Items of jewelry or art made of precious metals are hallmarked based on specific requirements either of the country of import or the place of manufacture. Such items, particularly those made of silver, gold, or platinum, are struck with an official mark (or series of marks), which guarantees fineness or purity of the metal used.
To determine the precious metal content in an item, certain non-destructive assay techniques are used. Two examples are the touchstone method (a very old assay method) and the X-ray fluorescence method (the modern, non-destructive assay method). While these assay methods are suited for finished goods (again because they are non-destructive), three other methods are more suited for raw precious metals:
Titration:
This assay method is one of the most widely used laboratory technique, which involves the analysis and determination of unknown concentration of a given reactant. It is used to assay silver bullion or stock. In this method, a reagent (titrant) of a known volume and concentration is utilized to react with a solution of the substance being analyzed (titrand), whose concentration is unknown.
With the use of an instrument called burette (to add the titrant), the exact consumed amount, on reaching the endpoint, can be determined. The endpoint refers to the point at which the assay is complete. The completion is signaled by an indicator. There are at least four types of titration. These are acid-base, redox, complexometric, and Zeta potential.
Cupellation:
This technique is considered the most exact, elaborate, and destructive method of assay and is best suited for gold bullion or stock. Also called fire assay, the method involves treating ores or alloyed metals under high temperatures and carefully controlled operations to separate gold from base metals (copper, zinc, or lead), which may be present in the ore. Once the base metals are heated at high temperatures, the gold (as well as the other precious metals that may be present) remains apart and the other non-precious metals react forming other compounds. Cupellation basically has two processes: large scale and small scale.
Spectrometry:
This method is best used to assay platinum bullion or stock. With the use of a spectrometer or spectrograph, the amount or concentration of a given substance is assessed. The substances are identified through the spectrum they absorb or emit. This assay method has several types. These include absorption, fluorescence, X-ray, flame, visible, ultraviolet, infrared, photoemission, Mossbauer, nuclear magnetic resonance, and Raman.
Again, the X-ray fluorescence (XRF) method is the modern assay method widely used today for analyzing precious metals, including (besides silver, gold, and platinum) rhenium, ruthenium, iridium, and palladium. As a non-destructive assay method, XRF can identify various elements in a substance (in fact, even in powder and liquid ones) within a few minutes.
One of the most important aspects in any of the assay methods pertains to the accurate determination of the composition of a substance at various points in the process. Controlling and minimizing metal losses can only be achieved through close monitoring of the composition of the work in progress.
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