A Textbook of Assaying | Page 9

percentage error, often
overlooked, is important. If 0.5 gram of silver be cupelled with 20
grams of lead, there may be obtained a button of 0.495 gram; the
absolute loss is 0.005 gram, and this equals 1 per cent. of the silver
present. Similarly, cupelling 0.1 gram, the resulting button may be
0.098; the absolute loss is only 0.002 gram, but this equals 2 per cent.
of the silver present. In the same way the student should see that the
two results, 91.5 per cent. and 92.0 per cent., are really more
concordant than the results 9.1 per cent. and 9.2 per cent.
A device often adopted in practice where a large number of assays of
one kind are made, and the report is given as so many ounces or
pounds to the ton, is that known as the assay ton. The assay ton may be
any arbitrary and convenient weight, but its subdivisions must bear to
it the same relations as pounds and ounces bear to the actual ton. On
the other hand, in a laboratory where many kinds of work are
performed, different sets of weights of this kind would only tend to
confusion, even if they were not unnecessary. With a set of gram
weights and its subdivisions anything may be done. If it is desired to
report as pounds to the ton, then, since there are 2240 lbs. to the ton, a
weight of 2.240 grams may be taken as the assay ton, and each 0.001
gram yielded will equal 1 lb., or 22.4 grams may represent the ton, and
each 0.01 gram a pound. Similarly, since there are 32,666.6 ozs. troy to
the ton; if we take 32.6667 grams as the assay ton, each 0.001 gram
will equal 1 oz. to the ton. In some cases it may be convenient to have,
in addition to the usual gram weights, one or other of the "assay tons"
mentioned above, but generally it is better to work on a purely decimal
system, and convert when required into ounces per ton, &c., either by
actual calculation or by reference to a set of tables.
PRACTICAL EXERCISES.

The student should practise such calculations as the following:--
1. Calculate the percentages in the following cases:-- (a) Ore taken, 2
grams; copper found, 0.2155. (b) " 1.5 gram; iron found, 0.8340. (c) "
30 grams; lead found, 23.2.
2. Calculate the parts per thousand in the following:-- (a) Bullion taken,
1.1 gram; silver found, 1.017. (b) " 1.14 gram; silver found, 1.026. (c) "
0.6 gram; gold found, 0.5500.
3. Calculate parts per 100,000 in the following:-- (a) Ore taken, 20
grams; silver found, 0.0075. (b) " 50 grams; gold found, 0.0026. (c)
Water taken, 500 c.c.; solids found, 0.1205.
4. Calculate cwts. to the ton in the following:-- (a) Ore taken, 5 grams;
tin found, 2.816. (b) " 5 grams; tin found, 3.128. (c) An ore with 68.2
per cent. of tin.
5. Calculate lbs. to the ton in the following:-- (a) An ore with 3.28 per
cent. oxide of tin. (b) Ore taken, 20 grams; oxide of tin found, 1.67.
6. Calculate ozs. (troy) to the ton in the following:-- (a) Ore taken, 50
grams; gold found, 0.0035. (b) " 20 grams; silver found, 0.0287. (c) "
25 grains; silver found, 0.0164.
7. Calculate in grains per gallon:-- (a) 0.51 gram per litre. (b) 24.6
parts per 100,000. (c) Solution taken, 100 c.c.; copper found, 0.0045
gram. (c) " 50 c.c.; iron found, 0.165 gram.
8. Convert into ozs. (troy) per ton:-- (a) 7 loths per centner. (b) 30
grams per quintal. (c) 15 parts per 100,000.
FOOTNOTES:
[1] Ether or carbon bisulphide.
[2] Such substances are best dried by pressing between folds of dry
filter-paper.

CHAPTER II.
METHODS OF ASSAYING.--DRY GRAVIMETRIC METHODS.
The methods of assaying are best classed under two heads, Gravimetric
and Volumetric, in the former of which the final results are weighed,
whilst in the latter they are measured. A commoner and older division
is expressed in the terms much used in practice--wet assays and dry
assays. Wet assays include all those in which solvents, &c. (liquid at
the ordinary temperature), are mainly used; and dry assays, those in
which solid re-agents are almost exclusively employed. Dry assays
form a branch of gravimetric work, and we shall include under this
head all those assays requiring the help of a wind furnace. Wet assays,
as generally understood, would include not only those which we class
as wet gravimetric assays, but also all the volumetric processes.
~Gravimetric Methods~ aim at the separation of the substance from the
other matters present in the ore, so that it may be weighed; and,
therefore, they must yield the whole of the substance in a pure state. It
is not necessary that a