Guide
How gold purity is tested: methods and their limits
Touchstone, acid test, carat meter, XRF and fire assay compared: what each gold purity test shows, how reliable it is and what to ask a buyer.
- No single quick test is perfect. Surface plating can mislead some methods.
- Touchstone and acid tests are quick scratch tests, useful but approximate.
- Electronic testers and XRF read the surface quickly without damaging the piece.
- Fire assay is the most accurate method but destroys a sample.
The common methods
| Method | How it works | Good for | Limit |
|---|---|---|---|
| Touchstone and acid | A scratch of the gold is compared with reference streaks and reacted with acid. | Quick checks on a small sample | Approximate, depends on skill |
| Electronic carat meter | Measures an electrical property of the metal. | Fast reading without damage | Can be thrown off by plating or shape |
| XRF analyser | X-rays reveal the metals present at the surface. | Accurate, non-destructive readings | Reads only the surface layer |
| Fire assay | A sample is melted and refined to measure the gold left. | Most accurate result | Destroys a small sample |
Why surface readings can mislead
Some tests read only the outer layer. A piece that is gold plated over a cheaper core can look like gold on the surface. For valuable pieces, a buyer may confirm with a second method before paying.
What to ask any gold buyer
- Which method are you using?
- Can I watch the test and see the reading?
- What karat does it show, and is that written on the slip?
- What happens if I disagree with the result?
Testing at ClearKarat
At ClearKarat the purity test is done at your side, the result is explained, and the karat is written on your slip. You can ask for testing without selling and decide afterward. Read more on our Plain Sight Promise page, or learn what the karat numbers mean in 22K, 24K and 18K gold explained.
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