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

26 ENAMELS AND ENAMELLING.
into contact therewith, and change into soda crystals, which'
dissolve in the water, calcined soda can be used for purposes
for which a solution of soda is required, and is often purchased,,
in preference to the crystals, in order to save expense of dead
freight on the water of crystallisation.
For enamel manufacturers, of course, water is a useless
encumbrance ; and they, therefore, buy calcined soda exclusively.
As, however, the ordinary calcined soda, even when
perfectly white in appearance, always contains a notable proportion
of iron compounds which spoil the colour of enamel
by producing a yellow tinge, this soda can only be used for
enamels wherein colour is an unimportant consideration.
Enamel Soda.
The trade in soda for enamel-making having developed
considerably, alkali makers now comply with the demand for
a non-ferruginous soda and supply, under the name of " enamel
soda," a product which may be regarded as perfectly free from
iron. To this end the soda is purified by frequent crystal-
lisations until the most delicate reagents fail to reveal the
presence of iron, except in traces, the product being afterwards
spread out in heated rooms until thoroughly dry.
Enamel soda is, it is true, dearer than the ordinary kind ;
but it furnishes a pure white enamel, whereas the ordinary
soda, even when fairly pure, gives rise to a yellowish tinge,
the extent of which depends on the amount of iron present.
Kelp.
This article of commerce is an impure soda obtained by
calcining
seaweed on the Irish and Scottish coasts. As it
contains a number of other salts in addition to sodium car-
bonate it is unsuitable for any but the commoner kinds of
enamel. Since the development of the manufacture of soda
from common salt, the trade in this impure soda has greatly
declined.