Furnace – MARK 3 (by )

Ok! Right!

Furnace Mark 1 was made in a coffee can, and was a bit too small. The crucible it was designed for was too small to pour some of the things we decided we wanted.

So we made the Mark 2, which was bigger. And then it was nearly ruined when a crucible (thin steel cup, since I can't find any iron pipes anywhere, and couldn't weld until recently) leaked molten Al everywhere, so we moved up to Salamander crucibles. And then we had to make crucible tongs, since we couldn't fit them with lifting lugs. And the Mark 2 turned out to not have enough clearance to fit the tongs around a crucible inside, as well as the inside having gotten rather crumbly and fallen to bits a lot. We tried to chop it down the side so one half of it (except for the base) could be removed to get the tongs in, but it just turned to power.

So on to Mark 3. For a start, it needs to be bigger. But maybe it can be smarter, too.

For the last two furnaces, we've lined them with a mixture of concrete, bentonite fireclay, and perlite. Apparently the concrete gives it strength, the perlite makes it a good insulator, and the bentonite makes it withstand the heat.

However, I've tried an experiment: I made cakes of perlite+bentonite and perlite+concrete alone, and blasted them with a torch until they glowed yellow. The concrete one was indeed harder than the bentonite one, but heavier - and after blasting, it did just crumble into powder. I think this explains why our existing furnaces have tended to go crumbly inside. The bentonite one was plenty strong enough as it was, and after firing, a lot stronger than the concrete.

So I'm planning to make the next furnace with an outer layer of perlite+bentonite mix - basically perlite with the minimum amount of bentonite required to make it stick together; it's a bit like the inside of a Toffee Crisp bar. The result is light, lacking cement, and a great thermal insulator.

However, this stuff does not take to being knocked very well. So I want an inner layer to protect it; something that does not require any heat insulating properties, but that will remain strong even at high temperatures. Obviously, cement is even less useful here than in the outer lining; it's most likely going to have to be a material similar to what crucibles are made of. Or firebricks.

Therefore, I've made a few experimental slabs of material; when they're all dry, I'll fire them, and then see what's the strongest. The mixtures are:

  1. Sand+bentonite, 1:1
  2. Sand+bentonite, 2:1
  3. Sand+bentonite+graphite, 2:1:1
  4. Bentonite+graphite, 3:1

The graphite powder (also known as plumbago) is interesting. It's sometimes a silvery black powder. Now, if you mix some bentonite with water, it becomes a sticky mass that's really quite hard to stir into a uniform mixture. However, just add a little bit of graphite and... instead of a thick sticky mass, it's a puddle of quite watery black liquid. The lubricating properties of the graphite appear to completely destroy the stickiness of the clay.

As I understand it, the strongest result should be a sand+bentonite mixture, since the right ratio of sand to clay will result in the sand grains being stuck together by clay, in the same way that the strongest mortar is made by having sand grains held together by cement. I've tried introducing graphite, however, since apparently crucibles can be made from a 50:50 fireclay/graphite mixture. I want to see what the benefit of this is; however, I've already found that it's a lot easier to mix and work with graphite added!

Anyway, when I have the ideal inner lining mix, I plan to cast lots of flat 'tiles' of it and bake and fire them independently, then stick them into the furnace with fireclay grouting. That way I think they'll have more leeway to expand and contract without cracking, both during their construction and when they're in use. Also, any that do break can be replaced.

Also - I've found a supplier for refactory insulating wool, which can form another layer of insulation outside the outer one (um... outermost layer?). And they also sell ITC-100, a magical stuff that you paint on the inside of your furnace that then reflects a huge proportion of the radiated heat back in. With my existing two-layer system and then these extra provisions, I hope to have the most efficient backyard furnace ever!!!


  • By Derek, Mon 14th Feb 2005 @ 6:08 pm

    well, the most efficient backyard furnace ever would probably have a heat exchanger on the exhaust gasses to pre-heat the air (saves another 30-50% of the energy). Maybe something to include in the Mark IV.

  • By Derek, Mon 14th Feb 2005 @ 6:17 pm

    well, the most efficient backyard furnace ever would probably have a heat exchanger on the exhaust gasses to pre-heat the air (saves another 30-50% of the energy). Maybe something to include in the Mark IV.

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