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

82 ENAMELS AND ENAMELLING.
under examination is fit for our purposes or not. In the
enamel mass manganese is converted into silicate, and as a
rule the small quantity of manganese present in potash does
not affect the properties of the enamel.
The alkaline earths lime, chalk, gypsum, dolomite, argil-
laceous limestone, barytes, witherite,
etc. are examined in the
same manner, by heating in the reducing flame and noting
the colour. As, however, many
of these substances are in-
fusible before the blowpipe, recourse is had to anhydrous
borax in order to obtain transparent samples.
When carbonates are fused along with borax, the bead is
at first turbid from the accompanying bubbles of carbon
dioxide liberated by the boric acid ; in such event the bead
must be maintained in a state of fusion until all the gas has
O
escaped and the mass becomes quite clear.
The enamel fluxes borax, boric acid, fluor spar and cullet
may all be tested for iron in the above manner, all that is
usually necessary being to fuse them, without admixture, and
examine their behaviour alternately in the reducing and
oxidising flames.
Testing Silica.
This substance, in the state of quartz meal and sand, is
quite infusible by itself, and consequently has to be fused
with an alkali, when tested for impurities, ordinary soda (of
course free from iron) being generally employed on account
of the fusibility of its bead. The mixture must be heated
until all the carbon dioxide has been driven off, leaving the
bead quite clear.
If the test fail to reveal iron in the silica or other sub-
stance under examination, but leaves the bead grey, yellow,
or brownish after heating in the reduction flame, this colora-
tion may be due to the presence of finely divided carbon.
This mav be readily confirmed by reheating the bead with an