Bismuth subcarbonate is formed by dissolving bismuth in a mixture of hydrogen peroxide and hydrochloric acid to form a colorless solution of bismuth(III) chloride. This is then reacted with sodium bicarbonate to precipitate white bismuth subcarbonate. The precipitate is filtered and dried. Some of this precipitate is placed on a piece of aluminium foil and heated. It turns black on the edges, then melts. The flame coming off the molten lump is slightly green. Once the lump cools, it is a yellowish color on the outside.
I then took another piece of dried bismuth subcarbonate and placed it on an iron loop. Again, it turned black and melted. I heated it until it was bright red hot and splashed it on the cold stove surface. The bead cooled to a yellow sphere (flat on one side) with bismuth metal visible on one end. It appears that the molten bismuth trioxide reacted with the iron wire to form iron(III) oxide and bismuth metal, which has a low melting point.
The bismuth subcarbonate decomposed to bismuth trioxide and carbon dioxide when heated. Bismuth trioxide melts at 817 C.
Here is the bead with the bit of bismuth metal visible.