Gold extracted from mines is never pure. Whether it comes from a cyanide leaching circuit, a gravity concentrator, or a sluice box, raw mine gold contains silver, copper, iron, and other impurities in varying proportions. Before a mine’s output becomes an investment bar or coin, it passes through industrial refining processes that remove these impurities to produce metal of defined, certified purity.
Understanding refining helps investors know what the markings on their bullion mean, why purity levels differ between products, and how the gold supply chain maintains the trust that makes gold fungible across international markets.
The Starting Point: Doré Bars
Most large-scale mines produce their gold in the form of doré bars — crude alloy ingots cast at the mine site.
What Is Doré?
Doré is an intermediate product containing:
- Gold: 70–95% (varies significantly by deposit mineralogy)
- Silver: 5–25% (silver is commonly associated with gold deposits)
- Trace impurities: Copper, iron, lead, zinc, base metals
Doré bars typically weigh 15–30 kg each. They are fire-assayed (analyzed) at the mine to determine their exact composition before shipment, which determines the value credited to the mine.
Why Doré Instead of Pure Gold?
Mines produce doré rather than pure gold for practical reasons:
- On-site refining is expensive — full refining requires sophisticated industrial infrastructure
- Security during transport — doré is valuable but not investment-grade; it requires assay before trading
- Economies of scale — large centralized refineries process many mines’ doré more efficiently than individual mine facilities could
Doré bars are typically transported by armored carrier directly to accredited refineries.
The Refining Process: Overview
All gold refining involves separating gold from other metals. The challenge is gold’s chemical stability — it doesn’t react easily, which is the same property that makes it valuable as money. Refiners exploit specific reactions that do affect gold to selectively separate it from impurities.
The two dominant industrial refining processes are:
| Process | Output Purity | Speed | Relative Cost |
|---|---|---|---|
| Miller Chlorination | 99.5% (.995 fine) | Hours | Lower |
| Wohlwill Electrolytic | 99.99% (.9999 fine) | Days | Higher |
These are often used in sequence: Miller first, then Wohlwill for the highest-purity output.
The Miller Chlorination Process
Developed by Francis Beaufort Miller (Australian assayer) in 1867, the Miller process remains the standard first-stage refining method for most large refineries.
How It Works
- Doré is melted in a crucible furnace at temperatures above gold’s melting point (1,064°C)
- Chlorine gas is bubbled through the molten metal
- Chlorine reacts preferentially with silver, copper, zinc, and other base metals — forming metal chlorides that are lighter than gold and rise to the surface as a slag
- Gold does not react with chlorine under these conditions — it remains in the melt
- The chloride slag is skimmed off, removing most impurities
- The remaining gold is poured into bars
Output
Miller process gold is approximately 99.5% pure (.995 fineness). This meets the minimum purity threshold for investment-grade gold bars under most standards, including the IRS definition for precious metals IRAs.
ℹ Note
The Miller process is fast — a single batch typically completes in under an hour — but it cannot remove silver completely from gold. This is why LBMA Good Delivery bars require only .995 fineness (the Miller process minimum), while premium investment bars at .9999 require the additional Wohlwill step.
However, silver is only partially removed — the Miller process leaves residual silver in the gold. For applications requiring higher purity (electronics, .9999 fine bars), a second refining stage is needed.
By-Products
The chloride slag removed during the Miller process contains silver chloride. Refineries process this slag separately to recover the silver — a valuable by-product revenue stream.
The Wohlwill Electrolytic Process
Developed by Emil Wohlwill in Hamburg in 1874, the Wohlwill process uses electrochemistry to produce the highest-purity gold available commercially.
How It Works
The Wohlwill process is an electrolytic procedure — it uses electrical current to drive a chemical reaction:
- The impure gold (from the Miller process or other sources) is cast into anodes (positive electrodes) at approximately 99.5% purity
- An electrolytic cell is filled with hydrochloric acid solution containing dissolved gold chloride
- Electrical current is applied: The impure gold anode dissolves into solution; pure gold deposits on the cathode (negative electrode)
- Selectivity: Gold deposits selectively on the cathode. Silver and other impurities from the anode either stay in solution or form anode slimes at the bottom of the cell
- The cathode deposits — essentially pure gold — are periodically harvested
- Cathode gold is melted and cast into bars or granules
Why Electrolytic Refining Achieves Higher Purity
The electrochemical process is extremely selective. Gold’s deposition potential differs sufficiently from other metals that the deposited gold is consistently above 99.99% pure. Four-nines (.9999) gold — the standard for investment bars from top refineries — comes from the Wohlwill process or equivalent techniques.
Some refineries produce five-nines (.99999) gold for specialized industrial applications, but this level of purity is unusual and commands a significant premium.
The Anode Slimes
As the impure anode dissolves, insoluble materials collect at the cell bottom as “anode slimes.” These slimes contain:
- Silver (as silver chloride)
- Platinum group metals (PGMs)
- Small amounts of other metals
Processing the anode slimes is a significant revenue source — platinum group metals (platinum, palladium, rhodium) are more valuable per ounce than gold and are recovered as by-products of gold refining.
✓ Pro Tip
When evaluating gold mining stocks, pay attention to by-product revenue. Refineries that recover platinum group metals from anode slimes and silver from chloride slag can significantly offset their processing costs, improving overall profitability.
From Refined Gold to Finished Products
After the Wohlwill process, refined gold emerges as cathode deposits or granules. Converting this to finished investment products involves additional steps:
Bar Production
- Melting: Refined gold is melted in induction furnaces
- Alloying (if required): Some products (22K coins, jewelry) require controlled addition of alloy metals at this stage
- Casting: Molten metal is poured into precision molds or continuously cast
- Assaying: Each bar or batch is sampled and fire-assayed to verify composition
- Stamping/Minting: Weight, purity, serial number, refinery mark, and assay office mark are applied
- Packaging: Investment bars are typically sealed in tamper-evident packaging with assay certificates
Coin Minting
National mints receive refined gold (typically as granules or small bars) from accredited refineries. Minting adds:
- Precisely controlled alloying (for 22K coins like the American Gold Eagle)
- Coining press striking with dies
- Quality inspection and certification
- Packaging and authentication features
Fire Assay: Verifying Gold Purity
The fire assay is the gold standard (literally) for verifying gold composition. It has been used for over 3,000 years and remains the definitive method.
The Process
- A small sample (~0.25 grams) is taken from the material to be tested
- The sample is mixed with lead oxide (litharge) and other reagents, then wrapped in a lead foil “cupel”
- The package is melted in a furnace — the lead and base metals oxidize and are absorbed into the porous cupel; gold and silver remain as a tiny “bead”
- The bead is rolled into a thin foil and treated with nitric acid, which dissolves silver but not gold
- The remaining gold is weighed precisely
The entire process is destructive (the sample is consumed), but extraordinarily accurate. Refineries fire-assay every production batch to verify output purity before shipping.
The fire assay -- melting a sample with lead oxide to isolate gold and silver -- has been used for over 3,000 years and remains the definitive method for verifying gold purity, accurate to parts per thousand.
LBMA Accreditation: The International Trust Standard
The London Bullion Market Association (LBMA) maintains the Good Delivery List — a roster of refineries whose gold bars are accepted in the global professional market without further assay.
What Good Delivery Means
A “Good Delivery” bar from an LBMA-accredited refiner:
- Has guaranteed minimum purity of 99.5% (.995 fine) for gold
- Has documented provenance and chain of custody
- Is accepted at face value by central banks, bullion banks, and institutional traders worldwide
- Can settle LBMA trades without re-assay
Non-accredited gold must be assayed at each point of sale — adding cost, time, and uncertainty. This is why LBMA accreditation is economically essential for industrial-scale refiners.
★ Important
For investors, buying bars from LBMA-accredited refiners is not just a quality preference — it is a liquidity decision. LBMA Good Delivery bars can be sold to any major dealer worldwide without additional testing, while non-accredited bars may require costly independent assay before a dealer will purchase them.
Current Accredited Refiners (Selected Major Names)
Europe/Middle East:
- PAMP (Switzerland) — MKS Group
- Valcambi (Switzerland)
- Argor-Heraeus (Switzerland)
- Umicore (Belgium)
- Emirates Gold (UAE)
Americas:
- Royal Canadian Mint
- Asahi Refining (USA/Canada)
- Metalor Technologies (Switzerland, with US operations)
Asia-Pacific:
- Perth Mint (Australia)
- Nadir Metal Refinery (Turkey)
- Various Chinese refiners
The LBMA Good Delivery List is publicly available and updated regularly.
Recycled gold from jewelry, industrial scrap, and dental materials accounts for approximately 25-30% of annual gold supply -- roughly 1,100-1,200 tonnes per year, refined through the same processes as mine gold.
Refining Recycled Gold
Not all gold entering refineries comes from mines. Recycled gold — from jewelry, industrial sources, and dental scrap — constitutes approximately 25-30% of annual gold supply (roughly 1,100-1,200 tonnes per year).
Sources of Recycled Gold
Jewelry scrap: Old or broken jewelry returned to jewelers or sold to pawnbrokers ultimately reaches refineries. Gold content varies by karat, and the jewelry is alloyed with silver, copper, and zinc.
Industrial scrap: Electronic circuit boards, contacts, and components contain gold in small quantities. Specialized e-waste recyclers process these materials and sell the recovered gold to refineries.
Dental gold: Gold dental crowns and fillings, recovered from dental offices or after cremation, are refined back to pure gold.
Coins and bars: Investment gold sold back to dealers is typically melted and refined again to reset the chain of custody and produce new standardized products.
The Refining Process for Scrap
Scrap gold typically goes through the same refining processes as doré. However, scrap may be pre-processed:
- Inquartation: Scrap alloys with insufficient gold content are diluted with additional silver to reach a ratio where nitric acid can dissolve the silver without attacking the gold (the “parting” step)
- Cupellation: Fire assay-based separation to remove base metals before dissolution
High-gold scrap (like old coins at 90% fineness) can go directly to electrolytic refining.
⚠ Warning
If you sell gold jewelry or scrap to a refiner, expect to receive payment based on the assayed gold content minus a refining charge (typically 1-5%). Disreputable buyers may under-assay your gold or charge excessive fees. Always get multiple quotes and request a copy of the assay results before accepting payment.
Reading Refinery Marks
Investment bars from accredited refineries display standardized markings that identify their provenance and purity:
Example: 1 oz PAMP Suisse bar
PAMP SUISSE— refinery name1 OZ FINE GOLD 999.9— weight, metal, and finenessCHI 999.9— Swiss assay office mark- Serial number (unique to the bar)
- Unique design (Fortuna motif)
- Assay certificate in the packaging
The serial number is critical: it links the specific bar to the refinery’s production records and, for premium products like PAMP’s VeriSure bars, to a digital authentication system.