As a precious metal, gold commands a high price. Methods for testing gold purity vary in accuracy, cost, and convenience. They include fire assay, ICP spectrometry, XRF, touchstone and acid testing.
Fire Assay
A fire assay consists of taking a scraping of the ring, weighing it, purifying it with heat and acid, and reweighing what is left, which should be pure gold. The fire assay is the most accurate test of how much of the ring is gold.
ICP
Inductively coupled plasma spectrometry is the second most accurate test, but it's too expensive to be practical. It measures the concentration of gold dissolved in an acid solution and requires other gold samples for comparison.
XRF
X-ray fluorescence, a quick and nondestructive method that analyzes the intensity of X-rays emitted by gold, loses accuracy when testing non-flat, chemically treated or gold electroplated surfaces, which distort and scatter the X-rays. XRF takes only several minutes, which has led to its popularity as a gold assay.
Touchstone Testing
Testing gold with a touchstone or sheet of ground black glass is an ancient but less-accurate method. The tester scrapes the ring on the stone and compares the color of the streak to streaks of known alloys.
Acid Test Kits
Testing with acid solutions, a simple but also less-accurate method, consists of scraping the ring and applying acids of different solutions that identify carat strength by a color reaction.
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