How Touchstone Acid Testing Works
The touchstone method involves pressing the gold piece firmly against a dark basalt or jasper stone to leave a visible streak of metal. The tester then applies a drop of nitric acid (or aqua regia for higher purities) to the streak and observes the reaction. Different gold purities dissolve the streak at different rates, allowing an experienced tester to estimate purity by comparison with reference streaks.
It is a fast and inexpensive technique requiring minimal equipment. For this reason, small or informal gold buyers in Chennai still use it as their primary method. However, interpreting the result is subjective — it depends on the tester's experience, the quality of the acids used, the lighting conditions, and how cleanly the streak was applied.
The Margin of Error and Why It Hurts Sellers
Portable XRF machines achieve accuracy of ±0.1–0.5 fineness points under controlled conditions. Touchstone acid testing, even in experienced hands, typically varies by ±15–30 fineness points. For 916-purity gold, this means a touchstone test might return a reading anywhere from 886 to 946.
Buyers who rely on touchstone testing have an incentive to resolve this uncertainty by choosing the lower end of their estimate — it protects their margin. If a 916 piece is assessed at 875, the price difference at ₹7,200 per gram (999 base rate) is: 916 × ₹7,200 = ₹6,595 vs 875 × ₹7,200 = ₹6,300 — a loss of ₹295 per gram. For a 50-gram lot, that is ₹14,750 less than a fair assessment would deliver.
Warning: If a buyer performs a touchstone acid test out of your sight — in a back room or at a separate counter — you have no way to verify what the test showed or whether it was performed at all. Always insist the testing happens in front of you on equipment whose readout you can see.
How Unethical Buyers Exploit This Method
Sellers unfamiliar with gold testing have no reliable way to challenge a touchstone result. The test is visual and fast; a buyer can claim the acid reaction indicated a lower purity and most sellers lack the knowledge to dispute it. This information asymmetry is routinely exploited in less transparent transactions.
Unethical approaches include using degraded or improperly formulated acid (which skews results), applying extra acid to dissolve a genuine 916 streak until it resembles an 875 reaction, or simply stating a lower purity verbally without showing any physical result. The antidote to all of these is requesting XRF testing and viewing the result yourself.
Why XRF Should Be the Standard for Sellers
XRF machines are no longer rare or prohibitively expensive. Most organised gold buying companies in Chennai equip both their offices and doorstep representatives with portable XRF devices. When a buyer tells you they only do acid testing, they are either underinvested in proper equipment or choosing a method that benefits their margin over your payout.
You are entitled to ask for XRF testing before you accept any offer. If the buyer cannot provide it, the practical advice is to seek a buyer who can. The incremental recovery from accurate testing on a 100-gram lot can easily exceed ₹30,000 — far more than the inconvenience of switching to a different buyer.
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