Coin Conservation versus Intervention


Living in a world of ideas, generated by curiosity, has its advantages and disadvantages. There’s no difference in how I’ve approached coin collecting, numismatic research and notions of doctoring, restoring and conserving said coins. Sometimes I need to be brought back on track with an application of logic and reflections on processes and results obtained. This article is intended to do exactly that. I have Susan Maltby, an art conservator and regular contributor to Coinworld to thank for seeding an article I knew I needed to write. This article is focusing more on the steps of intervention that have occured in recent years, leading to the superficial damage of a bronze coin recently; the 1919 penny imaged above.

The Importance of a Library

Top End Coins has made some inroads into historical aspects of Australian pre-decimal currency production; aspects of metallurgy and hardness testing being the most recent article of research. The results come about from connecting dots; the dots are articles and research accumulated in TEC’s library and then those thoughts swirl around in my head until a potential arises; which is then followed up by putting pen to paper. Tempering intellectual arrogance with some feedback from fellow researchers, numismatists and experts in other fields then helps to confirm that I am at least on the right path. I am thankful to Fred Lever, Neal Effendi, Andrew Crellin and others in this regard.

At the risk of starring in my own Monty Python skit, I realised that I needed to start accumulating fact based evidence in the field of coin conservation and intervention. So I reached out to Susan Maltby. Readers may not recognise the name, but anyone that has ever sought out conservation information on the net has likely read her articles on I am wary of the ephemeral nature of the internet as – for example – JNAA recently displaced its website and some article links in the library are no longer valid. For this reason I have begun another chapter in The TEC Library encompassing Restoration and Preservation. There you’ll find a number of articles in PDF downloadable form, with the original URL accompanying. I’ll continue to add to the library collection as time permits.

Coin Conservation versus Intervention

Google located Susan on Linkedin and I made the leap and joined up (I’m so over Facebook to the point that I have also joined X). Susan quickly put me on notice that her Coin World articles focus on “preventative conservation”, not “intervention” (thus the title “coin conversation versus intervention”). That led me to reflect on what I have been doing with coins and what did I want to accomplish with various interventions that I have experimented with in recent years. Susan replied to my mention of alkaline substances by querying whether its use was in relation to paper. I can see from a web search that the biggest consumer of Sodium Hydroxide includes the pulp and paper industry, but that is certainly not the case here.

The Road to Numismatic Hell is paved with Coin Interventions

The results of various experiments would take an entire blog post to catalogue. So this brief overview will be followed up with exactly that in due course. However, I wanted to share one of the earliest concrete examples of a coin intervention. I was trying to remember how I got on this path and then recalled this auction purchase of 2021 (below). Described as a lightly toned EF Centenary Florin, I encountered my first real example of PVC damage.

The coin looked pretty ordinary in the catalogue, the green splotchy films were evidence of PVC. However, the auctioneer had overlooked one important facet of the coin. It was a partially prepared proof created for export to Williams, that NZ dealer who had collector contacts globally. It is known as a type II proof-like. On the basis of that understanding I successfully bidded for the coin, warts and all.

The 1st Intervention – Silver Dip that Sucker!

The photo above (using axial lighting techniques) shows the result after initial silver dipping. The gunky green film was removed and outstanding fields and details were revealed. The only issue is glaringly visible – PVC damage that had eaten into the metal. The dipping hadn’t removed the PVC from this particular area and furthermore, rapid oxidation resulted. I likely tried a 2nd dipping and – realising that I could not reverse the damage – tucked the coin away in a drawer. By the way, Susan has covered the issue of damage that can result from acidified thirourea solutions aka silver dips. I must admit that I’ve significantly reduced the dipping of coins. I also now have a systemic method of monitoring purchased coins for evidence of previous dipping. Another article to pen in the future.

The 2nd Intervention – the Discovery of Acetone

After some months of playing around with cheap concoctions of nail polish remover from my local Chemist Warehouse, I discovered pure Acetone available at the local auto shop. The salesperson jokingly asked “You’re not using this to make meth, are you?”. My reply….


Seriously though, Breaking Bad and coin doctoring have done more to spark my interest in chemistry than high school ever did. Acetone is a highly volatile, organic solvent for the removal of gels, waxes, resins, adhesives, polishes etc. So I began soaking coins in acetone. I can attest to the removal of stick tape adhesive and other gunk However, I couldn’t satisfactorily deal with the PVC damage.

The 3rd Intervention – from Acid to Alkaline

I’d already found little use for acetic acid, which I’d thought might be more useful than hydrochloric and other acids. Acids really are out when it comes to bronze alloys. However someone reached out to me and suggested alkaloids as a cleaning agent. So I tried Sodium Hydroxide aka drain cleaner from Bunnings. One of the first experiments was on the following half penny, which I had soaked in acetone and was chipping away at the oxidation mark below the date.

Lo and behold, I realised that the whole surface was covered in grime as a toothpick started revealing (left image, below date). A quick soaking of sodium hydroxide with an ear cleaning bud just melted that layer of grime off effortlessly. I was hooked on my latest friend in chemical intervention! The PVC damaged florin was given a whirl with the alkaline cleaner, followed by a coating of Verdi-Chem and then sent off to PCGS. It came back MS66, one of the finest (if not the finest) type II proof-like variant.

Coin Restauration versus Intervention

Chipping away with acetone and a toothpick (left), before discovering alkaloid cleaning products which removed a layer of grime with ease.

When Coin Intervention becomes Damage

In the past week it was noted that a coin in the storage drawer had changed – significantly. The 1919 Penny graded AU58 was purchased in the study of a potential strike characteristic to denote Perth Mint production in the early years of Australian bronze. It came with typical grime that had been smeared over the obverse fields and stuck in the reverse legends. So this author whipped out the earbud and gave the surfaces a good cleaning up with sodium hydroxide a couple of months ago. The image below (left) is the coin just after intervention. The right image is what was noted a week ago.

Coin Conservation versus Intervention

What Happened – Theory and Literature

The outcome above has led to some reading on metal reactivity to various chemicals. From an Egyptian Engineering Journal I’ve acquired a very basic understanding of Potassium Hydroxide (KOH) and Sodium Hydroxide (NaOH) as alkaline agents of corrosion. Note in the stats below (from the article link above) the relative weight of oxygen remaining after corrosion tests for those two alkaline substances. What this indicates is that a passivation layer of copper oxide (CuO) readily re-establishes following corrosion. This coin was briefly wiped, then rinsed with tap water and then soaked in acetone to eat up the residual H2O moisture. The colour change is indicative of minute corrosion occuring to the passivation layer and underlying metal.

Coin restauration versus intervention

Click on the image to enlarge in new tab

Connecting Previous Research to Coin Conservation versus Intervention

A passivation layer is the initial oxidation of a bronze alloy surface that is invisible to the human eye (it’s very thin). The mechanics of oxidation has been previously written on by this author, as an introduction to the physics of Thin Film Interference aka the rainbow toning of Morgan Dollars and Perth Mint bronze. Alloys also play a role in the varied colours of bronze coinage in Australian pre-decimal currency.

However, this change to a lighter tan colour has me raising questions over which component of the alloy has corroded. Knowing that 1919 is Phosphor Bronze (approx. 95/5/.2 Cu/Sn/P), I have to wonder if the surface layer of Cu was adversely corroded in favour of the tin (Sn), a silver, lustrous metal. Alloy mixing errors are an ideal manner in which to observe the respective oxidation behaviours of various metals, like this 1941 penny below. More research is required to better answer this question of colour.

It is noted that high concentrations of NaOH also increase the rate of corrosion. The solution used had been sitting in a glass jar for some time; it is conceivable that the concentration of hydrogen ions (H+) have increased (with moisture loss) to create a higher pH level than what I originally mixed. It’s also worthy of note that hydrochloric acid wears down passive oxide layers (and dissolves bronze) by the combined action of hydrogen and chloride ions. This is in contrast to the alkalines, which only have the one ion species attacking the metal.

NaOH stronger at higher pH levels than KOH

Incremental Steps to Improved Coin Intevention

Finally, astute readers of this post might have noted that there is a better ‘hydroxide’ than Sodium. Potassium Hydroxide (KOH) is less corrosive to bronze alloys than NaOH, particularly in low doses. The above chart measures various concentrations of an electrolyte solution the metals were subjected to over a number of weeks. Intervening on a coin’s surface is a brief exposure by comparison. The chart below highlighted the differences in metal reactivity to various liquid agents and can be viewed by clicking on the image (page opens in new tab).


KOH is less destructive to Bronze than NaOH. Click on the image to see source page. Opens in new tab.

Coin Intervention is not Conservation

Obviously the subject of coin conservation versus intervention has not been resolved in this article. What the author is attempting to resolve is how the escalation of increasing intervention led to superficial damage of a bronze coin. While the research and writing of this article has consumed an entire day, the summary and results of processes and reactive agents used to-date are a good summary. This is the first foray into layman’s chemistry of coin intervention that will be continued.

Further areas of study will be to determine the utility of alkaline substances in the restoration of other metals, like this base metal half dollar below. The coin had been kept in a collection of circulated material, but came up nicely after a quick wipeover with NaOH. I did not take a photo immediately after intervention, so am unable to determine if the minute corrosion spots visible under axial lighting are from this intervention or a result of prolonged exposure to Darwin’s humid environment and previous handling.

A Final Warning on Confusing Coin Intervention with Coin Conservation

For the first time I’ve put down a substantive article on methods of intervention accumulated to-date. The author is of little doubt some will read this and suffer Prometheus’s fate. That’s the guy on the left, getting his liver torn out by an eagle on a daily basis because he thought he could whip some chemicals up and start cleaning coins, just like that! The author acknowledges considerable expense in gaining the knowledge and experience to-date. Intervention is not recommended unless that person has a lot of patience and some cheap coins to experiment with.

Not only did Zeus chain Prometheus to suffer aoens of suffering, he also sent him Pandora to create misery. A collector’s better-half will definitely create misery if one is foolish enough to dive into conservation and intervention without understanding the risks and pitfalls. I hope that by padding the library with suitable material and informing collectors through further articles, some informed judgement can be applied to their collections.

I hope to have feedback and further useful links from experts like Susan Maltby to help guide what is a hodgepodge of various methods – as outlandish as spraying coins with WD40 – into a safer method of conservation and restoration for Australian coin collectors. Constructive feedback and useful links are always welcome in comments below. Cheers, Les.