Researching ancient repairs

Conservation isn't all slow, precise work in clean studios-these pictures are from a recent session of casting experiments for my MA work, when I spent two hours standing over a furnace casting 8 different sample of tin bronze at temperatures of up to 1180c.

I'm not just doing this because I like fire-I'm researching an ancient repair technique known as casting-on or casting-in.

I don't intend to carry out these repairs on objects, or suggest than anyone else does-there are all kind of reasons not to-but I'm researching the technique because understanding how objects have been made and repaired in the past helps us to care for them for the future and sheds light on the context in which they were made.

In order to explain the technique I have to start at the beginning, with lost was casting. Lost wax casting, in case you have never encountered it, is an amazingly simple, subtle, ancient, modern, brilliant technique that has fascinated me since childhood. The basic concept goes like this: A model is made in wax, and then encased (invested) in clay or plaster. Once the investment has dried the wax is melted out through a hole in the plaster, leaving a cavity the exact shape of the model. The investment is fired to remove moisture and traces of wax, and molten metal poured into the mould cavity. The mould is broken away to reveal (hopefully) a metal copy of that original wax model, perfect in every tiny detail. This is how many sculptures are cast: jewellery, machine parts, gold teeth and many, many other things.

A founder who breaks the mould and finds the casting is not perfect is going to need a pretty serious defect to persuade her to start again from scratch. Not only does it waste a huge amount of time, effort and fuel and destroy your profit margin, you would also have no guarantee that your next attempt will come out any better. Producing a bronze without a single fin, crack, air bubble, pit, inclusion, potato, breakthough, or area of porosity is close to impossible so if you can, you work with what you've got. Even if your bronze is perfect, it's quite possible you cast it in sections and need to put them together.

In a modern foundry most assembly and repair work is done by TIG (tungsten inert gas) welding but historic foundries employed a range of techniques. In a cast-in repair new metal is cast directly in to a void in the object. These repairs turn up all over the place. A German monk writing around 1120AD under the name 'Theophilus' (his real name may have been Roger, but no one's quite sure) gave detailed instructions for using the technique to replace a missing turret on a censer of architectural design. His instructions could be summed up as:

  1. Clean and roughen the area you need to cast on to
  2. Model a new piece onto it in wax
  3. Build a clay mould around the wax piece, dry it and burn out the wax
  4. Cast new metal into the clay mould

There are examples of repairs like this in many objects from all over the world, and I am experimenting to see if and how the repair bonds the rest of the object. I have cast 8 bronze tokens and I am now casting new bronze onto them by different methods at different temperatures between 1080°c and 1180°c. If the new bronze stays attached to the old in any of the samples I will cut sections from them and use a microscope to examine the crystalline structure of the metal at the join in order to understand how and why they have bonded.