, 

 which cannot adhere to the glaze by themfelves. 



I have obferved that other colours are like wife prepared, Colours to be 

 which being laid upon the general furface, are fufed by the halted" by 1 the 

 fame fire as bakes the porcelain. Thefe colours are but few ; firftfire are few. 

 becaufe there are few metallic oxides that can fupport fuch a 



fire " 



