by J.C. Levesque
Reprinted from The CN Journal September 1978 pg 309
The manufacture of a die is an exacting task, and the expert management of each phase of its creation has considerable influence on the future performance of the die. The forging, turning, striking, hardening, and tempering of dies are of paramount importance, and the slightest variation in any step drastically affects the life and striking quality of the die. At the Royal Mint experiments were almost constantly in progress after 1870 to judge the merits of these stages of production, and modifications were often made when a particular technique was found to im¬prove the performance of a die.
We all have witnessed, at one time or another, the physical deterioration on a die as shown by its being transferred to the coins struck from it. The commonest phenomenon by far is the die crack or die break. The causes of die failure are at¬tributable to a number of factors both human and technical. Although it would be far beyond the scope of this article to delve into the profound technical and metallurgical reasons for deterioration of a die, we will nevertheless endeavour to present a brief outline of the major causes of die deterioration and, equally im¬portant, the reasons why some dies lasted longer than others.
Quality of Steel: This was perhaps the most crucial aspect of die production, and the slightest variation from the standard chemical composition of the steel would result in inferior dies readily susceptible to cracking. The various con¬stituents of die steel are, in descending order of amount present in the steel, iron, carbon, manganese, and silicon, with occasional minute traces of sulphur, phosphorus, and copper. The key element in determining the quality of the steel is carbon. It was found that a steel which would be best suited for coining purposes must contain from 0.60 to 0.75 of 1% of carbon. A variation of 0.03% would affect the tensile strength of the steel.
Many steel bars purchased for die manufacture had to be returned to the suppliers as unsatisfactory. The problem of inferior die steel was a serious one in the 1870s and afterwards, and the Royal Mint experienced considerable difficulty in obtaining good die steel. The quality of bars in a given shipment would vary in physical and chemical properties, and sometimes a given bar was not of uniform quality, which produced dies of varying quality. Sometimes small cracks appeared on the surface of some dies while they were being tempered, which rendered them unfit for use. Other dies developed cracks soon after being installed in the coining presses, and occasionally a die was condemned after only a single coin was struck. In a few exceptional cases, on the other hand, a sound die would strike as many as 200,000 coins before being retired.
The Coining Press: It was soon found that the Uhlhorn lever press was superior to the old-fashioned screw press with atmospheric pressure. The lever press struck coins by prolonged compression which gently squeezed the design into the blank, while the screw press struck with a rapid blow. Dies used with the former were less likely to deteriorate through prolonged use, while those used with the latter could not withstand the constant pounding and thus cracked easily. The number of dies required by the Uhlhorn press was considerably less, and the quantity of defective coins produced was noticeably diminished. The average number of pieces struck from a pair of dies in a screw press, although the type of steel used for dies depended on the kind of coining press used, since a certain composition of steel ideally suited to the lever press might not be suitable at all for the screw press. Nevertheless, the advanced construction of the Uhlhorn press proved to be highly satisfactory, and in later years the use of the screw press for general coinage was gradually eliminated.
Re-sinking of worn Dies: Beginning in 1870, worn dies, which had lost their original sharpness of detail but were otherwise sound, were often softened, re-sunk, and again prepared for coining. Evidently these reprocessed dies did not strike as many coins as their newer counterparts. However, considering that a new life was given to an otherwise unsuitable die, the number of coins struck both before and after re-sinking was significant. The average number of coins struck from a pair of such dies after re-sinking was 25,000.
Hardening of Dies: Hardening is a delicate operation and requires much care, having a significant influence on the physical characteristics of the steel. The limit of elasticity of the metal is affected the most. The dies are placed in containers with animal charcoal or wood ashes and heated in a furnace. The quality of the steel was greatly influenced by the chemical composition of the charcoal, largely due to the presence or absence of nitrogen. By 1870 much progress had been made in the techniques of hardening dies, upon which the sharpness of impression and the speed of coining greatly depended. It was found that, instead of instantly heating the die to a high temperature, as was done before, it was better to heat it slowly to the required temperature before plunging it into cold water. The dies were subjected to less heat than before, and the time in which they were kept in the furnace was reduced, which thus effected a great savings in fuel. This process hardened the die to a remarkable degree and increased its sharpness of detail.
Changes in the Shape of Dies: Late in 1873 the Royal Mint modified the shape of some parts of the dies. The most significant change was the shortening of the neck, which gave greater strength and durability. This increased the number of coins struck from a given pair of dies to over 70,000, a considerable improvement over preceding years. Another advantage was the reduction in the number of dies required for the presses.
Inexperience of Workers: Die making requires meticulous and expert care. The inexperience of newly-employed workers in the Die Department greatly influenced the future quality of the dies. Should any step of the process be performed less carefully, the die so produced would deteriorate more readily during use. Even if the dies were carefully made, inexperience on the part of the operators of the Uhlhorn lever presses would significantly affect the life of a die by reducing the number of strikes it could bear.
Reduction of defective Dies: Since the great majority of defective dies had only hair-line cracks extending along the periphery of the face, it was found that these could be salvaged by reducing their size by re-turning them in a lathe to remove any damage or imperfections. The face of the die was then softened, smoothed, and re-sunk with a punch for a smaller coin. This reduction considerably reduced the life of the die, since the crystalline structure of the die steel had been somewhat impaired by the previous striking.
The Size of the Dies: Larger denominations require increased striking pressure, and the area of stress of the die is greater. As a result the resistance of large dies to prolonged use is noticeably less and therefore their life is significantly shorter than that of their smaller counterparts.
Forging of the Dies: Experiments were conducted in 1879 to judge the effectiveness of hand forging of dies and forging by steam hammer. In order to standardise striking conditions, the dies tested were used in the Uhlhorn lever presses. It was found that dies forged under the steam hammer were better than those forged by hand, such that those forged by the steam hammer struck more than twice as many coins as those forged by hand. Dies forged by the steam hammer struck 83,244 coins per pair of dies, whereas handforged dies struck 36,919 coins per pair of dies. Evidently the steam hammer produced a physically stronger die.
Yearly Average Number of Coins struck per Pair of Dies: The following table shows the average number of coins struck from a given pair of dies for the British Imperial coinage from 1869 to 1880.
1869: 43,894 1873: 16,938 1877: 20,894
1870: 38,367 1874: 26,162 1878: 24,265
1871: 37,522 1875: 27,356 1879: 30,521
1872: 32,532 1876: 17,681 1880: 53,807
These averages depend on any number of the previously mentioned factors. The most significant of these is the number of worn dies which had been re-sunk, since such dies had a much shorter life. It should be noted that, although these figures are only for the Imperial coinage, the averages for any of the Canadian and Colonial issues would not have varied significantly. These figures may be of some use, albeit limited, to the student of die varieties in estimating the approximate number of coins struck from a single pair of dies.
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