ZAFFRE. SMALT. 221 
performed by criminals who are condemned to it for 
the commission of crimes which, by the laws of the 
country, have deserved the punishment of death. 
As a metal, cobalt was unknown till the year 1733, 
when it was discovered by a celebrated Swedish chemist 
whose name was Brant. In its metallic state it is not 
employed in the useful arts ; but in a state of oxide it 
is found extremely valuable in the colouring of porce- 
lain, in painting, enamelling, and for other purposes. 
Cobalt and ultramarine form the most permanent 
blue colours with which we are acquainted. The old 
painters generally used them for the representation of 
the sky and of blue drapery, and this is the reason why 
these parts in some old pictures have been found so 
much more durable than any others. 
Zaffre is an oxide of cobalt mixed with about three 
times its own weight of calcined and pounded flint. 
It has been chiefly imported into this country from 
Saxony and Bohemia, but it is now also manufactured 
from cobalt dug from mines in the Mendip Hills and 
in Cornwall. In Staffordshire there are several persons 
who carry on a considerable trade in preparing this co- 
lour for the earthen-ware manufacturers of that county. 
This substance is extremely valuable for the co- 
louring of porcelain and glass ; as it resists without 
change, the effects of the most intense heat. Hence 
also it is advantageously used for giving various shades 
of blue to enamels, and to glass manufactured in imi- 
tation of lapis lazuli, turquoise, sapphire, and various 
precious stones. So intense is the colour imparted by 
it that a single grain of zaffre will give a full blue tint 
to 240 grains of glass. 
Smalt is a kind of glass, of dark blue colour, formed 
by melting zaffre with three parts of sand and one of 
potash ; when this substance is ground to a coarse 
powder, it has the name of strewing- smalt, and is much 
used by sign painters, as an ornamental filling up of the 
vacant space betwixt the letters of signs. In Germany 
it is frequently employed instead of sand for the purpose 
