Bench Volcanos, Renate Sommer

    To cut a long story as short as possible, I was in the act of
pouring molten silver into an ingot mold (as I have done countless
times in the past) when there was a sudden and loud bang, the air
above me was full of glowing fragments of molten silver, the
crucible was flying through the air having been blown off it's
handle and my torch was snuffed out. 

Hi Renate, My best guess is water. Your ingot has been wet or moist
on the inside. When I was an apprentice in the early sixties, my
master taught me to run few drops of oil on the inside of the ingot,
and then wipe it thoroughly clean each time before filling it with
molten metal. Yours, Jon Holm Bornholm, Denmark