Streak Test
Also: Gold Assay, Touchstone Test, Acid Streak Test
The streak test is a classical chemical method for determining the fineness of gold alloys, in which a metal streak on a touchstone is compared with acid solutions of known concentration.
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The streak test is one of the oldest methods of gold assaying 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 shale) to produce a visible metal streak. This streak is then moistened with a test acid of known concentration. The reaction – discolouration, dissolution, or absence of change – reveals the fineness of the alloy.
Step-by-Step Procedure
- Preparation: Clean the touchstone (water or alcohol); the surface must be matte.
- Draw the streak: Rub the test piece firmly across the stone until a clear, even metal streak appears.
- Reference streak: Place a streak from a set of test needles of known fineness (e.g. 585, 750, 999) alongside.
- Apply acid: Add the appropriate acid solution drop by drop to 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 clean and note the result.
Acid Solutions by Fineness
| Test acid | Concentration (nitric/hydrochloric acid) | Target gold range |
|---|---|---|
| 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-karat / fine gold |
Gold alloys with a high fineness withstand weaker acids completely, while lower-grade alloys (e.g. 333 gold) react even to dilute nitric acid. Aqua regia – the only reagent that dissolves fine gold – is reserved for borderline fineness cases above 900.
Strengths and Limitations
Advantages:
- No electricity, no expensive equipment – usable anywhere
- Immediate result for experienced testers
- Minimal accessories required (touchstone + acid set)
- Near-non-destructive (only minimal material abrasion)
Limitations:
- Requires practice and a steady hand; beginner errors lead to misdiagnosis
- Accuracy of ± 5–10 parts per thousand is sufficient for many purposes, but not for assay-grade measurements
- On plated items (e.g. gold-filled / doublé), 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 determination or X-ray fluorescence analysis is advisable
- Acids are corrosive – safety goggles and gloves are mandatory
Streak Test vs. Modern Methods
For private individuals, dealers, and pawnbrokers, the streak test remains a tried-and-tested quick screening method. Those requiring greater certainty – for example when purchasing larger quantities of scrap gold or where tungsten forgery is suspected – should corroborate the result with a second method: Archimedean density determination, X-ray fluorescence (XRF), or ultrasound testing. The current gold price and the resulting material value can be calculated using the Melt Value Calculator on the basis of the determined fineness.
Key Takeaway
The streak test is a robust, low-cost initial screening tool for gold alloys. It delivers a reliable fineness estimate within minutes – but requires practice, appropriate acids, and an awareness of its limitations. As a standalone method it is adequate for low-stakes purchases; for high-value individual pieces, more precise methods should complement the picture.