Acid Test
Also: Acid Test, Touchstone Test, Streak Test with Acid
The acid test is a chemical procedure for determining the fineness of gold and silver alloys, in which test 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 resistantly the sample reacts. For an initial assessment when buying scrap gold or unknown pieces of jewellery, it remains a practical aid - but requires expertise and protective equipment.
Operating 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. The test is carried out in two variants:
1. Streak test on the touchstone (lydite) A black basalt or lydite stone takes up a metal abrasion. A drop of the matching test 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 slightly scratched or ground spot on the object. A protective layer (gold plating, silver plating) must be penetrated in doing so.
Test acids and their reactions for gold
The logic of the acid test: if the streak does not react to the matching test acid, the fineness is at least at that level. If a reaction occurs, it is below.
| Test acid | Target fineness | Streak stable at ... | Reaction (dissolution/discolouration) at ... |
|---|---|---|---|
| 8-carat acid | 333/1000 | >= 333 (8 ct and higher) | < 333 (fake or base metal) |
| 14-carat acid | 585/1000 | >= 585 (14 ct and higher) | < 585 (e.g. 8-ct pieces, fake) |
| 18-carat acid | 750/1000 | >= 750 (18 ct and higher) | < 750 (e.g. 14-ct pieces) |
| 22-carat acid | 916/1000 | >= 916 (22 ct and higher) | < 916 (e.g. 18-ct pieces) |
You can view the current gold price on this site at any time - relevant as soon as you wish to convert the determined fineness into a melt value.
Silver testing
For silver, diluted nitric acid (HNO3) is used. Pure or high-alloy silver (e.g. Britannia silver 958) reacts more slowly and weakly than low-alloy 835 silver or nickel silver (which contains no silver at all and foams strongly).
Step-by-step procedure
- Put on protective equipment - gloves and safety goggles are mandatory; test acids are corrosive.
- Prepare the touchstone - clean the surface, apply the metal streak.
- Dose the acid - apply a single drop of the matching test acid.
- Observe the reaction - read colour and intensity within 10-60 seconds.
- Compare with a reference streak - placing a streak of an alloy of known content beside it considerably improves accuracy.
- Neutralise and dispose of residues - rinse with water, dispose of acid residues in accordance with hazardous substance regulations.
You can then use the determined fineness in the melt value calculator to calculate the material value of your piece.
Limits of the method
The acid test provides an estimate, not a precision measurement. Important limitations:
- Gold-plated or plated objects (e.g. doublet/gold doublet) can deceive the surface if not scratched deeply enough.
- Tungsten counterfeits (tungsten counterfeit) with a genuine gold coating are not reliably detected.
- The method minimally destroys the surface and is therefore unsuitable for high-value collector coins or proof mintings.
- Sources of error in the streak test: too little abrasion, contaminated touchstone, outdated acid.
For a non-destructive and more precise analysis, X-ray fluorescence analysis (XRF) or density measurement by Archimedes is recommended. For coins, the coin authenticity check, which evaluates weight and dimensions, is a useful complement.
Legal note
The acid test does not replace official stamping by a recognised refinery. For resale or tax classification as investment gold (VAT exemption from 995/1000), certified testing procedures are required. Note: not tax or investment advice.
In brief
The acid test is a fast, low-cost aid for estimating the fineness of gold and silver - ideal for an initial overview when buying, but no substitute for instrumental analysis methods on high-value or suspicious objects. Anyone who wants to calculate the material value after testing uses the melt value calculator.