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An assay in refining or mining is an investigative analytical procedure that determines the amount of precious metals in a sample. An assay can also determine other elements and type of compounds in a sample, especially in mining where the composition of an ore can decide how it is best processed.
The most common assay in refining is the fire assay but other methods is starting to gain foothold in the business. XRF for fast assaying when buying scrap and various spectroscopic methods (Atomic Absorption or Mass spectroscopy) for determination of PGM:s.
A simple method used for a long time is the touch stone for determine fineness of gold objects.
A company or person doing assays is called an assayer. For a list of list of assayers, look here.
(Main article fire assay)
With fire assay a sample is crushed (when possible), mixed with a flux containing among other things litharge and a reducing component.
When fired, the litharge is turned into metallic lead that collects the precious metals in the sample and coalesces into a liquid in the bottom of the crucible. Oxides and other contamination is forming a slag with the other components of the flux.
The melt is poured into a cone mold where the lead forms a button in the bottom of the cone. The button is formed with a hammer and the slag is brushed off, then put into a cupel and into an oven. At 950 °C the lead is melted and in an oxidizing atmosphere it is oxidized into litharge and absorbed into the cupel. Left behind will be a small bead containing the gold, silver and PGM:s of the sample.
The bead is further treated to separate the metals and then weighed with a precision scale.
Fire assay on solid metal objects could reach an accuracy of 1 part in 10,000, while on ore the accuracy can be as low as 3 to 5 percent.
(Main article Testing carat gold with a touchstone)
Touching gold is the oldest way to test the fineness of gold. It is a comparison test where an object with an unknown karat is rubbed against a dark stone to create a line of gold. Parallel lines are made with different karat gold alloys. A test acid streak is drawn across the gold streaks and the speed which the gold dissolves is compared. With different strength acids and mixtures combined with a set of test needles of known composition a precision of 1/4 to 1/2 karat can be achieved.
Alloys above 22k isn't possible to test with a touch stone and acids.
White gold alloys are harder to test than yellow gold alloys.
- Patrick Litte in his assay shop, shows the final flash of the gold at the end.
- GRF : The assay / refining lab.....second time around, plans and procedures
- GRF : Some fire assay discussions
- Wikipedia : Wikipedia : Metallurgical assay
- GOLD Bulletin : Touching gold
- Mining and metallurgy : Analytical methods
- Internet Archive : The Sampling and Assay of the Precious Metals by Ernest Alfred Smith, 1913