Today another in my ongoing, seldom-updated series of posts about building my own backyard foundry. Today I’ll describe how the final step works: actually melting and pouring the metal. First, see my previous post on how to make a green sand mold.
Start by assembling all the equipment you’ll need in one place, on a day with no chance of rain. (Click on any photo for a larger version.)
Going from left to right:
- I’m wearing safety goggles, a leather apron, cotton shirt and trousers, and leather boots without laces with the trouser legs around them. You want the boots to protect you from accidentally splashed molten metal, but also want to be able to kick them off in a hurry if they’re on fire. I wear natural fibers for the rest of my clothing; synthetic fibers can melt onto you when exposed to high heat and produce a nasty burn.
- The mold is in a deep iron tray – an old baking pan. If the mold separates then the aluminum will spill into the tray, rather than onto my concrete driveway. Concrete can explode when heated rapidly.
- My ingot tray, a rusty old muffin tin, is also on an iron slab, this one the former top of a table saw that I found on the side of the road.
- The white can contains broken up bits of scrap aluminum, old sprues and ingots to re-melt.
- The furnace and lid; note that the furnace is strapped to a hand truck for ease of moving it around; since it weighs 100 pounds.
- A bag of charcoal. The grocery store was out of wood charcoal, which is preferable because it leaves less “clinker” — tiny bits of stone in the ash — than briquettes, which are compressed wood and coal dust.
- A bucket half full of sand. In the event of a fire caused by spilling liquid metal, you always want to put it out with sand, not water. Water will turn to steam and launch the molten metal off in a random direction.
- Crucible lifters (in the bucket) — two iron rods with the ends bent into hooks that fit into the loops on the top of the crucible.
- Crucible pourer (in the bucket) — a thick iron rod with a bolt through it, so that I can lift the crucible by its loops and then pour it
- Stir stick (in the bucket) — an iron rod to move things around inside the furnace
- A long-handled spoon, to skim the dross off the top of the melt.
- A pair of pliers. In an emergency if I need to move the crucible in a hurry I can pick it up with the pliers.
- The crucible — a metal pipe with loops bolted to the opening — is in the bucket right now as well.
- Paper, lighter fluid and matches to get the furnace going
- A tub containing aluminum-foil-wrapped packets of KCl and Na2CO3, used to flux the melt and reduce dissolved hydrogen. Today I’m melting very clean aluminum so I’m not anticipating that either will have much of an effect, but it doesn’t hurt.
- Leather gloves and a face shield, for later.
- In the foreground, a thrift store hair dryer. In retrospect I realize that I’ve created a tripping hazard by running the hose between the furnace tools and the mold; next time I’ll remember to run the hose the long way around. But I am never going to be walking backwards carrying the crucible, so it won’t be too bad.
I put a layer of charcoal with some lighter fluid and paper in the furnace, get it lit, and a few minutes later, turn the air on low:
Note that I have removed the lighter fluid from the area. Let’s keep the flammable liquids away from the liquid metal, eh?
A few minutes later the layer of charcoal at the bottom of the furnace is burning nicely, so I turn off the air and put the crucible in the furnace.
I then surround the crucible with additional charcoal, put the lid on the furnace, turn the air back on, and put an aluminum tube into the crucible through the hole in the lid. There’s also a small amount of scrap aluminum in the crucible already. You don’t want to fill the crucible with big pieces of aluminum as it will expand as it heats, and possibly damage the crucible. It’s less of a concern with an iron crucible like mine; ceramic crucibles are inelastic and can easily crack if they’re full of expanding metal.
The fire will be smoky for some time as it is still low-temperature and there’s a lot of new charcoal to catch fire. Note that I have removed the charcoal bag from the area. I’m planning on melting a lot of aluminum today — part of the point of today’s melt is to find out just how much I can fit into the crucible — so I will need to add more charcoal halfway through. But I don’t want the bag cluttering up the area.
Ten minutes later the exhaust is much cleaner and hotter. Aluminum is not like iron, which gradually gets softer and more malleable as it heats up. Rather, aluminum is more like water ice: as it approaches the melting point it suddenly starts fracturing easily, then it gets into a slushy state, and then it becomes a liquid. Ten minutes in, the aluminum in the furnace is not liquid but I could tear it with my stir stick more easily than I could tear paper. The tube has started to collapse into the furnace under its own weight.
I put on my gloves and face shield and add more bits of scrap. Less than three minutes later the tubes have melted completely. The liquid aluminum has a huge surface area on the red-hot crucible bottom, and transfers heat very quickly to the solid aluminum tubes sinking into it. Eight minutes later I have melted an entire lawn chair and am out of tubes, so I start on sprues and ingots, which take longer because they have much less surface area per unit mass. At this point the charcoal has burned down quite a bit so I add some more around the crucible.
Aluminum tubing is easy to find as scrap and in fact for today’s melt I didn’t even have to leave my property; someone left a broken lawn chair on my front lawn. Normally I am irritated when people leave trash on my lawn but I will gladly accept broken lawn chairs! The alloy is designed for easy extrusion, not for metalworking, so it is not ideal, but you can’t beat the price, and it usually is not painted. Paint (or vinyl coating) makes toxic fumes and causes the melt to take on hydrogen gas, which then forms bubbles in the casting. The Na2CO3 will help remove the hydrogen, but I’d rather not go there in the first place if I can avoid it.
Twenty minutes later I have melted enough sprues and ingots that the crucible is almost completely full. I take the lid off, skim the dross — the bits of aluminum oxide and impurities that float — off the top and put it in the ingot tray to cool; this will be trash. (Trying to recover aluminum from aluminum oxide is not really worth it when aluminum is so plentiful.) I add some flux and degassing powder, and then turn the air blast on to maximum to get the last bit of heat out of the charcoal. I remove the cover from the mold. A minute or so later, I’m ready to pour; I remove the lid, take off the last bits of dross, and get ready to take the very full crucible out. Here’s what the inside of the furnace looks like:
This isn’t quite as hot as I would like — I’d prefer that the crucible be more obviously red-hot — but this was the largest melt I’ve done and it’s taken longer than usual. As you can see, almost all the fuel is used up. It’s hot enough to pour and so I’m going for it. I very carefully lift the crucible out of the furnace and put it down in the sand bucket:
And then lift it right back up again with the pourer:
And then pour into the mold; the remainder goes in the muffin tin.
Then wait for it to cool, dump the sand back into the sand bin and extract the casting from the sand. The result was 1632 grams of aluminum, so now I know the maximum mass of aluminum I can melt with this crucible and furnace.