Enamels and enamelling; an introduction to the preparation and ...

TESTING THE RAW MATERIALS AND ENAMEL MASS. 83
addition of saltpetre this time in the oxidising flame. In
case the colour is the result of carbon or organic matter, these
bodies will be consumed by the oxygen liberated from the
saltpetre, and a perfectly colourless bead will be left on cooling.
Testing Lead Preparations.
The object of testing preparations of lead litharge,
minium, white lead and lead sulphate is twofold, namely,
for the presence of copper or antimony as accidental im-
purities, and for adulterants, which not infrequently are
intentionally added, especially to minium and white lead.
For the detection of adulterants it is generally sufficient
to reduce the minium or white lead to the metallic state on
charcoal before the blowpipe. If the resulting bead exhibits
the ductility of lead, and no residue is left behind 011 the
charcoal, the substance may be considered as pure. If, how-
have been
ever, the bead is brittle, the white lead may
of which can be
adulterated with zinc white, the presence
-ascertained with certainty by reheating the bead in the
oxidising flame. Under these conditions lead yields a yellow
incrustation deposited on the charcoal in the immediate
vicinity of the bead ; whilst zinc oxide, which is more volatile,
will be deposited a little farther away, and will besides show
up yellow when hot, turning pure white on cooling.
A white infusible residue left behind on the reduction of
white lead almost invariably consists of barytes, the powder
in that case exhibiting a crystalline structure when examined
under a magnifying glass. A red residue from the reduction
of minium indicates ferric oxide, or, more frequently, brick
dust. To distinguish between them,
fuse with a borax bead
in the reducing flame : a green coloration points to ferric oxide.
A similar coloration is, it is true, furnished by brick dust, but
in the latter case a much higher temperature is needed to fuse
the mass than when pure ferric oxide is present.