(09 July 1999)
After approximately 250 hours of mechanical work using simple tools and a Cavitron ultrasonic dental tool, all of the concretions were removed from the cauldron. One large concretion which contained fish bones, wood, textiles, leather fragments and decayed vegetable matter was successfully removed from the bottom of the cauldron as one large piece and will be treated later. The cauldron has two handles in good condition and when the exterior of the bottom was exposed, one leg could be seen, still attached to the main body. This foot is complete but broken and another leg (found in another concretion) is broken and missing the foot as seen in the photograph. A third incomplete leg was found attached to a base fragment in a very large concretion which had been excavated the year after the main body of the cauldron was found. This last sherd was removed from that concretion and treated along with the main body of the object. It was felt that this sherd would be very important in providing as much strength as possible when the cauldron was bonded back together. The cauldron is nearly complete, except for parts of the rim, small areas of the bottom and part of the side. In total, there were approximately 30 sherds. The surface appears in good condition although there are no markings except for the usual casting marks on the sides and some unusual casting marks on the bottom. A small sample of the metal was analysed as a grey cast iron which is now virtually 100% corrosion product. The cauldron, which weighed at least 70 to 80 kg. when it arrived in its concretion, now weighs only a few kilograms.
After removal of the concretions, the body of the cauldron and the fragments were washed in de-ionized water and the total conductivity, as well as the chloride content of the water was monitored for many months. Once we were satisfied that the chlorides and other salts had been substantially removed, the cauldron was allowed to slowly air dry. If they had not been removed as much as possible, the chlorides would have reacted with the iron to form more corrosion products. Although the cauldron was only brushed with acetone to help with removal of the water, the smaller sherds could be immersed in acetone then air dried over several weeks. One infra-red heat lamp was placed near the cauldron to aid with the drying.
Once the cauldron was dry, we felt that the body could be too weak to support itself since it was no longer solid metal. It had many large cracks in it, some of which are visible in the photograph. It was also missing part of one side from a point near the base up to the rim which would lower its overall strength considerably. It was decided that consolidation would be needed and possibly some form of filling in of the missing area on the side.
Several choices of consolidants from epoxies, polyesters to methacrylates were considered. Due to its reversibility and well-known characteristics, B-72, a poly(methyl acrylate) and poly(ethyl methacrylate) copolymer, 3% in toluene was eventually chosen as a standard treatment. Several coats were applied by brush to both sides of the main body until no more absorption was seen and the smaller sherds were simply immersed in the solution. After drying, some of the sherds were rebonded to the main body using a liquid polyester resin which was also run into the many cracks in the main body. The cauldron was placed on a curved Ethafoam mount to support it from the bottom yet still allow one to see the complete leg, visible in the right of the photograph. A coating of a hard microcrystalline wax was applied to the surface and the cauldron appeared to be stable. It remained in this condition for several weeks and was even sent to Montreal for exhibit during an archaeological conference. After it was returned, the last of the sherds were attached to the side.
However, a month later it was noticed that the bottom was suddenly starting to crack in hundreds of places and both it and the sides were starting to warp and sag. These were new cracks, not visible during final treatment nor even after the exhibition in Montreal. When these new cracks were discovered, it was thought that the body of the cauldron, being almost entirely graphite, simply could not support itself and was possibly in danger of completely collapsing into thousands of small fragments. The B-72 consolidant had either not penetrated the metal enough, was not strong enough or both. There were no visible signs of corrosion or salts in the cracks or on the surface.
A larger Ethafoam base was made to better support the bottom and two types of fast-setting polyester resin adhesives were run into the new cracks and any of the old cracks which had grown. At that time, the cracks were changing so rapidly that the cauldron had to be monitored at least twice a day. However, after each application of the polyester resins, the rate of new cracking seemed to diminish and the pot eventually needed to be monitored only once a week and then just once a month with few new cracks developing. Most of the cracks were along the base where the sides meet it or on the side which had the broken sherds. We considered making various internal supports but since these would have obscured the interior we decided to continue to apply resin as needed since it seemed to be working. Some cracks eventually grew from being almost invisible to 2 millimetres wide and a few sherds warped very badly. None of the sherds have separated off but some are now slightly out of position causing poor joins. In the last two months only a few tiny surface cracks have appeared.
It is difficult to determine why the cauldron body cracked as there are still no signs of corrosion or other problems. Vibration during the removal of the hard concretions may have weakened the structure on a minute scale not previously visible, and when the cauldron was allowed to stand for a long period of time, the graphitized metal may have slipped along these micro-cracks. As approximately 25% of the side of the cauldron is made up of bonded sherds, the geometry of the main part (sides are very tapered inwardly) is such that it wants to collapse in on itself. After drying of the cauldron, consolidation was done using B-72 since epoxy is not easily reversible and was thought not necessary at the time, however, perhaps its greater strength might have helped. Also, infilling the missing areas of the side would have completed the round shape of the cauldron which would have stopped the sagging of the sherds towards the missing areas. Even though the cracking has substantially stopped, the cauldron may still require an internal support and infilling if it is to be handled frequently or is ever put on display. It has been a very challenging artifact.
Douglas Beaton,
Conservator, Parks Canada,
Ontario Service Centre
