Gold is a highly useful and applicable precious metal. Due to its softness, it is often alloyed, or combined, with other stronger metals in order to make items such as jewelry. The purity of gold is measured by karats; pure gold has 24 karats. In the united states, gold must have this karat number printed upon the surface; however, fraud is common due to its high value. There are many methods to test gold purity. These methods range in cost and complexity, depending on the precision needed and the nature of the samples used.
Preliminary Tests
There are gold imitators with very similar physical characteristics, such as pyrite, that are sold as the real thing. In order to avoid spending unnecessary money on advanced purity tests, first try the following tests with your specimen. Scratch the sample on a piece of unglazed porcelain (found in common hardware stores) and examine the streak. If it is dark, it is most likely pyrite. If it is golden yellow, it is gold. Next, scratch the sample against a piece of glass. Since gold is softer than glass, it should not be able to leave a scratch mark. Finally, hold the sample next to a magnet. If the magnet attracts it, it is not pure gold. None of these tests are able to determine pure gold and/or the complete absence of gold, but they are useful as preliminary tests.
Acid/Sctratch Testing
Acid/scratch testing is a cheaper option among more exact gold purity tests, able to determine the purity of a sample with relatively high precision. This may be done at home by purchasing an acid-testing kit or in a professional laboratory. The test is performed in a series of trials with gold needles at varying increments of karat level. Scratch a needle and your sample against a touchstone, and apply a low-level acid that corresponds to the karat level being tested. This acid will change the color of the scratches; if the two colors match, your sample has the approximate karat level of the gold needle used in that particular trial.
Machine Testers
More expensive methods of purity testing involve the use of machine testers that evaluate the purity of a gold sample with the highest precision. The most common of these are electronic gold testers, which exist in a variety of forms, both personal and commercial as well as in the laboratory for professional appraisal. Electronic testers use drops of liquid solution to scan the sample, producing a digital read of the purity. A much more expensive method, found only in laboratories, is the x-ray fluorescence test, which uses x-rays to perform a wavelength dispersion analysis, which determines not only the karat level but additional information about the composition of your gold.
Fire Assay
The fire assay is usually used to determine the purity of larger samples of gold. This method is not often used on items such as coins or jewelry, because it is a destructive process which melts down gold and separates it from the other materials of the sample. In this process, the sample is heated to extremely high temperatures and fused with lead and silver, which act as the agents that separate and collect the pure gold at the end of the process. This product is then weighed to determine the purity of the original sample.
