Letter-shape variations of Australian George V pennies


In his little book, "The 1965 Australian Coin Varieties Catalogue", John Dean describes a particular variation on the reverses of Australian George V pennies. This variation affects shape of the letters forming the legend "COMMONWEALTH OF AUSTRALIA". On some coins the seriffed letters I, H, T, M, L , E, A and R have flat bases whereas on others the bases of the letters are distinctly concave. Dean also mentions this variation on the 1931 florin.


Flat- and curved-base letteringFlat- and curved-base lettering
Flat- and curved-base lettering


According to Dean's catalogue, the curved/flat-base lettering occurs in George V pennies as follows:
1911 - flat
1912 - flat
1913 - flat
1914 - flat
1915 - flat
1916 - flat
1917 - flat
1918 - flat
1919 - both
1920 - both
1921 - flat
1922 - both
1923 - both
1924 - curved
1925 - curved
1926 - curved
1927 - curved
1928 - curved
1929 - curved
1930 - curved
1931 - curved
1932 - curved
1933 - curved
1934 - curved
1935 - curved
1936 - curved



There seems to be a definite pattern here with the years 1919 to 1923 representing some sort of a transition period.
As a novice I have not (yet) read much Australian numismatic literature but it seems to me that this easily-discernable variation in lettering types has been ignored by numismatists, variety seekers and catalogue compilers alike. That struck me as odd, and still does.
Because of my interest in die varieties I had ordered back issues of some journals of the Australian Numismatic Society and the Numismatic Association of Australia, largely to read about Paul Holland's work in classifying penny and halfpenny varieties. I couldn't believe that Paul would ignore the subject and I was curious to see what he had to say.
At about the same time as I ordered the journals, I had been examining several George V florins under the microscope and I had noticed a similar variation of lettering styles. Reflecting on the similarity, I had formed a conjecture as to how the florin variety arose and wondered if it might apply to the pennies as well. My curiosity about Paul Holland's treatment of the subject was intensified. When the journals arrived early in January 2000, I was rewarded with the following observation (JANS 1993 page 16):
... it should be noted that significant variations also occur in the shape of the lettering on the reverses of George V pennies. In John Dean's earlier work ... these were classified as "flat base letter" and "curved base letter" reverses. While some of these variations may have arisen during the striking of the coins, some are so pronounced and prevalent that it seems likely that they arose during some earlier stage in die production (i.e. at the level of hubs used to prepare working dies or from derivative master dies.) ...
Paul's remarks do not contradict my conjecture, and even go so far as to support them. If he had said much more, he would have given the explanation himself. Since he did not, I am emboldened to publish.

The aim of this document is to offer an answer to all of the following questions:
How did the different letter shapes arise?
It is the premise of this document that flat-base letters are the original form and that curved-base letters are a deviation which arose through differential plastic flow of metal under pressure. This premise will be examined in more detail later.

Why do all early (pre-1919) pennies have flat-based letters while all later (post 1923) pennies have curved-base letters?
This is really a subject for more research. At the moment I offer the speculation that curved base lettering developed progressively during the production of derivative master dies.

Why is the 1931 florin the only one to show curved-base letters?
It isn't. In the preamble I hinted at the answer to this question. Although John Dean mentioned it only for the 1931 florin the variation can be observed on other florins. To be fair, the curvature on most florins is slight whereas on some of the '31 florins it is very pronounced:-

Curvature of letter bases on a 1931 florinCurvature of letter bases on a 1931 florinCurvature of letter bases on a 1931 florin
Curvature of letter bases on a 1931 florin


Why is the variation confined to George V coins? Curved-base lettering has only been documented (by John Dean) George V pennies (and the '31 florin). The lettering on the coins of George V is characterized by wide serifs so any curvature in the bases of those letters is clearly visible.
Later issues have legends with sans-serif lettering so any curvature is less visible. Also, according to the conjecture set forth in this document, the serifs play an important role in the development of the curvature.
Why is the variation not observed in halfpennies and other coins? If one accepts the differential plastic flow hypothesis (which is still to be explained) then one would expect curved-base letters to occur in a wide variety of coins rather than be confined to the pennies. This expectation is confirmed by observation.
If you look at a selection of pre-1940 halfpennies under a microscope, many of the coins are seen to have curved-base letters.


Curved-base letters on a 1939 ½d reverse
Curved-base letters on a 1939 ½d reverse


Other examples of curvature:

Curved-base lettering on a 1935 ½d obverse
Curved-base lettering on a 1935 ½d obverse 


Curvature on the tops of the letters on a 1938 threepence.
Curvature on the tops of the letters on a 1938 three pence. 



Plastic flow

When a coin is being struck, the flat metallic blank (planchet) is subjected to such high pressure that the metal behaves like a viscous liquid and metal flows from the field portions of the coin to the relief portions. This is the mechanism whereby the design cut on the surface of the die is transferred to the coin. Just as an example, assume we're going to make a "coin" with an extremely simple design, like this:

Plastic flow 


The next diagram shows, in cross-section, a pair of dies pressing a planchet. The arrows represent the flow of metal from the field portions to the relief portions of the coin during the strike. The flow is grossly exaggerated but I am attempting to illustrate how metal is squeezed from the field portions of a coin into the relief portions.

Plastic flow 


Real coins have more complex designs and in much lower relief but the principle is the same. Now let us digress for a moment and imagine the effect of striking a planchet with perfectly flat dies. If the diameter of the planchet is such that it fits snugly in the collar, then not much will happen when the planchet is struck.
Should the diameter be somewhat smaller then the planchet will spread under the pressure of the strike. This means that there is a general flow of metal from the centre towards the rim.
There is some observational evidence to suggest that such outward flow occurred during the minting of Australian pre-decimal coins. The evidence is most visible on high-grade silver coins but can also be seen on bronze issues and takes the form of radial strain lines running from relief features towards the rim, especially and most significantly rim-side of the legend.
The strain lines appear to be formed when the metal flows beneath the lettering while the metal forming the letters themselves is held in place by the die



Differential Plastic Flow

If the flow of metal from the field to the relief portions of a coin under the pressure of striking is combined with a general rimward flow of metal then the pattern of flow will vary according to the contours of the die. Let us examine what would happen to the metal in the foot of a serifed letter T such as found on George V coins. The diagram below shows a cutaway portion of a coin featuring just the bottom part of the letter. The arrow represents the direction of metal flow (assuming that there is indeed such a flow).


Differential Plastic Flow 


Imagine a cross-section through the serif of the T during coining when the die is in place and the metal flow is occurring. In region A the flow is unconstrained but in region B the metal has nowhere to go; it is held in place by the die. The result is a shear zone between regions A and B where the metal in A slips past the metal in region B. The surface metal in region C develops strain lines which can be seen under magnification. 


Differential Plastic Flow 


Now imagine a cross-section through the stem of the T.


Differential Plastic Flow 


As before, the metal in region A is unconstrained and the metal in region B cannot move because of the presence of the die. However there is a stronger tendency for the metal in region C to be dragged along with A because there is a much longer contact zone between the metal in the letter and that in the substrate. The result is that there is a tendency for the metal in the stem of the letter to flow along with the metal below, forming a separation zone, D.
Looking down on the letter we see that whereas the metal in the stem of the T is able to follow the overall flow, that in the serifs cannot. There result of this differential plastic flow is that the foot of the T develops a curvature.


Differential Plastic Flow 


From the foregoing explanation, it would seem that the development of curved-base lettering occurs during coining. There are some difficulties with such a conclusion.
Firstly, it requires that there be an outward flow of metal during a strike. That can only happen if the blank is significantly smaller in diameter than the finished coin or if there are significant relief features around the edge of the coin to be filled by the strike. It is unlikely that small-diameter blanks were ever used; the rims would be obliterated during the early part of the strike and then have to be re-formed by plastic flow during the latter part of the strike. The only relief feature on the rim which needs to be formed is the beading and the volume of metal therein doesn't seem to be great enough to provide sufficient flow to induce much curvature in the letter bases. (That is a purely intuitive assessment and subject to correction.)
Secondly, it is not clear that the strain lines observed on high-grade coins were actually formed by plastic flow during the strike. Close examination of the strain lines suggests that they may really be imprints from the die!



What really happened

The whole differential plastic flow conjecture hinges on the outward migration of metal during a strike. While there is probably some outward flow as rim beads are formed, it is (intuitively) unlikely that there would be sufficient flow to account for the degree of curvature exhibited in some coins (e.g. the 1931 florin shown earlier). Whereas some curvature may occur during a coining strike, it is likely that the principal cause lies elsewhere. Dies are made from punches (hobs) in a hobbing press and punches are made from dies.
Suppose you want to duplicate a master die. You use it to create a punch and then use the punch to create another die. This process is quite common and is frequently used to change the date on a coin.
Take a 1933 punch, carefully shave off the last 3, make a new "193_" die and then hand-punch or engrave a 4 onto it to make a 1934 die. What happens in a hobbing press is significantly different from what happens in a coining press. The pressure is greater (because the metal is steel rather than one of the softer coin alloys), the strike is very much slower, and there is no collar to prevent spreading. The following diagrams are taken from page 163 of Cooper, Denis R., The Art and Craft of Coinmaking - A History of Minting Technology, (Spink & Son, London, 1988).


Stages in the hobbing of a working die 
Stages in the hobbing of a working die 


First blow impression.
1. First blow impression. 


2. Annealing 


Second blow impression.
3. Second blow impression. 


 Frustum formed.
 4. Frustum formed.


5. Re-annealing.


Third and final blow.
Third and final blow. 


Now we come to the end of the matter. The pressing of a punch from a die is analogous to the striking of a coin except that there is ample opportunity for unconstrained radial plastic flow.
In conjunction with the serifs and other limiting features, the opportunity for differential plastic flow arises, generating curvature of letter bases and similar deformations in the process.


Further investigation

The single most interesting thing to do now would be to inspect any applicable dies and punches to determine whether they show curved-base lettering. Another thing to check would be the existence or absence of strain lines near the periphery of the tool. Another interesting study would be to try to correlate the curvature with die production but that is probably better left until after any inspection. Should the dies and punches for a year in which the coins exhibit curved-base lettering show no evidence of such letter forms then it would strongly indicate that curvature arises during coining.