Thanks once again to all who gave suggestions as to why the
explosion occurred in my ingot mould.
I am now completely convinced that it was ambient moisture that
caused the problem. I only use propane gas and the ingot mould is
coated with soot from a candle.
I finally bucked up the courage to remelt some scraps and spent much
longer than usual heating the mould. As I first heated it I noticed
a moisture line receding down the side. For some reason or other the
thing attracts moisture … and yet not enough to cause rust. We
live and learn.