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 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 Madurai 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. 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 in Madurai, 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.
Why XRF Should Be the Standard for Madurai Sellers
XRF machines are no longer rare or prohibitively expensive. Most organised gold buying companies in Madurai 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, 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 in Madurai.
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