Newsgroups: rec.crafts.metalworking
Subject: Re: Oven/Furnace, advice?
From: pjd@craycos.com (Phil Duclos)
Date: 11 Aug 1994 17:04:42 -0600

	Not sure where oven/furnace/kiln boundary is either but I have
built a few kilns so maybe I can help.

	Insulating (not structural) firebrick is used inside. This is
lightweight clay based brick with some silica/other added for fire
resistance. It is rated by the max temp it will survive. I forget who
makes it but the ratings go something like K-12 (1200F) thru K-32
(3600F). Higher temps cost more money and translate into a "sandier"
brick. K-12 are quite soft and lightweight. Easy to cut with a hacksaw
or file. Note that you are cutting an abrasive so expect some serious
wear on the tool.

	The outer shell better be metal as some heat gets through. I
used light gauge galvanized sheet. A shear and brake work best, but
snips and a 2x4 can work also. I used #6 sheet metal screws. One of the
biggest decisions is whether to have a "north-south" door or an
"east-west" door. The problem with a "north-south" door is that the
inside of the door, which is hot, is directly below your hand when
inserting/removing items from the kiln. The "east-west" door requires
side clearance. Take your pick. I used a piano hinge for the door.

	You'll need some resistance wire for the heating element. We
call it "nichrome" (nickel-chromium) wire here and almost nobody in
today's typical hardware store has heard of it. I found some in one of
the few remaining traditional hardware stores, you know, the ones where
not everything is wrapped in plastic? Electronic scrap yards are
another good source. In a pinch a wire specialty dealer or industrial
supply should have some. I used 18ga wire which had a resistance of 25
ohms per foot. I decided that I wanted a 10amp max draw at 120 volts
which resulted in 48ft of wire. I wound it on a mandrel resulting in a
1/2" OD coil about 3ft long. It is very stiff and winding it up
definitely hardens it. Be careful!

	The large kiln I made is approx 9d X 12w X 9h. I used two
heating elements in series and ran it off 240V. The bricks I used were
9 X 4 1/2 X 2 1/4. I chose dimensions which required as few cuts as
possible. Pick your own size.

	I laid out the bricks which constituted the interior walls and
cut slots about 3/4" deep for the heating coils to go into. The heating
coils will anneal and stretch when heated so its important to have a
way to hold them up and firmly in place. You really don't want contact
between the work and the coils.  The ends of the heating coils were
unwound a bit and stuck out the back where they attached to copper wire
with screws and nuts.

	A door handle and magnetic door latch finished it off.

	I have seen kilns where the bricks were pinned together to lock
them in place. I haven't tried this but it should work OK provided the
wire is quite stiff and fairly small gauge.

	I used "kiln cement" on one kiln to lock the bricks in place.
Bad mistake.  The cement dried and shrunk, which pulled the edge of the
brick off. It continues to flake off to this day, much to my
exasperation.

	The kilns I have made easily attain 1400F. The large one is a
little overpowered by the twin heating elements. Its temp swing is
larger than I would like once it gets up close to 1400F. I should
change the programming of the controller, but I'm lazy. Oh yeah, I used
a "K" thermocouple and small panel mount temp controller. Total cost
~$200, purchased from Thermx Southwest.  The controller is mounted
separately from the kiln although beneath would have worked OK, I guess.

	If you want a really big "oven" you might look for a "Blue M"
brand oven.  They are used for burnin, etc and might go up to 700F, I
don't know.

	Anybody need a heat treat furnace with inside dimensions of 9 X
2 X 5 and built like the proverbial brick outhouse? I've got one and
want to unload it.  email for details. cheap, works good! pick it up!