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

SPECIALTIES IN ENAMELLING. 169
cal difficulties, which nevertheless may be overcome by means
of the process now about to be described. After pickling in
fairly strong acid, the tubes are scrubbed clean with cylindrical
brushes, and similar brushes are employed to apply the
coating of enamel. Firing
is effected in muffles of suitable
length, but not wider than is necessary for the accommodation
of several tubes, laid side by side on supports
hollowed out
for their reception, so as to facilitate handling by means of
suitable tongs.
The ground enamel is fired at a high temperature and the
attendant must exercise care, by turning the tubes, to ensure
uniform heating throughout. This turning must be performed
more frequently during the firing of the cover enamel, to
prevent the latter from running and forming a coating of
irregular thickness.
Enamelled iron pipes are also excellent for conveying the
acid water of mines, as they remain unaltered for years, where-
as ordinary iron pipes are corroded in a very short time.
Of course the enamel employed for such tubes must be of
such constitution that it can effectually withstand chemical
influences. Amtmann of Gleiwitz uses the following formula
for enamel for this purpose :
Ground Enamel.
Quartz
34 parts.
Borax 15
Soda 2
Felspar
Borax ....
Cover EnauieL
\
34 parts.