A gold acid kit is a set of bottled chemical reagents used in conjunction with a touchstone to assess the karat purity of gold alloys by observing the rate and character of acid attack on a metal streak. Kits typically contain separate acid solutions calibrated to discriminate at the principal commercial karat thresholds — 10K (41.7% gold), 14K (58.5%), 18K (75%), and 22K (91.7%) — along with a fine-grained black siliceous stone, classically Lydian stone or a similar basalt-type material, on which the metal is rubbed to deposit a thin streak for testing. The method is inexpensive, requires no electrical equipment, and remains in daily use by jewellers, pawnbrokers, estate dealers, and customs officers worldwide for rapid field identification.

Principle of Operation

The underlying chemistry is straightforward. Gold is chemically noble: it resists attack by single mineral acids, including nitric acid (aqua fortis), at ambient temperature. Base metals commonly alloyed with gold — copper, silver, zinc, and nickel — are dissolved or discoloured by nitric acid at varying rates depending on their proportion. The testing acids in a standard kit are therefore graduated solutions of nitric acid, sometimes with small additions of hydrochloric acid for the higher-karat grades, formulated so that an alloy below the stated karat will show visible effervescence, discolouration, or complete dissolution of the streak, while an alloy at or above that karat will remain largely intact.

For alloys above approximately 18K, a stronger reagent is required. The 22K acid, and some specialist 18K formulations, incorporate hydrochloric acid alongside nitric acid to approach the composition of aqua regia (typically a 3:1 molar ratio of hydrochloric to nitric acid), the only common acid mixture capable of dissolving pure gold. The tester therefore works upward through the acid sequence: if the streak survives the 14K acid but dissolves in the 18K acid, the alloy is assessed as lying between those two grades.

The Touchstone

The touchstone is integral to the method. Its surface must be fine-grained enough to retain a coherent metal streak, chemically inert to the test acids, and dark enough to allow visual reading of the streak colour. Traditional Lydian stone — a dense, fine-grained black chert or basanite quarried historically from the Tmolus range in Lydia (present-day western Turkey) — fulfilled all these requirements and gave the technique its ancient pedigree. Modern kits typically supply a synthetic or natural black siliceous tile of comparable grain. The stone is cleaned between tests with a damp cloth or a brief application of the lowest-grade acid to remove residual streaks.

Procedure

• Rub the item firmly against the touchstone to deposit a visible streak approximately 10–15 mm long.
• Apply one drop of the lowest-karat acid appropriate to the suspected alloy.
• Observe the reaction over 30–60 seconds: vigorous effervescence or disappearance of the streak indicates the alloy is below that karat; little or no reaction suggests it meets or exceeds it.
• If the streak survives, apply the next higher acid and repeat.
• The karat range is bracketed between the highest acid the streak survives and the lowest acid that dissolves it.

Experienced testers also read streak colour as a preliminary guide: a reddish streak suggests a high-copper alloy (common in 10K and some 14K yellow gold), a pale or whitish streak may indicate a white-gold or high-silver alloy, and a rich yellow streak is consistent with higher-karat or lower-copper compositions.

Limitations and Accuracy

Acid testing is a comparative, semi-quantitative technique rather than a precise analytical method. Results depend on the freshness and concentration of the acid solutions — reagents degrade over time, particularly once opened, and kits should be replaced or replenished periodically. Surface plating or gold-filled construction can produce a false positive, since only the outermost layer is sampled by the streak; a deeper file mark tested separately is the standard countermeasure. White-gold alloys containing palladium or platinum may resist acid attack at concentrations that would dissolve a lower-karat yellow-gold alloy, potentially overstating purity. For definitive assay — particularly in commercial or legal contexts — fire assay or X-ray fluorescence (XRF) spectrometry remains the authoritative standard.

Acid kits also carry safety considerations: nitric acid and aqua-regia solutions are corrosive and produce toxic nitrogen oxide fumes on reaction. Proper ventilation, acid-resistant gloves, and eye protection are standard precautions.

Place in the Trade

Despite the increasing availability of portable XRF analysers, the gold acid kit retains a firm place in the trade on grounds of cost and simplicity. A complete kit is available for a modest outlay, requires no calibration, and can be operated without specialist training. For the pawnbroker assessing a walk-in item, the estate dealer sorting a mixed lot, or the jeweller verifying an unmarked finding, it provides a reliable first-pass result in under two minutes. It is best understood as a screening tool that narrows the field of inquiry, with more rigorous analytical methods reserved for items of significant value or disputed provenance.