With the furnace body complete and set aside to dry, you can start on
the furnaces lid. Start with a 1" tall section cut off a 12"
diameter sonotube. An easy way to do this is, add a band of
1" wide tape to the top of the tube then cut along the bottom edge with
a box cutter. Now lay the section of sonotube on a plastic
bag on a perfectly flat surface. Place a plastic yogurt cup,
or something similar, in the center to form the exhaust hole.
Add refractory until the tube is ~ 3/4 full. Now is
a good time to pack down the material to assure there are no large air
pockets. With the first layer packed down, add several pieces
of wire for support. I used stainless steel safety wire, but
a cut up metal coat hanger would work fine also.
Finish
filling the sonotube, and then begin building a raised center that is
7" in diameter. You can free hand this part as it is not an
essential part of the sealing system. What is important is
that the 1" thick lid top is perfectly flat on both sides. Before
covering the lid to dry I added a few small sections of safety wire to
connect the raised section to the main lid body. After two
days of misting the lid and having it covered, you can peel out the
plastic yogurt cup by hand.
About 5
days after construction, depending on weather conditions, you can light
the furnace. Pack the bottom of the furnace with charcoal
brickets on a bed of crumpled up newspaper and light the
furnace. If the refractory starts to steam, it was not cured
enough for this step. Remove the charcoal with a small shovel
or tongs and wait another day. If no steam is present, keep
adding charcoal until you have a healthy bed of coals. Let
this bed of coals burn several hours to cure the refractory as well as
burn off the sonotube and wood plugs connecting the furnace to the
burner chamber.
After
several hours the two top plugs were completely burned out.
The bottom plug was still in place but this is to be
expected. When the burner is lit the bottom plug will burn
out in a matter of minutes. Between the burning out the sonotube and
plugs and firing the furnace fully, you should wait over night.
Place the lid on the furnace and let it cool slowly until
morning.
So it's
the big day and you cannot wait to fire up your new foundry.
Before you can do this there are a few additional steps to
preform. First make sure you are in an open area with nothing
overhead. If you built your furnace on a cart, assure the
wheels are chocked and the cart/wagon is stable. With this
done take a 3" diameter piece of aluminum duct and secure one end of it
to your blower. If you do not have a dedicated blower, a leaf
blower or the exhaust on a shop vacuum will work fine. Now
using a hose clamp, clamp the other end of the duct to the cast iron T
on the burner box. Tighten the clamp just enough to allow you to slip
the duct on and off of the T with some resistance. Remove the duct and
blower and set it aside for now. Next you need to hook up
your fuel line. For this I tapped a 3/8 compression fitting
into the bottom of a one gallon plastic fuel can.
Check all
your connections one last time, and you are now ready to begin.
Start by building a small fire in the burner chamber with the
cap and duct off. The best way to accomplish this is with
newspaper and 3-4 charcoal brickets. Once you have an
established ember bed assure the fuel line is closed and fill the waste
oil tank.
Now
slightly open the fuel valve, to allow a small amount of oil to
drip onto the fire. Let it to burn at this rate for a few
minutes to allow the plastic coffee can liner to burn off.
During this time and all other times, stay out of the fumes!
While you are waiting, screw on the cap and place the lid on the
furnace body.
Once you
are ready, open the oil line ~ half way and connect the blower hose
back on. STAND
BACK! Now turn on the blower. If
everything is correct you will hear the furnace ignite. From this point on welder's
gloves and a face shield are a must!! Adjust the fuel trim until all
three burner holes have yellow orange flame as shown below.
After
only a few minutes my refractory started to glow a bright red.
This was way too hot for the aluminum I intended to melt.
If the furnace starts to get too warm simply reduce the
amount of oil via the needle valve.
Below is
a 5oz melt of aluminum which took about 5 minutes in a ceramic
crucible. Remember never to open the lid of the furnace with
the oil or blower on.
Below is
a picture of the dross I removed from the top of the melt.
Dross forms when metals oxidizes before they burn. Depending
on the metal dross can usually be fluxed out or simply skimmed off the
top.
In the
next guide I will highlight mold construction, and other basic
techniques. For now though you have the basic know how on
constructing a simple furnace. Please be safe and treat every
surface as if it's hot.
