[Illustration: FIG. 2.]
A bruising plate, like that in fig. 2, is convenient for general office
work. The slab is of cast iron, about an inch thick. It is firmly
supported on a solid block of wood, and pivoted for convenience in
emptying. The bruising-hammer is steel-faced, about 4 inches square, and
1-1/2 inch thick. The block is firmly fixed to a small table or tressel,
so that the slab is about 2 feet 6 inches from the ground. The slab is
cleaned, and the sample collected with the help of a stiff-haired brush.
~Drying: Determination of Moisture.~--In practice, the moisture is
generally determined by the samplers, and the proportion is specified in
grains per pound on the label attached to the sample when it reaches the
assay office. The method adopted is usually to dry 1 lb. = 7000 grs. of
the ore in a frying-pan heated over a gas flame, or in an ordinary oven,
until a cold bright piece of metal or glass is no longer damped when
held over it. The loss of weight in grains = moisture.
Properly, however, this work should be done by the assayer, if only for
the following reason. It is assumed that the dry ore of the sampler and
of the assayer are the same thing; according to the nature of the ore,
this may or may not be the case. The assayer, however, uses the sample
which he has dried for his moisture-determination, as the dry ore on
which he makes his other assays, and no variation in moisture would
influence the other and more important determinations.
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