The acid test for gold
Practical guide on using test acids to identify karats.
At a glanceThe acid touchstone assay is the oldest, most reliable rapid testing method in the goldsmith's trade. Used daily by pawnshops, jewelers, and scrap gold buyers in Switzerland, this technique determines karat fineness within seconds using calibrated acid solutions. This guide details correct test execution and chemical reading.
The Touchstone Principle
The method relies on rubbing a minute metallic streak onto a fine-grained black stone slab called a touchstone (lydite, black jasper, or polished basanite). The matte black background provides optimal visual contrast.
Test acids are precisely formulated blends of nitric acid ($HNO_3$), hydrochloric acid ($HCl$), and distilled water:
- Fine gold is completely immune to pure nitric acid.
- Base alloying metals dissolve and oxidize away.
- If the streak resists 18k acid, the metal contains at least 750‰ pure gold.
Step-by-Step Testing Protocol
Step 1: The Protective File Notch (Beating Gold Plate)
Never test an item directly on its surface! High-grade rolled gold or heavy 20-micron gold plating will deposit pure gold on the stone that resists 18k acid. Use a fine needle file in an inconspicuous spot to cut past surface plating into core metal.
Step 2: Drawing the Streak
Firmly rub the filed notch against the touchstone to deposit a dense, uniform metallic stroke about 15 mm long.
Step 3: Drawing Reference Streaks (Touch Needles)
Alongside your test sample, draw parallel reference streaks using gold needles (*touchaux*) of known fineness (e.g., 14k and 18k).
Step 4: Applying Test Acid
Apply a drop of calibrated acid across the streaks, typically beginning with 18k acid.
Visual Interpretation Grid
| Test Acid Applied | Material Tested | Immediate Reaction (0 to 10 seconds) | Final Diagnosis |
|---|---|---|---|
| 18k Acid (750‰) | Genuine 18k Gold | Zero reaction. Streak remains sharp, bright yellow. | Fineness ≥ 750‰ confirmed |
| 18k Acid (750‰) | 14k Gold (585‰) | Streak turns brownish and dissolves slowly over 15 seconds. | Fineness between 14k and 18k |
| 18k Acid (750‰) | 9k Gold (375‰) | Streak vanishes rapidly, turning red-brown. | Fineness below 14k (retest with 9k acid) |
| 18k Acid (750‰) | Brass / Gold Plating | Instant fizzing. Streak disappears in 1 second with greenish bubble. | Fake / Base metal |
| 24k Acid (Aqua Regia) | Fine Gold 999‰ | Cold: no reaction. Dissolves only when warmed. | Fine 24k gold |
| Silver Test Acid | 800–999‰ Silver | Streak instantly turns deep blood-red / ruby-red. | Genuine solid silver |
Touchstone Maintenance
- Neutralize acid: Blot acid off immediately and wipe the stone with a cloth dipped in sodium bicarbonate solution.
- Resurfacing: When saturated with streaks, lap the stone under water with fine 800-grit wet sandpaper.
- Oiling: Rub a drop of light mineral oil into the stone to restore deep black luster.
Frequently Asked Questions (FAQ): Acid Testing
Does the acid damage the jewelry piece?
No, because acid is applied only to the streak on the stone, never onto the item itself. Only the tiny file notch leaves a minor mark on a concealed area.
What is the shelf life of test acids?
Test acids degrade over time when exposed to heat and light. Replace bottles every 12 to 18 months for reliable readings.
Buy Investment Gold in Switzerland (VAT-Exempt)
Buy and store physical gold in secure Zurich or Geneva vaults at direct live LBMA spot rates.