|Torch Fire Assay
|Jensen Technologies Lab has developed a Torch Fire Assaying Procedure.
Torch Fire Assaying Technique
Charles Butler at Butler Lab has developed an assay procedure to be used as a supplement
to furnace assaying of the noble metal ores. It is quick and inexpensive, with most assays
requiring 15 to 20 minutes. (Reprinted from Butler Lab Website www.butlerlab.com)
It is an excellent prospect tool and field guide for the drilling crews. It can also be used to
assay mill run samples and tails, leach solutions, chemical precipitates and any of the noble
metallic salts, including silver, gold and the platinum group metals. It is ideal for assaying
wet or dry concentrates from one's recovery system and the flow water being used in said
system. This water, when working with ores carrying water soluble values, will carry away a
portion of one's values unless steps are taken to save them.
The infinite number of possible torch flame settings allows one to create the reductive or
oxidizing flames when they are required. As a result, only two fluxes are required and we
seldom have a failure. When difficulties do arise, we are made aware of it within a minute or
so, allowing the assayer a chance to make any needed adjustments of the torch, or the
addition of either type flux.
1. When assaying for the "noble metals", in order to free our metallic values, one must
create a liquid fusion of the "assay charge". We do this by mixing our crushed sample with
the proper assay flux and placing same inside a crucible. Then, with our oxy-acetylene torch
adjusted to the proper oxygen to acetylene ratio, we create the "reductive flame" required to
liquefy the assay. The intense heat from the torch, applied directly down over the assay,
creates temperatures in excess of 2,000 degrees centigrade, enabling one to recover all of
the noble metals, if present.
2. One needs a "collector" to collect and hold any liquefied gold, silver or platinum group
values the tested sample may have contained. We accomplish this with one of our flux
ingredients, "litharge" a lead oxide powder. When performing correctly, heat from our
reductive torch flame changes this litharge to liquefied lead. This lead alloys, or combines
with all the molten noble metals, except osmium, one of the platinum groups. When assaying
specifically for the platinum metals, most assayers add a silver "inquart", either as foil or
wire, to act as their collector, as silver does alloy with all platinum group metals. In addition,
the extreme effects the PGM's have on the silver "beads", makes for easy recognition that
platinum metals are present.
3. The fusion procedure must produce a 1/8 to 5/32 inch diameter lead "button". This lead
will start to appear as real small, shiny beads, within the first 30 to 40 seconds, after
applying the reductive torch flame to the assay charge, if everything is working as planned.
Failure to observe any tiny lead beads, indicates "trouble". A few difficult ores do require the
adding of a second measure of assay flux. When fusion is complete (everything liquefied)
and all lead has settled to the bottom of the crucible, we dump the "slag", leaving the lead
button in the crucible for "scorification".
4. This step is the scorifying of the lead button to remove any "base metals" (metals other
than noble metals) that were picked up by the molten lead. This is performed by adding a
small pinch of borax glass, then applying a highly oxidizing torch flame. The lead button will
catch on fire, burning at over 3,000 degrees centigrade. This high heat changes all base
metals to "scoria" or ash, which joins the molten borax glass as a slag, for disposal. The
somewhat smaller lead button, containing any noble metal values that were present, is
dumped out onto an aluminum plate for cooling.
PGM Dore Bead
5. The next step is to "cupel" the button, to dispose of the lead. Our cupels are small, bone
ash cups. Their porosity adsorbs the liquid litharge, created by the oxidizing of the lead
button, with the oxidizing torch flame. Cupelling is complete, when the button, growing
smaller, becomes a bead, refusing to reduce in size any further. After the torch is
withdrawn, most beads will "blick", or flash, just as they begin to cool, at the very instant
they change from liquid to a solid. These beads are referred to as "dore beads", as they may
contain any or all of the noble metals, silver, gold, platinum, palladium, iridium, osmium,
rhodium and ruthenium. The assayer must determine which, with additional testing.
All measurements and computations are based on the volumetric principal, instead of by
weight. This, since scales sufficiently accurate for this work would be very expensive and
inappropriate for outdoor use. A small, accurately calibrated spoon is used to measure out
the material to be tested. A spoon five times larger is used for measuring the flux. The noble
metals are recovered as a small, round bead, its diameter is measured with a 40X power,
optical comparator, rather than weigh them, to determine the value of the ore which
produced them. Through measurement and the use of our "discoloring solution", most ores
can be evaluated. However, some beads do require "parting" with nitric acid and distilled
water to remove the silver, then rebeaded, before their ore can be evaluated. The discoloring
procedure lets us know when parting is necessary. Actually, parting is seldom required, as
at least 90% of one's tests can be evaluated through use of natural observation and the
"Bead Discoloring Procedure". It readily identifies all beads containing 75%, or more, silver
and lets one know the approximate silver content, ranging from 75% to 100%. The results are
expressed in "Troy Ounces" of silver or gold and platinum group metals per ton of materials
The following facts, comparing the torch assay procedure with the conventional, furnace fire
assay, will further indicate the different ways in which this technique has advanced the art
for the prospector and miner.
l. There is no massive furnace to be heated, only a small assay weighing a few grams and a
two ounce crucible.
2. The "fusion charge", in the conventional assay, just lies there, within the crucible, without
benefit of any appreciable mechanical stirring to assure that the molten lead will come into
contact with the entire melt, during fusion. In our method, the crucible is manually rocked
around in a circular motion, and in addition, the flame is used to stir the melt, assuring
complete alloying of the lead and any noble metals contained within.
3. The method eliminates removal of any tiny beads from the cupels, cleaning and weighing
them. This is a tedious task, at best.
4. The entire process is carried out under one's visual observation, from start to finish. This
allows for the immediate adjustment, if necessary to correct for conditions caused by some
5. The light weight, mobile equipment can be taken to a field site if needed.
6. With the Torch Assay Procedure, one can develop, prove and promote his gold, silver or
7. Probably, the most important feature is the fact that one receives an answer within
minutes, instead of hours, days, or weeks.
J-Tech has developed a full scale Fire Assay capability. Please, inquire for availability.
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