Streak Test
Also: gold acid test, touchstone test, touch-needle test
The streak test is a classic chemical method for determining the fineness of gold alloys, in which a metal streak drawn on a touchstone is compared with acid solutions of known concentration.
The streak test is one of the oldest methods of gold testing and has been used since antiquity. The piece to be tested is rubbed against a touchstone (classically a Lydian stone, a black, fine-grained siliceous slate) to leave a visible metal streak. This streak is then wetted with a testing acid of known concentration. The reaction — discolouration, dissolution or no change — reveals the fineness of the alloy.
Step by step
- Preparation: Clean the touchstone (water or alcohol); the surface must be matt.
- Draw the streak: Rub the test piece firmly across the stone until a clear, even metal streak appears.
- Comparison streak: Draw a streak beside it using a set of touch-needles of known fineness (e.g. 585, 750, 999).
- Apply acid: Drop the appropriate acid solution onto both streaks.
- Read the reaction: After 15–30 seconds, assess the change in the streak — colour, rate of dissolution and residue are compared.
- Clean and record: Wipe the touchstone and note the result.
Acid solutions by fineness
| Testing acid | Concentration (nitric/hydrochloric) | Gold target range |
|---|---|---|
| 9-carat acid | dilute nitric acid (~10–15%) | Up to 375 fineness |
| 14-carat acid | medium nitric acid (~20–30%) | 375–585 |
| 18-carat acid | concentrated nitric acid (~45%) | 585–750 |
| 24-carat acid / aqua regia | hydrochloric + nitric acid (3:1) | High carat / fine gold |
High-fineness gold alloys resist weaker acids entirely, while lower-grade alloys (e.g. 8-carat gold) already react to dilute nitric acid. Aqua regia — the only agent that dissolves fine gold — is used only for borderline finenesses above 900.
Strengths and limits of the method
Advantages:
- No electricity, no expensive equipment — usable anywhere
- Instant result for experienced testers
- Minimal kit (touchstone + acid set)
- Almost non-destructive (only minimal material abrasion)
Limits:
- Requires practice and a steady hand; beginner errors lead to misdiagnoses
- Accuracy of ± 5–10 thousandths is enough for many purposes, but not for assay-grade measurements
- On plated pieces (e.g. rolled gold) the streak can look deceptively correct if the layer is thick enough
- Silver and platinum are harder to grade than gold; for platinum a supplementary density measurement or X-ray fluorescence analysis is advisable
- Acids are corrosive — safety goggles and gloves are mandatory
Streak test vs. modern methods
For private individuals, buyers and pawnbrokers the streak test remains a proven quick check. Anyone needing greater certainty — for example when buying larger quantities of scrap gold or where tungsten counterfeits are suspected — should confirm the results with a second method: density measurement by Archimedes, X-ray fluorescence (XRF) or ultrasonic testing. The current gold price and the resulting material value are calculated by the melt value calculator based on the determined fineness.
In brief
The streak test is a robust, low-cost initial screening for gold alloys. Within minutes it gives a reliable estimate of fineness — but it requires practice, suitable acids and awareness of its limits. As a standalone method it suffices for uncritical purchases; for valuable individual pieces, more precise methods round out the picture.