The Lid:
The lid for the furnace comes first. Cut a piece of sheet steel about 3" wide,
by roughly 3" longer than the circumference of the bucket. This will be something
like a strip of steel four feet long. If you want, add two
sort of tongues for attaching a handle to -- mine are about 1 1/2" high lobes,
rising from the strip, with a 3/32" hole drilled in the top of each. Curve the strip into a
circle, rivet the ends together (overlap of 1") and drill
a couple holes evenly along the circle. Strand wire back and forth through these
holes so it's like a tennis racquet. This will reinforce the cement. Put a steel
can with both ends cut out in the center (this will be the vent/exhaust pipe)
and fill up to the brim with refractory (see below.) Bake at 250 F for 2 hours,
then at 450 for three hours.
After four years of use, my lid has seriously cracked and pieces are falling out of
it. This is partly because the cure method is not sufficient. When I rebuild it
I will use two layers of 1/2" hardware cloth to supplement the wire reinforcement
in the lid. After three years of use, the can fell out of the lid, because of loss
of material nearby; that's actually been nice because there's enough space to get
a poker down into the crucible without lifting the top off.
The Bucket:
As I said, this is a big industrial bucket, more than knee-high
and about 18" wide. It has a hole cut in the side, maybe 1" diameter, 2
1/2" from the bottom, for the burner to go in. I cut it with a dremel tool, although it
would have been easy enough to punch a bunch of holes and connect them with a
knife. Put a piece of duct tape over the hole to seal it for now.
Next we have to pack the refractory into the bucket. First, cut two plywood
circles to have an outer diameter about
three inches less than the inner diameter of the bucket, and wrap a piece of
thin sheet steel around them to form a cylinder. This goes in the bucket, with
spacers of wood to center it, and the refractory is packed in.
The refractory comprises 1/3 fire clay, 2/3 fine silica sand, some broken-up
firebricks (optional) with only enough water to make it cohesive. You want it
as dry as possible. Definitely NOT sloppy. Ram up the refractory into the spaces with a
broom handle, to mash it, 2" deep, raise the spacers 2" and ram in beneath them,
repeat. When you get to the top, pull off the piece of duct tape over the burner hole
near the bottom and put a piece of pipe in, through the clay, into the inside.
You will later run air through this. Three or four air inlets make the burning
more even and efficient, but are more complex to make. Remove the cylinder (pull
the plywood out, roll the steel and remove it) add about 2" of clay to the very
bottom. You can also add a fire brick to the bottom, to give a good solid base
for crucibles to sit on.
The correct way to cure a furnace is to put a heating plate in it and slowly increase
the heat over a period of eight hours until there is no longer steam coming out, then
fire up the burner and run it up until the walls glow. If you haven't built the burner
you can cure it the way I did: yank the forms, light a fire with newspaper and kindling
down in there, put a hair dryer in the side, start blasting away, and begin dropping
charcoal briquets in there. I actually never had a problem with my furnace casing;
it has some big cracks in the side but it's also been hit by two cars, so I'm fairly
happy with its solidity, and the quickness of the cure was sure nice. However, if
it gets rained on, it dissolves. This is a clear sign that I did a very poor job of
curing it. But, hey, it works.
This page written on 12/15/00, last modified 23 May 2023.