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

36 ENAMELS AND ENAMELLING.
shaken up with fuming nitric acid in a vessel of glass or
glazed porcelain, whereupon a brisk reaction occurs, the nitric
acid being decomposed, with evolution of brown fumes, whilst
the tin is converted into a white powder tin oxide which
settles down to the bottom of the vessel. As soon as the
action of the nitric acid has abated, a fresh quantity is added,
and is followed by further gradual additions until finally the
reaction ceases altogether.
Owing
to the violence of the reaction the acid must be
added in small quantities at a time, as otherwise the contents
of the vessel will come over ;
the work should also be done in
the open air or under a chimney with a good draught, as the
brown fumes (nitric oxide and hyponitrous acid) cause great
inconvenience to the respiratory organs.
After the reaction is at an end, the vessel is left at rest for
a day, whereupon the supernatant acid (which may
be used
over again) is carefully decanted from the white deposit,
and the latter is washed several times with water, by
sedimentation. To remove the final traces of nitric acid, the
tin oxide precipitate is afterwards repeatedly washed in a
glass funnel and then dried, the funnel being meanwhile
kept covered over with paper
to exclude dust.
The product thus obtained is of excellent quality, but is
too expensive for any but the finest enamels, which are required
to remain perfectly white or be tinged with very delicate
colours.
To prepare tin oxide by the dry method, the process em-
ployed by manufacturers of tin ash is imitated, the metal
being fused in a flat stoneware pan
and afterwards raised to
a strong red heat. The rapidity of the oxidation is in direct
proportion to the temperature
to which the molten metal is
exposed.
At first the surface of the metal becomes covered with a
blackish pellicle, consisting of tin protoxide (stannous oxide),