This is an attempt to duplicate the results of Dean Crawford's coin-colouring experiment which has been bothering me for a few hours (see my text 'Artefact Hunting, the "Lesser of Two Evils"? More on "Fragmentation"'). I really do not see how the chemistry of what he describes works. On my way back from the shops yesterday I put my hand in my pocket and pulled out a handful of loose change and found that some of the coins were almost straight from the mint. That gave me the idea of trying to duplicate the "kitchen sink experiment" which the metal detectorist used to "prove" to Mr Hooker's readers that artefacts will rapidly self destruct in agricultural soil. Readers of my blog will know that I am sceptical of such generalisations. So, willing to put my money where my mouth is, I decided to put it to the test. I have some fertiliser from my vegetable patch which though it is unbranded, so is Mr Crawfords who just used some random material he found. His is white granules, mine is too. His is good for plants, mine is too. On the label, my fertiliser says it is composed primarily of potassium nitrate, urea and has potassium pentaoxide in it, the latter is nasty stuff - I guess that's why the label says to use gloves (coins beware) it is highly corrosive to metals, maybe we'll get copper phosphate?
Experiment 1: Warsaw Chamber of Numismodeath
Here is a photo of the ingredients, on a tekkie-style camo jacket (which I happen to have at home) for authenticity. The vessel representing "Gloucestershire" is rather a nice piece of plastic kitsch. It's All Saints Day coming up and in Poland we buy all sorts of crap to decorate the family graves with on that day, so the supermarkets have a variety of these special candle holders... I thought this was quite suitable for the "Warsaw Chamber of Numismodeath" as I called my experiment.
Above I picture the coins, Mr Crawford had four, I thought I'd be more ambitious (besides which having them touch each in wet soil other gives all sorts of potential for electrochemical cells to be set up). There's a wider range here, from mint fresh to dark brown. I degreased them in ethyl alcohol (which I happen to have hanging round the house).
So first I tipped the soil into "Gloucestershire". This took a bit of time as I wanted to make sure there were no worms in there as they'd drown. I thought a depth of seven centimetres would be adequate.
Then I added the coins. Mr Crawford's photo shows his lying on the soil surface. I cannot see how this would work, at the first rainfall, much of the fertiliser would be washed off the upper surface and the lower face would not be wetted. So I decided (to give the chemicals a chance to work) I'd bury some about a centimetre down, and leave some on the surface (see photos). I had a few qualms about one of them, a French frank which was aesthetically worn and patinated, qquite a nice thing. But then, putting my money where my mouth is, my thesis is that coin is not going to come out of my jar looking anything like Mr Crawford's. Let's see.
Now I intend to keep an eye on the jar, it is on my windowsill (outdoors in November it could well freeze and chemical reactions would stop), I'll keep it moist and then in three weeks (so about the 26th November we'll open it up and see whether my coins look anything like Dean Crawford's and whether that French frank is still in a collectable condition.
Experiment 2: The Warsaw Dungeon of Numismodeath
The second experiment mirrors more closely what Mr Crawford shows in his photo, where he indicated that he tipped fresh dry fertiliser grains onto dry-looking earth. There does not seem to be much point trying to duplicate the colour changes he documents by doing that, moisture surely has to be present. Again, with winter approaching, in Poland one cannot count on free moisture in November, so I decided to construct a humidity chamber.
I decided to buy a more aggressive fertiliser for this experiment. A trip to the garden centre revealed that not all manufacturers tell you what is actually in the box, but of those that did Biopon Autumn grass fertiliser looked to be the nastiest for buried metals (and by all accounts your skin). It's got all sorts of stuff in it, Phosphorus pentoxide, potassium oxide, calcium and magnesium oxide, sulphur trioxide. I reckon if that does not destroy the coins, not much else will.
UPDATE 3rd November 2015
The flood in Gloucestershire (experiment one) has abated, the coins are now clustering in nicely damp soil, Mr Crawford will be glad to hear that there is already a light green powdery efflorescence on the uppermost edge of one of them, and some interesting minute spots of what looks like redeposited copper where it touches another one below it. I have sealed the top of the vessel off with four layers of polythene and we'll start counting the three weeks for both experiments from today (24th November).