Archaeological spoil management with regard to metal detector use

in #history4 years ago (edited)

Whilst metal detectors have been with us for over fifty years, their use has been mainly confined to the amateur hobbyist.
Over the last decade, we have seen them being increasingly used in archaeological excavations, but in a limited and haphazard fashion. Though many archaeologists have come to see the metal detector as a powerful and useful tool, many more refuse to permit their use, based on their perceptions and prejudices.
Leaving the politics and controversy aside, this article will concentrate solely on the way metal detectors can be used in an archaeological setting and how to get the best from them.

The machines

Contrary to popular belief amongst the archaeological communities, nearly all commercial metal detectors available today, despite the claims of their manufacturers, are very limited in regard to depth. The most modern commercially available machine at the time of writing would still find it difficult to find a denarius size coin below a depth of 25 cm. Most have much less ability.
Despite these limitations, there are three very clear situations in which the detector becomes a practical tool to the archaeologist, these being:
 The checking of ground level before excavation starts
 The checking of each level as excavation progresses
 The searching of spoil for metal artefacts that may have been missed

Metal detector practise

It has often been the case that where detectors have been used on excavations, it has been very limited, almost as an afterthought. Setting up archaeological excavations to take into consideration the use of detectors from the onset as accepted routine is absolutely essential to get the best results.
There have been many digs, particularly in the UK over the last few years, where, due to the cost of conservation, the archaeologists have been instructed to not recover more artefacts than is necessary to give the site it’s context, which while understandable from a financial viewpoint, is abhorrent to those who view every ancient artefact as a valuable contribution to our knowledge base. This has meant that detector use has been discouraged or abandoned on many digs.
Despite these setbacks, the detector is proving to be essential in many areas of archaeological use.
Some digs are careful, systematic affairs with the spoil sifted or washed to extract the smallest artefacts. Most though are rescue digs with time limits, where sifting and washing are not options. It is in this area that the detector shows it’s worth.
In many digs, the excavation may be simply a trench or two with the spoil to one side, making detecting a simple affair, especially if used continually from the onset, to make sure all the spoil is scanned.
Many rescue digs though, are over a large area and spoil from the excavations can rapidly grow and become problematical, often requiring its removal by truck to other locations.
In the summer of 2010, the archaeological department of the town museum in Vinkovci, Croatia, was engaged in a rescue dig over 1200 m2. Volunteer detectorists came late to the scene, when the spoil heaps were considerable and being periodically taken away in trucks. Despite that, many coins and artefacts were detected and recovered, making up a large percentage of the total metal finds from the site.
Following this initial trial, detectors were used on all subsequent rescue excavations on possible metal age sites.

Fitting in with the existing system

As elsewhere in Europe, the Museum of Vinkovci had a set procedure when carrying out excavations. The soil layers would be systematically dug out by spade and shovel and loaded into wheelbarrows. The soil in the wheelbarrows was examined to extract any visible artefacts then the spoil dumped off the area of interest and forgotten. Often the soil was in hard clumps or sticky mud, reducing the ability of the technicians to spot smaller artefacts. The use of detectors allowed the operator to recover a very high amount of missed items, but due to the system, the finds had no context. Something had to change.
It was after the discovery in 2011 of a large 8th century Avar culture graveyard by detectorists that the beginnings of a workable system took shape. The graves were being dug systematically by teams, but multiple graves were being dug at any given time and the workers would put the spoil into any wheelbarrow that was near and dump the spoil anywhere around the perimeter of the site. Quickly a system was adopted between the lead archaeologist and the detectorist where each grave was assigned its own barrow and the number of graves being dug was limited by the amount of barrows available. Each barrow was assigned a certain area to dump and this allowed the detectorist to very quickly return recovered items to the grave they came from. The initial difficulties due to the possible embarrassment of the technicians in charge of examining the soil in the barrows was soon overcome and it became clear that all overlooked metal items were being found and then returned to the exact place they were dug from. In practise, the technicians became even more vigilant, rather than less so, when detectors were in use.
Another problem was that workers would often dump their personal rubbish, such as silver foil, bottle caps and cigarette packets, into the barrows, causing wasted time and frustration for the detector operator, though this practise ceased when the problems caused were pointed out to those responsible.
There was still room for improvement though, and the changes outlined below resulted in detector finds becoming over 90% of all recovered metal artefacts on most subsequent digs.

The employment of a detectorist from the very start of the dig allowed the operator to organise the position of spoil heaps and the way they were managed. The ground would be scanned for signals that may cause interference and either cleared the ground signals or relocated the allocated dump site before the first barrow load arrived.
As the areas being dug were usually in assigned sections, then the same sections would be duplicated in spoil areas and the workers instructed to strictly dump the spoil in the area allocated.
The use of stakes or signs with the name or number of the section being dug is recommended at all times but essential when the detectorist on the spoil heap is out of sight of the dig area, such as the cellar of a building or around a corner and not able to see which barrow came from which area of the dig.
It proved important that constant scanning should take place so that there was no build up of unscanned waste resulting in artefacts becoming too deep to detect. This problem was further reduced by encouraging the workers to make a heap as they went along, to about the height of 1.5m, then dumping the soil down the sides of the heap they made (Pic.1). This spreads out the soil thinly and also made other, non metallic objects, such as pottery, glass and bone, visible to the naked eye.
To further reduce possible confusion, found artefacts are placed in plastic bags with the appropriate designation or handed to the archaeologist when found, whichever is the local practice.
Many urban digs are in confined spaces, which do not allow the possibility of setting out controlled dumping areas. Less space means that the spoil is usually dumped in one place and constantly removed by mechanical diggers. This scenario is the most difficult for the detector operator, who must scan constantly to have any chance to ensure that all the spoil is searched.
These simple practises by the town museum of Vinkovci have resulted in an incredibly higher amount of artefacts being recovered, with minimum interference in the process of excavation and recording and the only increase in cost to the project is the employment of a single detector operator.
A further use of the detector is to pinpoint undug metal artefacts as each new layer is revealed and marking their positions (pic.5). This helps ensure that care will be taken when that spot is reached and is far more valuable to the archaeological process as the item can be recorded in situ. This also proved valuable in the excavation of the previously mentioned Avar graveyard as the detectorist was able to judge depth of grave goods and their positions within the graves, helping reduce excavation time to such an extent, that the dig finished a month ahead of schedule.

Pic.1. A worker dumps his barrow load of spoil down the slope, spreading out the soil for examination. Note the stake with the designation of the section being dug.

Pic 2. & 3. Items found by detector in spoil from archaeological digs.

Pic.4. Scanning archaeological waste from urban dig site, March 2014.

Pic.5. Shallow 8th century grave with metal signals marked.

© Emma Stephanie Gaunt 2019