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

ENAMELLING ARTICLES OF SHEET METAL. 161
result is that when the last portions of enamel set hard,
tensions arise between the particles, and fine cracks are
formed ; these usually radiate from the rivets and in length
exceed the diameter of the latter three- or four-fold. The
same thing happens in the case of lapped joints, the cracks
making their appearance on both sides of, and perpendicular
to, the joint.
It was owing to the occurrence of these inconveniences
that the production of perfect ware of enamelled sheet iron
was so retarded as to be looked upon for a long time as
impossible. There are, however, two very effective means of
reducing the formation of the aforesaid cracks to a practically
negligible minimum ; the one consists in exerting the
utmost care to make the enamel coating as thin as possible,
whilst the second is to ensure very protracted and uniform
cooling down on the removal of the ware from the muffle.
It will be readily understood that a very thin coating of
enamel will be the better able to follow the expansion and
contraction of the underlying metal the slower the cooling
process is carried out, since in the latter case the vitreous
coating acquires a comparatively high degree of elasticity, and
consequently is enabled to better withstand rapid changes of
temperature.
Moreover, it must not be forgotten that the durability of
the enamel is always greatly increased when the cooling
process is conducted gradually ; the so-called "toughened glass"
(verre trempty, which is so resistant that it does not crack
when molten lead is poured into a cold vessel made of the
material in question, is prepared by simply cooling the glass
ware very slowly indeed.
From the explanation given above, the intelligent manu-
facturer, who does not shirk the trouble of instructing his
workmen thoroughly, will find no difficulty in producing fault-
less enamelled sheet-iron ware. Furthermore, since in many
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