Acid Test
Also: Acid testing, Touchstone test, Streak test with acid
The acid test is a chemical method 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 resistant the sample. 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 method relies 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 the touchstone (Lydian stone) A black basalt or Lydian stone takes up an abrasion of the metal. A drop of the appropriate 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 dropped directly onto a lightly scratched or ground spot on the object. A protective layer (gold or silver plating) must be penetrated in the process.
Test acids and their reactions with 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 stays 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 look up the current gold price on this site at any time – relevant once you wish to convert the determined fineness into a melt value.
Silver testing
For silver, diluted nitric acid (HNO₃) is used. Pure or high-grade silver (e.g. Britannia silver 958) reacts more slowly and more weakly than lower-grade 835 silver or nickel silver (which contains no silver at all and froths vigorously).
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 a metal streak.
- Dose the acid – apply a single drop of the appropriate test acid.
- 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 content alongside greatly 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.
Limitations of the method
The acid test provides an estimate, not a precision measurement. Important limitations:
- Gold-plated or clad objects (e.g. rolled gold / gold doublé) can deceive at the surface if not scratched deeply enough.
- Tungsten counterfeits (tungsten counterfeit) with a genuine gold coating are not reliably detected.
- The method damages the surface minimally and is therefore unsuitable for high-value collector coins or proof strikes.
- Sources of error in the streak test: too little abrasion, a contaminated touchstone, aged 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 hallmarking by a recognised refinery. For resale or the tax classification as investment gold (VAT exemption from 995/1000), certified test procedures are required. Note: this is not tax or investment advice.
In brief
The acid test is a quick, low-cost aid for estimating the fineness of gold and silver – ideal for a first overview when buying, but no substitute for instrumental analysis methods on high-value or suspect objects. Those who wish to calculate the material value after testing use the melt-value calculator.