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

160 ENAMELS AND ENAMELLING.
ascertain by experiment the most suitable mass for the iron
with which he has to deal, since the sheet metal supplied by
different rolling mills exhibits divergent degrees of expansion.
The reason for the necessity of a high coefficient of expan-
sion in enamels for such ware is not far to seek, and resides
in the thinness of the sheet metal employed. The walls of
sheet-iron ware seldom exceed ^th of an inch in thickness,
whereas the thinnest utensils of cast iron are twice or three
times as thick. Consequently a sheet-iron vessel, when heated,
rapidly becomes hot all the way through, and therefore expands
hence if the enamel is unable to follow this
very quickly;
movement it must necessarily tear away from the metal.
To counteract this inconvenience as it must be regarded
from the present point of view of rapid conduction of heat,
it is customary to enamel the utensils on the outside as well,
the result of which treatment enamels, as vitreous bodies,
being bad conductors of heat is to prevent the rapid heating
through of the metal, and thereby to a certain extent protects
the inside enamel from disruption.
Sheet-iron ware may be divided into two classes according
to the methods of manufacture adopted, viz., those made
entirely out of a single piece, such as very flat stewpans ;
and
those wherein several pieces are joined together by lapping or
riveting. Those of the first class offer no special difficulty to
the uniform fusion of the enamel coating, provided the metal
is of the same thickness throughout; but lapped or riveted
articles are harder to deal with, and require very skilful
handling in order to produce a good class ware, free from a
large percentage of " outshots ".
This is due to the circumstance that the metal is much
thicker at the lapped or riveted joints than anywhere else,
which thicker places remain hot much longer than the rest
when taken out of the muffle, and consequently the enamel
there is still fluid after the remainder has become solid. The