Composition of Enamel. 403 
and spread itself, like a cloud, beyond the limits pre- 
scribed for it. 
A mixture of yellow and blue is also used to produce 
green. Those who paint figures or portraits employ 
glass composed in this manner; but those who paint 
glazed vessels, either earthen -ware or porcelain, employ 
in general copper green. 
Independently of the beautiful green colour produced 
by oxydated copper, it produces also a very beautiful 
red colour; but I do not know that it is employed on ena- 
mel. This beautiful red colour, produced by copper, is 
exceedingly fugitive. The oxyd of copper gives red 
only when it contains very little oxygen, and approaches 
near to the state of a regulus. Notwithstanding the dif- 
ficulty of employing this oxyd for a red colour, a method 
has been found to stain transparent glass with different 
shades of a very beautiful red colour by means of calx 
of copper. The process is as follows : You do not em- 
ploy the calx of copper pure, but add to it calx of iron, 
which for that purpose must not be too much calcined ; 
you add also a very small quantity of calx of copper to 
the mass of glass which you are desirous of tin going. 
This glass at first must have only a very slight tinge of 
green, inclining to yellow. When the glass has that 
colour you make it pass to red, and even a very dark 
red, by mixing with it red tartar in powder, and even 
tallow. You must mix this matter well in the glass, and 
it will assume a very dark red colour. The glass swells 
up very much by this addition. Before it is worked it 
must be suffered to settle, and become compact ; but as 
soon as it has fully assumed the colour it must be imme- 
diately worked, for the colour does not remain long, and 
even often disappears while working; but it may be re- 
stored by heating the glass at the flame of a lamp. It is 
exceedingly difficult to make this colour well ; but when 
