The mixture of common salt and vitriol is charged into the salt-cake
pot, which is heated by a fire below. When from two-thirds to
three-quarters of the hydrochloric acid has been expelled from the
charge, the mass acquires the consistence of thick dough, and at this
stage it is raked out of the pan on to the roasting hearth alongside,
where the decomposition is completed by means of flames playing directly
on to the top of the charge. The hydrochloric acid evolved during the
process is condensed in much the same manner as in the process of
Hargreaves & Robinson previously described. It is a curious fact that in
the earlier years of the Leblanc process, hydrochloric acid, or "spirits
of salt," as it is frequently called, was a by-product that required all
the vigilance of the alkali-works inspectors to prevent it being allowed
to escape from the chimneys in more than a certain small regulated
amount. Now, it is the principal product; indeed, the Leblanc alkali
maker may be said to subsist on that hydrochloric acid, as his chief
instrument for producing chloride of lime or bleaching powder.

Mechanical furnaces are now used to a large extent for the salt-cake
process. They consist broadly of a large revolving furnace-hearth or
bed, on to which the mixture of salt and vitriol is charged, and on
which it is continuously agitated, and gradually moved to the place of
discharge, by rakes or the like, operated by suitable machinery.

The next stage of the Leblanc process is the manufacture of "black ash,"
or crude sodium carbonate. This is usually done in large cylindrical
revolving furnaces, through, which flames from a fire-grate, or from the
burning of gaseous fuel, pass; the waste heat is utilised for boiling
down "black ash" liquor, obtained by lixiviating the black ash. A
mixture of salt-cake, limestone or chalk (calcium carbonate), and
powdered coal or coal slack is charged into the revolving cylinder;