Fire Assaying
The Department highly recommends
analysis of samples by the fire assay method to determine
precious metal content. Fire assaying, in use for thousands of
years, still stands the test of time. The following summary of the process is
principally taken from "Assaying," by Jim Steinberg in
Mining Artifact Collector. The article was written from a
historical viewpoint, but the process is the same as the fire
assaying of today except for the use of electronic balances,
mechanical pulverizers, and so forth.
In Webster's dictionary assay is defined as
follows: in metallurgy, the determination of the quantity of any
particular metal in an ore or alloy; especially, the
determination of the quantity of gold or silver in coin or
bullion.
While the most common definitions of the word
assay do revolve around the determination of gold or silver in
ore or alloys, assay is itself a much broader subject which
involves the quantitative analysis of chemical substances both
organic and inorganic. The primary interest of this article is
the assay of metalliferous ores. Because even this is a broad
subject which has filled a large number of books, the highlights
of fire assay by the scorification process in gold bearing
samples are going to be summarized here.
Initially, the sample must be reduced to a
powder so that it can be tested. This powder is often called
"pulp" and the scales to weigh it are called "pulp
scales." The assayer begins by running the sample through a
crusher. With many crushers, the fineness of the output is
adjustable. The sample is still not sufficiently fine after
initial crushing, so the assayer then puts it onto a "buck
board" for further pulverization under a muller that rubs
the material into a finer state with a sliding motion. Harder
samples are made finer using a device called a "rocker"
that uses a heavier weight upon the sample being pulverized.
Assayers doing a smaller volume of work might use an iron mortar
and pestle, although it requires considerably more effort.
As the pulverization of the ore sample
proceeds, the assayer mixes and then divides the sample into
smaller and smaller portions until he has reduced the amount of
the sample to the size that he will actually process. This can be
done manually or by using devices designed to assist in the
sampling process. This is done to assure a uniformity within the
sample and to increase the accuracy of the assay to be performed.
When the sample has been sufficiently
pulverized it must be run through sieves of the appropriate size.
Material that does not pass through must be further ground until
the entire sample will pass through the sieve. What has passed
through the sieves must then be carefully mixed and then stored
in a marked container. The contents of these containers should
not be shaken or agitated as this can cause the materials to
begin stratifying according to their masses and upset the
accuracy of the process.
From various parts of the container, selected
portions of the sample are taken and weighed. The weighed sample
is then placed in a scorifier, a dish that can sustain the heat
of the assayers oven. Along with the sample litharge (a form of
lead), various chemicals that will assist in allowing the metals
in the sample to separate from the slag are included.
This mixture is roasted in the assayer's oven
until the melted slag completely covers the lead bead that forms
in the scorifier.
The sample in the scorifier is next poured into
the cup of the scorification mold. Here it is allowed to remain
until it is cold. Once cold, the sample is removed from the mold.
It is cone-shaped, with the metal at the apex of the cone and the
slag forming the bottom. The metal part, or lead button, is
detached from the slag. This button may then be hammered into a
cube with no sharp corners.
The button is placed into a cupel of
appropriate size that is then placed in the furnace. Cupels are
comprised of a material called bone ash. When it has come up to
heat, the button is placed in the cupel. In this process lead and
other impurities within the button are both oxidized and driven
into the material of the cupel itself. A good cupel is capable of
absorbing its own weight in litharge (the lead in the sample).
The metal in the cupel melts and will be observed to become
smaller as the process proceeds.
Towards the end of the process, the surface
tension of the metal will draw it into the shape of a bead. The
bead will appear to be in rapid motion and at the moment the
process is complete, an optical energy release will sometimes be
visible as a "flash" or "blick." At this
point, the cupelation is complete and the cupel with its bead may
be removed from the oven.
Now the bead is removed from the cupel. The
composition of the bead should now be gold and silver. The bead
is weighed in a type of scale made specifically for this task
called a button scale. Button scales, because they are measuring
something so small, must also be very accurate and are thus
always enclosed, while analytical or pulp scales do not always
require enclosure. Weighing the bead has shown how much metal is
there, but has not told how much is gold and how much is silver.
The next step of assaying is called
"parting." In this step the gold and silver are
separated from each other by solution. The weighed bead is
flattened, placed in a porcelain capsule and treated with a
solution of water and nitric acid. Once reaction begins, the
capsule is warmed. Silver in the bead forms a solution of silver
nitrate that is carefully washed away until only the gold, if
any, remains. This is gently dried in the porcelain capsule and
then removed.
The final sample of gold is again weighed in
the button balance, unless it is too small to be weighed, in
which case it is simply described as a "trace" or
"color." From the weight of this bead the assayer will
then calculate the gold and silver ore value per ton of ore. The
assayer may use a special set of assay ton weights when weighing
the gold to more easily calculate the assay value of the ore.
In Summary, Fire Assaying is a three-step process:
- Fusion - The sample is mixed with flux, then heated
to 1850° F. A slag containing the unwanted elements and a lead
button containing the gold and silver are formed.
- Cupeling - The lead button is heated
and oxidized in a bone ash cupel that adsorbs the lead oxide,
leaving a precious metal bead in the cupel.
- Parting and Weighing - In this part
of the process, the gold is separated from the silver. Two
weighing steps are involved.
Discussion of Fire Assaying's Purported Problems
Fire assaying is a series of chemical
steps that takes advantage of the precious metal's chemical
behavior. Those who claim they have non-fire assayable gold are
saying they have a substance that chemically does not behave like
gold. Arguments used to explain why fire assay is not applicable
to their "Colloidal" or "Micron" gold
generally fall into one of the three categories discussed below.
- "The particles are so small they vaporize
and so are not in the button."
- 1850° is below the melting point of gold. Even
if the temperature goes above 1850° the vapor pressure of gold
is small. so very little is lost. H2O, for example,
has vapor pressure 6 orders of magnitude higher.
- "Small particles of gold float on the
surface of water so they float on the slag."
- This ignores the process that goes on. It is
not dependent on gravity. The PbO2, now Pb, dissolves
the gold. It is the Pb that collects at the bottom of the
crucible.
- "Interfering elements mask the gold."
- The London Mint ran an assay of 1000 mg
tellurium, 1 g Au, 25 g Pb and skipped the fusion step! Even so
the "worst" they could do was to lose about half the
gold. These conditions are highly unlikely in a rock sample. What
about the platinum group metals? These, if present, report with
the gold in the bead.
Conclusion
Fire assaying, in use for thousands of years, still stands
the test of time.
For more information about Arizona assayers, see
Registered Assayers.
To see the results of a survey of various assay labs conducted
by the Bureau of Land Management, see
BLM Assay Laboratory Report
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