Conserving Britain’s biggest Iron Age hoard
How do you begin to analyse a corroded mass of almost 70,000 Celtic coins? Neil Mahrer guides us through the painstaking process of conserving the largest coin hoard yet found in the British Isles, and what has been discovered so far.
Metal-detectorists Reg Mead and Richard Miles had been investigating the same field on the Channel Island of Jersey for three decades, recording occasional stray coin finds throughout this period. In May 2012, though, they discovered a rather more substantial concentration and, realising the significance of what they had found, immediately refilled their hole and contacted Jersey Heritage, the professional museum body on the island. A team of local archaeologists, with me as conservator, was assembled to investigate the discovery, but we little suspected how important a find this would prove to be. When we began our excavation four weeks later, we only initially opened a 2m by 2m hole because we were expecting to uncover a small hoard of perhaps a few hundred coins, possibly in a pot. In fact, it soon became apparent that the hoard was much larger – indeed, it wasn’t until the end of the second day that we managed to expose all of its edges. Thousands of coins, fused together by a layer of green corrosion, formed a mass measuring approximately 140cm by 70cm, and 15cm thick. Even at this early stage, it was possible to identify many of its contents as silver/copper alloy staters and quarter staters made by the Coriosolitae tribe of Celts from the neighbouring French coast, and dating to around the middle of the 1st century BC.
Given the hoard’s clear importance, it was decided to excavate it intact in a single block. This was no small task: in order to keep the coins damp during their recovery, a thin (10-30mm) layer of earth was left on the hoard’s top and side surfaces, and we then dug down below the level of its base all around it, leaving the mass on a layer of earth about 200mm tall. This was to act as both a supporting base and as a reservoir of water in order to buffer the hoard itself. As an extra precaution, hoard and base alike were wrapped in cling film when not being worked on. The next stage involved particularly delicate work: to ensure the hoard’s safe extraction from the trench, we needed to reduce the hoard’s contact with the ground as far as possible, so that it wouldn’t be stuck hard to the earth underneath it when the time came for the lift. To that end, the earth layer beneath the coin block was removed as far as it was believed safe to do so, leaving it intact for the full width of the hoard where it touched the coins, but tapering down like an inverted cone. Hand tools were then used to dig four evenly spaced parallel tunnels, each 120mm in diameter, through the earth that remained under the hoard.
It was then time to bring in rather heavier equipment: the hoard and its surrounding earth weighed around a ton, so it would only be leaving its 2,000-year-old hiding place with the help of a crane. A metal scaffold structure was constructed to fit closely around the hoard, with four nylon belts fitted through the tunnels underneath it and around the scaffold so that it could all be lifted out by crane. The belts would support the hoard’s weight as fully and evenly as possible, while the crane’s chains and hooks touched only the metal structure. Layers of foam and plywood shielded the hoard’s top edge from being compressed by the belts, and once the block had been raised about 30cm, another foam layer on a specially constructed wooden pallet was placed beneath. Finally, this protective sandwich was lifted by an extending crane on to a flatbed truck, and the hoard was carefully transported to our laboratory for further investigation.
In the first two years after the discovery of the hoard (now known as Le Câtillon II; the first Le Câtillon hoard, found in 1957, comprised c.2,500 Celtic coins), Jersey Heritage was granted permission both to remove overlying earth to reveal its surface fully, and to perform non-invasive research to reveal more about its make-up, but we could not at this stage take apart the mass of coins to examine them individually. We discussed the possibility of X-raying the entire block with colleagues at Southampton University, but the mass of coins was too large for the available equipment. Limited X-ray fluorescence (XRF) analysis of exposed surfaces was undertaken by Karl Harrison and Andrew Shortland of Cranfield University, however, and we were also able to carry out some microscopic studies of organic materials in the block’s surface, revealing various plant and animal remains.
What quickly became clear, though, was that the contents of the hoard were more varied than at first thought: partially exposed gold torques, silver bracelets, pieces of flat gold sheet, glass beads, and many other artefacts were identified scattered amid the coins on its surface. As a whole, the assemblage was such an impressive object that we initially considered preserving the whole block intact, but in light of its eclectic make-up it was decided that we should instead disassemble the hoard on an object-by-object level in order to reveal all of its contents.
Once funding had been obtained, this work began in 2014. It was a painstaking process, conserving and recording each piece as it was removed from the block, and, while the conservation work followed conventional best practice, it was complemented by an innovative method of recording the finds using a cutting-edge piece of kit called a Faro Edge six-axis metrology arm, which had a contact point probe and a scanning laser head. The laser was used to record scans of the hoard’s surfaces and its contents at regular stages of the work, while the physical contact probe head was used daily to record the position of every single coin and all the other finds on the hoard’s surface to fractions of a centimetre in three dimensions. This meant that, even after the hoard had been taken apart, future researchers would have a virtual model of the mass as found, in which the coin positions would be linked to their database records of tribe, chronology, and so on.
This was a key theme: we were keen that our work should allow the maximum amount of research, both during the disassembly phase and for many years to come, so we took advice too on the recognition, extraction, and preservation of samples of interesting soil, organics, and corrosion that we encountered during the process. Nor did we deconstruct the entire block: a solid, full height 15cm by 15cm by 15cm section of the hoard was left intact with all its earth and organics remaining in place between the coins, while 1,500 more coins – still only a small portion of the whole – were left untreated as a research archive. There will be plenty for future researchers to study, giving them the benefit of future advances in research techniques.
In that spirit, we began our work. The team comprised three professionals – two conservation assistants, Viki Le Quelenec and Georgia Kelly, both of whom were qualified archaeologists, supervised by me as conservator – and we were supported by a diverse range of professional advisors and 20 parttime volunteers who did much of the hands-on work. Olga Finch, the archaeology curator for Jersey Heritage, acted as the archaeology/ research advisor, and the two finders, Reg Mead and Richard Miles, acted as principal coin identifiers, their work overseen by Philip de Jersey, Guernsey Museum Service archaeologist. So, how did the work progress?
The hoard block was disassembled one coin at a time. This process involved a team of two people: one to physically remove the coin and operate the metrology arm that measured the coin position, while the other operated the computer linked to the arm and assigned a sequential number to each new coin, before bagging it. Plotting each coin only took a few seconds, touching the point probe three times across its centre axis, and three more times around its edge, to position it in three dimensions using Geomagic software – using this method, little by little we were able to build up a three-dimensional map of the coins. We also used laser scanning to record the position of newly uncovered torques and other items, so that they could be placed precisely in the final three-dimensional virtual hoard model.
Helpfully, the act of physically separating the coins proved easier than had been envisaged. They were connected to each other only by the fusion of their copper corrosion products, and this bond fortunately proved weak – it was always possible to remove a coin one at a time using a metal hand tool, usually a specially blunted knife, without causing any damage. As for the other artefacts, sufficiently robust jewellery and other objects were cleared of all surrounding coins and then the number around their base was also reduced. At some point in this process, they usually came loose, and if not they would be gently manipulated by hand until they came free.
As the hoard was potentially full of important organic material, we had to keep it wet at all times. This was done in two ways. At night, or during longer periods when the hoard was not being worked on, it was kept tightly wrapped in layers of cling film. The hoard spent the majority of 2012-2014 swaddled in this way, kept in a cold store at 4ºC, and during the following period, when it was being excavated in a publicly viewable open lab, it was kept wrapped at night but had to be actively moistened when it was on display. While working on the hoard, we made use of hand sprayers containing de-ionised water, but when the mass was left alone for any length of time we set up an ultrasonic humidifier to produce a stream of cold, de-ionised water-saturated air that exited from a series of holes in a special device positioned along one edge of the hoard so that the vapour slowly rolled over its surface.