The scratch test, more properly called the touchstone test, is the traditional non-destructive method for estimating the karat fineness of a gold article. The piece is rubbed against a fine-grained dark stone — historically a basanite or schist touchstone — to leave a streak of metal, and the streak is then tested with calibrated nitric acid solutions of varying strengths. The reaction of the streak to each acid grade indicates the approximate karat fineness.

Procedure

The tester holds the article and rubs it firmly against the touchstone to deposit a visible metal streak roughly a centimetre long. A series of reference streaks are placed alongside, made with calibrated needles of known fineness (10K, 14K, 18K, 22K, 24K). Drops of acid solutions formulated for each karat range are applied to the streak. A 14K acid will dissolve streaks below 14K but leave a 14K-or-higher streak intact; a 18K acid will dissolve streaks below 18K. The pattern of dissolution across the acid grades brackets the actual fineness.

Sensitivity and limits

The touchstone test is approximate. A skilled operator can routinely distinguish between adjacent karat grades (10K versus 14K, 14K versus 18K), but cannot reliably separate fine increments such as 14K from 14.5K. The test cannot detect plated gold reliably unless the plating wears through during the streak operation, so the tester typically files a small notch on a hidden surface to expose interior alloy before streaking. The test cannot distinguish gold-content alloys from gold-clad or gold-filled articles unless the test surface is broken through to substrate.

Modern alternatives

Contemporary trade testing has largely shifted to handheld X-ray fluorescence (XRF) instruments for routine verification, which provide non-destructive elemental analysis to within fractions of a karat in seconds and additionally identify the alloy components (silver, copper, palladium, zinc) that affect colour and value. Fire assay, the destructive method involving cupellation in a furnace, remains the legal reference standard in disputes and is required for high-precision determination. The touchstone test continues in use as a fast initial screen, particularly in pawnbroking, scrap-buying and field operations where instrument access is impractical, and as a teaching tool in jewellery training programmes.