Coloured Perth Pennies – A tentatively definitive Answer

Intro

After years of hypothesises and conjecture, a couple of pieces of key evidence suggest that a cement mixer is responsible for those colourful Perth Mint bronzes. That’s not a punch line. The Perth Mint records clearly indicate a cement mixer was purchased to acid rinse planchets prior to striking. Sulfuric acid is the probable agent. This author has purchased PCGS no grade Australian bronze and compared to dozens of high-grade half pennies. The visual similarities suggest that the acid rinse is the likely culprit of Perth Mint rainbow toned bronze.

Colour as key

Coins and Banknote (2022) published the initial foray into the role colour plays in determining alloy in Australian pre-decimal bronze (see references & further reading). For this essay, the introductory elements will be skipped for concrete examples of how understanding colour can improve perception and arbitrage in collecting.

Image 1 above shows a PCGS image of a 1917 Calcutta penny, most certainly made of phosphor bronze. Phosphor bronze is about 95% copper (Cu), 4.5% tin (Sn) and a smidgen of phosphor (P). The coin on the right is PCGS Genuine UNC Details (91 – Questionable Colour). The link to the still active PCGS cert can be found in further reading. It sat in my office for several years until I cracked the slab, rinsed it in acetone and then left it for a couple of years to naturally tone up.

Time and oxygen did the rest. Whatever impeded the natural toning of the coin gave way to thin film interference – aka toning. The coin subsequently graded mint state 62. I knew the coin was phosphor bronze. PCGS provided examples of high-grade phosphor bronze pennies like the above. What it took me some years to understand is that someone (at the Calcutta Mint?) impeded oxidation of that 1916 penny. Once I understood the role of ‘stable patination’ (Google it) in metal preservation, I naturally had to experiment. Hypothesis confirmed in this case.

Image 2 above illustrates the plethora of colour evident in the various alloys of 1943 Melbourne half pennies. The black background was one of several No Grade Questionable Colour half pennies purchased from an American Ebay seller. I was curious as to why a US dealer was holding a stock of non-graded Australian coins returned in those large “Do Not Grade” SAFLIPS, used when PCGS doesn’t want to touch the coin with a bargepole.

I am quite confident in suggesting that coin no. 3 above has been subject to an acid wash, probably sulfuric. The reasoning will be provided later in this essay. It’s the range of colours here that are indicative of changing levels of alloy in the coin. Tin (Sn) and zinc (Zn) are lighter coloured metals. Increasing quantities of those two metals versus copper results in lighter colour composition in the respective alloy. As for the acid dipped coin, note that PCGS specifically mentions loss of lustre due to dipping as a reason to provide a No Grade result. The coin has a dull surface, which is visible in the comparative image.

It’s rare that I see dipped silver coins suffering such surface damage, but bronze coins are significantly easier to perceive after years of observation. I will take this opportunity to correct one rookie error made in that 2022 CAB article. The No Grade 1950 half penny above right is suffering from an acid wash and the dull surface is compared to the Borg pedigree on the left. Post mint attempts at acid washing/dipping bronze typically ends in visible damage, with loss of lustre being the most obvious sign. Caveat Emptor.

I need to take readers through the personally learned experiences of sulphur, because the chemistry creates different outcomes. Above is a Centenary Florin recently photographed in a collection of 30+ commemorative florins. What I noted when assembling the image after was the dull surfaces (right) – it stood out in contrast to many other polished finishes that came up nicely when photographed with ambient light. A check of the PCGS certificate shows the coin at time of grading (above left); absolutely stunning! Susan Maltby – a Canadian conservationist I’ve contacted over the years, has more to say on adsorbed thiourea and re-corrosion (3). As an aside, research in conservation offers some avenues of research I’d like to pursue in reducing the re-corrosion issue (4).

I had to laugh when Rob Ensleigh and Eric Eigner asked if this 1937 Crown was my handiwork at the Epping Fair. Why yes, it is! However, this spectacular outcome is notably different to most experiments in one regard. I’d just moved from the NT to Newcastle region in April 2024 when we had that spectacular rainfall that caused so much inundation and damage up and down NSW. This coin had been forgotten about under the house and was subjected to uncontrolled, high humidity levels for about six months.  I was pleasantly surprised by the results. It also hints at what sulphur can do under certain conditions.

Natural vs Artificial Toning

Follow me on Top End Coins and you’ll be shown coins that are suspected of being artificially toned, like the 1958 Perth proof example below. Lollipop rainbow colours are mythical unicorns in late period bronze. The chart on the right from the rocket scientist (2) – illustrating by mathematics the optical perception of colour created by depth of patination is ordered from top to bottom by way of procession for natural colouring. Let’s compare with an example in my possession further below – the 1958 Melbourne proof penny.

It’s probably this 1958M proof penny (below) that started my research into colour. Below left is the coin imaged in 2021. The peachy orange and ruddy brown (layers A-E on the Sunnywood chart) mottled with blue on the high points (F) are in 2023 being surpassed by cyan and mint green (G-H), especially on the obverse. That is natural progression, with atomic bonding beginning in the high points and steps (the ridges in the rim, portrait, lettering etc). Remember that the chart above was created for silver Morgan dollars. The colours on bronze are a little different but the physics of toning remain the same.

Note two years later (above right) that the 2^nd cycle yellows are advancing. There are no oxidation burns on the coin below either; highly suggestive of direct contact with sulphur in the  58 Perth PCGS example illustrated above. The difference between the 1937 crown and this 1958M penny is that natural toning is slow, methodical and follows a set order of colours. The 37 Crown is a riot of colour! The lollipop 58 Perth penny has colours out of sync. Sunset yellow and orange (J-K, middle 2^nd cycle) have preceded the cyan and mint green. I suspect controlled, forced toning with sulphur. If someone can prove me wrong then I will happily acknowledge the error in a future article.

Damage or Enhancement by Sulphur?

Let’s drill into a layman’s explanation of the chemistry of corrosion. Canadian conservationists are one of the better references I’ve read to understand the chemistry of brass (5) and conservation of metals (6). I encourage collectors to read article 5 because the 1943M half penny I pointed to above (black background in a line of suspects) is 96.7% Cu and 3.2% Zn – aka brass! Necessity is the mother of invention and it appears that tin was scarce or diverted to other efforts in the war. The article has good illustrations of dezincification, which brings out the salmon colour of copper in the alloy, very similar to the PCGS No Grade 1964 half penny below. Note that the surface lustre has been stripped on this coin; the dull surface on the reverse is easily seen in this image.

Among recently gifted half pennies was the following example and one of the rare occasions I perceived the effects of sulphur on the obverse. The salmon pink is the result of alloy reacting with sulphur – drawing out the copper – but the lustre indicates that mint processes likely produced this outcome. From Kurz:

“The thin film we are talking about here is a layer of molecular compounds that form naturally at the coin’s surface over time. Silver coins are typically struck from an alloy of composition 90% silver – 10% copper. Both silver and copper are somewhat reactive with both oxygen and sulfur. As a result, under natural storage conditions (unless the coin is stored in a vacuum or a completely inert environment such as a helium or argon atmosphere), a layer of “patina” or “toning” will form at the surface of the coin, comprising molecules of silver sulfide, copper oxide, etc.”

Sulphur in the Minting Process

I’ve read numerous articles on metallurgy; finding information specific to copper is more difficult. Gowland (7) mentions in the chemical properties of copper (p.47) that “when heated to redness with access of air, as in annealing sheets, etc., a dark coloured scale is formed, which consists almost wholly of cuprous oxide. It may be removed by plunging the copper whilst red hot into cold water; in practice the water used contains sulphuric acid.” University of Wollongong PhD candidate H. Yasbandha provides an excellent overview of the preparation of coin blanks at the Royal Australian Mint in the image below and thesis in further reading (8).

It was Gary Tate who provided me the following screenshot from the digitised 1952 Perth Mint report. It was a “gotcha” moment; incidentally some of the finest rainbow pennies to pass through my hands are dated 1951 and 1952. Further research into the outcomes determined by the Mint Master and continuation or changes in practice over the final 12 years of the Perth Mint would make for useful reading. Some sort of acid wash was applied until Perth ceased minting bronze as seen in the 1961 half penny illustrated earlier.

We can discern the likely nature of the wash by the black marks visible on the otherwise colourful half penny. Even a very dilute sulphur rinse is sufficient to seed the surface of the metal. If you’ve looked at Sunnywood’s colour classification chart (2), you’ll see glossy black and dull black are terminal references for toned coins. The spots likely represent the incompletely dried remains of planchet rinse. They have subsequently seeded the coin’s surface and passed through the various colours until thickening as black marks as visible today. That is to say, the thickness of the patination is sufficient to absorb light, rather than reflect it. Note also that the black marks are erratic, unlike the black dots on the (probably) artificially toned 1958 Perth Penny. Those proof planchets would NOT have been tumble dried in a concrete mixer!

References and Further Reading

1. https://topendcoins.com.au/wp-content/uploads/2021/03/Australian_Bronze_2.pdf
2. http://www.jhonecash.com/research/sunnywood_classification.asp
3. https://www.coinworld.com/news/us-coins/conservator-continues-review-of-coin-cleaning-survey.html
4. https://www.tandfonline.com/doi/full/10.1080/00393630.2020.1773056
5. https://www.canada.ca/en/conservation-institute/services/conservation-preservation-publications/canadian-conservation-institute-notes/dezincification-brass.html
6. https://www.canada.ca/en/conservation-institute/services/conservation-preservation-publications/canadian-conservation-institute-notes.html
7. Gowland, W 1914, The Metallurgy of The Non-Ferrous metals. Charles Griffin and Company LTD.
8. Yasbandha, H. 2001, Surface Engineering of Coinage Dies. University of Wollongong Thesis Collection: https://ro.uow.edu.au/articles/thesis/Surface_engineering_of_coinage_dies/27646125?file=50347077