Intro

Metallurgy has been the major attraction for this author and dealer in the collection of pre-decimal currency. 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 with other numismatic researchers and extensive reading on topics of metallurgy, this essay suggests that the various dots and punches found on 1919 and 1920 pennies are metallurgy hardness tests. 

Pre-reading Holland

Holland’s article is suggested pre-reading to better understanding this article. On page 42, Holland (1) 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 public as those dots “are typically small and easily obscured”. This author concurs with that observation, keeping in mind that:

  • 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

It is suggested that where previous research has potentially drawn erroneous conclusions is by focusing on the placement of dots as mint markings. Australia experience in the minting of coinage 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 (2), a random sample of output would have sufficed to inform Melbourne Mint staff on Sydney Mint’s final product. So why dot these different locations? 

Fred Lever’s Engineering Mindset

This author has been collecting since 2017 and notes that the very thorough guide to 1920 pennies authored by Fred Lever and Neal Effendi (3) was uploaded to Top End Coins’s library in 2019. So it’s been about five years of communication with an engineer who did his apprenticeship in a workshop. Fred’s influence has had a fundamental role in shaping the research and outcomes arising in this essay. 

A piece of steel smashing out thousands of coins needs to be hard, but not brittle. The science of creating that type of die steel is a subject for another article. The Melbourne Mint’s very pressing requirement to make their own dies necessitated learning where the sweet spot was. Thus – as Fred pointed out – there arose a need for material testing.

Correspondence from the Royal Mint in July 1919 (3) provided instructions to Melbourne Mint staff on the requirements for manufacturing, finishing and testing dies created from the tools provided by the Royal Mint. It was previously suggested by this author that the Rockwell tester was used by Melbourne Mint staff in 1919 and 1920. This is incorrect as Mike Diamond of Error-ref.com pointed out in private correspondence; the Rockwell tester wasn’t patented until 1919.

In the Royal Mint’s provision of instructions to the Melbourne Mint, reference was made to hardness testing… “which should be about 100 on the Scleroscope test.” (3). The problem with the Scleroscope is that the evidence doesn’t back up what is essentially a non-destructive ‘bounce’ test. Many dots plague the 1919 and 1920 pennies reverse dies, indicative of spherical indentation. The Vickers and Rockwell tests do not yet exist in commercial production; this leaves the Brinell hardness tester as likely author of many dots.

Briggs & Confirmation in Literature

Dr David Briggs has studied and published extensively on Australian pre-decimal coinage at the Melbourne Mint. He has supplied information in correspondence that medal and token maker Stokes & Son were assisting the Melbourne Mint in their early endeavours. The Brinell tester was in circulation at the time; Stokes & Son had the skilled workers to provide knowledge and testing equipment to the Melbourne Mint. We can have greater confidence in suggesting that a number of 1919 and 1920 reverse penny dies were struck with a Brinell Hardness Tester.

What is hardness? “Hardness is a characteristic of a material, not a fundamental physical property. It is defined as the resistance to indentation, and it is determined by measuring the permanent depth of the indentation. More simply put, when using a fixed force (load) and a given indenter, the smaller the indentation, the harder the material. Indentation hardness value is obtained by measuring the depth or the area of the indentation.” (5)

The simple schematic below (courtesy of Researchgate.net) is sufficient for this article in illustrating the nature of this test. Further reading can be found in Tabor’s Hardness of Metals (6), archived online.

 

Where things got interesting in Tabor’s work is on page 15 of the book. The Brinell test creates deformations. They are illustrated below and termed ‘piling-up’ and ‘sinking-in’.

That piling-up deformation above is the indenter ball pressing into the die metal, creating a rim. When that deformed die strikes a planchet, it will create the opposition impression – a slightly raised dot with a moat around it. Below is what that looks like on a coin – the dot above scroll mark placement on a 1920 double dot reverse.

Image 2 – 1920 double dot penny, upper dot. The test mark looks a lot better than the lower dot.

1920 Double Dots – When 2 Mint Marks Were Made

The dot below scroll (image 3) on this 1920 double dot displays an extensive moat around the dot. The deformation is significantly larger than on the upper dot (image 2). What’s the likelihood that the dot below was struck and deemed inconclusive or alternatively, issues were identified with the steel? Even to this author’s inexperienced eyes that moat appears to be too large and erratic, relative to other testing marks. It is suggested that this is the reason a single double dot die exists in Australian bronze.

A comparison of the images above and below is suggestive that – while the marks may have been strategically placed, they are testing marks, not mint marks. More importantly, once a test has been conducted, if found inconclusive, another location must be used to conduct the test a 2nd time. Hello double dot! 

Image 3 – 1920 double dot penny, lower dot with non-concave and extensive moat around the testing mark.

Perhaps an apprentice or inexperienced worker that made a hash of it and was ordered to repeat the test? Alternatively, perhaps the deformation may have led the shopfloor boss to question the die’s tensile strength and a second test was conducted? Tabor (6 p.14) writes of technical difficulties in hardness testing that was researched and corrected some years after Melbourne Mint had conducted their first hardness tests. One could anticipate learning curves that required additional steps be taken, like a second hardness test on the die face. Note similar deformations on the upper AND lower dot of the 1919 double dot penny (image 4).

Image 4 – another wonky hardness test above the scroll on this 1919 double dot penny. Lower dot looks a bit better.

1920 Double Dot – Other Potential Reasons for these Mint Marks

Some emails sent to other numismatists over the 2023 Christmas period follow. Note the highlighted text emphasising changes made to alloys over the period, including 1919.

Email 1:

Hi Neal,

You mentioned Dave Briggs memory of an alloy change in 1921, which you believed was in 1922. I believe I have an answer and further avenue of research on this subject. 

… just started XRFing the 1922 pennies again and noted the 511 Phosphor Bronze analysis with the addition of Nickel. 

I just received a stack of 1920 to further the research there and hit a few of them with the XRF for a 510 Phosphor Bronze result – no nickel. 

…have a small stack of 1921 pennies, some showing Nickel and others the same alloy as 1920. I’ll see if I can determine which mint added the nickel as I believe the stretched deformed rims in 1921 is not Melbourne’s handiwork. That might permit to differentiate suppliers or a changing recipe from a single supplier.

The small stack of 22’s that have dropped the Phosphorus and switched to Cu/Sn/Zn show no discernible difference in date. This date spread is something I will have to familiarise myself with later. 

Two high grade 1916’s in my possession return 510 phosphor bronze readings, BUT with the addition of Nickel. 

The single 1920 double dot not slabbed in my possession shows the addition of nickel… I wasn’t expecting that. Therefore, I will need to find some more double dots to test. (Subsequent testing of two additional 1920 double dots confirmed the different alloy)

Email 2:

Pulled out a bag of 30 odd 1919 pennies for a quick squiz. See attached image. The pile on the left is Phosphor bronze with trace Nickel. Pile on the right is Cu/Sn/Zn with Ni. 

Three on left in foreground are high grade, which I’ve been using for strike characteristics. Those three also show signs of rusting, probably rusty dies. 

Thus, a rough and ready guide of 10-15% of total 1919 population is showing an alloy that was apparently mandated after 1919. That alloy is comparable to the divergence witnessed in 1922, when Phosphor bronze with Ni ceases. 

Email 3:

Afternoon,

An unusual (actually unique) alloy find this afternoon – a 1920 1st Indian Obverse penny with a Cu/Sn/Zn alloy mix. Note the slightly unusual colour.  

Previous analysis showed 1921 pennies switching to this alloy – approx 40% of the coins in my possession. 1919 is also infrequently found in this alloy. I suspect Perth Mint, but am wondering if MM tested some bars of alternate alloy at some point?

Neal – could you provide me with the written communications from the RM to the MM concerning suggested/mandated alloy changes please?

Effendi Providing a Valuable Clue

To make this exercise more challenging, Neal Effendi showed me an apparently London supplied 1920 obverse die delivered sometime after Calcutta first supplied their initial tools and dies. The 1st Calcutta supplied master obverse die shows the King’s crown with the 4th bead that is incomplete and joined by a line to the 3rd bead (image 5, left side). 1921/22 Indian Obverse pennies (image 5, right side) do not display this beading defect. Thus it is suggested that Sydney Mint did not supply Perth Mint with surplus obverse dies. Sharples (4) did not differentiate obverse dies when he wrote his article; the minor die identification flaw marking was likely overlooked. Stay tuned for a future article. 

Image 5 – Indian Obverse die with pearl defect above. Below, another supplied Indian obverse die with full pearls.

Discussion

The literature and email texts above illustrate a period of experimentation in alloys by Melbourne mint and potentially Perth. This is quite understandable, given the learning curve faced by mint personnel in the manufacturing of steel dies and the modern minting of coins. All this is taking place in the expansive requirements for low value currency post-WWI and in the anticipated changeover to square nickel-based coinage which never eventuated. 

It’s also useful to remember that state governments are paying for the minting of these coins while the Commonwealth pocketed the seigniorage. So while a silicon bronze alloy might be optimal from a quality control perspective (a single example has been brought to light by XRF analysis to-date), the Mint Master would be responding to state treasury costings. Exotic, expensive alloys are likely to be extremely rare.

That’s conceivably one of the reasons the much quoted 97.5/2/0.5% Cu/Zn/Sn alloy in guide books doesn’t materialise until after World War Two. Phosphor bronze (Cu/Sn/P) appears to be the common offering or preferred alloy in the early stages of Australian bronze manufacturing. More research is required to understand why.

Further research is also required on the die steels produced throughout the period. Andrew Crellin has made mention of it in emails. Teasing apart the mint marks as testing marks placed strategically on the die face to differentiate varying alloys and/or die steel is something requiring further investigation.

Another potential reason for different Sydney and Melbourne die markers was to ensure the correctly fitted dies for the older presses at Sydney were easy to identify, following annealing and hardening. Recall that the Calcutta Mint was unaware of coining press details (for Sydney?) so sent dies unhardened (1). A look at the fittings for the various Taylor & Challen presses exported to Australia would be a useful exercise. The author does not believe that all coins show evidence of hardness testing. It is suggested that the dies were simply punched for correct handling to the appropriate mint. This will be the subject of a future article.

The Next Article? – FBL vs CBL

Neal Effendi made a pertinent and interesting commentary in recent missives. Because the Bombay branch of the Royal Mint had delivered 6 pairs of hardened 1920 penny dies ready for use, Neal has been looking for 6 No Dot (ND) pennies distinguishable by die characteristics. The easiest of die characteristics to distinguish a pre-hardened die is flat based letters (FBL). The coins from an FBL die look different from the cloned dies that have curved based letters (CBL). That led this author to take a closer look at the most notorious of dotted 1920 bronze pennies and another article is born. 

Initial Conclusions and further Research

The literature suggests that some dots on the 1919 and 1920 pennies were likely metal hardness testing. An initial – and superficial – analysis of various dated pennies tentatively suggest that the various placed dots denote different alloys. It is conceivable that mint staff may have placed those test marks to monitor circulation of the various alloys used in circulating coins. What those changes in alloy mean is research for a future essay.

A larger scale analysis of hundreds of coins from 1919 to 1922 or maybe 1923 is necessary. This will permit the author to tease out the changes in alloy and determine what role (if any) those dots played as marker of those different alloys. The availability of early bronze die steels for XRF analysis would be of great assistance in furthering the role that die steel played in metal hardness testing. 

References & Further Reading

  1. Holland, Paul. Die pairings, curved-base letters and dots: why are George V pennies so complex? https://numismatics.org.au/naa-journal/2017/
  2. Robinson, Leslie. 1882 Melbourne Half Sovereign – The Sixth Head. https://topendcoins.com.au/wp-content/uploads/2020/04/1882m-the-sixth-head.pdf
  3. Effendi, Neal & Lever, Fred 2015. An Examination of the Australian 1920 Penny. https://topendcoins.com.au/wp-content/uploads/2019/01/1920-complete-story.pdf
  4. Sharples, John. Penny Reverse Master Dies of George V: https://numismatics.org.au/wp-content/uploads/2021/06/Vol-6-Article-3.pdf
  5. Brinell Hardness Testing. https://www.hardnesstesters.com/test-types/brinell-hardness-testing
  6. Tabor, D 1951. The Hardness of Metals. Cambridge. https://archive.org/details/TaborHardnessOfMetals/page/n1/mode/2up