1920 Dots – Mint Marks Part Two


In the first part of this article, all the previous literature offering convoluted theories of mint marks were ignored. In its place, a focus on the metallurgy and manufacturing of dies in a workshop unfamiliar with said processes was made. Evidence has come to light that supports this approach. Part two offers an updated analysis, backed up with some promising literature on this specialised subject and some surprising XRF analysis results.

Why Test the Metal?

A piece of steel smashing out thousands of coins needs to be hard, but not brittle. The science of creating that particular 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 the need for material testing.

Correspondence from the Royal Mint in July 1919 provided instructions to Melbourne Mint staff on the requirements for manufacturing, finishing and testing dies created from the tools provided by the Royal Mint. In part one, this author suggested that the Rockwell tester was used in 1919 and 1920. This is incorrect – the Rockwell tester was not yet commercially available.

Mint Marks Made by the Scleroscope Hardness Tester?

In locating correspondence from the Royal Mint in JNAA Vol. 6, reference is made to hardness testing… “which should be about 100 on the Scleroscope test.” The problem with the Scleroscope is that the evidence doesn’t back up what is 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.

1920 Dots Likely Made by the Brinell Hardness Tester

Experienced numismatic researcher Neil Effendi asked my thoughts on the Royal Mint suggesting a Scleroscope test, yet Melbourne Mint appeared to have used a spherical indenter instead. Unless we can access Melbourne Mint records or the Mint Master’s diary and find written evidence of their intent, I find it more productive to look at what the evidence suggests. edit: Dr David Briggs has studied and published extensively on Australian pre-decimal coinage at the Melbourne Mint. He has supplied information 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 materials 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.

From a specialist in material testing: “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.”

The simple schematic below is sufficient for this article in illustrating the nature of this test. Further reading can be had in Tabor (1951) “The Hardness Of Metals”, archived online.

Image 1. Ball indentation on left, machine schematic on the right (courtesy Researchgate.net)

Image 1. Ball indentation on left, Brinell hardness testing machine schematic on the right (courtesy Researchgate.net)

1920 Mint Marks Straight out of the Literature

I want to cut straight to the chase. 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’.

Image 2. Illustration of indentor deformation resulting from hardness testing. From D. Tabor’s “The Hardness of Metals”

That piling-up deformation above is the indenter ball pressing into the die metal, creating a rim. When the 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 in a 1920 double dot reverse.

1920 Double Dots – When 2 Mint Marks Were Made

The correlation between the two images is suggestive that – while the marks may have been strategically placed – they are testing marks, not mint marks. In the case of a double dot reverse, once a test has been conducted, if found inconclusive, another location must be used to conduct the test a 2nd time.

Image 3. An inverse image of the deformation "piling up" illusrated in Image 2.

Image 3. An inverse image of the deformation “piling up” illusrated in Image 2. Double dot 1920 penny.

Similarly, the dot below scroll on this 1920 double dot displays an extensive moat around the dot. The deformation is significantly larger on this dot. What’s the likelihood that the dot below was struck and deemed inconclusive? Even to this author’s inexperienced eyes that moat appears to be too large.

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? That’s so much easier than trying to imagine this was a special mint mark or an incorrectly placed mark.

Image 4. An inverse image of the deformation "piling up" illustrated in Image 2. Note the large, irregular moat.

Image 4. An inverse image of the deformation “piling up” illustrated in Image 2. Note the large, irregular moat. Double dot 1920 penny.

1920 Double Dots – Other Potential Reasons for these Mint Marks

Time constraints require this author to cut and paste some emails sent to other numismatists over the Christmas period. Note the highlighted text illustrating 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 different 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 discernable 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. 

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. 

1920 Dots – Mint Marks Part Three TBC

What needs to be followed up on to wrap up this subject:

  1. A complete XRF analysis of pennies from 1910 through 1932. There is clearly chop and change in what the various mints choose to alloy as a planchet. These choices do not adhere to Royal Mint requirements. This will be another blog post subject in its own right.
  2. An understanding of what these various alloys mean in metallurgy. For example, modern Nickel alloy producers advertise the “malleability” and “toughness” of Nickel. This needs to be put in context of the requirements and output of the various mints during the period being discussed. This author’s interest in metallurgy will be another subject for a separate post.
  3. Confirmation of experimentation in die steel production. Andrew Crellin has made mention of it in the email chain over the Christmas period. Teasing apart the mint marks as testing marks placed strategically on the die face to differentiate varying alloys and varying steel types is something requiring further investigation.
  4. Written confirmation from Melbourne Mint and Royal Mint communications. While researchers like Neal Effendi and Dr David Briggs provide a level of confidence in quoting, nothing beats source material.

The points above will – given TEC’s intention to move to NSW shortly – take time to realise. Thus the reader may be waiting awhile for the sequel to this story. Nonetheless, it is suggested here that metallurgy drives the so-called Mint Mark Dots saga. The various alloys that are found between mintages AND in single dates; along with the study of strike characteristics posted in other blog posts is arguably a promising avenue of research. It will hopefully clarify the nature of the mint marks and permit collectors and dealers to accurately identify the sources of various mintages in the future.