You probably already know this, but one can’t be too careful. Molten aluminum is DANGEROUS! 1220°F will seriously put a damper on your fun if it gets on you. Wear gloves, safety glasses, jeans, and heavy boots. Don’t do anything stupid. There we go. Done with the “safety talk”!
With your mold ready to go, light your furnace. Your furnace could be a paint can with a hole near the bottom of the side for a metal pipe to provide extra air from a ShopVac blower. Place charcoal in the paint can, light it, turn on the ShopVac, and you have a furnace! You could also build something more complicated like what I have in the video – a helium tank lined with refractory and capped with a refractory lid. David Nash on YouTube has some good tutorials on doing this kind of furnace if you are interested.
With your furnace lit, place aluminum in your crucible and put the crucible in the furnace. If you use a paint can furnace, you could also use a soup can crucible. No modifications needed. However, I bought a clay-graphite crucible from http://www.budgetcastingsupply.com/. If you used a soup can crucible, you can just pick it up from the furnace with pliers from the furnace. However, since I have a more fragile crucible, I got a friend to weld some crucible tongs from mild steel stock.
Phewft! By the time you’re done reading all that, your aluminum should be melted! Make sure that you put enough aluminum into the crucible to fill up the entire slingshot mold. Once it is all molten, remove the crucible from the furnace. Add about 1/2 tbsp. each of washing soda (sodium carbonate) and table salt (sodium chloride) to the melt to remove gas bubbles and slag, respectively. Skim off the created slag with a spoon or, like I have, a flat steel rod.
This is the best part! It is time to pour the molten aluminum! As you pour, make sure to admire the one-of-a-kind shiny liquid streaming from your crucible. To actually pour the aluminum, pick up your crucible with your tongs/pliers and bring it over to your mold. Evenly and steadily pour the aluminum into the sprue (the cut-off soup can), filling the sprue at least 1/2 way full. The vaporizing Styrofoam may make a fun little burst of flames or smell awful, but you are doing this outside… right? Pour excess aluminum out of the crucible onto some dry dirt or sand, then let everything cool for at least half an hour.
While you let the aluminum and the furnace cool, you may want to clean up. My forearms resembled those of a coal miner when I was done casting. The video of me pouring my slingshot is below:
