How Gold & Silver Coins Are Made - PERTH MINT BEHIND THE SCENES

The Coin Minting Process at the Perth Mint

Key Concepts:

• Annealing: A heat treatment process used to soften metal, making it more malleable for shaping.
• Gauging: The process of reducing metal to a precise thickness, crucial for coin production.
• Blanking: Punching out circular discs (blanks) from metal strips, which will become coins.
• Pickling: Using an acid bath to remove a small amount of metal from blanks to achieve precise weight.
• Burnishing: Polishing and cleaning metal blanks to prepare them for minting.
• Planchet: A blank disc of metal ready to be struck with a design to become a coin.
• Karat: A unit of purity for gold, with 24 karat being pure gold.
• Hydraulic Ram: A machine using hydraulic pressure to shape metal.
• Die: A hardened metal stamp used to imprint the design onto a coin.

I. Initial Metal Preparation

The process begins with refined gold and silver, typically arriving at the Perth Mint in large bars. For silver, 1,000-pound bars are cut into smaller pieces and melted down into “silver logs.” These logs are further cut and processed. Gold undergoes a similar process, being melted into large bars and then cut into smaller, manageable pieces. Importantly, all incoming metal is refined to 24 karat purity. To create alloys like 22 karat gold, copper (and sometimes silver) is carefully mixed in at a separate station to maintain the purity of the 24 karat stock.

II. Strip Production: Breaking Down the Metal

The next stage involves reducing the metal into thinner strips suitable for blanking. Robbie Stewart, General Manager of Manufacturing, explains that a 50-year-old German machine breaks down cast gold bars into thinner strips. This process work-hardens the metal, necessitating annealing – heating the gold to approximately 700°C in specialized ovens to restore its malleability.

For silver, logs (billets) are heated to around 600°C and forced through a hydraulic ram, creating thin coils. These coils are then further thinned using a gauging machine. This machine utilizes a laser to precisely measure the thickness of the silver, rolling it back and forth until the correct gauge is achieved. This laser-based gauging system represents a significant improvement over the older “Farmer Norton” method, which relied on manual micrometer measurements and was prone to errors, resulting in higher reject rates.

III. Platinum Processing & Manual Techniques

Unlike gold and silver, platinum processing is less automated. Platinum strips are manually fed through the gauging machine to achieve the desired thickness. This manual process is considered more tedious and challenging than silver blank production.

IV. Blanking & Weight Control

The metal strips, whether gold, silver, or platinum, are then fed into blanking machines. These machines punch out circular blanks of various sizes. A crucial step is weight control. Robots weigh each blank individually. Blanks that are too light are remelted. Those slightly overweight undergo a “pickling” process.

Robbie Stewart details the pickling process: blanks are immersed in an acid bath for a short period to remove a small amount of metal, bringing them within the acceptable weight range. This is particularly important for gold and platinum to avoid giving away excess metal value. The removed gold is recovered and reused.

V. Blank Finishing: Burnishing & Preparation for Minting

After pickling, blanks are burnished – polished and cleaned – using demineralized water, a special soap, and stainless steel shot in a process lasting 2-3 hours. This prepares the blanks for the final minting stage. Silver blanks emerging from the pickling bath have a unique frosted or dull appearance.

VI. Minting & Specialized Techniques

Silver blanks proceed through a series of pickling and cleaning stations before being minted. The minting process itself is highly efficient, with machines capable of producing 9,000 coins per hour.

Ultra-high relief coins require a unique two-step process. They are initially minted oversized and then the outer portion is clipped away, leaving only the central, high-relief design. This method prevents die cracking and ensures accurate alignment. Coins with a plain edge are likely produced using this oversized-and-clipped technique.

The world-famous silver kangaroos are minted using standard minting machinery.

VII. Gold Coin Production: A Manual Process

Notably, the production of gold coins and bars is largely manual. Each blank must be physically placed into a hydraulic press for minting. After minting, gold coins undergo a final acid bath to remove any residue, ensuring a perfect finish.

VIII. Finishing & Quality Control

Following minting, coins may receive additional finishes, such as colorization or gilding (rose gold and 24 karat gold). The Perth Mint maintains comprehensive in-house capabilities, including collar production and die creation. A rigorous quality control process ensures that only flawless coins are released. Any coin exhibiting even minor imperfections ("tech marks") is rejected.

IX. Data & Statistics

• The gauging machine utilizes a laser for thickness measurement, improving accuracy compared to the older micrometer method.
• The minting machine can produce 9,000 coins per hour.
• The annealing oven reaches temperatures of approximately 700°C.
• Silver billets are heated to around 600°C.

X. Synthesis & Conclusion

The Perth Mint’s coin production process is a fascinating blend of traditional craftsmanship and modern technology. From the initial refining of metals to the final polishing of finished coins, meticulous attention to detail and quality control are paramount. The process highlights the importance of precise metal preparation, accurate gauging, and rigorous weight control, particularly through the innovative use of pickling and laser-based measurement systems. The continued reliance on manual processes for gold coin production underscores the commitment to producing high-quality, valuable coins. The entire operation demonstrates a vertically integrated approach, with the Perth Mint controlling every aspect of coin production in-house.