Acid Test
Also: Acid Test, Touchstone Test, Streak Test with Acid
The acid test is a chemical method for determining the fineness of gold and silver alloys, in which testing acids produce a characteristic colour reaction on the metal surface or on a touchstone.
The acid test is one of the oldest and most widely used methods for determining the fineness of precious metals on the spot. It exploits the differing chemical resistance of gold and silver alloys to acids: the higher the precious metal content, the more resistant the sample will be. For a first assessment when purchasing scrap gold or unknown jewellery pieces it remains a practical tool – but it requires expertise and protective equipment.
Working Principle
The procedure is based on the property of acids to attack base alloying metals (copper, zinc, nickel), while pure gold is not dissolved even by concentrated nitric acid. Testing is carried out in two variants:
1. Streak test on a touchstone (lydite) A black basalt or lydite stone picks up a metal streak. A drop of the appropriate testing acid is applied to this streak. The colour reaction of the streak reveals the approximate fineness.
2. Direct drop on the metal surface Less gentle but more precise: the acid is dripped directly onto a lightly scratched or filed area of the object. Any surface coating (gold plating, silver plating) must be penetrated first.
Testing Acids and Their Reactions with Gold
The logic of the acid test: if the streak shows no reaction to the matching testing acid, the fineness is at least at that level. If a reaction occurs, the fineness is below it.
| Testing acid | Target fineness | Streak remains stable at … | Reaction (dissolution/discolouration) at … |
|---|---|---|---|
| 8-carat acid | 333/1000 | ≥ 333 (8 ct and above) | < 333 (fake or base metal) |
| 14-carat acid | 585/1000 | ≥ 585 (14 ct and above) | < 585 (e.g. 8-ct pieces, fakes) |
| 18-carat acid | 750/1000 | ≥ 750 (18 ct and above) | < 750 (e.g. 14-ct pieces) |
| 22-carat acid | 916/1000 | ≥ 916 (22 ct and above) | < 916 (e.g. 18-ct pieces) |
The current gold price can be checked at any time on this site – relevant once you wish to convert the fineness established into a melt value.
Silver Testing
For silver, diluted nitric acid (HNO₃) is used. Pure or high-fineness silver (e.g. Britannia silver 958) reacts more slowly and less vigorously than low-fineness 835 silver or nickel silver (which contains no silver at all and effervesces strongly).
Step-by-Step Procedure
- Put on protective equipment – gloves and safety goggles are mandatory; testing acids are corrosive.
- Prepare the touchstone – clean the surface, apply a metal streak.
- Apply the acid – place a single drop of the appropriate testing acid on the streak.
- Observe the reaction – read off colour and intensity within 10–60 seconds.
- Compare with a reference streak – placing a streak from an alloy of known fineness alongside greatly improves accuracy.
- Neutralise and dispose of residues – rinse with water, dispose of acid residues in accordance with hazardous substance regulations.
The fineness established can then be entered into the melt value calculator to calculate the material value of your piece.
Limitations of the Method
The acid test provides an estimate, not a precision measurement. Important limitations:
- Gold-plated or clad objects (e.g. gold-filled/double) can deceive at the surface if not scratched deeply enough.
- Tungsten forgeries (tungsten forgery) with a genuine gold cladding are not reliably detected.
- The method causes minimal surface damage and is therefore unsuitable for high-value collector coins or proof strikes.
- Sources of error in the streak test: insufficient metal abrasion, a contaminated touchstone, aged acid.
For a non-destructive and more precise analysis, X-ray fluorescence analysis (XRF) or the density test by Archimedes is recommended. For coins, the coin authenticity check that evaluates weight and dimensions can serve as a useful complement.
Legal Note
The acid test does not replace official hallmarking by a recognised refinery. For resale or VAT classification as investment gold (VAT exemption from 995/1000), certified testing procedures are required. Note: not tax or investment advice.
Key Takeaway
The acid test is a quick, low-cost aid for estimating the fineness of gold and silver – ideal for a first assessment when purchasing, but no substitute for instrumental analysis methods when dealing with high-value or suspect objects. Anyone wishing to calculate the material value after testing can use the melt value calculator.