Chemistry

Metalworking what is an Assay



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Assaying is the analysis of ore, metal or alloy. The discipline is known as metallurgical assay.


There are several uses for assaying metals, both before and after refining it.  For example, samples are always taken from potential mines, to be sure of the quality of the desired metal, to verify the mining of said metal will be profitable.  These samples are then sent or taken to an assay office, where the sample (s) will be analyzed in any of several different ways.


While there are many areas of the world that have gold, one of the very precious metals, not every site will prove to have sufficient quantities or quality for mining.  Samples of rocks are taken, ground down, and the entire sample will be submitted to an assay laboratory for in depth examination to determine how much, if any, gold exists in said sample.  The report will give the estimated quantity of the metal, as well as use the ground rocks as the base for estimating how much metal one can expect to retrieve per ton of rock or soil mined.


Of the many areas that have gold, silver, or even uranium, etcetera, not all areas contain sufficient quantities to make it profitable for mining on a commercial or wholesale level.  Sometimes, the metal may be inaccessible to a point of being too costly to justify the initial investment, or it may be exceedingly dangerous, making mining to risky for safety standards to be satisfied. 


The risks involved, both physical as well as financial will also be affected by the price on the world market, which may make the mining more profitable at one time and not at another time.


There are different methods for assaying metal in its different forms, such as gold ore, or a finished piece of gold jewelry, or even gold coins, gold bars.


The testing methods are each called by different names, such as the Touchstone method, more suited for very valuable pieces, which would be damaged if more destructive methods were to be used, such as cutting or scraping or drilling.  Fire assay/cupellation for example, would be completely destructive in such a case as find jewelry.  This is used for assaying ore, not already refined products.  The fire assay or cupellation method is very accurate, to one part in ten thousand.


X-ray fluorescence on the other hand is a non-destructive technique that is suited to normal assaying.  It is non-destructive, and it is typically accurate to two to five parts per thousand.  This process is well suited to pieces that are relatively flat, and may cover a large surface.  One can also measure the content of other metals present in the sample.  This process does not work well, and is not indicated for items that have a chemically treated surface or electroplating.


Wikipedia, the online encyclopedia, says the following about methods of assaying and marking of gold:

“Method details for various fire assay procedures vary, but concentration and separation chemistry typically comply with traditions set by Bugby or Shepard & Dietrich in the early 20th century.”


More modern variations that may differ from the standards of assaying are viewed with caution, and the standard traditional tests have a long history of accuracy, and of reliability.  So-called “special,” or newer methods have frequently had reduced accuracy.


Jewelry made of gold, of course is usually made up of gold with added metals to make it more durable, and to affect the color of the finished piece.  Most gold jewelry may range from 10 karat, 14 karat, 18 karat, or twenty-four karat, which would be considered pure gold.  In some countries, such as in Mexico, they also have gold jewelry made which is marked 12 karat gold.

For follow-up research, the following are considered to be excellent sources for expert information:

Bugby, Edward E. A Textbook of Fire Assay 3rd ed (1940), Colorado School of mines Press, Golden Colorado.

Fulton, H.C., A Manual of Fire Assaying, McGraw-Hill Book Company, Inc., New York, NY, 1911.

Lenahan, W.C. and Murry-Smith, R. de L., Assay and Analytical Practice in the South African Mining Industry, South African Institute of Mining and Metallurgy, Johannesburg, South Africa, 1986.

Shepard & Dietrich, A Textbook of Fire Assaying, McGraw-Hill Book Company, 1940.

Taylor, P.R. (ed.), Prisbrey, K.A., Williams, J.F., Sampling, Preparation, Fire Assaying, and Chemical Analysis of Gold and Silver Ores and Concentrates, Department of Mining, Engineering and Metallurgy, University of Idaho, 1981.

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