Gold Purity Testing: Acid Test vs XRF vs Touchstone
| Financial Services
Key Highlights
- Purity is a credit input, not a lab detail: Every karat point an appraiser gets wrong moves the loan-to-value calculation and the maximum amount a customer can draw.
- Three methods dominate the branch: touchstone and acid test (traditional, low-cost), XRF spectrometry (electronic, non-destructive), and fire assay (lab-grade, destructive, used mainly to settle disputes).
- No single method is sufficient alone: touchstone is operator-dependent, XRF reads only the surface - gold-plated fraud defeats it unless the item is filed or cut first.
- RBI's 2025 directions make the method a compliance artifact: the assaying procedure and assayer certification criteria must now be named in the board-approved credit policy.
Why the Purity Testing Method a Branch Uses Is a Credit Decision, Not Just an Operations One
Every gold loan runs on a simple chain: purity determines net fine gold weight, net fine gold weight at the prevailing rate determines value, and value capped by the regulatory LTV ceiling determines the maximum loan amount. Purity testing sits at the very start of that chain, which means an error there does not stay contained to the appraisal counter - it propagates directly into the sanctioned loan amount. Over-estimate purity and the NBFC is under-collateralized from day one; under-estimate it and a genuine customer is shortchanged and likely to walk to a competitor. Neither failure mode is acceptable at scale, which is why the method a branch uses to test purity deserves as much scrutiny as the credit policy itself.
We covered the documentation side of this in our Gold Loan SOP Checklist for New NBFCs - valuation methodology and assayer onboarding are named RBI requirements a new entrant must have before its first branch opens. This post goes one level deeper: the three methods branches actually use to test purity, how they compare, and why the strongest branches don't rely on just one of them.
Comparing the Three Methods
Touchstone and acid test, XRF spectrometry, and fire assay are the three methods in active use across Indian gold loan branches and jewellery hallmarking centres. Here's how they compare on the dimensions that matter for branch operations.
| Method | How It Works | Speed & Cost | Accuracy | Best Fit |
|---|---|---|---|---|
| Touchstone + Acid Test | Item is rubbed on a basalt touchstone to leave a streak, graded acid (10K–22K) is applied, and the streak's reaction is read visually against known karat marks | Under a minute per item; lowest equipment cost of the three, but consumes acid and touchstone over time | Approximate - typically within 1–2 karats in a trained appraiser's hands, but reads only the surface layer | High-volume walk-in branches as the fast first-pass screen |
| XRF (X-Ray Fluorescence) Spectrometer | A low-energy X-ray beam excites the metal's atoms; the emitted fluorescence spectrum is measured and matched to elemental composition electronically | 15–30 seconds per reading; higher upfront equipment cost, near-zero consumable cost, needs periodic calibration | High precision on composition - but reads only the top few microns, so gold-plated or gold-filled items can return a false high reading unless filed or cut first | Branches processing higher ticket sizes, or as the electronic cross-check on every touchstone result |
| Fire Assay (Cupellation) | A weighed sample is fused with lead and fluxes, cupelled to separate base metals, then parted to isolate pure gold, which is weighed directly | Hours to complete, requires a laboratory setup and trained chemist; the item is partially consumed in the process | The reference standard against which every other method is judged - destructive but definitive | Referee assay for disputed valuations, regulatory audits, or high-value / bulk lots, not routine branch appraisal |
Four Things Worth a Deeper Look
1. Touchstone and Acid Test: Cheap, Fast, Operator-Dependent
Touchstone and acid testing has been the default at Indian gold loan counters for decades because it's fast, inexpensive, and needs no electricity or calibration schedule. Its weakness is that the result depends entirely on the appraiser's eye - reading the colour reaction against a karat scale is a skill that improves with training and degrades with fatigue, rushed volume, or a poorly lit counter. It's also fundamentally a surface test: an item plated or filled with high-karat gold over a base metal core can pass a rushed acid test if the appraiser doesn't nick or file through the surface first. This is precisely the control our internal controls piece flags under "standardized valuation methodology" - the method only holds up if the procedure and the filing step are enforced consistently across every branch and every appraiser, not left to individual judgment.
2. XRF: Non-Destructive Accuracy, With a Surface-Reading Caveat
XRF spectrometers have become the standard upgrade for NBFCs scaling branch networks, because they give a repeatable, operator-independent composition reading in under 30 seconds without damaging the item - a meaningful advantage when the same jewellery may need to be returned to the customer intact on closure. The caveat that catches NBFCs new to XRF is that, like the acid test, it only reads the outer few microns of the item. A well-executed gold-plated or gold-filled fraud will still return a deceptively high XRF reading unless the appraiser files or clips into the item first to expose the base metal - meaning XRF replaces the guesswork in reading the result, but not the physical step of breaching the surface. XRF equipment also drifts and needs periodic calibration against certified reference standards; an uncalibrated unit gives confident-looking readings that are quietly wrong, which is more dangerous than an appraiser who knows to double-check.
3. Fire Assay: The Reference Standard Branches Rarely Use Themselves
Fire assay remains the most accurate method available and is treated as the referee standard when a valuation is formally disputed, when regulators or auditors sample-test a branch's appraisals, or when bulk gold from an auction lot needs a definitive purity certificate. It's rarely run at the branch level because it destroys part of the sample and requires laboratory infrastructure most branches don't have - instead, NBFCs typically route disputed or high-value cases to an empanelled assaying lab. Knowing which items warrant that escalation, and having a documented process for it, is itself part of the SOP a new entrant needs to define before launch.
4. Building a Two-Method Cross-Check Into the SOP
The NBFCs with the fewest purity-related disputes and shortages don't pick one method - they pair a fast first-pass method with an independent electronic check on every loan above a defined ticket size, and route anything inconsistent to a senior appraiser or referee assay. A common pattern: touchstone and acid test as the initial screen, XRF as the mandatory second check before disbursement, and the two results cross-verified by different individuals under the maker-checker control covered in our internal controls piece. Where and how that appraisal happens also depends on the physical counter and vault setup - see our companion piece on branch vault security best practices for how a secure, CCTV-covered appraisal counter fits into the same workflow.
Key Takeaways
No single purity testing method is sufficient on its own - each has a blind spot, and the branches that avoid purity-related losses are the ones that pair methods rather than rely on one.
- Touchstone and acid: fast and cheap, but accuracy depends on the appraiser and it reads only the surface
- XRF: precise and non-destructive, but still a surface reading unless the item is filed first, and needs regular calibration
- Fire assay: the definitive reference standard, reserved for disputes and referee cases, not routine branch use
- Cross-check, don't choose: pairing a fast method with an independent electronic check, under maker-checker, catches what either method misses alone
Standing up a reliable, well-documented purity testing process - the right equipment, calibration schedule, appraiser certification, and cross-check discipline - is exactly the kind of operating-model work Technovative Consulting does with gold loan NBFCs and banks. We help with gold audit team and assaying process setup, training programs for appraisers and branch managers, and core process design and SOP execution to get purity testing right from the first branch.