Numismatic Feature - Proof Coins

PROOF COINS - The Quest for Perfection

The modern process of minting of proof coins of pristine quality truly epitomises this ancient art. The process from initial drawing to completed proof coin is a protracted one involving several hundred hours of work.

The Design

The first step is to produce a drawing of the proposed design. The artist needs to master two special techniques - the ability to create a design which works within a circular framework and which may be translated effectively into a coin many times smaller than the original drawing.

From the drawing the artist makes a three dimensional plaster model, usually around 20-25 cm in diameter. A soft rubber mould is taken from the plaster model.
To make the 'pattern' from the mould, epoxy resin mixed with a hardener is poured into the mould and allowed to cure.

The Steel Tool

The pattern is placed on a reducing machine in order to trace the design on to the face of a small steel billet. Once the reduction ratio required to give the diameter for the coin is set, the design is traced in a slow spiral. The rotating cutter traces a similar but smaller path on the face of the steel billet, cutting the design in miniature to create a 'reduction punch'.

This process normally takes around forty eight hours and is repeated to achieve the sharp detail that is required. The result of this process, the 'reduction punch' is then machined, hardened and tempered for use in the next stage.

The Master Tool

In a process known as hobbing, an hydraulic press, exerting many tonnes of pressure, is used to press the reduction punch several times into the cone-shaped end of a prepared steel billet. Several pressings, interspersed with machining and softening, may be necessary.

In addition, the tool-maker moves metal into the weak spots of the design between each pressing by gently 'tapping up', using a hammer and a punch. The hard steel tool produced is the 'matrix'. The engraver uses hand gravers and small punches to bring up the detail, then works on the table - the flat background to the design - to remove any tool marks and, if necessary, adds the beaded edging used in some designs.
The matrix is used to produce the working punch (the master die), using the same process as before. The master die is then hobbed into other steel billets to produce the working dies.

The Proof Die

It is generally accepted that a proof coin has the design 'frosted' and the table area highly polished. To achieve this, the die has to undergo special preparations. First the table area of the die is given a light polish and the face of the die is covered with transparent adhesive tape. Using a surgical knife blade, the tape covering the design is carefully cut away. The exposed design is then sandblasted using a fine grade grit to give the frosted effect.

The remainder of the tape is removed and the table area of the die is polished, using various grades of wooden 'lapping' sticks and diamond paste, until it has a mirror-like finish. To protect the surface, the face of the die is then hard chrome plated to a depth of 3-4 microns.

The Proof Blank

The surfaces of the blanks to be used for proof coins are specially prepared, in a buffing process, to produce a mirror-like finish. To ensure that the blanks going forward for coining are within the weight tolerance allowed by legislation, each blank is individually weighed in a computer-controlled balance.

Great care is taken throughout the procedure to ensure that blanks are not damaged in any way. During the cleaning and weighing of the blanks, tongs with specially protected jaws are used. Thereafter, operators only handle the blanks and coins by gripping the edge while wearing finger cots and gloves.

Minting the Proof Coins

The press is set up with a reverse die and an obverse die in the top and bottom die holders and a collar with a serrated bore manufactured to give the correct number and configuration of mills (serrations) around the edge of the coins.

The blanks are placed one at a time over the collar and the press activated to strike the coin. Each coin is struck at least twice by the proof dies to enhance the clarity of the design.

At this stage each coin is subjected to rigorous inspection, under magnification. The slightest imperfection on the polished table or a bright spot appearing in a frosted area means rejection.

Dies are cleaned between coinings, as even the smallest amount of dust on the surface during pressing could mean rejection of the coin. A further close examination of each coin is made by specialist inspectors before the coin is sealed in its protective acrylic capsule and packaged ready for issue to collectors

Sources:
'The Tenth Proof Issue of The Australian Koala Platinum Coins' booklet issued by the Perth Mint - 1997.
'How Are Coins Produced ? booklet by the Royal Australian Mint, 1994