You will require some basic materials for this project.
First and foremost is a propane torch with a blowtorch (standard) type
tip and a propane bottle to match. These are common items at any hardware
store. MAPP torches may also work. A MAPP/Oxy torch is probably a bad idea,
but if you had one of those you wouldn't need a furnace in the first place.
You will also need one empty coffee can or other large container. What
you use is irrelevant only so long as it's round and metal, big enough
to fit your blade in, and of a disposition for you to knock a hole in it.
Finally, some lava rocks (for gas grills) and a scrap piece of metal
window screen material, and a normal rock or chunk of slate about as wide
as your can will be needed. For the lava rocks, get the 'natural' and not
the pressed and formed ones. For the window screen, make sure it's as wide
in one direction as your can is tall and make sure it isn't nylon, which
will at first smell very bad and second melt away when you fire up your
furnace.
Begin by punching a hole just large enough for the
tip of your propane torch to fit through in the side of the can. Where
you punch the hole depends on what you plan to use for furnace for. If
you're going to be doing throwing knives punch the hole near the bottom
of the can so the tip of the knife (inserted point first in the can) will
be heated. If you plan to do normal knives you wish to heat the entire
blade and the hole should be made nearer the center. You can use a drill
for this if you're not some kind of luddite. Otherwise a can opener or
big nail will serve you nicely.
The reason you bought natural lava rocks is pressed
lava rocks are all the same size: Namely, too large. Since we're building
this furnace small we will need small rocks. There will be a myriad of
both large and small rocks in your bag. Weed out the ones that are about
the size of marbles and set them aside. If you run out of small rocks you
can make more small rocks out of the large ones by employing the services
of your driveway and a hammer. You're going to need to fill a large portion
of the can with rocks. Gather a bunch.
The construction begins. Roll your window screen
into a cylinder with a diameter of no more than two inches. The one I use
is one and one quarter inches. Place it in the center of your can and fill
the gap between the screen and the walls of the can with lava rocks. If
you aren't too horribly uncoordinated you will wind up with a lining of
rocks retained by a piece of window screen with a nice hollow in the middle.
Your furnace is now ready for use.
To use it, just fire up the torch and stick the
nozzle about a half an inch into the hole in the can. With any luck the
flame will not go out and your furnace will begin to heat up. Put the rock
over the top of the can leaving a one inch open gap or so to retain the
heat that would otherwise escape from the top of the furnace. Let it heat
up for a few minutes before you throw in the blade. Then simply insert
your blade, point first, into the hollow in the furnace. Replace the heat
retaining rock the best you can (the end of your knife sticking out of
the can may get in the way). Given time a large portion of the blade will
be glowing and ready for quenching. Take it out with some tongs or pliers
and have at it, quickly and before the steel cools.
Caveats. Obviously, being metal and plugged into
a propane torch your can will be inclined to get very hot. It shouldn't
melt, but the following stipulations apply: First, it would stand to reason
that if the furnace can make your blade hot enough for hardening it can
make itself hot enough to cause pain. Do not touch the can while the furnace
is running and give it a good ten minutes before you try to pick it up
after you turn it off. Second, the can will get more than hot enough to
scorch countertops and wood, so do not place it on any of these surfaces
if you value them.
After running a few blades your window screen might
degrade. If your screen is looking sorry just replace it. If you have a
rock so close to the hole in the can that it extinguishes the torch's flame
every time you plug it into the can just push it out of the way with any
handy solid object.
The thought of heating large blades has crossed
my mind more than once. In general you can't heat a blade much longer than
four inches in a furnace like this. Even if you do find a can tall enough
to fit anything larger in the heat will be spread too much and the blade
will not be heated sufficiently. Perhaps the enterprising bladesmith could
add additional propane torches in different locations to power a larger
furnace.
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