I need to make some small parts. Ideally they would be made of steel.
And I'd really like to setup a home melting furnace, but my landlord
would not share my enthusiasm.

Low temperature metals are a possibility, like pewter, whitemetal, or
solder. However these aren't very strong.

I can do lost wax or RTV mold making. What's the toughest non-metal
material that I could pour into it?

The toughest thing I can think of is JB-Weld glue. It's just a little
too thick and doesn't flow very well. Epoxy is pretty weak. Then
there's urethane plastic...

Can anyone recommend a tough material for pouring into a gravity mold?

Thanks for your help.