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

90 ENAMELS AND ENAMELLING.
in this respect is the ash of certain lignites, which, in addition-
to carbon, contains a considerable quantity of ferric oxide
capable of colouring
the enamel mass.
Where the draught in the enamel smelting furnace is
defective, the products of combustion are likely to be highly
charged with reducing gases, which will have an injurious
effect on many coloured enamels ; for instance, the emerald
green shade produced by cupric oxide will, under such con-
ditions, be invariably discoloured owing to the reduction of
the cupric oxide to cuprous oxide.
Consequently, care must be taken to select a fuel that yields
no flying ash, and to construct the furnace in such a manner
as to ensure an efficient draught in order that the fuel may
be completely consumed to carbon dioxide and water vapour.
The fuel at the disposal of the enamel-maker includes
wood, peat, lignite, coal, wood charcoal, coke, and,
in some
places, anthracite.
Under ordinary circumstances wood is too expensive for
use as fuel in smelting enamels, and is moreover attended
with the inconvenience that it burns away very quickly, so
that the fire requires frequent attention to keep it up.
Peat is of low calorific value, yields a large proportion of
ash, and is difficult to use in furnaces on account of its bulky
nature.
Wood charcoal, especially from hard woods such as beech,
forms an excellent fuel in every respect, the ash being small,
the combustion perfect, and the heating power considerable.
Unfortunately in most places it is too expensive for our purpose,
and it is only where wood is abundant that the use of
charcoal is feasible. By reason of its high content of potassium
carbonate, the ash from wood or charcoal can be utilised as a
source of potash for the preparation of common enamels.
When lignite is used the dense, dark-coloured kinds should
be chosen, as these alone have a suitable calorific power, the
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