This week’s objects….
This week, Curator of Archaeology James has brought out a very interesting and significant hoard from the late Bronze Age/early Iron Age. The Sompting hoard was discovered in 1946 in Sompting and includes 17 socketed axe heads, a cauldron, shield boss and fragments of other cauldrons. Axe hoards were not uncommon for this period, and it is not known exactly why they were buried, but there are many theories.
The Sompting Hoard
I will be assessing 13 of the axe heads to see what condition they are in, if they are stable and if they need any conservation treatment. These would have been manufactured around 700 – 600 BC (approximately 2500 years ago!) and they were cast in a copper alloy, although without analysis we cannot tell the exact composition. (See previous post on why we use the term copper alloy). We can, however, tell from the green corrosion covering the axes, that the base metal is copper, because this is typical of copper corrosion.
What we can tell from an initial assessment
An initial check tells me that the axe heads are still in good condition, with only surface corrosion affecting them. This comes in the form of a hard green crystal-like corrosion which is difficult to remove manually. Some of the axes have an underlying layer of black or brown corrosion which is also difficult to remove without damaging the objects. These are often the first corrosion products to form, being basic copper oxides, produced when the copper reacts with oxygen over a long period.
We can also see that some of the axes are sharpened, and have been used, and some have not. In fact we can see the scratches made by the person who sharpened the axes, 2500 years ago. All the axes have casting seams meaning they were mass produced from moulds. Casting seams are where the molten metal has seeped into the cracks along the sides of the mould – and some have evidently come from the same mould as they have the same surface pattern.
A closer look shows us that some of the axes are miscasts or ‘seconds’. One from a set of four identical ones, is missing a collar. Two others have holes in the sides which look to be badly cast rather than the result of corrosion. The details on a few of the axes are not very clear, this could be a result of corrosion, or again badly cast and not up to the founder’s standards.
To clean or not to clean
That is always the question a conservator needs to ask. The initial desire is to see a polished metal surface, however in this case that surface doesn’t exist, due to corrosion. A corrosion layer is made when the surface metal, and the elements in the burial environment react together, to form a new product – corrosion. Often, if you try to remove the green corrosion you may only reveal a layer of red corrosion underneath, which has eaten into the surface, destroying surface details. The further you try to remove these layers, the deeper you will dig below the original surface, destroying any chance of retaining original details.
With many archaeological metal objects there may not be much of the original metal left, and by removing all the corrosion you will simply end up with an unrecognisable lump of the core metal. If there are still details, they may be made of corrosion now, instead of original metal, and in the corrosion surface we can sometimes see details of wear or use, which would all be removed if the corrosion was removed.
There are chemical methods that conservators can use to remove copper corrosion, however in this case, the objects are stable and I will carefully use mechanical methods to bring out the surface details. I will also remove the compacted chalk on the surface, and again using my conservator super-powers, mechanically remove some of the areas of green corrosion which are obscuring the beautiful details.
The Portable Antiquities Scheme website has a number of guides for finders, on simple treatments for metal objects. This guide lists do’s and dont’s, and also discusses the considerations and dangers of cleaning your object.