Removing Verdigris From A Postumus Antoninianus

So, I am not in the habit of cleaning coins, but the verdigris on this one worried me enough to make an exception. A large blue-green blob, and several smaller, on a billon coin that is mostly copper. Not only does it deteriorate the general impression of the coin, I also suspect it would spread and possibly ultimately destroy the coin. So I decided to try a treatment with sodium sesquicarbonate, and document what happened.

This is what the coin looked like when I got it. It is a Roman antoninianus by Postumus, reverse showing Neptune, issued 262 AD. Nominally 20 % silver. It fills a hole in my Postumus collection, and I acquired it because I could see that beneath the verdigris, it is pretty well preserved.

Original condition:



A close up of the worst area shows what we are dealing with:



I made a sodium sesquicarbonate solution by mixing 10.6 g of sodium carbonate (washing soda) and 8.4 g of sodium bicarbonate (baking soda) dissolved in 100 ml of water. It should be distilled water, but I decided Swedish tap water was good enough (I also wanted to check if that assumption was true ). The recipe is from online, somewhere ... and nominally makes a fairly strong solution, about 15 %. There was some precipitation despite stirring and careful heating, so the actual strength would be less.

Into the solution Postumus goes, and then I wait.

After two days, the solution had a light blue color. In school chemistry classes, I learnt that a solution with copper turned blue ... so a bit worried that I might be slowly dissolving my 1760 year old coin, I take it out of the solution and put it under the loupe (showing only obverse and the "blob area"):



A close inspection and comparison with images of the original state calms my mind - it doesn't look like the coin material itself is hurt. But there is a clear effect on the verdigris, which is encouraging. So back into the sodium sesquicarbonate he dives, poor Postumus.

Four days later, that is after 6 days in the solution, this is what it looks like:



Now the verdigris is really recessing, and perhaps more interesting, the coin now looks silvery! I did not expect that, but I don't mind the effect ...

After another 6 days, all in all 12 days:



Things are certainly going on . After 6 more days, total 18 days:



It's beginning to look pretty good. Patience for another 6 days, in total 24 days in the solution and it looks like this:





Practically everything gone, one little tiny barely visible green spot left of the large blob!

I don't know what you think (tell me ), but I am much more happy with my Postumus now that I have cleaned his nose and shoulder. He has gone into an airtight capsule where he can rest among his fellow antoniniani without feeling embarassed about his condition.

So what happened to Postumus during those 24 days? Well, he lost a bit of weight, 0.05 grams to be precise, down from 3.32 g to 3.27 g. I like to calculate things, so just for fun I tried to estimate how many molecular reactions per second took place while dissolving those 0.05 g in 24 x 24 hours. The number I arrived at is a staggering 80 trillion reactions per second! (My calculation is in the footnote below.) Quite some action for a 1760 year old guy ...

Note: The precise chemical reaction is beyond my high shool chemistry knowledge, but I understand (from Wikipedia) that the blue-green gunk can in fact consist of several different chemical compounds: copper acetate, copper carbonate, copper chloride. For simplicity, assume we are dealing with copper acetate only. That has formula Cu(CH3COO)2 and molar mass 181.63 g/mol. Thus 0.05 g is 2.7x10^-4 mol, or 1.62x10^20 acetate molecules (multiplying with the Avogadro constant, 6.022×10^23). That number of molecules is then dissolved in 24x24x60x60 seconds, i.e., 2,073,600 s, which makes 7.81x10^13 dissolved every second. Round off to 80x10^12, since we have only one significant digit in the input data.