Friday, 27 November 2015

Artificial Fertliser (1)

Attentive readers may remember that 24th November was the date I set for examining the contents of the "Two Warsaw Chambers of Numismodeath" (PACHI blog Thursday, 29 October 2015). This was an attempt to duplicate the results of Dean Crawford's "kitchen sink" coin-colouring experiment which seemed to me suspicious (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 worked and wanted to duplicate his "experiment" to see what would happen. They involved burying fresh and patinated coins in artificial fertiliser for three weeks (as Mr Crawford had described) to see what degree of damage was caused. I'll do this in two posts, for the experimental methodology (so to speak) see the earlier post, for the summary of results you can skip to the third post in the series.

[UPDATE... This post was to be illustrated with photos showing the excavation of the containers, I bought a fresh memory card for that... sadly I neglected to read the packaging, a memory card is a memory card, no? Well, except if its a memory card that needs formatting before use. So, not a single photo is now recoverable. I'll post up pictures of the raw coins when I have calmed down].   

Let us start with Experiment 1: Warsaw Chamber of Numismodeath
In this the coins were in wet soil saturated with a 3% fertiliser solution.The plastic foil was removed and the soil was found to be still damp (and wet at the base of the jar). There was a faint hydrogen sulphide smell from the damp soil. I originally had intended to smash the jar and excavate it from the side, but it was too cold outdoors and this would not have been appreciated by the indoor staff, so I decided to scoop it out from the top. In the end this did not make much of a difference, the coins in the lower levels (buried a centimetre down) were not very much affected by the experience. A huge surprise was the bimetallic 2zl piece which exhibited no trace of any corrosion, but it was one of the deeper ones.

The condition of the coins from the lowest level on excavation was quite surprising, they mostly had their brassy metallic surface intact.  There were four one grosz coins and a 2zl piece (bimetallic) which had gone in 'mint fresh', and a 1997 and 2009 coin which had been patinated. The 'mint' coins came out with a brassy surface, but when they dried, a grey-pink surface devekloped. This I think is the crystallisation of something from the soil water, but it seems quite firmly bonded to the coin. In several areas the coins had the mint-lustre preserved in small patches. It is difficult to be sure without a microscope, but there seem locally to be minute specks of redposited copper (?) in this crust, but it is difficult to see because of a white crystalline (laminar) material which crystallises out in the crust in the same areas (and comes off on the hands). I do not know what this is yet. The surface of several of the coins seems etched slightly, it is matted. This was more noticeable in the two older coins.

Six coins were at a slightly shallower level, just below the surface. They exhibit the same characteristics, but the etching is more pronounced. They include kopejki which had lost their smooth patina and now have an etched metallic surface. In both cases the amount of metal lost is clearly minimal (I should have weighed them before the experiment, but due to the accretion of the crust, the results would not have been very reliable as a measurement of the effect).

It was the six coins in the topmost layer of the experimental humidity chamber which more closely duplicated Mr Crawford's results. Three of them were more or less in the same state as those just described from lower down, and this included one of the kopiejeks which had had a nice smooth patina. That was now gone and the surface was etched. It is not clear whether the very light pitting is new, or whether the solution had stripped out corrosion caused by a century in the soil. None of the new coins put n the same solution showed this effect, and even from this group, local areas of mint lustre were preserved. The French frank had been a nice coin when it went in, now - to use the correct archaeological terminology - it s pretty knackered. The smooth black patina has gone though some patches remain, giving it a blotchy appearance. There is however a deposit of dark green on the surface, when I excavated the coin this had been a thick crust about 3mm thick on both faces of the coin, copper salts had clearly migrated from the object into the surrounding soil, this crust was however not at all cohesive and crumbled when the object was dipped into water to clean off the soil. The other coin which exhibited the same effect was not washed, and contained a small 1967 kopiejka in the crust. This was interesting, they were not touching, but where they overlapped the smaller coin had the original patina preserved, but where the coin projected beyond the other, the surface was etched. The green crust formed a bridge between the two coins suggesting that what was drawing the copper out of the one coin was the presence of the other. This suggestion is supported by the fact that the green crust was mainly on the upper face of the larger coin, but underneath it was only a ring around the edge.

I have not cleaned the coins I obtained more rigorously, I will keep them in the as-excavated state in case I can ever get an analysis done of some of the corrosion products. These coins are not as collectable as they would be with some other surface (and I am sure you can tart them up using easily available materials, that's what dealers sell glass bristle brushes [ouch] and renaissance wax [uh-oh] for).  Despite that, they are perfectly legible and retain all f their value as archaeological evidence despite the bad treatment they have received.

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