How Gold Bars Are Assayed and Certified in Vaults: LBMA Standards, Hallmarks and Bar Lists

Before a gold bar enters a vault and becomes part of global wholesale settlement, it goes through a chain of testing, marking, and tracking that has been refined over 250 years. The modern LBMA good-delivery system formalizes practices that began with the founding of London's bullion market in the 18th century. Understanding how this works explains why some gold trades at premium prices instantly while other gold sits in dealer inventories for months.

The LBMA good-delivery specification

Step 1: refining and casting

Raw gold (from mine dore or recycled material) is melted and refined through one of several processes (Miller chlorination, Wohlwill electrolysis, or aqua regia) to remove impurities. The refined gold is then cast into the standard bar shape. Major LBMA-accredited refiners include Valcambi, PAMP, Argor-Heraeus and Metalor of Switzerland, Heraeus of Germany, Rand Refinery of South Africa, and Tanaka and Asahi of Japan.

Step 2: assay or purity testing

Assay is the formal determination of fineness. Modern refiners use multiple methods, often in parallel for redundancy.

• Fire assay or cupellation is the traditional gold-standard method, dating to ancient times. Samples are melted with lead and bone-ash; impurities oxidize and are absorbed. Accuracy is around 0.01 percent.
• X-ray fluorescence or XRF is non-destructive surface analysis. Fast but only tests the top few micrometers. Useful for quick verification but not authoritative.
• Inductively coupled plasma or ICP is chemical analysis with extremely high precision near 0.001 percent. Used for high-value verification.
• Gravimetric analysis is a weight-based purity test using density (gold's specific gravity is 19.32). Used for cross-verification.
• Density and conductivity testing are portable verification methods for field testing.

Step 3: hallmark and serial number stamping

After assay confirms compliance, the refiner stamps the bar with mandatory marks: refiner name (or logo), serial number (unique), year of casting, fineness (such as 999.9), and weight to the troy ounce. The marks are pressed into the surface using hardened steel dies. LBMA requires the marks to be deep enough to be readable for decades and not easily removable.

Step 4: chain of custody begins

Once stamped, the bar enters the chain of custody. It is photographed, weighed, and assigned a unique entry in the refiner's bar list. The bar list typically includes refiner name, serial number, weight in troy ounces, fineness, casting year, and the immediate owner. When the bar transfers to a vault, the bar list updates with the vault location and account holder.

Step 5: vault entry

LBMA-approved vaults in London include those operated by HSBC, JPMorgan, Brink's, Loomis, Malca-Amit, and the Bank of England. On entry, bars are independently re-weighed and inspected. The vault's bar list records the exact bar, its dimensions, the date of entry, the owner, and any subsequent custody changes. Bars that fail vault inspection (defects, weight outside tolerance) are rejected and returned to the refiner.

What chain of integrity means in practice

LBMA gold has chain of integrity, meaning every bar has been traceable through the LBMA system since refining. Once a bar leaves the LBMA system (sold to a private buyer, broken into smaller bars, melted down), it loses chain of integrity. Reintroducing such gold to the LBMA system requires re-refining and re-assay by an accredited refiner, at significant cost.

Bar lists: the audit trail

Allocated gold accounts come with a bar list, the exact list of bars assigned to the owner. Major gold ETFs (GLD, IAU, PHYS, SGOL) publish their bar lists publicly. ETF holders can see the exact serial numbers, weights, and refiners of bars backing their shares. This level of transparency is unusual among financial instruments and is one of the strongest arguments for gold's institutional credibility.

Counterfeit prevention

• Tungsten core fakes: tungsten has density similar to gold (19.25 vs 19.32). Detected by deep drilling, ultrasound, or precise dimensional measurement.
• Plated bars: gold-plated tungsten or lead. Detected by sonic test, scratch test, or XRF beyond surface depth.
• Wrong-weight bars: detected by precise scales.
• Hallmark forgery: detected by comparing stamps to refiner exemplars.
• Reused serial numbers: detected by cross-checking against refiner registries.

Retail bars vs good-delivery bars

How buyers verify a bar

1. 1.Check refiner is on LBMA accredited list (published on lbma.org.uk).
2. 2.Confirm hallmark, serial number, fineness, and weight are clearly stamped.
3. 3.Verify weight on calibrated scale to the gram or better.
4. 4.Use XRF for surface fineness confirmation.
5. 5.For high-value purchases, request fire assay or independent third-party verification.
6. 6.For LBMA bars, request the bar list entry showing serial number and provenance.
7. 7.Cross-check serial number against the refiner's records if accessible.

The economics of LBMA accreditation

Becoming an LBMA-accredited refiner requires production capacity of at least 10 tonnes per year, demonstrated quality control, financial stability, and ongoing audit. Maintaining the accreditation requires periodic LBMA reviews. The accreditation is valuable because LBMA gold trades at lower spreads and higher liquidity than non-LBMA gold.

Frequently asked questions

What is LBMA good-delivery gold?

Gold bars produced by LBMA-accredited refiners that meet specific weight, purity, and marking standards. These bars are the global wholesale standard and trade at the lowest spreads.

How is gold purity tested?

Traditional fire assay remains the gold standard for high-precision testing. Modern methods include XRF (surface, fast), ICP (chemical, ultra-precise), and density tests for cross-verification.

What does chain of integrity mean?

It means an LBMA bar has remained within the LBMA-approved vault system since refining, with continuous custody records. Once that chain breaks, the bar must be re-refined to re-enter the system.

Can I see the bar list for my gold ETF?

Yes for most major ETFs. GLD, IAU, PHYS, SGOL all publish bar lists. You can search by serial number and see refiner, weight, and fineness for every bar backing your shares.

What is fire assay?

An ancient and still-authoritative purity test. A gold sample is melted with lead and bone-ash; impurities oxidize and are absorbed by the ash. The remaining pure gold is weighed and compared to the starting sample.

How do you detect tungsten-filled fake bars?

Ultrasonic testing reveals internal voids or different material density. Deep drilling samples the core. Precise dimensional measurement combined with weight identifies anomalies. XRF alone is not sufficient because it only tests surface layers.

Why are retail bars more expensive per ounce?

Smaller bars cost more per ounce to produce, mint, and package. Premiums on a 1g bar can be 15 percent over spot, while a 12.5 kg good-delivery bar trades within 0.5 percent of spot.

Where can I find the list of LBMA refiners?

The official list is at lbma.org.uk under Good Delivery. It includes current and former refiners. Always verify your bar's refiner against the current list.

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