Why tin? Tin melts very low in temperature. Basically, the lowest melting point of any useful, non-toxic metal(*). It melts around 500 degrees, is very castable, and looks brilliant. There's only one real good reason not to use it: it's relatively expensive. I got mine from Rotometals in San Leandro for around $10/lb. Its density is similar to lead.
(*) Tin may cause some illnesses, this is debated. work it as if it does. Do your own research.
Step 1: Safety!
Tin is cooler than basically any other metal you'll ever work with, that means that if you screw up, you'll probably survive and have to deal with the hospital, burn wards, grievous scars, and potentially being disfigured or crippled for the rest of your life. Always have full face protection, arm and body protection that buttons at the neck, and closed toe shoes.
Respirator might not be a bad idea either, as who knows what those stray 1 or 2 percent are in the 98-99% pure Tin. I suggest doing things outside.
Pretty much, dress like you're welding, only with a clear visor. Check out my fashion statement picture for an idea. Do any of this at your own risk, perform your own safety assessments. Only you are responsible for the consequences of your actions.
Step 2: Mold
I had a bunch of gypsum cement hanging around, so I made my molds out of it. It was a horrific and almost dangerous failure. Don't do it.
So after that, I got myself some steel. In this case, obtanium resulted in some 1/4" wall that fit my needs. After dialing the tig welder up to its maximum (and finding out that its cooling fan has 3 levels!), I had a mold.
The downside of a steel mold is that the steel holds heat so much longer than the tin, so it takes so much longer to cool. casting sands are a good option too, if you can get some.
Step 3: Melt That Tin: Get Ready
For a "furnace", I used an old campstove I found hanging around. It was propane fired, and I've got propane around. I'd suggest doing this in a place with adequate ventilation. Outside worked for me, but I still threw on the respirator, just because it is fun.
For temperature control, I used a Fluke IR thermometer, which was reasonably accurate, even with the shiny surface of the tin. I also took a lot of readings of temperatures of the pot and the mold to make sure the pot wouldn't melt and that the mold was appropriately hot.
At this point, don your safety gear, just because. Fire up the stove, throw a blingot into the crucible, and let it heat up over a low-to-medium heat. It'll start to puddle shortly. Once it puddles, you can dial up the heat, and add more tin. It's a lot like melting snow for water, if you've ever been snow camping.
Step 4: Melt That Tin, for Real!
I should say, before you start, it's good to figure out by displacement the volume of metal you will need, then calculate the weight, and add by weight. Tin slumps a lot, because it shrinks when it cools, a considerable amount. Heating your mold and planning for this make this not a problem.
Tin conducts heat really well, so it melts pretty much only after the whole chunk is just barely below melting temperature... That probably doesn't make much sense on its own, but you'll notice how it doesn't melt, doesn't melt, then just slumps and goes liquid really quickly.
Step 5: Melt It and Skim It
What you want is for all of the metal to be fluid, no lumpy bits, so get yourself a tool for messing with the molten metal. I used a bent stainless steel spoon. With a MIG-Welding glove gloved hand, carefully stir the metal and feel for any chunks. Also, be aware of the crap on top.
Skimming the crap:
Your spoon is "cold". It's hot enough to burn you, but it is still cold enough to instantly solidify the liquid metal it contacts. We'll use this to our advantage to grab up some of the blackish-yellowish scum floating on the top by grabbing some of the metal under it as well. Feel free to try and stir all the scum to one spot, then pull the spoon out, let it cool, then fish out the scum. It'll take some practice, so be patient, don't let the metal get too hot, and BE CAREFUL, and don't put your face over the molten metal.
Step 6: Pour and Let Cool.
If your mold is good, it should be like pouring liquor. If your mold has any moisture in it, or anything that changes phase below the melting point of tin, get ready for some flying molten metal.
Then, let it cool, let it cool a lot.
So before you think you're clever, the Tin conducts heat well, so you can check the temperature of the tin, and get a good idea, right? Wrong. Your mold material doesn't conduct heat very well at all, so it will be smouldering hot for a very long time. Let it sit a while.
Step 7: Extract, Finish!
I was doing a very simple casting (a block), so I didn't need any fancy finishing or anything, but at this point, feel free to polish or paint the surface. Go crazy, you've made something of metal!