This operation is the next in course, and is performed in what is known
at the sugar factory as the strike pan, a large air tight iron vessel
from which the air and vapor are almost exhausted by means of a suitable
pump and condensing apparatus. As is the case with the saccharine juices
of other plants, the sugar from sorghum crystallizes best at medium
temperature.
The process of boiling to grain may be described as follows: A portion
of the sirup is taken into the pan, and boiled rapidly _in vacuo_ to the
crystallizing density. If in a sirup the molecules of sugar are brought
sufficiently near to each other through concentration--the removal of
the dissolving liquid--these molecules attract each other so strongly as
to overcome the separating power of the solvent, and they unite to form
crystals. Sugar is much more soluble at high than at low temperatures,
the heat acting in this as in almost all cases as a repulsive force
among the molecules. It is therefore necessary to maintain a high vacuum
in order to boil at a low temperature, in boiling to grain. When the
proper density is reached the crystals sometimes fail to appear, and a
fresh portion of cold sirup is allowed to enter the pan. This must not
be sufficient in amount to reduce the density of the contents of the pan
below that at which crystallization may take place. This cold sirup
causes a sudden though slight reduction in temperature, which may so
reduce the repulsive forces as to allow the attraction among the
molecules to prevail, resulting in the inception of crystallization. To
discover this requires the keenest observation. When beginning to form,
the crystals are too minute to show either form or size, even when
viewed through a strong magnifying glass. There is to be seen simply a
very delicate cloud. The inexperienced observer would entirely overlook
this cloud, his attention probably being directed to some curious
globular and annular objects, which I have nowhere seen explained. Very
soon after the sample from the pan is placed upon glass for observation,
the surface becomes cooled and somewhat hardened. As the cooling
proceeds below the surface, contraction ensues, and consequently a
wrinkling of the surface, causing a shimmer of the light in a very
attractive manner. This, too, is likely to attract more attention than
the delicate, thin cloud of crystals, and may be even confounded with
the reflection and refraction of light, by which alone the minute
crystals are determined. The practical operator learns to disregard all
other attractions, and to look for the cloud and its peculiarities. When
the contents of the pan have again reached the proper density, another
portion of sirup is added. The sugar which this contains is attracted to
the crystals already formed, and goes to enlarge these rather than to
form new crystals, provided the first are sufficiently numerous to
receive the sugar as rapidly as it can crystallize.
The contents of the pan are repeatedly brought to the proper density,
and fresh sirup added as above described until the desired size of grain
is obtained, or until the pan is full. Good management should bring
about these two conditions at the same time. If a sufficient number of
crystals has not been started at the beginning of the operation to
receive the sugar from the sirup added, a fresh crop of crystals will be
started at such time as the crystallization becomes too rapid to be
accommodated on the surfaces of the grain already formed. The older and
larger crystals grow more rapidly, by reason of their greater attractive
force, than the newer and smaller ones on succeeding additions of sirup,
so that the disparity in size will increase as the work proceeds. This
condition is by all means to be avoided, since it entails serious
difficulties on the process of separating the sugar from the molasses.
In case this second crop of crystals, called "false grain" or "mush
sugar" has appeared, the sugar boiler must act upon his judgment, guided
by his experience as to what is to be done. He may take enough thin
sirup into the pan to dissolve all of the crystals and begin again, or,
if very skillful, he may so force the growth of the false grain as to
bring it up to a size that can be worked.
The completion of the work in the strike pan leaves the sugar mixed with
molasses. This mixture is called _malada_ or _masscuite_. It may be
drawn off into iron sugar wagons and set in the hot room above
mentioned, in which case still more of the sugar which remains in the
uncrystallized state generally joins the crystals, somewhat increasing
the yield of "first sugars." At the proper time these sugar wagons are
emptied into a mixing machine, where the mass is brought to a uniform
consistency. If the sugar wagons are not used, the strike pan is emptied
directly into the mixer.