What precisely do you mean by burn.
If it was getting hot enough to melt the porcelain you'd have no engine well before that, as it would have melted first.
Porcelain is normally made from Kaolin, which is aluminum hydrosilicate, this is fused with white quartz, well specifically feldspar, at a temperature of 1300-1400 C.
Even soft procelain needs 1100C to melt.
It can be pitting or something else though, really need to see a plug.
tabetha