Assaying and Acid Testing Gold

Testing for gold content, fineness or purity.

There are a number of methods for determining the gold, silver, platinum and palladium content, or fineness, of precious metals.

Fire Assay or Cupellation Method

The most accurate method to assay gold, with an accuracy of 2-3 parts per ten thousand (0.02%), is the Fire Assay (Cupellation) method. This involves taking a small sample from the article, typically about 250 milligrams, weighing it accurately, wrapping it in lead foil with some added silver, cupelling it in a furnace at about 1100C to remove all base metals and then placing the resulting gold-silver alloy button in nitric acid to dissolve out the silver and re-weighing the resulting pure gold. This is the standard reference technique used by the national Assay laboratories worldwide for Hallmarking and is covered in the International Standard, ISO 11426:1993. Fire assay is also used to determine the silver content in precious metals alloys.

Precious metal items of art or jewelry are frequently hallmarked (depending upon the requirements of the laws of either the place of manufacture or the place of import). Where required to be hallmarked, semi-finished precious metal items of art or jewelry pass through the official testing channels where they are analyzed or assayed for precious metal content. While different nations permit a variety of legally acceptable fineness's, the assayer is actually testing to determine that the fineness of the product conforms with the statement or claim of fineness that the maker has claimed (usually by stamping a number such as 750 for 18k gold) on the item. Fire assay is better suited for the assay of bullion and gold stocks rather than works or art or jewelry because it is a completely destructive method.

Touchstone or Acid Testing for Gold, Silver, Platinum and Palladium

Touchstone testing is an ancient method for measuring gold content whereby a rubbing of the item is made on a special touchstone alongside rubbings of known reference samples and treated with nitric acid and a mixture of hydrochloric and nitric acids (Aqua Regia). The colour change and rate of dissolution of the reacted area is compared to that of the reference sample. It is accurate to about 15 parts per thousand and is useful as a sorting test to differentiate between different karats.

The touchstone method is particularly suited to the testing of very valuable pieces, for which sampling by destructive means, such as scraping, cutting or drilling is unacceptable.

Aqua Regia

Aqua regia or aqua regis (Latin for royal water or king's water) is a highly corrosive, fuming yellow or red solution, also called nitro-hydrochloric acid. The mixture is formed by freshly mixing concentrated nitric acid and concentrated hydrochloric acid, usually in a volumetric ratio of 1:3 respectively, although other ratios are made and used to test varying purities of gold. It was named so because it can dissolve the so-called "royal metals," or noble metals, gold and platinum.

X-Ray Fluorescence

The modern X-ray fluorescence is also a non-destructive technique that is suitable for normal assaying requirements. It typically has an accuracy of 2 to 5 parts per thousand and is well-suited to relatively flat and large surfaces. It is a quick technique taking about three minutes, and the results can be automatically printed out by computer. It also measures the content of the other alloying metals present. It is not indicated, however, for articles with chemical surface treatment or electroplating.

X-ray fluorescence (XRF) is the emission of characteristic "secondary" (or fluorescent) X-rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays. The phenomenon is widely used for elemental analysis and chemical analysis, particularly in the investigation of metals.

Electronic Gold Testers

Electronic Gold Testers are portable devices that use the capacitance decay principle to determine karat. Accuracy is poor, being correct to only 1-2 karat (4-8%) and is compromised if the surface is gold-plated. It is useful only as a crude sorting test.