The steel 'scales', or basically rusts in big flat sheets from the intense heating/cooling cycles. This eventually wears down the thickness of the crucible walls, but mine are around 1/4" thick or so (I believe it's schedule 40 pipe). On the outside, the scale isn't so much of an issue, but on the inside it can pollute your melt a little. I tend to heat the crucible empty to good red-orange heat, then lay it on it's side in the sand pit and scrape out the worst of the scale with my skimming tool. Then it's back into the furnace and it gets charged with aluminum. It takes more fuel to get the crucibles up to temperature, but it decreases melt time on subsequent melts. I generally try to pour at least 4-5 crucibles (2-4 hours worth of foundry work) if I'm going to fire it up. Takes more propane for the first melt than the subsequent melts, so it's more fuel-economical.
On my 3" crucible I welded short (3/4" long, 1/2" diameter) 'lugs' to the top on the sides for a lifting/pouring shank to get ahold of, and a ring at the base so I can tilt the crucible via a handle with a small hook at the end. For the larger crucible, I'll probably use two rings on the top to lift, and a third at the bottom to tip the crucible.
The steel crucibles will rust like crazy. The heating/cooling cycles, plus the scale, flux and degassing chemicals. Molten aluminum dissolves steel very slowly, so you do pollute your melt a little bit, but I've never noticed any drastic changes in the aluminum's machineability or properties. Eventually, the crucibles will get to a point where they are unsafe to use. Toss 'em, and make new ones. I've got maybe 70-80 melts on my steel pipe crucible, and it's still good to go.
I prefer to keep my aluminums separated by three different types. The first is cast aluminum, cylinder heads, transmission housings, alternator housings, etc. These are generally sand-cast parts and the aluminum has a high silicon content that helps it flow better and take up more detail.
The next is die-cast aluminum. Hard drive housings (I'm a network and systems admin, I get dead drives by the metric ton...), some car parts, etc. Die cast aluminum parts generally have very finely finished surfaces, completely unlike a sand cast part. Typically very smooth surface, thin parting lines (if any), and numerous small cast features. Die cast aluminum is lower in Silicon than sand cast aluminum, as the dies are filled by hydraulic pressure vs hydrostatic pressure.
The last is extruded aluminum. Most aluminum bar stock is extruded aluminum. If it machines well, it should cast decently, although tossing in some good high silicon cast aluminum helps it fill fine details. Ladders, aluminum channel, heatsinks, etc. Any aluminum I can't readily identify goes into this bucket. It isn't bad, and still makes decent castings, but I prefer good cast aluminum for castings that have thin sections or lots of detail.
I don't recommend melting pop cans or aluminum swarf, though. Pop cans are fairly pure aluminum, and the resulting castings can be softer than expected. It's also dangerous as heck. If there's *any* liquid left in the cans, it can cause a steam explosion that will splatter molten aluminum everywhere. Also, the cans are thin, and oxidize a lot unless you shove them under the surface, which can cause the above scenario. Swarf is much the same - It's so thin it oxidizes more than it melts, the oils and whatnot cause more dross, and the burning oils in the melt increase the hydrogen content, causing porosity in the castings. The swarf can be compressed into a block, but I'm still leery of a boiling-liquid explosion sending molten aluminum everywhere.
Lastly, don't try to use a steel crucible for brass/bronze or cast iron. The brass/bronze will do a wonderful job of brazing itself to the crucible, and I don't trust the mechanical strength of the steel crucible at cast iron temperatures. Cast iron, brass, and bronze are about 3x as heavy as aluminum. So a #10 crucible (10 pounds of aluminum), weighs around 30-ish pounds when full of cast iron. At the end of a 4' pouring shank, that's a considerable amount of weight. And trust me, 4' is barely long enough. Cast iron is nearly white hot, and the radiant heat it throws off is *insane!*
Sorry for the long post, but I just enjoy sharing what I know about foundry work. It's enjoyable in itself, and greatly expands the possibilities for a hobby machinist with a limited budget. Feel free to ask more questions, and I'll try to get some pics of my equipment in the next day or so.
Thanks!
