Q From Will Thomas: Where did the expression acid test come from? In the back of my mind — way back — it originally had something to do with testing diamonds to see if they were real. It’s an expression I don’t often hear anymore. Clearly it means the definitive test.
A That’s right. As you may guess, the phrase started its life in science but was popularised. Chemists have been making acid tests for centuries and, in particular, the newspapers and books of the later nineteenth century are full of references, mainly in connection with tests to identify suspected adulteration of foods such as bread, butter or milk or to prove to customers that they are pure. One American advertiser around 1900 made great play of the acid tests that had supposedly been done on his woollen goods to prove that they didn’t contain any cotton or other inferior materials.
So far as I know, nobody tests diamonds with acid. The most famous acid test — and almost certainly the oldest — is the one for gold. This relies on the fact that gold is insoluble in virtually every acid (except the famous aqua regia) and that it takes only a moment to check a sample with a strong acid to learn if it’s genuine. As an example, The Merchants’ Magazine of New York commented in 1849 on items made from a new alloy resembling gold, “It would be difficult for the most practised eye to discover they were not gold, without having recourse to the acid test.”
The first recorded examples of the figurative expression come from the US in the middle of the nineteenth century, though it’s more than likely substantially older. The earliest I’ve found is in an advertisement supplement in the issue of the Columbia Reporter of Wisconsin for 18 November 1845: “Twenty-four years of service demonstrates his ability to stand the acid test, as Gibson’s Soap Polish has done for over thirty years.”