Sunday 24 August 2014

The Tekkie Myth of Finds 'Rescue'

The sockpuppet "K.P.Volkswagen" responds on Andy Baines' blog to David Knell's "(the danger posed by chemical fertilisers appears to be largely an urban myth)" with a remarkable text:
maybe you read some articles on the effects of acidification or corrosive properties on metalic (sic) objects due to chemicla (sic) nitrate fertilizers, funny how finds do not come out of the ground perfectly preserved and some seem to suffer from surface degration (sic) including Victorian/modern finds. Anway you forgot about the result of damage caused by mechanization, broken parts of the same item.
Acidification. Maybe "KPVW" would like to read up about corrosion processes of metals in soils. As one may imagine, there is a lot of it, both dealing with archaeological material in the soil as well as commercial objects (for example buried pipelines).

The result of corrosion mechanisms - both chemical and biologically induced is the production of hydrogen ions, the result of putting a non-passivating metal object in the soil is an automatic increase in hydrogen ions (an acidic halo) around it. This is why organic materials are sometimes preserved in the immediate vicinity of iron for example.  So basically those buried Roman metal objects have been sitting in an acid bath of their own making two thousand years. The problem is where there is oxygen available, the hydrogen ions form water and then more of them are produced to keep the equilibrium, that's why iron objects from sandy coil have thicker crud crusts than those from clayey soils, its the degree to which the oxygen gets down to them. So KPVW just saying "acidification" explains nothing.

Soil pH (click to enlarge)
What I think has misled this milieu is a lot of literature that was being produced back in the 1970s (a lot of it by Scandinavians) about the effects of acid rain on architectural monuments, corroding the stonework and the metal roofs of buildings (but in the case of the metals, that is a different, fully aerobic, corrosion process). This was aimed, sad to say, at the chemical industries of central Europe, the chimney emissions of which at that time were not filtered, and blew right across the Baltic and killed their forests. There were some Danish and Swedish archaeologists who jumped onto that band-wagon and tried to show the same thing was happening to buried archaeological metals. To cut a long story short, the case is far from proven, and the rate of research in this particular area has slowed down in past decades. Air quality in the UK is now vastly better than it was a few decades ago.

Main distribution of Bronze Age to early
medieval PAS-recorded finds over
PAS affordances surface.

So to come back to ground, the ground in PASlandia... the colourful picture to the right is a map of soil pH in England and Wales. Red and orange are acidic, blue is alkaline, a pH of 4.5 and lower (orange and red on the map) is regarded in corrosion science as corrosive. As for the "Tekkies to the rescue" model, on the left is a map showing where PAS-recorded finds (many from metal detecting) are coming from. It will come as no surprise that there is a huge gap between what detectorists say they do, and what they do. Most of the PAS finds are coming from the regions of the country with alkaline and neutral soils, and not from the areas where finds are "threatened" according to the "acid" evoked by "K.P.Volkswagen".

Chemical nitrate fertilizers
The first thing that needs saying is that as we all learnt at school (Nitrogen cycle), soils naturally contain nitrates. Without them, a soil will not support any plant growth. The problem for farmers is that with intensive cropping, they remove more than the atmosphere and biological agents can put back, so they add some themselves. There is of course zero difference between a nitrate molecule produced naturally, or one produced in a factory. Animal excreta, spread on the fields as manure contains exactly the same nitrate molecules as white powder scattered from the back of a tractor. This "artificial fertiliser" phobia is simply that, an irrational phobia - jolly helpful for those who want to make profits from "Organic food" (and if it worries you, buy Polish vegetables and consume less chemicals see here).

All chemical nitrate fertilisers cost money, growers therefore use them economically, applying only as much as is necessary to get good crop growth. What is needed is taken up by the plant (that which is not lost to denitrification). In good farming practices there should be little surplus, nitrate levels in the soils should not rise more than their natural levels and there are in the UK (and EU generally) strict controls on farming practices to control the amount of (highly soluble) nitrates entering streams and groundwater. Nitrate fertilisers are being gradually replaced by organic nitrogen containing compounds, most commonly Urea (carbamide CO(NH2)2). I would have thought metal detectorists being great mates with find-donating farmers would know this. Anyway, what role do nitrates have in corrosion of buried metals? For a starter in industry they are used as corrosion inhibitors (phosphates too in the case of ferrous objects), but of course in the soil, it is far more complex than that. Soil acidity and aeration are both effects that will affect the rate at which nitrates react with copper alloys, as is the buildup of a protective corrosion product layer on the object, and its coherence. Of course once again, the Roman coins and nails have been sitting in a dynamic soil solution chock-a-block full of nitrates(and much else) for two thousand years, why suddenly there should be a need to hoik them all out now beats me.

"Finds do not come out of the ground perfectly preserved Funny that. Drop a wrought iron nail in the ground and it corrodes. Fancy that. Copper clad coins too. Jumping Jehosophats, who'd have thought it? ("KPVW" might be interested to know that this is the first topic of lesson one of archaeology 101 just in case there are any students in the first year who'd not realised that. It's page two of the reading stage four 'Ladybird Book of Archaeology'). This of course has nothing to do with agriculture, corrosion happens almost everywhere except the desert and Outer Space.

"some seem to suffer from surface degredation"
("including Victorian/modern finds"). Well, fancy that too !! That surface that is in contact with corrosive environment is the one that corrodes! Amazing. And things that have been in the ground any time, why, fancy them coming out looking different from when they were put in. What amazing stuff you can "learn" when you take up metal detecting, all those things that seemed so difficult in school suddenly, when you've got a rusty nail, there in yer 'and, all come to life! Queen Victoria died in 1902, one hundred years ago. A kinky pink Volkswagen buried in a field a hundred years (or even thirty) would "seem to have" have a pretty degraded surface when dug up. Believe me. This again is nothing to do with agriculture or artificial fertilisers.

Well, let's have a look at some of those objects with "degraded surfaces" - like a few taken randomly from the PAS blog feed about Archaeology Day. Well, true, these copper alloy objects have lost their brassy glint, so in that sense they are "degraded" if you are a collector that wants nice shiny baubles in the display case. As an archaeologist (and one with a background precisely in metals), I'd say they are not in bad nick.  Like the many hundreds of thousands of objects from metal detecting on the PAS database, like an equally large number of metal detecting finds that never made it to the PAS but are emptied from the archaeological record straight onto EBay.

I think archaeologists and collectors/dealers look at objects in a different way. I see a heavily corroded metal object, and it's a decision whether for the report it needs cleaning, or maybe just an X-ray, or whether we can get the information needed from it without either, followed by preventive conservation. I might wonder why this one has thick grey lumpy crud on it, while another three from the same context have blue-green smooth crud. Has there been a change in burial environment, and if so, what?  A collector is interested in how nice it will look on the photo he posts bragging about it in the 'Metal Detecting R Us' forum, or in the display case doubling as a glass-topped coffee table in the living room. My 'thick-grey-crud-which-could-be-taphonomic-information'  is for him disfiguring and most likely has the collector reaching for the coca-cola or Harpic to strip it down to the bright shiny metal which is more displayable. I am pretty sure that a lot of this collector-talk about "degraded surfaces" and "bad condition" is due to looking with a collector's eye for what would be considered a "nice patina" (or failing that, 'cabuinet toning' of a metallic surface).

Also we should remember that today's metal detectors are capable of finding more than would be spotted by an eyes-only digger in the old days. I have seen it myself on-site, they can find metal objects corroded beyond all visual recognition (lead and pewter finds in particular). Artefact hunters and archaeologists with metal detectors are finding objects corroded far beyond what people were picking up a few decades ago. That does not mean that this corrosion is necessarily happening now.

 "Damage caused by mechanization, broken parts of the same item". Firstly, horse drawn ploughs were quite capable of breaking artefacts, we have flints from ancient buried soils in Roman lynchets on the Downs showing this, the problem is not "mechanization" and its not new. Again, many Roman objects in the ploughsoil were dropped in ancient fields and have been in the ploughsoil several centuries, yet survive.

In reality, small compact objects in soil (like pebbles and bits of stick) are not invariably sliced through every time a plough comes near them. Unless blocked from moving by something bigger and firmly seated, most will, like the soil itself, 'flow' around the share as it moves through the soil. That's what many of the artefacts we recover now have been doing once, twice or more a year since they were dropped in fields centuries ago. If what collectors are saying was true, there should be zero undamaged artefacts in fields older than King George V for metal detectorists to find, and any fool can see that this is quite simply not the situation.

I think this case is (a) overstated and (b) misunderstood. If you look at reports of rally finds (the first Water Newton one for example), you see hundreds of artefacts illustrated which are NOT plough damaged and a small percentage which seem to be, but then when that damage occurred (in fields which have been under plough since the eighteenth century at the very latest), cannot be determined. This is no argument for hoiking all finds (and just the collectable metal ones) out now.

We should move away from object-centred arguments. A coin with a big nick in the edge or a wide fresh plough scratch on one face is still a coin of Edward I of moneyer "Fred on Lond" - it is less collectable, but its the same piece of archaeological information. This knife is the same piece of information if found whole (it was) or in pieces (as I've photoshopped it here). You can still argue if that really is a Roman knife, or an early Medieval one, even when its in pieces, and even if some of the bits are missing. Again, not as nice as a collectable (not that many artefact hunters collect utilitarian iron artefacts, they throw them away or discriminate them out), but again, this "threat" is seen mainly from a collector's point of view, not that of archaeological information.

I am of course not saying finds in ploughsoil are not in any danger at all (or that the problems mentioned by metal detectorist "KPVW" are the only factors we need to be discussing). I believe, however, that a cold look at the claims that are made by artefact collectors and their supporters in favour of hoiking it all out now - of which this one is typical - are vastly overstated. They are not only shallow and utterly unconvincing, but they are also object-centred rather than taking a wider look. Furthermore, they are self-serving arguments wholly subservient to the interests of the collector.

Finally, if the archaeological evidential values of stuff ARE being damaged on sites by changes in pH, chemical properties, mechanical movement or whatever (including metal detecting hoiking), then simply hoiking random collectables out and scattering them in personal collections, some of which may be recorded, is not the only option. Once a responsible metal detectorist has identified a threat to a field or part of a farm with which he or she is familiar, we could adopt three tactics :
1) Recover all of the evidence by means of a proper survey (including metal detectorists) using the methodology clearly outlined in Our Portable Past with the results properly archived, curated and written up,
2) Monitor such sites to  decide at what stage intervention is necessary, but until then preventing it from being altered by random private depletion,
3) Attempt to mitigate the problem, for example by inducing changes in the farming regimes, but again there is no point in doing this if the site is saved (at somebody's expense) from one type of depletion, but subjected to another (random artefact hunting for  private entertainment and profit). 

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