1920 Dots – A Revised Theory of Mint Marks

Edit 21 May 23.

From Mike Diamond of Error-ref.com via email:

“The Rockwell hardness tester wasn’t patented until 1919.  It’s highly unlikely that the Australian Mint was using one in 1920.  I don’t know what is responsible for the raised dots.  Evidently something was being pressed into the die face. Another type of hardness test?”


Metallurgy has been the major attraction for this author and dealer in the collection of pre-decimal currency; as opposed to other collectables like banknotes and stamps. Like many other collectors, Dr Paul Holland has been a source of understanding in early Australian pre-decimal bronze mintages in various JNAA essays. Arguably the most significant writing of Dr Holland on early Australian bronze is to be found in Vol.28 (1). Following discussion and research on topics of metallurgy, this essay suggests that the various dots and punches found on 1919 and 1920 pennies are simply metallurgy hardness tests. The question that follows from this likelihood is what determination (if any) was made to send certain dies to the older gold presses at the Sydney Mint?

A Brief Overview

The link to Holland’s article is worth following up in better understanding this article. On page 42, Holland writes that “to the author, this suggests the dots had a more transient utility… as identifying markers… for internal use by the mint” than for the general public as those dots “are typically small and easily obscured”. This author is agreed with that observation on the following premises:

  • The Treasury issued the directive that mint marks were not to be used

  • Melbourne Mint staff had no experience in manufacturing and hardening die steel

I suggest that where previous research has potentially drawn erroneous conclusions is by determining the placement of dots to be marks to illustrate which mint produced what. Logic suggests that there would be no need for such internal markers. Sydney Mint was a gold minting operation, with old presses that were insufficient for striking hard bronze alloy. Like the Trial of the Pyx (see previous half sovereign research by this author), a random sample of output would have sufficed to inform Melbourne Mint staff on Sydney Mint’s final product. So why the dots?

Enter the Engineer

It was in private conversation with Fred Lever (2) that the suggestion of Rockwell hardness testing marks arose in relation to the dots. Fred writes that he is an engineer. Dr Paul Holland writes that he is a scientist. Dr Holland’s study of hydrodynamic flow of metal during hypervelocity impact is crucial to assist in understanding early Australian efforts in manufacturing hubs and dies. Fred however metaphorically placed me on the floor of a metal workshop inexperienced in the manufacturing and hardening of steel. This led me to explore the techniques and processes for determining metal hardness in steel manufacturing processes.

These efforts to better understand metallurgy come about from previous research that led to the publication of 1882 Melbourne, The Sixth Head Half Sovereign (3), which led to Marsh’s The Gold Sovereign 2021 ed. recognising 1882M as a unique sixth head half sovereign. The Royal Mint was experiencing the same difficulties as Melbourne Mint was to appreciate 70 years later. William Wyon, Chief Engraver of the Royal Mint, gave evidence before the Royal Mint Commission in 1848 saying: “with respect to the dies, every die is perfected by the graver… and in fact made an original before it is hardened; so that, in case of a failure of the original matrix , a die could be converted… and used… to obtain puncheons from it”. (4, p.26).

While the hand engraving of each individual die of unknown tensile strength is a salutary process that produced incredible numismatic outcomes – particularly in Australian gold half sovereigns –  millions of pennies required for the growing post-war economy necessitated modern manufacturing processes.

Enter the Rockwell Hardness Tester

The types of dots visible on 1919 and 1920 pennies could not be correlated to the Rockwell tester UNTIL I saw the following Youtube video today. Click on the image below to watch the video in another tab. 

The lecturer in the video is – per his website – an instructor of mechanical design and inspection at Hudson Valley Community College in Troy New York (https://deanodell.com). In the above image we can see the diamond tipped indenter on the left for harder materials and the ball bearing tipped indenter for accurate readings of softer metals. It was after 4:00 minutes in the video that the ‘penny dropped’ (pun intended) on the various dots.

The lecturer states that using the diamond tipped indenter in a given testing mode (mode A) may lead to further tests in mode C with the ball bearing tester for greater accuracy. In paraphrasing Dr Holland, Occam’s razor suggests that the simplest explanation likely accounts for the facts and I believe we can finally address 1919 & 1920 pennies. It was the combination of indent markings that could not be adequately explained until now. 

One Dot at a Time Ralph

Here’s a handful of images (courtesy of PCGS) used to illustrate various dots. From top left to bottom right: 1. 1920 dot over bottom scroll (DOBS) 2. 1920 dot under bottom scroll (DUBS) 3. 1919 lower dot of double dot 4. 1919 DUBS 5. 1920 dot above top scroll (DATS) 6. 1920 DUBS 7 & 8. 1920 double dot. 

So what can we deduce from the Rockwell Tester and processes likely to be in place on a shop floor like Melbourne Mint? The Rockwell tester comes with various indenters – extremely hard conical cone and various ball bearing sizes. A glance at the above images suggest the hard point is indented on coins 1, 2 and possibly 6. A larger ball bearing test may have been conducted on coin 3. Smaller ball bearing tests perhaps on coins 4, 5, 7 & 8. Click on this link to see what conical ball bearing sizes are available for modern testing. A closer look at the historical indenters available in the early 20th Century will perhaps elucidate on the matter. 

As the lecturer warned in the video, accurate baseline testing is necessary, along with appropriate test settings and rear pendular weights to ensure accuracy in the final result. It takes little imagination to suggest that the huge learning curve in steel hardening, skillset of mint workers and apprentices tasked with this measuring and use (or misuse) of various testing indenters over these two years gave rise to the indents collectors appreciate today. One very important point to remember – once an indent has been made by the tester, no further testing can be conducted on that same point. A second indent must be made elsewhere! Helloooo double dots.

Did an unstable placement of the die on the testing anvil result in slippage that created the egg shaped indents seen on a couple of dies? It seems plausible. A dot above top scroll might have looked like the bottom scroll as the die was centered on the testing anvil. It’s easy to imagine in the time pressed environment that Melbourne Mint staff were required to work in. Given that DATS coins are Melbourne minted, it would appear to be nothing more than a worker error. To know more about the creation of said dies could help identify why each individual coin was put through such a hardness test.

That Raises the Next Question

It’s the question that drives me – if hardness tests were being strategically placed on the dies at different locations, were they simply serving as reference marks or was Melbourne Mint manufacturing and hardening various types of steel to different tensile qualities? I suspect clues are to be found in other literature – possibly written by Holland – as I recall reading that 1930 and 1931 was a time of further experimentation at the Melbourne Mint. The collapse in Treasury demand for coinage led to Melbourne Mint testing various steels for the manufacturing of dies. That is literature I will have to chase down again and see what inferences can be made from another great period of numismatic variety. 

Not the Final Word

This long overdue post is simply the starter on a subject that has long held my attention. The suggestion that the marks simply illustrate engineering processes is not a difficult one to make. It’s whether the marks were for identification purposes or did the older presses at Sydney Mint require a different quality of die from the more modern Melbourne Mint presses? Sydney pushed its old dies on Perth Mint following a decision to discontinue bronze minting; yet Perth Mint was also equipped for the pressing of softer gold coins. Is there a technical reason for the passage of Sydney dies to Perth or was it simple expediency? Probably the latter but I will look into the matter. 

I was surprised to find a Facebook group centered around a very experienced American numismatist still producing reproduction dies. One can see in that FB post illustrated above the complexities and decisions that go into making die steel. I might have to request access to Melbourne Mint records to be able to further my own research into steels and hardness outcomes sought by Melbourne Mint. The recollection of failed dies in 1931 (1931 Indian obverse dropped 1!) suggests that even ten years later, mint staff knowledge of die steel was still a work in progress. Perhaps it might be possible to write a technical essay on die steel manufacturing from the 1920’s through 1930’s one day. I do know from the XRF scanner in my office that the classic bronze alloy mix of 97.5/2.0/.5  CU/SN/ZN doesn’t stabilise until after World War II. It’d be nice to have a readable book explaining the reasons for such metal variety throughout that period. 

This post is written with gratitude to Fred Lever for generously providing his time and knowledge. 

List of References:

  1. Holland, Paul – Die pairings, curved-base letters and dots: why are George V
    pennies so complex? https://numismatics.org.au/wp-content/uploads/2021/06/naa-journal-vol-28-holland.pdf

  2. Lever, Fred – Private Emails 2018 – 2023.

  3. Robinson, Leslie – 1882 Melbourne, The Sixth Head Half Sovereign – https://topendcoins.com.au/wp-content/uploads/2020/04/1882m-the-sixth-head.pdf 

  4. Fearon, Daniel – The Sovereign; The World’s Most Famous Coin; A History and Price Guide. Hillden Publications, 2001.