A touchstone is a small tablet of dark stone such as fieldstone, slate or lydite. It has a finely grained surface on which soft metals leave a visible trace when they are rubbed against it.
The age-old touchstone method is particularly suited to the testing of pieces suggested to contain precious metals, for which sampling by destructive means, such as scraping, cutting or drilling is unacceptable. A rubbing of the item is made on the stone, treated with acids and the resulting color compared to references. Differences in precious metal content as small as 10 to 20 parts per thousand can often be established with confidence by the test. It is not indicated for use with white gold, for example, since the color variation among white gold alloys is almost unperceivable
The fire-assay is the most elaborate but totally destructive assay method. Also called cupellation, with an accuracy of 1 part in 10,000. In this process the article is melted, the alloys separated and constituents weighed.
In this process, an alloy consisting of both noble and base metals is placed in a crucible. This mixture is then melted and allowed to freeze. When solidified, a button consisting of precious metals and lead can be removed from the slag of metal oxides and other materials. When assaying ores this slag layer can be quite massive, but when assaying karat gold, it is virtually non-existent. After cooling, the metals are placed in a special pot made of bone ash or clay called a cupel. Under high heat, lead turns to litharge, a lead oxide, which is absorbed by the cupel or lost to the atmosphere. At the end of the cupellation process, a button of pure gold and silver remains in the bottom of the cupel. The button is then placed in nitric acid to dissolve the silver, and the remaining pure (24k) gold weighed to determine the gold content present in the original starting material.
XRF Spectrometer (X-Ray Fluorescence)
The XRF method is widely used to measure the elemental composition of materials. It is a fast, non-destructive analysis method with an accuracy of 2 to 5 parts per thousand. When the analysis is complete the results can be printed out by computer. Depending on the application, XRF can be produced by using not only x-rays but also other primary excitation sources like alpha particles, protons or high-energy electron beams.
ICP Spectrometer (Inductively Coupled Plasma)
An ICP uses a high temperature Argon plasma to analyze element concentrations in liquid solutions. It is an optical emission (OES) system and uses the light emission from the high temperature excitation of the atoms to determine their concentration.
Typically a sample is weighed accurately, placed in a beaker and dissolved in aqua regia which is a mixture of hydrochloric and nitric acids. The solution is diluted and injected into the flame of an ICP spectrometer. The sample ionises and emits radiation which is analysed with a spectrometer. From the intensities of the radiation, the amount of precious metal in the solution can be calculated. The technique can produce an accuracy of about 10 parts per million.