The Minting of Gold

From Mine to Refinery

Mining and Ore Processing

Gold ore typically contains between 1 and 10 grams of gold per tonne of rock — meaning a tonne of ore yields less than a teaspoon of gold. After extraction, the ore is crushed and ground to a fine powder. The most common extraction method is cyanide leaching (the carbon-in-pulp or carbon-in-leach process), where a dilute sodium cyanide solution dissolves the gold, which is then adsorbed onto activated carbon and stripped in a high-temperature process. The result is a gold-bearing solution from which the metal is recovered by electrowinning or zinc precipitation.

The output of this process is doré — an alloy of gold and silver (and sometimes other metals) typically containing 70–95% gold. Doré bars are the raw material shipped from mines to refineries. They are not yet suitable for investment or jewellery — they must be refined to remove silver, base metals, and trace elements before they meet the purity standards required by markets.

The Refining Process

The Miller Process (99.5% purity)

Developed by Francis Bowyer Miller in 1867, the Miller Process is the most widely used industrial gold refining method. Chlorine gas is bubbled through molten doré at approximately 1,100°C. The chlorine reacts preferentially with silver and base metals (zinc, lead, copper) to form chloride compounds that float to the surface as slag and are skimmed off. Gold does not react with chlorine under these conditions and remains in the crucible.

The Miller Process produces gold of approximately 99.5% purity rapidly and cost-effectively — a typical batch of several hundred kilograms takes only a few hours. However, it cannot remove silver completely and cannot achieve the higher purities required for electronics or specialist investment products. For those applications, a second refining step is required.

The Wohlwill Process (99.99%+ purity)

Developed by Emil Wohlwill in 1874, the Wohlwill Process uses electrolysis to achieve the highest commercially available gold purity. The impure gold (typically already at 99.5% after Miller refining) is cast into anodes and suspended in a hydrochloric acid-gold chloride electrolyte solution. When electric current is applied, pure gold dissolves from the anode and deposits onto a thin gold foil cathode at 99.99% or greater purity.

The Wohlwill Process is slower than the Miller Process (24–48 hours per batch) and more expensive, but produces the four-nines (999.9) and five-nines (999.99) fine gold required by the LBMA Good Delivery standard, electronics manufacturers, and specialist mint products. Most LBMA-accredited refiners use a combination of both processes — Miller for bulk processing followed by Wohlwill for final purification.

Gold Purity Grades Explained

Gold purity is expressed as fineness — the proportion of pure gold by mass, measured in parts per thousand. A bar stamped "999.9" contains 999.9 grams of pure gold per kilogram, with 0.1 grams of allowable impurities. This system replaced the older karat (carat) system for bullion, though karats remain the standard for jewellery.

999.9 vs 999.99 — Does Five Nines Matter?

What the Difference Actually Means

The difference between 999.9 fine (four nines) and 999.99 fine (five nines) is 0.09 grams of impurities per kilogram of gold — less than a grain of rice. In a standard 400 troy ounce Good Delivery bar weighing approximately 12.4 kilograms, this amounts to approximately 1.1 grams of additional impurities in a four-nines bar compared to a five-nines bar.

For the overwhelming majority of investment purposes, this difference is irrelevant. Both grades are accepted by central banks, bullion markets, gold ETFs, and all major dealers. The gold price differential between four-nines and five-nines products is typically zero at the spot level — both trade at the same gold price benchmark.

When Five Nines Matters

Five-nines fine gold commands a meaningful premium and matters in three specific contexts:

1. Electronics and semiconductors: Microchip bonding wire, connector plating, and circuit board contacts require 99.999%+ purity because even trace impurities (parts per million of platinum, palladium, or silver) can affect electrical conductivity and corrosion resistance at the nanoscale.

2. Collector and prestige coins: The Royal Canadian Mint produces limited-edition Maple Leaf coins in 99.999% purity — the "five-nines Maple Leaf" — which command premiums of 5–15% over standard 99.99% Maple Leafs purely for the prestige of the specification.

3. Scientific and laboratory use: Chemical standards, radiation shielding applications, and certain medical devices require the highest available purity to eliminate interference from trace elements.

How Coins Are Minted

Blanking

Refined gold at the required purity is alloyed if necessary (for 22k coins, copper is added in precise proportions), then melted and cast into long strips of uniform thickness using continuous casting machines. Blanks — the disc-shaped coin "flans" — are punched from these strips using high-precision hydraulic presses. The blanks are weighed individually; any outside tolerance are recycled.

Annealing

Blanking work-hardens the metal, making it brittle and difficult to strike. Blanks are annealed — heated to 600–700°C in a controlled atmosphere furnace, then slowly cooled — to restore ductility and create the uniform crystalline structure required for sharp strike definition.

Burnishing and Cleaning

Annealed blanks are tumbled in a burnishing machine with steel balls and burnishing compound to remove oxides formed during annealing and to create a uniform surface finish. The blanks are then washed, dried, and inspected before striking.

Striking

Each blank passes through a coining press where upper and lower dies bearing the coin design strike the blank simultaneously with pressures of 50–200 tonnes. The metal flows under this extreme pressure to fill every detail of the die. Bullion coins are typically struck once; proof coins (the mirror-finish collector versions) are struck two or three times with specially polished dies to achieve their characteristic cameo appearance.

Assaying — Verifying Purity

Assaying is the analytical process of determining the precise metal content of a gold product. For LBMA Good Delivery accreditation and mint quality control, multiple assay methods are used:

The fire assay — also called cupellation — remains the ultimate legal standard for gold purity disputes. It has been used to test gold for over 2,500 years, with written records of the process dating to ancient Mesopotamia. Despite modern instrumental methods being faster and non-destructive, fire assay is irreplaceable as the reference standard because it directly measures pure gold content with the highest possible accuracy.

LBMA Good Delivery — The Global Standard

The LBMA Good Delivery list is the most important quality standard in the international gold market. A refiner or mint that achieves Good Delivery accreditation has demonstrated that its bars consistently meet the following specifications:

The LBMA publishes its Good Delivery List publicly — it includes approximately 150 accredited gold refiners worldwide. Bars from non-listed refiners, or bars that leave the approved vault system, must be re-assayed before re-entering the market. This "chain of integrity" is what gives LBMA Good Delivery bars their universal liquidity and acceptance by central banks, ETF custodians, and institutional traders globally.