Here are the steps I followed to turn them into ingots.
I do things a bit differently.
First, I flux, flux, and flux again while melting down whatever is in the pot at the moment. No, you don't have to do it, but you have choices:
- Get as much of the dirt, grit, rust, crap, etc as you can out of the alloy before it ever sees an ingot mould.
- Do it only once - in your nice casting pot, with metal you're about to try and make perfect bullets from. Don't mind if the bottom pour valve in your pot sticks.
- Don't worry too much about fluxing. Who cares...
It's about dirt - tin and other goodies do not become "un-alloyed; they're in solution, not suspension.
If you're going to do this more than once, it is not a bad thing to either make up a decent smelting pot or have somebody do it for you. Welders who work at pulp mills where they have access to stainless steel scrap pipe in big sizes should be high on your list. My pot looks pretty scabby and ugly, but it holds about 100 lbs of metal - it's not to hard to figure out how to have an easy pour stand built for the thing while you're at it. Melting the fruits of your scavenging down in large lots helps to keep alloys consistent.
While you're buttering up your welder buddy, have him make you some ingot moulds out of angle iron cut to the length that makes you happy with bar stock welded over the ends. They stand upright on the ground very nicely, ingots drop out cleanly, and they'll last forever. The ingots stack really nice as well. Mine are of about a size that three of them just fit in my 20 lb. furnace; I find tiny little ingots annoying to store, pack around, and work with while casting.
I don't add cold metal to molten alloy at any time while doing the smelting thing. Not ever. Not even once. I don't care how dry it looks or how dry you think you've kept it. All it takes is the tiniest little pocket of moisure to be lurking in there somewhere, and you will get a rapid education. Even bugs are bad news - which is how I got my education on moisture and molten lead, and why I don't cast anywhere that flying insects have access to.
Yes, it's tempting to use just a little bit of molten metal in the bottom to speed things up when melting the next bunch. The process of heating all those ingots up in an empty pot will evaporate any lurking water booby traps long before they have a chance to give you the lead shower treatment. I even get twitchy about adding poured ingots to my pot during a casting session. Which is why they get pre-heated in a bread pan in the oven while waiting their turn. Safer, and makes getting new metal up to casting temperatures a much faster proposition.