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

DECOLOEISING AGENTS. 77
Saltpetre or minium affords the simplest
means of de-
colorising enamel masses, because neither of these substances
produces any colour if used in excess, whereas manganese
dioxide stains the enamel violet. The following considerations
must, however, be borne in mind, namely :
an
excess of salt-
petre makes the enamel more refractory, owing to the
formation of potassium silicate, whereas minium increases the
FIG. 3.
fusibility of the mass, though diminishing its power of resisting
chemicals and precluding the employment of certain pigments.
Since, however, the amount of saltpetre required to produce
decoloration is usually so small that the refractory propert}^
of the resulting potassium silicate may be regarded as practi-
cally negligible, it is generally preferable to employ saltpetre
for this purpose.