ENAMELLING. 
which they pretend to give the receipt 
would never be fabricated. They only 
serve to shew an able practitioner the me- 
thod, and leave it to him to correct or make 
additions. This was found to be the case by 
Citizen Meraud, who was engaged to pre- 
pare them for the manufactory of S6vres. 
He was obliged to make the colours for 
painting on glass rather from his own expe- 
rience than from the instructions in the 
work just mentioned. 
The materials and fluxes which enter into 
the compositions of the colours for painting 
on glass are, in general, the same as those 
applied to porcelain. They vary only in 
their proportions ; but a great number of 
the colours used for enamel and porcelain 
cannot be applied on glass ; many of them, 
when seen by transmitted light, entirely 
change their aspect, and exhibit an ob- 
scure tint, which can be of no use when 
deprived of the white ground which throws 
them out. We shall point out these when 
we treat of the colours in particular. Those 
colours which can be used on this body 
sometimes change in the baking, and ac- 
quire a great transparency. They are ge- 
nerally beautiful only when placed between 
the eye and the light, and they answer the 
purpose intended in painting glass. 
There is more difficulty in baking plates 
of coloured glass than is commonly thought. 
The bending of the piece, and the alteration 
of the colours, are to be avoided. All the 
treatises we have consulted recommend 
the use of gypsum. This method some- 
times succeeded with Brougniart, but gene- 
rally the glass became white, and cracked 
in all directions. It appears, that the 
glasses which are too alkaline, and which 
are far the most common in clear white 
glasses, are attacked by the hot sulphuric 
acid of the sulphate of lime. He was able 
with ease to bake much larger glasses than 
any before painted, by placing them on 
very smooth plates of earth or unglazed por- 
celain. 
Concerning the several particular Colours. 
After having collected the several pheno- 
mena which each class of vitrifiable colours 
offer with regard to the bodies on which 
they are placed, we must shew the particular 
and most interesting phenomena which every 
principal species of colours employed on 
tender porcelain, on glass, and in the fire 
that bakes the porcelain present. 
Concerning the Reds, Purples, and Violets ob- 
tained from Gold. 
The carmine-red is obtained from the 
purple precipitate of Cassius. It is mixed 
with about six parts of its flux, and this 
mixture is directly employed without being 
first fused. It is then of a dirty violet, but 
acquires the beautiful carmine by baking. 
It is however very delicate ; a little too 
much heat or carbonated vapours easily 
spoil it ; yet it is more beautiful when baked 
with charcoal than with wood, , 
This colour, and the purple which differs 
little from it, as well as the shades which are 
obtained from their mixture with other co- 
lours, really change in all porcelains, and in 
the hands of all operators, But this is the 
only one which changes on hard porcelain. 
It may be replaced by a substitution of 
rose colour from iron, which does not 
change; so that by excluding from the 
pallet the carmine made from gold, and 
substituting the rose-coloured oxide ofiron 
here spoken of, we have a pallet composed 
of colours, none of which are subject to any 
remarkable change. The rose-coloured 
oxide of iron has been long known, but was 
not employed on enamel, because it is then 
subject to considerable change. Or per- 
haps, when the painters on enamel became 
painters on porcelain, they continued to 
work according to their ancient method. 
It might be supposed, that by previously 
reducing the colour named carmine, al- 
ready mixed with its solvent into a vitreous 
matter, the last tint would be obtained ; but 
the fire which must be used to melt this vi- 
treous mass destroys the red colour. Be- 
sides, it is found that to obtain this colour in 
perfection, it is necessary to pass it through 
the fire as little as possible. 
The carmine of tender porcelain is made 
of fulminating gold, gently decomposed, 
and muriate of silver ; there is no tin in it, 
which proves it is no\i necessary for the fa- 
brication of a purple colour that the oxide 
of this last metal, and that of gold, should be. 
combined. 
Violet is likewise obtained from the pur- 
ple oxide of gold. This colour proceeds 
from having a greater quantity of lead in 
the flux, and it is nearly of the same tint, 
whether crude or baked. 
These three colours totally disappear in 
the strong fire necessary to bake porcelain. 
Carmine and purple afforded, upon, glass, 
only tints of a dirty violet. The violet on 
the contrary has a beautiful effect, but is 
subject to change to blue. 
