Standard gold-filled wire has a base metal (eg a brass alloy) as it’s base with a thin layer of gold fused over the base. When you melt the tip to make a ball, you are MELTING the entire tip–both the brass and the gold. This forms a ball made of a new alloy consisting of both the brass and the gold. Thus your ball on the end of the gold filled wire will be a brass alloy with a bit of gold in it. It will tarnish over time. Note that this is totally different from soldering gold filled.
Janet in Jerusalem