A notch up the complexity scale is the touchstone test. Assayers and bench jewelers commonly use it to determine karatage. Touchstone testing compares how gold alloys of known and unknown fineness react to acid. For gold, the test is based on the fact that 24K gold resists all but the strongest acids. The purer the gold (the higher the karatage), the stronger the acid required to change its character or to dissolve it.
The principal of touchstone testing goes back to 600 BC. It’s from these ancient practices that we get the term “acid test.” To subject something to the acid test means to verify its value or quality.
The test, when applied to gold, requires a number of items:
- Bottles of nitric and hydrochloric acids
- A set of comparison test needles in a range of known white and yellow gold karatages (and other precious metals)
- A touchstone, usually a slab of smooth, black stone called basalt
When the bench jeweler in a store tests an item that comes in for repair, he begins by checking the quality stamp and noting the item’s heft, color, and luster. Based on these observations, he estimates the fineness of the metal. The touchstone test verifies or disproves the estimate.
First, the jeweler rubs a tiny part of the jewelry on the touchstone. This leaves a streak of metal on the stone. He has to be careful not to damage the customer’s jewelry. For example, if he’s testing a ring, he should never rub the prongs. A better choice would be to test the side of the shank, which can be buffed smooth when the test is complete- or some area that doesn’t show. It’s also important not to take rubbing from soldered joints because solders will contain different elements.
The bench jeweler then takes two test needles, one each of the estimated karatage and one of lower or higher quality than the piece being tested. He rubs streaks with these needles on either side of the original streak.
He then applies acid solutions to each streak. For higher karatages the solution is aqua regia (a mixture of hydrochloric and nitric acid). Aqua regia is the only acid that will dissolve gold completely. Nitric acid is used alone for 14K and lower.
The bench jeweler compares the reaction of the known streaks to the unknown. He looks for color changes and watches how quickly the streak disappears. The two streaks that react the same way are the same karatage.
The results of the touchstone test are a little imprecise. The test relies on the jeweler’s observations and experience. And very pure samples, such as 22K and 23K gold, cannot be tested precisely because they react similarly. The touchstone test is pretty accurate in the 10K to 18K range.
Platinum and tungsten carbide rings are tested using a more complex process. Most platinum and tungsten rings can usually detect it by its weight and color. If verifying platinum or tungsten wedding bands ever becomes an issue it is better to send it to a trade or repair shop that specializes in platinum or tungsten jewelry.
On the other hand, testing silver jewelry is relatively easy. Most bench jewelers have a bottle of Schwerter’s solution (a liquid chemical mixture) on hand. They put a drop of it on an inconspicuous part of the silver item to be tested. The resulting color change idicates how pure the silver is, according to the following:
Bright blood red = .995 pure silver (fine silver)
Dark red = .925 pure silver (sterling silver)
Very dark red = .900 pure silver (coin silver- cannot be marked silver in the US)
Brown = .800 pure silver (cannot be marked silver in the US)
Green = .500 pure silver (cannot be marked silver in the US)
The touchstone test and Schwerter’s solution are good for quick identifications, though moth tests should only be performed by experienced bench jewelers.
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