Cempojzt'ion of Enamel, It 



Veilow, or greenlfli. To preferve, therefore, the red colour 

 of this oxvd in the fire, it mull be prevented from vitrifying, 

 and abandoning its oxygen ; which may be accompUflied by 

 the method I have indicated. I have tried a variety of dif- 

 ferent fubftances to give it this fixity, but none of them fuc- 

 ceeded except akmi. Tiie dofes of alum and fulphat of iron 

 may be varied. The more alum you add, the paler will be 

 the colour. Three parts of alum to one of fulphat of iron 

 give a colour which approaches flefli colour. It is alum alfo 

 which gives this colour the. property of becoming fixed at a 

 very flrong heat. This colour may be employed on raw 

 enamel : it has much more fixity than the purple, but not io 

 much as the blu(? of cobalt. It may be wafhed to carry off the 

 fuperfluous faline matter, but it may be employed alfo with- 

 out edulcoration; in that flate it is even more fixed and mor^ 

 beautiful. It does not require much flux; the flux which 

 appeared to me to be belt fuitcd to it, is compofed of alum, 

 minium, marine fait, and enamel fand. This flux muft be 

 compounded in fuck a manner as to render it fufficiently ■ 

 fufible for its objeft : from two to three parts of it are 

 mixed with the colour. In general, three parts of flux are 

 ufed for one of colour: but this dofe may, and ought to be 

 varied according to the nature of the colour and the fliadc of 

 it required. Red calx of iron alone, when it enters into fufioix 

 with jrlafs, trives a colour which fecms to be black; but if 

 the colour be diluted with a fufhclent quantity of glafs, it at 

 lalt becomes of.a tranfparent yellow. Thus, the colour really 

 produced by calx of iron combined with glafs is a yellow 

 colour, but which being accumulated becomes fo dark that 

 it appears black. In the procefs above given for making the 

 red colour, the oxyd of iron does not fufe : and this is ths 

 cifenlial point; for, if this colour is carried in the fire to vi- 

 trification, it becomes black, or yellowlfli, and dlfappears if 

 the coat be thin, and the oxyd of iron prefent be only m a 

 fmall (luantity. 



Yclkiu. 



• Though yellow may be obtained in a dife(?t manner, com- 

 pound yellows are preferred ; bccaufe they are more certain 

 in their cnic% and more eaiily applied than the yellow, 

 C i which 



