The bronze knife – casting
It was very cold and misty on the 19th December 2016. It was the kind of freezing dampness that hangs in the air and slowly permeates your clothing and gnaws at your bones if you stay out in it for too long. But I didn’t mind the cold as I set out to make the short journey to Butser Ancient Farm. The day had arrived when I would get to play with a furnace and cast a bronze knife for the first time!
Arrival
Upon my arrival at the Farm, I was promptly taken to meet my mentor who was a kindly gentleman called James Clift. He showed me where we would be working and we discussed what we planned to achieve during the day.
The furnace had been set up by James in a simple open-fronted outbuilding. The furnace itself was a simple wire-framed affair lined with refractory.
We began work on creating the mould. James explained to me what the flask was and how best to pack the sand and create the cup. We used good old talc as a parting powder. If I’m honest, I can’t remember what casting sand we used. I don’t think I even asked on the day as the question didn’t cross my mind. I’m guessing it was Mansbond oil sand but I’m not certain. The pattern we used was of an antenna knife that was based upon a real bronze age design produced in Hallstatt, Austria in the late Bronze Age (circa 800-1200 BC – I believe this period is also known as the Urnfield Culture if I’ve done my homework well).
Warming up
With the mould ready for pouring, we broke for a very welcome cup of tea. You remember my earlier comment about the cold gnawing at your bones?! I was grateful for the hot drink and also the next task – feeding the furnace with fuel and oxygen.
A crucible was loaded with appropriate quantities of copper and tin in a 90/10 ratio. It was placed carefully into the furnace and lump-wood charcoal was packed in around it.
James had a dual-bellows setup with two smallish bellows connected to the top of a ‘Y’ shaped pipe. The bottom of the ‘Y’ fed into a small hole at the bottom of the furnace. Each bellow was pumped in turn … left, right, left, right, left, right, and so on. Charcoal was continually added to the furnace which crackled and popped satisfyingly as the air was forced in through the flames underneath. Small sparks escaped in the column of hot air that rose from the mini inferno. If the furnace itself wasn’t enough to expel the feeling of coldness, the act of pumping those bellows for the best part of two hours certainly did the trick.
Mirror
Eventually, the copper and tin had become molten. That beautiful mirror-like surface could be seen gently rippling inside the crucible. It was time to pour.
The cast was given a few moments to settle, after which we carefully parted the two halves of the flask. This was an exciting moment for me. This was the result of a morning’s hard graft and my very first experience of casting bronze.
To say I was very happy with the result would have been an understatement. This was very cool! The knife was removed from the mould with tongs and unceremoniously dunked into water to cool it down for handling.
Clean up
The next job, of course, was the clean up, however before I embarked upon this task, James explained to me how we could harden the edge of the blade. Using the rounded end of a ball pein hammer, I hammered the edge of the knife blade on an anvil. James likened this to compressing the metal crystals, compacting them, making the structure more dense and thereby making it harder. I’m no metallurgist but this made sense to my way of thinking. Once the hardening was completed, the filing and clean up began.
James furnished me with a simple, small file and I began the arduous task of cleanup. I began by filing off any burs along the blade’s edge and it very quickly became apparent to me that this was not going to be a quick job. James explained how we should use a course file to get the item into shape and then use successively finer grades of file and ultimately paper to gradually remove all the score marks from the previous, courser phase. Finally, we should use wire wool for the final stage of polish.
Power tools?
I asked James about the use of power tools to speed things up a little and he dismissed the idea stating that the finish was never the same and it’s easy to overdo things. Before you know it, you’ve removed too much material. I could appreciate his sentiment so no power tools were used in the production of this knife.
By the end of the day, most of the dull surface had been removed from the knife blade but the finish was still very course. Additionally, the blade edge would need considerable filing to create a keen edge. I knew I still had many hours ahead of me before I arrived at a finish I would be completely happy with.
Leaving Butser Ancient Farm that evening, I felt very happy with what I’d learnt that day and also with what I carried home with me. I had formally begun my relationship with bronze and my first artefact was taking shape nicely.
