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

156 ENAMELS AND ENAMELLING.
The chief physical influence to be resisted is that due to
extreme fluctuations of temperature ; and in this connection
the enamel can be very easily tested by heating it quickly,
followed by rapid cooling, the simplest plan being- to fill the
enamelled utensil with water, raising this to boiling point,
and then immersing the vessel in cold water. The enamel
mass, after this test has been several times repeated, should
still exhibit its original characteristics without alteration, and
no cracks should be visible.
The chief chemical influences to be resisted are acids and
alkalils, and the enamel may be considered as sufficiently proof
against the former if, when the enamelled ware has been filled
with boiling vinegar for about a quarter of an hour, the liquid
gives no precipitate on subsequent exposure
to a current of
sulphuretted hydrogen. The absence of such precipitate
indicates that the enamel has suffered no corrosion ; the
lustre should also remain unimpaired.
A very simple test for the presence of lead in an enamel
consists in moistening a portion of the enamel with strong
nitric acid, to destroy the upper layer.
After rinsing the spot
with water a drop of ammonium sulphide is allowed to fall
on the place, whereupon, in the absence of lead, no change of
colour will ensue ;
whereas, if lead be present, the spot originally
occupied by the nitric acid will immediately darken and
in a short time turn quite black. The opinion has already
been laid down that enamels for cooking utensils should be
free from lead, on account of the poisonous properties of that
metal, the dangers of which are intensified when the super-
ficial layer of enamel has become damaged or defective.