Q: I’m melting manganese bronze and it makes a lot of smoke when I pour it into my plaster investment molds. Then the castings seem to come out porous, black and pitted. What am I doing wrong?
A: Manganese bronze must be melted under a flux coating, as it will fume when exposed to air in its melted state. Degassing may also help. For fluxes and degassers, consult your local foundry supply. Always pour into a full pour cup to avoid entraining flux and dross. If the pour cup empties too rapidly, try restricting the size of the main gate.
Pitting and porosity may also be a result of poor gate/vent design. Bronze shrinks as it cools, and if an adequate reservoir of molten metal is not available for the part to draw from, the cooling metal will pull liquid metal from the wrong place, distorting the surface and causing characteristic pits. Most art foundries prefer to use silicon bronze, which melts more cleanly, with much less tendency to fume. Manganese should come out fairly bright; one of its best qualities is its resistance to firescaling.
by Andrew Werby