Art & Science of Coin‑making

Coins present an intriguing combination of expert craftsmanship and state‑of‑the‑art technology. It could be said they are the place where art meets science.

Modern coin-making, also known as minting, involves a number of stages.

Coin design & engraving

Every coin starts with an artist’s sketch. Product developers collaborate with coin designers and engineers to design an image that will translate well onto a coin.

Once the design is approved, a three-dimensional model is sculpted using either traditional plaster modelling or 3D modelling software. A milling machine is then used to create a synthetic model with a diameter of approximately 20cm.

A coin designer can spend many hours working on the model, refining the details to ensure the design can be manufactured to the highest quality.

It can take up to a year for a design concept to appear on a finished coin.

Dies

Once completed, the coin design is cut directly into tool steel using a computer-controlled engraving machine, creating what is known as a reduction punch or die. The reduction die contains a positive (raised or relief) impression of the complete design of the coin. The finishing touches are applied by a master engraver, who meticulously perfects the contours and fillet work.

The reduction die is then placed in a hydraulic press, where the design is transferred onto a softened steel block using 100 to 400 tonnes of pressure. This process is called hobbing. The result is a master die, containing a negative impression of the coin design.

Afterwards, the reduction die is hardened, and during a multi-stage pressing procedure a positive die is produced. During this process, the hardened steel is pressed into mild steel, whereby an exact copy of the coin image is transferred onto the latter. Finally, this positive die is used to produce the actual negative minting dies.

After pressing, these are turned into the correct shape, hardened, and chromium-plated to enhance durability. Depending upon the variety of coins being minted, the service life of a pair of minting dies generally suffices for producing up to one million coins.

Blanks

Before they are imprinted by dies in the coin-making process, unmarked coins are known as blanks. A blank must undergo many steps before a design is committed to its surface. Each stage is necessary to ensure longevity, precision and appearance.

Blanks are placed in the rimming machine to remove the rough edge left when the blanks were pressed out of metal sheets. The machine also adds a raised rim on the disc to ensure that the detail from the die will transfer flawlessly when the coin is struck. A rim can add 10 to 15 years to a coin’s life.

Blanks are burnished by tumbling for up to six hours with stainless steel beads of various sizes that wash and polish their surface. Each tumbler is used for a specific alloy to prevent cross-contamination. Blanks are then spread out evenly and thoroughly hand-dried with towels to prevent scratching.

After burnishing and prior to striking, the blanks go through a final inspection and degreasing. Imperfect blanks are removed for re-melting and re-use.

Coining

The coining process differs according to the type of coin being produced; circulation coins, collector coins or investment coins.

 

Circulation coins

Circulation coins are mass-produced from base metals such as copper, nickel and aluminium. They are used for everyday cash transactions.

After the mint’s fitters and machinists install the dies in the coin presses, blanks are fed through the press where they are struck simultaneously on both sides by the dies. Dies can strike coins with up to 200 tonnes of pressure, at a rate of up to 650 pieces per minute. Under the pressure of striking, metal particles spread and stretch. This results in a sharper rim, and the edge of the coin taking on any serration in the press’ collar. This is how coins get the grooves you often see around their edges.

Collector coins (Uncirculated)

When produced by Sovereign Mints, collector coins are legal tender but produced to be used as gifts or souvenirs, or as pieces for collections. Uncirculated coins are mass-produced like circulation coins, but at a slower rate, and have not been distributed or used as currency. The press for uncirculated coins has an automatic feed and can make up to 80 coins per minute or 20,000 coins per day.

 

Collector coins – Proof

Specially minted for collectors and usually sold in a velvet presentation case, proof coins also come with a certificate of authenticity. They are made in presses that strike with 180-360 tonnes of pressure. Each proof press is manually operated, and the blanks are struck 4-6 times each with specially polished dies. This creates softly glazed yet minutely detailed images that seem to hover over a mirrored field. Each proof die can strike approximately 250 coins before it has to be replaced.

Investment coins

Investment or bullion coins are the mint industry’s most valuable products. Made of precious metals with small mintages, they are true highlights of the industry’s commitment to artistic and engineering excellence. Investment coins are produced using a single stamp method and have either a heavy frost, light frost, or brilliant polish.

Medals

Many mints produce national medals to commemorate significant historical events or sites and to honor those whose superior deeds and achievements have enriched national history or the world.

Quality Control and Packaging

Circulating coins travel along a conveyor system, where they are counted and placed into small bags or sachets. The bags of circulating coins are sealed in boxes, which are stored in the mint’s vaults until they are securely delivered to banks.

Uncirculated coins are inspected by staff members who monitor the production conveyor belt as coins leave the press. Any coins that do not meet quality standards are removed.

Proof and bullion coins are individually inspected by the press operator, using eyeglasses and magnifying lamps to check for any flaws. The tiniest imperfection will result in the operator rejecting the coin, after which it will be sent to a foundry and melted down.

Uncirculated and proof coins are individually packaged and delivered to the mint’s shop or dealers who have commercial arrangements with the mint.