The
two essential things regarding every piece of metal offered in
payment of any dues were, first, the weight or quantity, next,
the fineness or purity of the same. The process of weighing
even the baser metals used in coining must be conducted by the
careful use of accurate scales, with precise notes of the
results. In precious metals, gold, silver, and their high
grade alloys, a very small variation in the fineness makes a
great difference in the value. Nothing is more essential than
the accurate determination of the weight of the sample and of
the metal obtained from it. It requires keen sight and most
delicate adjustment in the hand which manipulates the
Lilliputian scales of an Assayer's table. The smallest weight
used in the Mint is found in the Assay Room; it is the
thirteen-hundredth part of a grain, and can scarcely be seen
with the naked eye, unless on a white ground. The Assay
Department is strictly a technical and scientific branch of
the service.
The
Assaying Rooms
These
are on the second floor, in the southwest corner of the
building. In one of these are fires, stills, and other
appliances used in the delicate and complicated process of
assay, by which the specific standard of the fineness and
purity of the various metals are established and declared.
Assaying
Gold
The
gold is melted down and stirred, by which a complete mixture
is effected, so that an assay piece may be taken from any part
of the bar after it is cast. The piece taken for this purpose
is rolled out for the convenience of cutting. It is then taken
to an assay balance (sensible to the ten-thousandth of a half
gram or less), and from it is weighed a half gram, which is
the normal assay weight for gold, being about 7.7 grains troy.
This weight is stamped 1000; and all the lesser weights
(afterwards brought into requisition) are decimal divisions of
this weight, down to one ten-thousandth part.
Silver
is next weighed out for the quartation (alloying), and as the
assay piece, if standard, should contain 900-thousandths of
gold, there must be three times this weight, or
2700-thousandths of silver; and this is the quantity used. The
lead used for the cupellation is kept prepared in thin sheets,
cut in square pieces, which should each weigh about ten times
as much as the gold under assay. The lead is now rolled into
the form of a hollow cone; and into this are introduced the
assay gold and the quartation silver, when the lead is closed
around them and pressed into a ball. The furnace having been
properly heated, and the cupels placed in it and brought to
the same temperature, the leaden ball, with its contents, is
put into a cupel (a small cup made of burned bones, capable of
absorbing base metals), the furnace closed, and the operation
allowed to proceed, until all agitation is ceased to be
observed in the melted metal, and its surface has become
bright. This is an indication that the whole of the base
metals have been converted into oxides, and absorbed by the
cupel.
The
cupellation being thus finished, the metal is allowed to cool
slowly, and the disc or button which it forms is taken from
the cupel. The button is then flattened by a hammer; is
annealed by bringing it to a red heat; is laminated by passing
it between the rollers; is again annealed; and is rolled
loosely into a spiral or coil called a cornet. It is now ready
for the process of quartation. This was formerly effected in a
glass matrass, and that mode is still used occasionally, when
there are few assays. But a great improvement, first
introduced into this country by the Assayer in 1867, was
the--"platinum apparatus," invented in England. It
consists of a platinum vessel in which to boil the nitric
acid, which is to dissolve out the silver, and a small tray
containing a set of platinum thimbles with fine slits in the
bottom. In these the silver is taken out, by successive
supplies of nitric acid, without any decanting as in the case
of glass vessels. The cornets are also annealed in the
thimbles; in fact there is no shifting from the coiling to the
final weighing, which determines the fineness of the original
sample by proportionate weights in thousandths. In this
process extra care has to be taken in adding the proportion of
silver, as the "shaking" of any one cornet, might
damage the others.
Assaying
Silver
The
process of assaying silver differs from that of gold. To
obtain the assay sample, a little of the metals is dipped from
the pot and poured quickly into water, producing a
granulation, from portions of which that needed for assay is
taken. In the case of silver alloyed with copper there is
separation, to a greater or less degree, between the two
metals in the act of solidification. Thus an ingot or bar,
cooled in a mould, or any single piece cut from either, though
really 900-thousandths line on the average, will show such
variations, according to the place of cutting, as might exceed
the limits allowed by law. But the sudden chill produced by
throwing the liquid metal into water, yields a granulation of
entirely homogeneous mixture that the same fineness results,
whether by assaying a single granule, or part of one, or a
number.
From
this sample the weight of 1115 thousandths is taken; this is
dissolved in a glass bottle with nitric acid. The standard
solution of salt is introduced and chloride of silver is the
result, which contains of the metallic silver 1000 parts; this
is repeated until the addition of the salt water shows but a
faint trace of chloride below the upper surface of the liquid.
For instance: if three measures of the decimal solution have
been used with effect, the result will show that the 1115
parts of the piece contained 1003 of pure silver; and thus the
proportion of pure silver in the whole alloyed metal is
ascertained. Extensive knowledge and experience are required
in such matters as making the bone-ash cupels, fine proof gold
and silver, testing acids, and other special examinations and
operations. The Assayer must, himself, be familiar with all
the operations of minting, as critical questions are naturally
carried to him. The rendering of decisions upon counterfeit or
suspicious coins has long been a specialty in this department.
Once a year the President appoints a scientific commission to
examine the coins of the preceding year. There has never yet
been a Philadelphia coin found outside of the tolerance of
fineness.