Composition of Enarnel. 391 
But it is to be observed, that it is necessary to employ 
some further manipulations when you wish to have ena- 
mel proper for being applied on metal, and are desirous 
to give it all the perfection of which it is susceptible. In 
that case, you do not employ crude sand, but calcine it, 
in a strong heat, with a quarter of its weight of marine 
salt, either in a small quantity in a crucible, or on a large 
scale in a potter’s furnace. If you wish to have a very 
fusible enamel, you may even add minium, or lead cal- 
cined by the former operation, and nearly as much sea- 
salt, that is to say, a fourth. You then obtain a white 
mass half fused and porous, which you pulverise, and em- 
ploy in the composition of enamel instead of sand, and in 
the same proportions as sand : you may even diminish the 
quantity of this matter to 50 per cent, if you are desirous 
to obtain an enamel very fusible. This will depend also 
on the calcine employed ; for that which is most charged 
with tin is the least fusible. 
When you wish to have fluxes for the colours, you em- 
ploy the same compositions before mentioned, except that 
you put little or no tin into the lead. In the latter case 
you must generally employ minium. This flux is good 
for certain colours, but not for all. There are some 
which become tarnished by fluxes, that contain the oxyds 
of lead. In that case, you must make fluxes without 
oxyd of lead. Nitre and borax are generally used for 
making this glass, but you add no calx of tin. The fol- 
lowing are those which I have tried : 
Three parts of siliceous sand, one of chalk, and 
three of calcined borax, give a matter proper to be 
used as a flux for purples, blues, and other delicate 
colours. 
Three parts of white or flint glass, one of calcined bo- 
rax, a quarter of a part of nitre, one of the white oxyd of 
antimony made with nitre well washed, give an exceed 
