Torch Fire Assaying Technique
by Charles Butler, Butler Lab,

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.

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.

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 being assayed.

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.

1. 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 troublesome ores.

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 platinum property.

7. Probably, the most important feature is the fact that one receives an answer within minutes, instead of hours, days, or weeks.

J-Tech (Jensen Technologies, LLC) has developed a full scale Fire Assay capability. Please, inquire for availability.

Reprinted and edited from Charles Butler, Butler Lab,

This article was contributed by Jensen Technologies, LLC whose laboratory is located at 15985 S. Golden Rd, Unit H, Golden, CO 80401. Their phone number is (303) 478-3929 and their FAX number is (720) 542-3498. You may find out about their Laboratory Services by contacting by e-mail. Their website is where all the information is available.

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