Streak Test
Also: Gold test, Touchstone streak test, Touchstone test
The streak test is a classic chemical procedure for determining the fineness of gold alloys, in which a metal streak 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 on a touchstone (classically: Lydian stone, a black, fine-grained siliceous slate) to produce a visible metal streak. This streak is then wetted with a test acid of known concentration. The reaction – discolouration, dissolution or remaining intact – provides information about the fineness of the alloy.
Step-by-Step Procedure
- Preparation: clean the touchstone (water or alcohol), the surface must be matt.
- Draw a streak: rub the test piece firmly over the stone until a clear, even metal streak forms.
- Comparison streak: place a streak from a set of test needles of known fineness (e.g. 585, 750, 999) next to it.
- Apply acid: drip 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 document: wipe the touchstone, note the result.
Acid Solutions by Fineness
| Test acid | Concentration (nitric/hydrochloric acid) | Target range gold |
|---|---|---|
| 333 acid | dilute nitric acid (~10–15%) | Up to 375 fineness |
| 585 acid | medium nitric acid (~20–30%) | 375–585 |
| 750 acid | concentrated nitric acid (~45%) | 585–750 |
| 999 acid / aqua regia | hydrochloric + nitric acid (3:1) | High carat / fine gold |
Gold alloys with high fineness resist weaker acids completely, while lower-grade alloys (e.g. 333 gold) already react to dilute nitric acid. Aqua regia – the only agent that dissolves fine gold – is used only for fineness borderline cases above 900.
Strengths and Limits of the Method
Advantages:
- No electricity, no expensive device – usable anywhere
- Instant result for experienced testers
- Little equipment (touchstone + acid set)
- Low destructiveness (only minimal material abrasion)
Limits:
- Requires practice and a steady hand; beginner mistakes lead to misdiagnoses
- Accuracy of ± 5–10 thousandths is sufficient for many purposes, but not for assay-accurate measurements
- With plated pieces (e.g. Doublet/Gold Doublet) the streak can look deceptively correct if the layer is thick enough
- Silver and platinum are harder to classify than gold; for platinum a supplementary density measurement or X-ray fluorescence analysis is recommended
- Acids are corrosive – safety goggles and gloves are mandatory
Streak Test vs. Modern Procedures
For private individuals, buyers and pawnshops the streak test remains a proven quick test. Anyone needing higher certainty – for example when buying larger quantities of broken gold or when tungsten counterfeits are suspected – should back up the results with a second procedure: density measurement according to Archimedes, X-ray fluorescence (XRF) or ultrasonic testing. The Melt Value Calculator calculates the current gold price and the resulting material value on the basis of the determined fineness.
In Brief
The streak test is a robust, low-cost initial screening for gold alloys. It delivers a sound assessment of the fineness in minutes – but requires practice, suitable acids and knowledge of its limits. As the sole procedure it suffices for uncritical purchases; for valuable individual pieces, more precise methods complete the picture.