After the
concentration has proceeded far enough, the action of the heat is
suspended, and the reddish-brown, oily-looking liquid is drawn into
the vacuum-pan till it is about a third full; the concentration is
completed by boiling the juice in vacuo at a temperature of 150
degrees, and even lower. As the boiling proceeds, the sugar boiler
tests the contents of the pan by withdrawing a few drops, and
holding them up to the light on his finger; and, by certain minute
changes in their condition, he judges when it is time to add an
additional quantity. When the pan is full, the contents have
thickened into the consistency of thick gruel by the formation of
minute crystals, and are then allowed to descend into an heater,
where they are kept warm till they can be run into "forms" or tanks,
where they are allowed to granulate. The liquid, or molasses, which
remains after the first crystallization is returned to the vacuum
pan and reboiled, and this reboiling of the drainings is repeated
two or three times, with a gradually decreasing result in the
quality and quantity of the sugar. The last process, which is used
for getting rid of the treacle, is a most beautiful one. The mass
of sugar and treacle is put into what are called "centrifugal pans,"
which are drums about three feet in diameter and two feet high,
which make about 1,000 revolutions a minute.
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