GoldPaq

Have you ever wondered how jewelers and other goldsmiths determine how pure, or fine, gold is?

The process of measuring the “fineness,” or purity, of gold is referred to as assaying. There are many different methods of assaying the fineness of gold, some of which you can learn about here.

In times past, touchstone testing was employed to measure the gold content of gold items. In this method, the gold item is rubbed on a touchstone which also has rubbings of known reference samples and treated with acids. The gold content is judged by the color of the reacted area when compared to that of the reference sample. A highly inaccurate method, touchstone testing is rarely used in modern assaying.

The most accurate method of assaying gold is the fire assay or cupellation method. Historically following the touchstone testing method, this method has an accuracy of 2-3 parts per ten thousand (0.02%). For cupellation assaying, a small scraping of gold, around 250 milligrams, is taken from the gold item. It is then wrapped in lead foil with some added silver and cupelled in a furnace at about 1100?C to remove all base metals. The resulting gold-silver alloy is placed in nitric acid to dissolve out the silver, resulting in pure gold, which is then reweighed. This is the technique used by the assay laboratories worldwide for gold hallmarking, or stamping it with a gold content stamp.

Technology often employed in gold assaying nowadays, resulting in several assaying methods that are more science than art.

One of these methods is Inductively Coupled Plasma Spectrometry, or (ICP) which is nearly as accurate as fire assay, while also requiring a smaller sample of the gold to be assayed. About 20 milligrams of gold is needed for ICP, in which the sample is dissolved in acid and subjected to a sample analysis in an ICP spectrometer. The equipment to complete ICP testing involves scientific knowledge and expensive instruments, but is highly accurate. ICP is accepted for hallmarking purposes, and can also measure other alloying materials used in goldsmithing.

An exciting development in gold assaying is X-ray fluorescence (XRF), which is non-destructive enough that it can be used not only for assaying gold in manufacturing, but can also be used to certify gold content in retail outlets. However, it’s accuracy is affected by the items being x-rayed; it is most accurate under perfect conditions, or when the surface of the gold item being measured is relatively flat and sufficiently large enough for X-ray. On curved, detailed, or smaller surfaces, the X-rays are scattered an accuracy is reduced. It’s pluses are that it is a very quick way to assay gold, taking around three minutes, and can be used in tandem with a computer that can give a print out of the results, and that it can also be used to measure the content of alloying metals used in the gold piece. Negatives are the fact that is measures the gold content of a thin surface layer, which means that accuracy is affected when the gold item has a chemical surface treatment ? often used to enhance color. This measurement of thin surface layers of gold also means that the XRF process can be “fooled” by gold electroplated items.

XRF has become so popular ? and so reasonably priced ? that the technology is often employed by both assayers and retailers. Consumers can have their gold assayed in retail markets in a matter of minutes. Not only can they have the fineness of potential purchases tested, but for a small fee, many retailers are willing to use XRF to test items that the consumer already owns.

To recap, gold can be assayed in different ways, from simple touchstone testing to elaborate ICP assaying, from primitive to scientific. The most accurate method of gold assaying is one of the oldest, fire assaying, followed closely by ICP assaying. While XRF technology can be less accurate depending on several factors inherent to the piece being assayed, it is acceptable both in manufacturing and in retail.

Gold assaying gives both gold jewelry manufacturers and consumers alike proven methods of determining the fineness of gold pieces. However, it also protects consumers from buying gold that may not be as fine as the karat weight suggests.

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