Ask The Assayer


Dry Concentrators
RMC Services Page
Contact RMC
DCP Page
Schedule of Services
Mines For Sale
Metals Prices
Knelson Concentrators
Helpful Info





Leach Gold from ore's by using electro-chlorine. Leaches gold from ore at comparable cost to cyanide but it is no more toxic than a common household swimming pool       More Info


Now Available ! 

Get the most historic book ever written about gold and precious metals processing

De RE Metallica








Complex Ores :Myth's and Facts of Fire Assaying the Complex Ore

Definition Of the Word Assay: To perform a analysis on a given sample using standard well accepted techniques and procedures. The data derived from these analytical procedures should have the ability to be duplicated by the assayer and other assayers alike.

Fact: No Refinery in the world that writes checks for the purchase of precious metals relies on anything but standard well accepted assay procedures.

Fact: No profitable operational mine relies on anything but standard well accepted assay procedures.

     Some people theorize that a fire assay is a worthless exercise because it fails to recognize a multitude of problems that arise from analyzing "complex ores". The main problem with these assertions is that what they are calling complex ore's are usually simple and very workable (usually worthless) ores. It seems that when a company or individual receives one extremely high assay (usually high in Platinum Group Metals) and many low assays on the ore body in question, it evolves into a "complex ore". This will more often than not be used as a justification for the low assay. The mine principles go from being novices one minute, to well seasoned experts based on the wide variety of complex ore propaganda available to them. From sub micron metal clusters, amorphous colloidal silica particles, to unstable isotopes, the ideas are definitely out there to find. But the one FACT that remains constant is that NOT one person or company on the face of this earth has a profitable operation that is based on any these principles.

The term "complex ore" has been so misused by ignorant misguided people that it has retained a personality of it's own. To some people the term has a certain allure, a chance to be part interesting exciting opportunity. They feel that they are taking part in something historic, because they took part in or help fund someone that "cracks" the complex ore.

It is very interesting that some assayers and chemist are able to isolate these exotic complexes using instrument analysis or special leaches. When it comes to a instrument analysis  such as AA, ICP, or DCP the instrument compares the sample, to a KNOWN value of precious metals (standards). Standards are prepared by weighing real actual gold, silver, platinum, etc......The instrument only measures what real known values are in he standard and compares that to what real values are in the sample. If "complex", not yet fully discovered precious metal values are present in the ore, YOU WOULD NEED A COMPLEX  UNDISCOVERED STANDARD IN ORDER TO ANALYZE THE SAMPLE. No chemical supplier in the world sells complex standards for analyzing precious metals.

Well accepted definitions of a complex ore which can be difficult to assay

     A true complex ore can be thought of as a mineralogical trash pile. Ore bodies that are high in such reagent consuming minerals such as mixed sulfides of (iron, copper, tellurium, selenium) are traditionally known of as complex or refractory ores. Other elements such as nickel, chrome, lead and bismuth can lead to interference and some difficulty in an assay. But those who have experience with these ores or even have taken the time to read the textbooks of others who have had similar experiences, find that most all ores can be assayed by fire assaying. I personally have yet to see a ore whose recoverable values could not be determined by a properly performed fire assay (23 years and about 7,500 plus fire assays).

     It is very common for a real complex ore to not respond to standard fire assaying techniques. But the thing that people who misuse this premise leave out of the argument, is that it is usually very obvious why the fire assay was unsuccessful. An example of this would be a high iron sulfide ore which would be considered a true complex ore. If the sulfide ore is fluxed with the same amount of lead and borax glass as a normal ore the result will be a gooey fused mess. A competent assayer will use a flux designed for the "complex" sulfide ore to insure a proper assay. The true complex ore is one that can be difficult to assay, but those with experience and knowledge should be able to produce repeatable assays.

Hypothesis of why fire assay may not work

Listed below are some of the 'theories" of why fire assaying is unable to identify precious metals in certain ores. We'll refer to these ideas as theories, but really they are just ideas.

  • Precious Metals Have Volatilized in cupellation process !

    It is true and factual that some small percentages of precious metals tend to have some loss during the cupellation process. But this fact is often misused by some who claim 100 % of the precious metals will gas off during cupellation no matter how you flux or pre-treat a particular ore. If all of the precious metals volatilized off in the cupel then, we can assume that they were present in the lead dore' from the fusion. We then should be able to eliminate cupellation by dissolving the lead in nitric acid and be left with a gold insoluble. Any platinum group metals could be precipitated or analyzed by instrument for quantities present. 

    Another way to determine if values are volatilizing is to have a known inquart (preferably silver) in your assay. If something truly is present in the lead dore' that causes volatilization then your inquart will gas off along with any values that hypothetically are in the ore sample.

    And yes very rarely (3 times), I personally have seen inquarted samples (up to 1 gram of Silver) end up with no button whatsoever in the cupel. Usually this is caused by the flux and fusion time collecting just the right amounts of interferences into the lead dore'. The fusion pours nice and the lead dore' will appear clean. This phenomena happens due to collected amount of possibly copper, tin, bismuth, selenium, tellurium (each sample would be different). Usually the cupel is discolored or darkened due to the inference collected.

    Some main things to try in order to eliminate this from happening are, changing your sample size and flux. Roasting the ground ore prior to assay to remove any harmful sulfides which might be causing the problem. Dissolve your lead dore' in nitric acid to see if any amounts of precious metals are present. Scorification of the lead dore' is also a well accepted procedure to remove any un-desirable interferences before cupellation.


  • You need to inquart (seed) the sample with large amounts of precious metals!

Inquarting a fire assay with small amounts (5-10mg) of "gold free" silver inquarts is a well accepted procedure. But some people argue that in order to collect all the metals present, the sample needs to be inquarted with large amounts of precious metals. One scenario calls for the 1part ore to 1part silver inquart for assaying the ore. The main problem with this is usually .999 silver, purchased from a reputable refinery, is used in the assay. When this silver is used, it means that .1 % is not silver and most likely other precious metals. The .1 % of a ton equals 29 ounces of something per ton that is not pure silver. Lets say   10 % of the impurities are gold (usually higher for .999 silver) that would be 2.9 ounces of gold per every ton of .999 silver. If the sample was then inquarted on a one to one basis with the silver, a cupeled dore would be produced which would contain gold and precious metals from the silver.

A blank sample (using exact same flux and fusion times) but leaving the ore out, is a good way to determine approximately how much precious metal impurities are present in the silver. The dore produced from the blank should be tested exactly the same way as any dore' used to evaluate a ore sample, preferably side by side for comparison.

  • Precious metals did not collect in lead and remain in slag!

If the ore does contain precious metals, it is possible (in some cases) that if fluxed improperly some of the precious metals could remain in the fusion slag. But it would be extremely rare (almost impossible) if all precious metals in a sample (100%) reported to the slag and ended up being repelled by the by the lead. If something was present in the ore to cause detectable amounts of precious metals to report to the slag, then a properly done wet acid digest or neutron activation would show the values present.

  • Must re-melt your ground slag many times in order to break down the complexity of the sample !

Grinding your slag repeatedly is often recommended for problem "complex" ores in order to break down the complex bonds. Some "promotional assayers" go as far to say that re-fusing the slag will stabilize the unstable isotopes in the complex ore. If the ore did contain radioactive unstable isotopes of lets say gold, then the laws of physics state gold isotopes don't stabilize into other gold isotopes. They become other elements based on their decay methods.

  • Long fusion times and high temperatures !

Some people think that the lack of high temperatures and long fusion times are reasons that a fire assay does not recognize values in some ores. The argument is that high temperatures can break down the metal-metal bonds or metal clusters. Again these are ideas and no profitable mining company or refinery implements these to recover any extra precious metals that they feel are complex in nature.


It is well accepted by chemists and assayers alike, that if a ore will produce an accurate wet digest then it should be able to produce similar results when fire assayed. This being said, if a assay was obtained by wet chemistry instrument analysis methods of lets say 1.5 ounces of gold per ton of ore. Then similar results of 1 to 1.5 oz per ton should be able to be reproduced by fire assay. Often one or more fire assayers cannot duplicate the false positive that can be obtained by improperly performed wet instrument digest.

Use extreme caution when dealing (and investing large sums of money) with assayers who claim that any ore in question cannot or does not respond to fire assaying. Any ore no matter how "complex" in nature can be tested for precious metals by fire assaying. Wet digests are very accurate if done correctly, but the results should be in comparable ranges to the fire assay, and visa versa. The main thing is not to be duped by some part time chemist who thinks he's re-invented the wheel. Always try to get a second or even third opinion (from reputable labs) if you hear the term "Complex Ore, Will Not Respond To Fire Assay." 

2009 Rare Metals LLC.  









Have a Question? Can't find any specific info anywhere. Just send us a e-mail and we will be glad to try to answer your question or solve your problem.


Recover Gold Dry Using RMC's Dry Air Concentrators. Process 2-150 Tons per hour. Self Cleaning Adjustable Speed Concentrator.       More Info