From Touchstone to XRF: How Gold Testing Methods Evolved in India

Gold testing in India has a history spanning thousands of years. The methods have evolved from crude visual estimation to sophisticated instrumental analysis, but the underlying goal has remained constant: determining how much pure gold a sample contains.

Touchstone Testing: The Ancient Method

The touchstone method is the oldest known technique for estimating gold purity. Evidence of its use dates to the Indus Valley civilisation (approximately 2500–1900 BC), making it one of the oldest analytical methods in continuous use anywhere in the world.

The technique involves rubbing a gold article against a dark, fine-grained stone (typically a type of jasper or basalt) to leave a visible streak. The colour and character of the streak are compared against streaks from gold samples of known purity. Acid solutions (usually nitric acid) can be applied to the streaks to dissolve base metals, further refining the purity estimate.

Touchstone testing is rapid, inexpensive, and requires minimal equipment. However, it provides only an approximate estimate (typically within 1–2 karat) and depends heavily on the skill and experience of the tester.

Specific Gravity Testing

The specific gravity test uses the Archimedes principle to estimate purity. The article is weighed in air and then weighed while submerged in water. The ratio of these weights gives the specific gravity, which can be compared against the known density of pure gold (19.32 g/cm³).

If the measured specific gravity is significantly lower than expected for the declared purity, it suggests the article contains more base metal (or less dense material) than claimed.

The method is non-destructive and provides a useful screening check, but it cannot detect all forms of adulteration — particularly if the adulterant metal happens to have a similar density to gold.

Acid Testing

Chemical acid testing involves applying progressively stronger acid solutions to a sample or streak. Different acids dissolve different metals at different rates, allowing the tester to estimate the gold content based on which acids dissolve the sample and which do not.

This method is more precise than touchstone alone but is semi-destructive and produces results that still depend on the tester's interpretation.

XRF Spectrometry: The Modern Standard

X-Ray Fluorescence spectrometry transformed gold testing when it was adopted for hallmarking. XRF irradiates the sample with high-energy X-rays, causing each element to emit characteristic fluorescent X-rays that are measured by a detector. Software calculates the precise elemental composition.

XRF is non-destructive, provides results in seconds to minutes, and achieves accuracy within 0.1–0.5% for most precious metal alloys. It became the primary testing method in BIS hallmarking centres and is the workhorse of India's hallmarking infrastructure.

Fire Assay: The Referee Method

Fire assay (cupellation), dating back 5,000 years, remains the most accurate method for gold and silver analysis. It achieves accuracy to 0.01–0.03% by completely separating precious metals from base metals through high-temperature smelting.

In BIS hallmarking centres, fire assay is used when XRF results are borderline, when surface coatings are suspected, for high-value bullion testing, and as the definitive referee method in disputes. The standard IS 1418:2015 governs fire assay procedures in India.

The Two-Stage Protocol

Modern BIS hallmarking centres use both methods complementarily: XRF screens rapidly and non-destructively, while fire assay provides definitive confirmation when needed. This two-stage approach — rooted in methods spanning from the ancient world to the modern laboratory — delivers both efficiency and absolute accuracy.