398 
j Researches respecting the 
Med . 
We have no metallic oxyd capable of giving directly 
a fused red ; that is to say, we have no metallic calces 
which, entering into fusion, and combining under the 
form of transparent glass with fluxes or glass, give direct- 
ly a red colour. To obtain this colour it must be com- 
pounded different ways, as follows Take two parts, 
or two parts and a half (you may, however, take only 
one part,) of sulphat of iron and of sulphat of alumine^ 
fuse them together in their water of crystallisation, and 
take care to mix them well together. Continue to heat 
them to complete dryness $ then increase the fire so as to 
bring the mixture to a red heat. The last operation 
must be performed in a reverberating furnace. Keep the 
mixture red until it has every where assumed a beautiful 
red colour, which you may ascertain by taking out a lit- 
tle of it from time to time, and suffering it to cool in the 
air. You may then see whether the matter is sufficiently 
red: to judge of this it must be left to cool, because while 
hot it appears black. The red oxyds of iron give a red 
colour ; but this colour is exceedingly fugitive ; for, as 
soon as the oxyd of iron enters into fusion, the portion of 
oxygen which gives it its red colour leaves it, and it be- 
comes black, yellow, or greenish. To preserve, there- 
fore, the red colour of this oxyd in the fire, it must be pre- 
vented from vitrifying, and abandoning its oxygen ; which 
may be accomplished by the method I have indicated, I 
have tried a variety of different substances to give it this 
fixity, but none of them succeeded except alum. The 
doses of alum and sulphat of iron may be varied. The 
more alum you add, the paler will be the colour. Three 
parts of alum to one of sulphat of iron give a colour which 
approaches flesh colour. It is alum also which gives 
this colour the property of becoming fixed at a very strong 
