Originally Posted by
geargnasher
Springfield is right, it isn't lead, or tin, that's why it won't melt away. It is accumulated oxides of mostly tin, it starts a 'seed' in a shiny spot and just builds and builds a few molecules at a time from the flash-oxidized molten surface of the metal you pour into the cavities. It's the same sort of stubborn crud that clings to the sides of a casting furnace as the alloy level drops. A reducant like beeswax or paraffin quickly takes care of it once you get the mould up to the melt point of the elemental metal.
Being an oxide, and beeswax being a reducant as JonBinGlencoe pointed out above, you can heat the mould to the point that the tin would start to melt if it weren't oxidized, apply the wax to instantly reduce the oxides to elemental, liquid metal, and wipe them away.
It happens to me with brass moulds because they do like to be run hot, it IS worse with binary lead/tin alloys, and the brass moulds DO like to be run hotter/faster than other types which DOES exacerbate the oxide accumulation.
What I do to patina a brass mould for the first session is preheat it to casting temp in a mould oven and let it cool naturally a few times, then smoke the cavities lightly with a BBQ grill lighter before ever putting lead in there. That keeps those little soldered "seeds" from ever starting. After a few casting sessions, I wipe out the soot and leave them alone. This is the only time I ever smoke moulds because it can cause a lot of other problems, but it really helps break-in with brass.
Gear