Guide
Gold acid test kit: how the touchstone test works.
An acid kit tells you roughly what karat a piece of gold is by seeing which acids dissolve a streak of it. It's quick and cheap, but it reads only the surface, and the acids are dangerous.
Updated October 28, 2026
How it works
Pure gold resists all but the strongest acids, and the more gold an alloy holds, the stronger the acid it takes to dissolve it. Kits include bottles of nitric acid at measured strengths for the lower karats, and nitric and hydrochloric acid mixtures for the higher ones, plus a dark testing stone (a touchstone).
The test, as gemologists do it
- Check for base metal first. If a piece might be plated, file a small notch in an inconspicuous spot and put a drop of 18K acid in it. A strong, fizzing green reaction means base metal.
- Rub a streak of the piece on the stone, away from any solder joints. Rub streaks from reference needles of known karat (10K, 14K, 18K, and 22K) beside it.
- Apply the acid across the streaks and wait 20 to 40 seconds.
- Compare. If your streak survives an acid that dissolves a reference streak, your piece is finer than that karat. Rinse the stone in baking soda and water afterward.
White gold needs white-gold reference needles. The Gemological Institute of America's own worked example ends in "14-karat or slightly higher": the test gives a range, not an exact number.
What it can't tell you
- What's under the surface. Gold plating can be as thin as 0.175 microns, so a streak can be all gold while the piece is brass. The file notch matters.
- Exact purity. It narrows a piece to a karat range.
- Coins and bullion. Never file or acid-test a coin you mean to sell: the damage costs more than the test is worth. See is my gold coin real?
Safety
Gold testing acids are corrosive: one maker's data sheet lists 25 to 50% nitric acid, with warnings for severe skin burns and eye damage. Wear gloves and goggles, work with ventilation, keep the bottles away from sunlight and children, and neutralize spills with baking soda.
For silver, the common test solution contains potassium dichromate, which is a known carcinogen and fatal if inhaled. See the silver acid test.
Sources
- GIA, Bench tip: the touchstone method for testing karat gold.
- Karl Köhler GmbH, safety data sheet for gold and platinum test acids (2024); PubChem, potassium dichromate.
- 16 C.F.R. §23.4 (gold plating thickness).
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