A Textbook of Assaying | Page 5

of ore made up of parcels of
different weight and quality, each parcel should be separately sampled
and parts of each sample, bearing to each other the same proportion by
weight as the original parcels, should be taken and mixed. For example,
a lot of ore is made up of one parcel of A, 570 tons, one of B, 180 tons,
and another of C, 50 tons; a sample representing the whole may be got
by mixing 57 parts of a sample of A with 18 parts of a sample of B, and
5 parts of a sample of C.
[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. Some ores are
sent to the smelter with from 5 to 15 per cent. of adherent water. In
these cases it is best to spread out the sample, and taking equal portions
fairly at regular intervals, weigh into a Berlin dish 20 grams. This
should then be dried over a sand-bath, or if the ore is likely to be
injured by excess of heat, over a water-bath until the weight is constant.
The loss of weight multiplied by 5 gives the percentage of water
present.
Example:--
Weight of dish + wolfram 32.67 grms. " " dish 12.67 " ----- " " wolfram
20.00 "
" " dish + wolfram 32.67 " " " " dried 30.15 " ----- " " water 2.52 "
2.52 × 5 = 12.6 ~12.6%.~
There are other ores which are not apparently wet, but in the state
called "air-dried." It is easier to take fair samples of these, and,
consequently, it is not necessary to use so large a quantity as 20 grams.
But with a smaller quantity, extra precautions must be taken. All dry
solids at ordinary temperatures absorb moisture from the air. The
amount varies with the nature of the material and with the quantity of
surface exposed. Light bulky powders absorb more than heavy ones,
because of the greater condensing surface. It is on this account that it is
well to weigh substances, which have been dried, between close-fitting
watch-glasses. The method of determining moisture is to weigh out into
the glasses 5 grams of ore, and dry in the water-oven until there is no
further loss of weight. On taking the glasses out of the oven, they
should be at once closed, the clip put on, and after cooling in a
desiccator weighed. If after a second trial the loss is the same, or only

increased by a milligram, the determination is finished.
Example:--
Weight of glasses + pyrites 31.0470 grms. " " glasses 26.0470 " ------- "
" pyrites 5.0000 " " " glasses + pyrites, dried 1 hour 30.8965 " " " " "
dried 1-1/2 " 30.8957 " " " " " 31.0470 " " " " " dried 30.8957 " ------- "
" moisture 0.1513 "
0.1513 × 20 = 3.026 ~3.02%.~
[Illustration: FIG. 3.]
Sometimes it may be advisable to dry 10 grams, in which case
multiplying the loss by 10 will give the percentage. The dried ore
should be transferred to a weighing-tube (fig. 3), and reserved for the
subsequent determinations. The weighing-tube with the ore must be
marked, and kept in a desiccator.
Most ores and inorganic substances