Having decided that I should try and do alchemy and metal casting, I needed a source of heat. More than that, it had to be heat of the right sort, i.e. the right temperature in the right place when I wanted it. And able to use wood or charcoal.

Further restrictions included that a lot of venues wanted heat sources placed at least 18 inches above the ground to avoid scorching the grass, and that it had to be capable of being built, used, stripped down and packed away in a car by one person. Finally, it should at least not look modern. I have not put any pictures of real period furnaces in this post because mine is still annoyingly not right, but I cannot see how to make it just right without either a van to transport it or an event permitting me to turn up two days early to build a replica furnace from the raw ingredients. Which would be fun, but I’d rather get a TV camera crew to film it.

Taking all that into consideration, the first obvious solution was to use modern lightweight furnace bricks. Anything made out of bricks or stone and clay would be heavier and hold heat for longer, which is not helpful if you have to pack up a minibus for a 5 hour drive north. That decided I had to work out a way of hiding the modern bricks, since they would look out of place in a medieval camp. The very first covering I tried, just clay, dried and fell off the bricks in a couple of hours, which was rather embarrassing. This is the guts of the furnace, setup for distillation, the white bricks are rather too modern.
acidmakingfurnace front
(If you want a closer look, click on the photos to enlarge)

I first sourced a steel base from a friend, with legs which screwed on and off. It worked well for several years.
furnace base mark 1

Here’s the base and the second version of the outer coating. This consisted of chicken wire covered with clay, which naturally dried onto the wire and didn’t fall off. It had the advantage of looking rather rustic. What I didn’t think of doing, despite having information about that sort of thing, was mixing it with dung so that it didn’t dry and crack so much.
The drawback of it was that it had to be remade every event, and meant that it took 2 hours to get it ready for the public and often required patching the second day.
clay and chickenwire furnace covering 2007

Then, with the same base I tried using slabs of stone:
Furnace design 2008
Finally I thought of mixing clay with chopped string, drying it and firing it for use as slabs of clay, which would make setting up and taking down easier. Unfortunately they broke rather easily when being fired or transported, and although the shards were useful for covering some chicken wire or the top of the furnace, ultimately it came to nothing.
This took 2 and a bit seasons to work out, with the furnace first trialled at Blore Heath in September 2006, and all through 2007 and 2008 I tried these various possibilities.

In July 2007 I got hold of an archaeological report into excavations in medieval Worcester that had evidence for the use of tiles as an outer protective layer for a furnace. This sounded ideal, but a little adaptation was required in order to meet my needs. Of course the inner insulation would still be required, and many of the alchemical furnaces were made of bricks, just a single layer, which meant that the inside would be at say 500C and the outer much lower, which I wanted to avoid due to the transport issues already mentioned.

I got them made and collected them just before the start of the 2009 season.
Only they were getting just a bit heavy for the steel base and its small legs. You can see in this photo that I have wedged it all up with some lumps of wood:
furnace lanark 2008 with tiles

I also tried to cover up the base with some linen canvas, which did give it quite a good look, here at Stirling Castle when they were doing things related to John Damien, the physician and alchemist to James IV:

All of which necessitated an improvement, but the way forwards was now clear:
melting furnace Lanark 2010

Note the sheet of stone acting as a base. It is heavy but easily carried, especially since it split into two pieces after use on slightly uneven ground and being heated rather hot by the furnace. Even taking that into account, the 4inch squared baulks of timber mean it is more intrinsically stable even on bumpy ground, because of the way they are rigid and sit nicely on top of each other. Even better, the 60kg or so of tiles added to the weight and stability, meaning it was almost impossible to overturn.
This is the final version of the furnace. I can think of no further improvements for a single man portable furnace that is capable of different configurations. Of course I could get specific bricks and tiles to make a circular distillation furnace, or build a bronze melting furnace more like that of Biringuccio, but it would require several men to lift it in and out of a van for events, rather like the blacksmiths forges.
As it stands it is portable, stable, fireproof and meets whatever re-enactment safety standards you care to think of, as well as looking at least vaguely authentic.