I started building a Goodman-Holmes heat treat oven some years back, but never got terribly far with it, as I was to the point of puzzling over how best to cast the refractory cement (once the slabs are in place, the whole unit gets welded together, so replacement of the refractory is pretty much impossible). […]
I started building a Goodman-Holmes heat treat oven some years back, but never got terribly far with it, as I was to the point of puzzling over how best to cast the refractory cement (once the slabs are in place, the whole unit gets welded together, so replacement of the refractory is pretty much impossible). Plus, the refractory cement suggested in the book (sourced from McMaster-Carr) turned out not to be rated for direct contact with flame. I’ve since started looking more at just getting a kiln for the purposes of heat treating, as temperature control should be much more precise with an electric rather than gas system (plus never having to worry about running out of propane).
As such, I’ve been checking craigslist from time to time for kilns, and found one for $150 that looked like just the sort I’d find useful. Roomy interior, front opening door, 220VAC power. Little old lady drove it only to church on Sundays (actually, the seller’s aunt used it from time to time to fire ceramics – close enough). Looked like something out of the 1950s and even smelled like the piece of vintage lab equipment that it is. I bought it the next day and somehow managed to single-handedly manhandle it into the garage on its accompanying stand.
The only thing I’m not sure of is whether an electric kiln will be more cost effective than a gas oven. Probably, but the mere fact that the unit has a much larger interior than the gas unit already puts it ahead. It’s large enough to heat treat a big knife blade, which is a project I keep hoping to attempt one of these days (I’ve had the O-1 bar stock patiently waiting for me for a very long time).
After purchasing a length of appropriately sized (the meter goes up to a whopping 50 amps, after all) cable to form a suitable extension cord, I plugged the unit in and let it heat up.
It drew a constant 25A and I let the temperature climb all the way up. Note that this unit has two temperature settings: on and off, depending on whether you have the cord plugged in or not (no power switch).
I thought “what the heck” and let the kiln climb as high as it liked. At just over 2350°F the current draw fell drastically, which I figured was just some sort of transition point in the elements that caused a sudden increase in resistance. Well, in a sense I was correct – driving the elements to failure will indeed cause them to break, and you can’t beat the resistance of an air gap.
I heated up the broken ends with a propane torch and twisted them together as per the notes at handspiral.com, but it looks like I have a lot of additional breaks to track down and repair.