Composition of Enamel, 399 
heat This colour may be employed on raw enamel : it 
has much more fixity than the purple, but not so much as 
the blue of cobalt. It may be washed to carry off the su- 
perfluous saline matter, but it may be employed also 
without edulcoration $ in that state it is even more fixed 
and more beautiful. It does not require much flux ; the 
fiux which appeared to me to be best suited to it, is com- 
posed of alum, minium, marine salt, and enamel sand* 
This fiux must be compounded in such a manner as to 
render it sufficiently fusible for its object: from two to 
three parts of it are mixed with the colour. In general,, 
three parts of fiux are used for one of colour : but this dose 
may, and ought to be varied according to the nature of 
the colour and the shade of it required. Red calx of iron 
alone, when it enters into fusion with glass, gives a co- 
lour w hich seems to be black ; but if the colour be diluted 
with a sufficient quantity of glass, it at last becomes of a 
transparent yellow. Thus, the colour really produced 
by calx of iron combined with glass is a yellow colour, 
but which being accumulated becomes so dark that it ap- 
pears black. In the process above given for making the 
red colour, the oxyd of iron does not fuse : and this is the 
essential point ; for, if this colour is carried in the fire to 
vitrification, it becomes black, or yellowish, and disap- 
pears if the coat be thin, and the oxyd of iron present be 
only in a small quantity. 
Yellow . 
Though yellow may be obtained in a direct manner, 
compound yellows are preferred ; because they are more 
certain in their effect, and more easily applied than the 
yellow, which may be directly obtained from silver. The 
compound yellows are obtained in consequence of the 
same principles as the red colour of iron. For this pur- 
pose we employ metallic oxyds, the vitrification of which 
