This weekend I started building my new furnace after having gone to buy the fire bricks last weekend. I am using Pyrolite bricks from a place on Bruce McLaren drive (the famous NZ race driver/car builder – he started in an Austin 7!).
During the week I was looking for a suitable steel shell to use. I needed something about 400mm diameter. In the end I contacted Jensen metal products in Onehunga as they make air conditioning ducting. This comes in standard sizes so I arranged to get a 300mm long length in 400mm diameter. I assumed they’d just cut a piece of existing duct. My friend Mike, who lives just around the corner form them, nicely collected it for me. Turns out the firm was founded by his grandfather!
The pipe when it arrived turned out to be just rolled, galvanised steel spot welded together. They obviously made it specially. This was no problem, only cost me $10NZ, but if I had known I would have specified the exact diameter. The duct was a little larger than it needed to be.
That was no problem though. I still have on loan a spot welder (for my Austin 7 building) so I simple cut the cut the duct, tightened it up and spotted it back together. It’s definitely been rolled since when I cut out the previously spotted section it barely sprung open. I really should get or make a spot welder some time, they are incredibly handy.
I made it fit tightly around six bricks. I then cut a wooden base, a bit like a barrel end to fit in the bottom.
I also drilled holes for the vent and the tuyere (hole the burner sticks through – pronounced tweer) and a drain hole in the base. To drill the holes I bought the cheapest lock fitting kit I could get, about $8NZ from Bunnings. These come with a 22mm spade bit and a 52mm hole saw. I knew this would totally bugger them (and it did as the photos show) but it worked for this one off use. Using a power drill on a hammer setting works REALLY well with these pyrolite blocks. I needed to trim the bricks too so I used a diamond wheel in my angle grinder. It goes right through the things no problem.
Cutting and drilling. The hole saw did have teeth at one point!
I then put the base in the duct and added the bricks then filled the spaces behind them with mortar. I used ordinary brick mortar but with this special guacamole added. I can’t remember what the hell it is called but the brick people told me you add this to the mortar and it make it more heat proof and less prone to shrinkage. They use it (and the same bricks) in their pizza ovens. Since the mortar isn’t in contact with the flame I hope it will work OK.
Bricks in place, drain hole and tuyere with burner inserted.
I just put the mortar around the outside of the bricks and made sure it was well packed down. Now I need to let it dry for several weeks. I also need to work out a suitable lid mechanism (probably just a simple handle, these bricks are light). I will also make up a steel trolley on wheels to be able to move the thing around. A steel shell full of bricks and mortar is heavy! I messed up my first lid and drilled too large a hole so I chopped that one up to make some plinths to stand the crucibles on in the furnace.
The crucibles I made are 3.5 inches by 5.5 inches but then next ones will be slightly larger so the furnace was sized to them. I might add a steel hot face inside the furnace. Just a simple cylinder of steel inside the bricks to help protect them. Stainless steel might be good for that if I can get some.
I now need to let it set for two or three weeks which is plenty of time to make up a trolley and some handles for the lid slab. Can’t wait to fire this one up!