Copper alloy casting


Buckles and harness pendants made at Lanark festival of History.  Bronze was cast into clay moulds made with horse dung and sand.  Failed buckles can also be seen - failure can teach you much about the processes of making things.  The black substance around the buckles is the clay and horse dung mould material.  

My furnace setup.  Note the pair of bellows at once side.  The tiles around the furnace are based upon those known to have surrounded a  full sized bronze furnace of the late 14-15th centuries at Deansway, Worcester.  For safety, I carry a fire extinguisher, bag of sand and bucket of water to deal with any possible fire hazard.  The sand is to smother the hot centre of the furnace if necessary, since water and ceramics at one thousand degree's do not mix well.

This shows a wax model of a late medieval purse hanger. This is encased in clay, the wax melted out and the mould fired before brass is poured into it.

 

 

 

 

 

 

 

 

As far as I am aware I am the only person with a fully functioning, transportable and (fairly) authentic medieval bronze casting capability.  This is the result of four years of experiments and research in period texts, archaeological records and my own back garden.  The aim was to construct a furnace which was stable, safe to use, and capable of achieving temperatures of 1300C.  With it can produce replica artefacts and just as importantly, debris, which look similar to those that have been found by archaeologists. 

The furnace itself runs on charcoal and bellows, with one person required to operate the bellows.  In medieval times this would often have been an apprentice.  Although it would be an experience for the public pumping the bellows, they are rather close to the fire and there is a risk of sparks which is why the working area is roped off.

Making authentic medieval moulds to cast metal into can involve mixing horse dung, clay and sand, which is guaranteed to get a yuck out of most people (Although some children are anxious to have a go).  There were many recipes in use at the time, and I have found that simply mixing chopped wool with clay works as well.  This is formed around a lost wax original or by pressing the desired shape into the clay.  The next step is carefully drying and then firing the moulds.  Many larger moulds, for bells and cauldrons, were not fired to the ceramic sintering temperature (usually around 800C), which meant that they were flexible and yet strong, however on smaller moulds I find it easiest to fire them thoroughly in the charcoal which eliminates any possibility of water getting in and causing problems.  

After that the metal has to be melted in crucibles, I use graphitic clay ones which were state of the art for the late medieval period.  It also involves labouring on the bellows for a long period of time, careful positioning of the crucible within the furnace to catch the heat, and some luck.  There is a huge number of things which can go wrong, the most common problems I have had being the metal not being hot enough to cast, and the mould in-gate being too small so metal cannot run into it.  Mistakes occasionally show up in the archaeological record, with incomplete and broken castings having been found in London, ready to be re-melted.  The bigger mystery is why they were not re-melted, and instead thrown away into a pit to be found 500 years later. 

Such is the difficulty involved that it is clear why a seven year apprenticeship would be required to master every stage in the process, and during events I strive to get across the experience and skill required, and the human senses and skills learnt.  For example, in this period you can only judge the temperature of the furnace by the colour of the charcoal and flames coming out of it, there are no thermocouples to help.  Yet experienced furnace operators have been found to be able to judge temperature to within 20 or 30C by eye.  

By showing people how the technology of the period worked, they can gain a better appreciation of the skills of our ancestors, the advantages of modern technology and methods and hopefully awaken some interest in old fashioned skills which will broaden their horizons in the future.