A Textbook of Assaying | Page 4

of the whole (= 1/20th), if taken
in portions of equal weight and at frequent and regular intervals, will
represent the mass from which it was taken. Taking a heap of ore, A,
and selecting one out of every twenty spade-, bag-, barrow-, or
wagon-fuls, according to the quantity of stuff in the heap, there is
obtained a second heap, B, containing one-twentieth of the stuff of the
heap A. If we crush the stuff in B until this heap contains
approximately the same number of stones as A did--which means,
crushing every stone in B into about twenty pieces--B will become the
counterpart of A. Selecting in the same manner 5 per cent. of B, there is
got a third heap, C. This alternate reduction and pulverising must be
carried on until a sample of suitable size is obtained. This may be
expressed very clearly thus:--
A = 1000 tons of rocks and lumpy ore. B = 50 " " rough stones, 1/20th
of A. C = 2.5 " " small stones, 1/20th of B. D = 0.125 " " coarse powder,
1/20th of C.
[Illustration: FIG. 1.
CONE
PARTLY REDUCED CONE
PLAN OF FRUSTRUM DIVIDED.
ELEVATION OF FRUSTRUM DIVIDED.]
If the material to be sampled is already a dry powder, 5 per cent. of it
should be heaped in a cone; each lot being added on the apex of the
cone already formed, so that it may distribute itself by falling evenly in
all directions. When the cone is completed, convert it into a low
frustrum of a cone by drawing stuff uniformly and in a direct line from
the centre to the circumference. Draw two diameters at right angles to
each other, and reserving any two alternate quarters, reject the others.
Mix; and form another cone, and proceed until a sample is got of the
bulk required.

This is the usual plan, and all samples should be treated in this way
when the stuff is fine enough to fall evenly down the sides of a cone.
Samples as they reach the assay office are seldom in a fit state for the
work of the assayer; they are generally too coarse, and ought always to
be more than he wants for any particular determination. The portion he
requires should never be taken at hap-hazard; the sample must be
reduced systematically to the quantity required.
1. If the sample is a liquid: it is sufficient to shake the bottle, and take
out a measured or weighed quantity for the assay.
2. If a liquid with a solid in suspension: measure the whole of it. Filter.
Make up the filtrate with the wash-water or water to the original bulk.
Assay it. Dry and weigh the residue, and make a separate assay of it.
3. If of a creamy consistency, free from heavy particles: mix well;
spread out evenly on a glazed tile. Take up equal portions at equal
distances. Mix and assay.
4. If a mud of coarse and fine particles, or of particles of unequal
density: weigh and transfer to a porcelain dish, or weigh in the dish.
Dry at 100° C., weigh. Treat the residue as a solid capable of being
powdered.
5. If a solid capable of being powdered, or already powdered: heap up
into a cone; flatten with a spatula; divide along two diameters at right
angles, and carefully reject the whole of two alternate quarters,
brushing away any fine powder. Mix the other quarters, and repeat (if
necessary). For small quantities a fine state of division is essential.
6. If a solid with metallic particles: powder and pass through a sieve;
the metallic particles will not pass through. Weigh both portions and
assay separately. Sifting should be followed by a very thorough mixing.
7. If a metal or alloy in bar or ingot: clean the upper surface of the bar,
and bore through the bar. Use the borings. If the ingot or bar is small,
cut it through and file the section. Filings must be freed from fragments

of the file by means of a magnet; and from oil, if any be present, by
washing with a suitable solvent.[1] Where practicable, metals and
alloys are best sampled by melting and granulating. The student must
carefully avoid any chance of mixing dirt or particles of other samples
with the particular sample which he is preparing. One ore should be
done at a time, and when finished, it should be labelled and wrapped up,
or bottled, before starting on a fresh sample.
When an ore requires to be very finely ground in an agate mortar, it is
often advisable to mix with a little pure alcohol and rub until free from
grit; dry at 100° C. and mix well before weighing.
When an assay is required of a quantity