Streak Test
Also: Touchstone test, Acid streak test, Gold streak test
The streak test is a classic chemical method for determining the fineness of gold alloys, in which a streak of metal drawn on a touchstone is compared using acid solutions of known concentration.
The streak test is among the oldest methods of gold testing and has been in use since antiquity. The piece to be examined is rubbed against a touchstone (classically a Lydian stone, a black, fine-grained siliceous slate) to leave a visible streak of metal. This streak is then wetted with a testing acid of known concentration. The reaction — discoloration, dissolution or no change at all — 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 streak of metal appears.
- Comparison streak: draw a streak alongside it using a set of test needles of known fineness (e.g. 585, 750, 999).
- Apply the acid: drip the appropriate acid solution onto both streaks.
- Read the reaction: after 15–30 seconds judge how each streak has changed — colour, speed 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) | 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-carat / fine gold |
High-fineness gold alloys resist weaker acids completely, while lower-grade alloys (such as 333 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 an experienced tester
- Minimal kit (touchstone + acid set)
- Low destructiveness (only a slight material abrasion)
Limits:
- Requires practice and a steady hand; beginner mistakes lead to misdiagnosis
- Accuracy of ± 5–10 thousandths is enough for many purposes but not for assay-grade measurement
- On plated pieces (e.g. gold-filled/gold double) the streak can look deceptively correct if the layer is thick enough
- Silver and platinum are harder to grade than gold; for platinum a density test or X-ray fluorescence analysis is recommended as a supplement
- Acids are corrosive — safety goggles and gloves are mandatory
Streak test versus 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 broken gold or when tungsten forgeries are suspected — should back up the results with a second method: an Archimedean density test, X-ray fluorescence (XRF) or ultrasound testing. The current gold price, and the resulting material value, is calculated by the melt value calculator on the basis of the fineness determined.
Quick recap
The streak test is a robust, low-cost first screening for gold alloys. In minutes it delivers a dependable estimate of fineness — but it demands practice, the right acids and an awareness of its limits. As a standalone method it suffices for routine purchases; for valuable individual pieces, more precise methods round out the picture.