I got a 15lb champagne cooling bowl, it was damaged already (why I got it free) but it turns out it sells for $400 euros!
I melted it in a small cast iron pan and used the lee ingot mold. Fluted a few times and that's all. It should flow if you melt it in your bottom poor casting pot, pure tin however does not! I tried it with 95/5 solder and it was very thick and didn't come out the spout.
So short answer, treat it like normal lead for melting assuming it is actually pewter.