Acid Test
Also: Acid Testing, Touchstone Test, Streak Test with Acid
The acid test is a chemical method for gauging 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 among the oldest and most widely used ways to check the fineness of precious metals on the spot. It exploits the fact that gold and silver alloys resist acids differently: the greater the precious-metal content, the more resilient the sample. For a first read on scrap gold or unidentified jewellery at the point of purchase, it remains a practical tool — provided you have the know-how and the right protective equipment.
The working principle
The method relies on acids attacking base alloying metals (copper, zinc, nickel) while pure gold is not even dissolved by concentrated nitric acid. It is carried out in two ways:
1. Streak on the touchstone (lydite) A black basalt or lydite stone captures a smear of metal. A drop of the matching testing acid is placed on the streak, and the colour change of the streak reveals the approximate fineness.
2. A drop applied straight to the metal surface Less gentle but more precise: the acid goes directly onto a lightly scratched or filed spot on the object. Any protective coating (gold plating, silver plating) has to be broken through.
Testing acids and their reactions on gold
The logic of the acid test: if the streak does not react to the acid matched to it, the fineness is at least at that level. If it does react, the fineness is below it.
| Testing acid | Target fineness | Streak stays stable at … | Reaction (dissolving/discolouring) at … |
|---|---|---|---|
| 8-karat acid | 333/1000 | ≥ 333 (8 kt and higher) | < 333 (fake or base metal) |
| 14-karat acid | 585/1000 | ≥ 585 (14 kt and higher) | < 585 (e.g. 8 kt pieces, fake) |
| 18-karat acid | 750/1000 | ≥ 750 (18 kt and higher) | < 750 (e.g. 14 kt pieces) |
| 22-karat acid | 916/1000 | ≥ 916 (22 kt and higher) | < 916 (e.g. 18 kt pieces) |
You can pull up the current gold price on this site at any time — useful once you want to convert the fineness you have established into a melt value.
Testing silver
For silver, diluted nitric acid (HNO₃) is used. Pure or high-grade silver (for example Britannia silver 958) reacts more slowly and more faintly than low-grade 835 silver or nickel silver (which contains no silver at all and foams up vigorously).
Step-by-step procedure
- Put on protective gear — gloves and safety goggles are essential; testing acids are corrosive.
- Prepare the touchstone — clean the surface and apply the metal streak.
- Apply the acid — place a single drop of the matching testing acid.
- Watch the reaction — read colour and intensity within 10–60 seconds.
- Compare with a reference streak — placing a streak of known fineness alongside greatly improves accuracy.
- Neutralise and dispose of residues — rinse with water and dispose of acid remnants in line with hazardous-substance rules.
You can then feed the fineness you have determined into the melt value calculator to work out the material value of your piece.
Limits of the method
The acid test gives an estimate, not a precision measurement. Key caveats:
- Gold-plated or clad objects (for example gold filled / gold double) can mislead the surface reading if you do not scratch deep enough.
- Tungsten forgeries (tungsten forgery) with a genuine gold shell are not reliably detected.
- The method damages the surface, if only slightly, so it is unsuitable for valuable collector coins or proof strikes.
- Sources of error on the streak test: too little metal deposited, a contaminated touchstone, expired acid.
For a non-destructive and more accurate analysis, consider X-ray fluorescence (XRF) or a density test after Archimedes. For coins, the coin authenticity check, which evaluates weight and dimensions, is a useful complement.
Legal note
The acid test does not replace an official hallmark from a recognised refinery. For resale or for tax classification as investment gold (VAT-exempt from 995/1000), certified testing methods are required. In Ireland, investment gold is VAT-exempt under the VAT Consolidation Act 2010, whereas silver, platinum and palladium carry the standard 23% rate. Note: this is not tax 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 at purchase, but no substitute for instrument-based analysis on valuable or suspect items. If you want to calculate the material value after testing, use the melt value calculator.