Silicon bronze has a melting temperature high enough that it will
cause standard investments to flux (melt) if the section is thick
enough to hold the heat for enough time. So thicker sections will
get that nasty black rough surface. Try Satincast Xtreme it may hold
up a little better at that temperature. But for the best results you
need to move away from gypsum bonded investments. Either high temp
investments for platinum or shell casting.
James Binnion Metal Arts