chemical combination of lime with silica and alumina, or practically
of lime with dehydrated clay. In order to effect this, the usual
method is to place the mechanically mixed chalk and clay (technically
called slurry), in lumps varying in size, say, from 4 to 10 lb., in
kilns with alternate layers of coke, and raise the mass to a glowing
heat sufficient to effect the required combination, in the form of
very hard clinker. These kilns differ in capacity, but perhaps a fair
average size would be capable of producing about 30 tons of clinker,
requiring for the operation, say, from 60 to 70 tons of dried slurry,
with from 12 to 15 tons of coke or other fuel. The kiln, after being
thus loaded, is lighted by means of wood and shavings at the base,
and, as a matter of course, the lumps of slurry at the lower part of
the kiln are burned first, but the moisture and sulphurous gases
liberated by the heat are condensed by the cooler layers above, and
remain until the heat from combustion, gradually ascending, raises the
temperature to a sufficient degree to drive them further upward, until
at length they escape at the top of the kiln. The time occupied in
loading, burning, and drawing a kiln of 30 tons of clinker averages
about seven days. It will be readily understood that the outside of
the clinker so produced must have been subjected to a much greater
amount of heat then was necessary, before the center of such clinker
could have received sufficient to have produced the incipient fusion
necessary to effect the chemical combination of its ingredients; and
the result is not only a considerable waste of heat, but, as always
occurs, the clinker is not uniformly burnt, a portion of the outer
part has to be discarded as overburnt and useless, while the inner
part is not sufficiently burnt, and has to be reburned afterward.
Moreover, the clinker, which is of excessively hard character, has to
be reduced by means of a crusher to particles sufficiently small to be
admitted by the millstones, where it is ground into a fine powder, and