Andrew and Pete thanks for your posts, I now believe that some items of scrap had a magnesium content, and seem to now recall, that I somewhere I read of the dangers of melting scrap magnesium car wheels.
The tower video was very interesting and I had missed it. The night before I did my casting it had rained, and the ground on which the dross was emptied was damp concrete, and this contributed to the explosions.
The only unresolved problem is how to identify aluminium/magnesium alloy, as I have not come to any conclusions.
Cheers David