Building the Furnace

Choice of Furnace Container

Back in Dave's day just about everything came in 5 gallon metal cans. These days those products are shipped in plastic pails, and about the only thing that still comes in metal is roofing/driveway tar. As a consequence you won't find many 5 gallon metal cans, and if you did you probably wouldn't want to clean it out. So- what to do?

First of all- don't be tempted to use a plastic pail! During usage the outside of the furnace will get hot enough to melt and burn plastic, so plastic is out.

Any 5-7 gallon metal container will do the job. I bought a 6 gallon trash can at the hardware store, this may not be in keeping with the dumpster diving, build it from scrap mentality, but I got sick of looking for a suitable container in a trash heap.

In this picture you can see I added an extension to the lid so it would have greater thickness. I was concerned the lid would crack at the edges. I also added metal ears intended to allow lid removal with some sort of tool. These ears have been entirely useless and can be left off, although some normal handles on the lid would be useful.

Dimensions of Furnace Container

Furnaces shouldn't be significantly larger than the material they are trying to melt- a greater volume will take more time and fuel to reach working temperature. If you start with the goal of accommodating a certain crucible size, the furnace dimensions will follow from that:

Diameter of Furnace =
  diameter of crucible +
  space between crucible and furnace wall +
  thickness of furnace wall

Height of Furnace =
  height of crucible +
  height of fuel bed +
  thickness of furnace base

In my furnace I use a #6 crucible, which is around 6 inches in diameter and around 7 inches high. When the crucible is in the furnace there must be enough space to get the lift out tongs around the crucible. I gave it around 1.5 inches on both sides of the crucible. The refractory thickness is around 2 inches on both walls and base. A 6 gallon trash can measuring 12.5 inches in diameter and 13 inches high suits a #6 crucible quite well.

Refractory Mixes

Refractory mixes are a bit like your Auntie's apple pie recipe, everybody has one a bit different and relatively little is done to work out the quantitative merits of one versus the other. I won't add to the confusion by presenting yet another mix, but I do have the following information.

Dave's book says use silica sand. Where I come from silica is SiO2 which is the dominant component of Plain Old Sand. So can one use suitably clean sand for the refractory mix? It would appear so. I used Quikrete Medium grade (#60) sand and it looks to be A-Ok. If you are really obsessive you can probably pick up sand officially designated as silica sand at a pool supply shop.

I was able to find both fireclay and castable refractory mix (both in fine powder form) at a local pottery supply shop. The castable refractory ran around $90 for a 1001b bag. Fireclay was around $8 for 100lb. I went with fireclay. The fireclay is a very fine powder, so wear a face mask while you are handling it.

I used Dave's recipe for refractory which says two parts of sand to one part of fireclay mixed thoroughly. Dave never says if it is 2:1 by weight or by volume. I went with 2:1 by volume- which means less fireclay, since as a fine powder it has a much lower density than sand.

I was initially concerned that just fireclay and sand wouldn't give enough structural integrity to the furnace wall. This may be what leads people to add cement to the mix (which would seem a bad idea given the water combining chemistry of cement). Following the simple sand/fireclay mix has worked for me. The walls of the furnace had no tendency to slump after the refractory was rammed in tightly, and the furnace wall has no cracks after several firings.

Do take the time to mix the refractory thoroughly and carefully. If you have access to a cement mixer and don't need exercise, use it. Use the minimum of water necessary to have the refractory hold its shape in the form. Leaving it in a sealed container for days (or even weeks) actually seems to be a good idea. It gives the water a chance to evenly distribute through the clay and sand.

Other Construction Notes

Dave's book talks about a collapsible inner form that forms the inner wall of the furnace. Wrapping sheet metal around wooden circles and holding them with tape wasn't going well for me, so I used nails and built a non-collapsible inner form. This was a mistake. After the refractory is packed in, pulling out the inner form as one piece is very difficult. I finally managed to get the form work out, but it was tough (imagine your hand going into a maw of shredded sheet metal). A collapsible inner form makes life much easier.

The tuyere (air blast entry) form is supposed to be made from a split piece of piping. I just used a short length of pipe that was snug fit to the the longer pipe that is attached to the blower. The short length of pipe is left in the furnace wall permanently.

For the blower I purchased a cheap 2-speed hairdryer at Target. On a low fan speed the metal melts quite nicely- indeed I think a lower fan speed is preferable since at higher speeds more heat is blown out the top of the furnace.

In this picture we have the assembled furnace. The blower (hairdryer) has been press fitted (ie- jammed) into an ABS fitting that has been screwed onto the long blow pipe. The pipe is long enough to prevent plastic melting temperatures from migrating back to the blower end.