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

40 ENAMELS AND ENAMELLING.
on the character of enamel, we will first devote a little attention
to the chemical properties of this salt.
In point of chemical composition borax is a compound of
sodium, boron and oxygen, viz. : sodium-biborate (Na 2 B 4O 7 +
10H 2O), and forms prismatic crystals which dissolve in twelve
parts of cold, or two parts of hot, water. In addition to this
ordinary form a second variety
is known under the name of
octahedral borax, differing from the former by containing only
half as much water of crystallisation.
When heated, borax fuses at a moderate temperature and
very soon begins to froth up (intumesce) to such an extent
that a very large vessel must be employed or the mass will
overflow. In this process, water of crystallisation is given
off, and finally, a white, highly porous
borax is left.
residue of calcined
For the purposes of the enamel-maker calcined borax is
generally used, the frothing of the ordinary kind being a
source of great trouble in the melting process. While still
warm, the calcined borax is reduced to fine powder and stored
in properly closed vessels ;
otherwise it greedily absorbs mois-
ture from the air and changes back at least in part into
the crystalline form.
When calcined borax is heated to bright redness it finally
fuses into a colourless glassy mass, the so-called vitrified
borax, or borax glass.
For our purposes, however, this treat-
ment is unnecessary, the same result being produced in the
melting of the enamel.
Is is, however, always advisable to melt a small sample of
the borax before purchasing, in order to test for the presence
of pigmentary oxides. If pure, the borax will remain as clear
as glass, but if the fused mass exhibits a yellow tinge ferric
oxide is indicated ; copper
gives a green coloration, and
manganese compounds a violet shade.
From a chemical point of view the behaviour of borax