Be sure to carefully examine each photo and read the highlights, the majority of this instructable is explained and documented in photos. Any and all questions will be answered to the best of the ability, and if we don't know we'll do our best to find out.
This forge is a bit more durable, but much less portable than other models you may have seen, but it can also melt steel should you choose to buy a proper clay-graphite crucible. The ability to melt steel is given to the furnace by adding clay/brick walls to the top of the furnace as seen in step three, no extra equipment (aside from the mud/brick and crucible) is needed.
Step 1: Materials
2" Steel nipple
2" Steel end cap
Charcoal starter fluid
PCV pipe segment (~1.5" diameter)
Slag stick (A long bolt will work)
Proper safety equipment (eye protection, welding gloves, etc.)
Step 2: Construction of the Furnace
First, dig a small depression to sit the charcoal starter in, using the mud cover up all but one of the holes in the bottom. Fill the starter approx. 1/2 way with the charcoal. Take the blow dryer and duct tape a length of PVC pipe to it. Stick the end of the PVC in the one remaining hole and set the blow dryer on a brick or something else to keep it directly off the ground. Center the crucible (more on that later) in the furnace and fill in the sides of the furnace around the crucible with more charcoal. Soak with starter fluid and light a match.
We tried using a clay flowerpot but it cracked minutes into the first burn of the furnace. Not 100% on what happened there, either the pot wasn't completely clay, it wasn't properly fired, etc.
Step 3: Problems With the Ceramic/Soup Can Crucibles
The first crucible (a container used to hold the molten metal) we tried was a ceramic mug with the handle broken off. This lasted about 15 minutes before cracking, however because the coals were packed around it, the mug retained its shape, so at the end of the night and after melting ~20 cans, we had a good chunk of nearly pure aluminum in a puddle under the furnace. The slag was retained in the bottom of the mug, unable to pass through the cracks.
Following the instructions of other instructables and misc internet resources, we tried a soup can next. The soup can worked for a brief time of ~10 minutes before becoming extremely brittle. It broke while tapping the bottom of the can with the slag stick to try to get all the molten aluminum and slag out.
Step 4: Steel Crucible
We salvaged the nipple by turning it upside down and purchasing a new end cap. After heating it red-hot and using a hammer and chisel to knock off as much garbage as possible, the crucible was serviceable again. We also pounded a pour spout into the nipple for ease of filling smaller molds.
Its also interesting to note that when the steel was first placed into the furnace, it burned green. We believe its was the zinc used in the galvanization process that was causing the flames to turn green.
Step 5: Melting, Pouring, and Removing the Slag.
An easy pouring technique: Using the groove-join pliers, pull the crucible out of the furnace and set it on the ground. Open the pliers all the way, and grip the crucible around the middle of the nipple. This will allow you to turn the crucible all the way upside down should you wish. Of course, its best to wear gloves and eye protection during this time especially in case it splashes/other bad things happen.
Using scrap aluminum found around the dump or in the nearby woods as in our case, gives a much higher gross of aluminum with a lot less slag than soda cans. Go to your nearest dump/landfill/etc and get some scrap aluminum.
Slag is the non-aluminum material left over from melting down the aluminum. It could be other metals with a higher melting point, minerals, etc. The point is, is that it needs to be removed. Luckily for us, slag floats on top of the molten metal, so one takes the slag stick with the pliers, and scrapes the slag off the top of the pool of liquid aluminum. You'll know its slag because as it cools, it turns a dark gray color, whereas the aluminum will be shiny and bright.
Step 6: Final Remarks
Soda cans are not as horrible as most people say. While its true they contain a bit more slag than most other scrap metal, its easily removed, and after being melted down a second time to try to get rid of even more slag, they were reasonably pure. In short, for someone just starting out melting metals, soda cans are great. They're free, melt quickly and easily, and when you end up wasting 30 of them experimenting you don't feel bad about it.
When collecting scrap metal to melt down, be sure that you can actually fit the piece of metal into your crucible, or have a method of breaking it down. A dremel is invaluable in this aspect, with a cutting wheel and about ten minutes, I had a three foot piece of solid aluminum pipe broken down into manageable pieces. The little rubber/plastic caps on pipes melt easily enough, however the screws/rivets need to be cut out of the pipe (again by use of dremel) otherwise it turns into unnecessary slag.