Acid test

An acid test is a test used to confirm that a material such as jewellery is gold. Figuratively, it is a definitive test which is destructive to imitations. Generally, it may be any scientific or metallurgical test which uses acid.

The figurative meaning of the expression, where it is applied to tests of character, or destructive tests to other materials, became popular during and after the California Gold Rush[1]

An acid test was used during the California Gold Rush of the 1850s by gold seekers and gold traders to quickly and incontestably find whether a given metal, excavated or found, was gold or a mere base metal. The metal would be exposed to a strong acid (typically by adding a small drop of acid to the surface). Unlike base metals, precious metals, including gold, do not corrode easily. Thus, base metals would begin to fizz or bubble when immersed in acid (most often nitric acid), while precious metals would remain unaffected. Hence, a clearly visible result is obtained on the spot, with no need for further testing. Nitric acid was available for the production of explosive material. For these reasons, the acid test was already the most popular way to test for gold, since it was decisive, immediate, cheap and simple to perform.

An analogy has since been drawn from this historical test to more modern quality tests, such as the three Acid tests (Acid1, Acid2 and Acid3) which are designed to test web browsers for compliance with current web standards. In very much the same manner, an acid test is the common name given by economists to a test to verify a business' liquidity. These uses drop the original implication that false materials will be destroyed by the test.

Since the end of the 19th century, "acid test" has been a colloquialism for a "sign of approval" or "verified" and "tested" products.