I have had this happen to me twice, and been in the room when it
happened to other people two other times. Have not had the bucket
split, but have spread copper, shakudo, and/or shibuichi bits around
the area. So far I have not taken the time to experiment and figure
out the why. My guess is that it was due to either:
pouring the copper to slowly into the water.
having the crucible to close or to far away from the top of the
water surface when pouring.
Either way it looked like water got encased inside the molten metal.
(Similar to the water occasionally found in streling casting grain.)
The resulting super heated water, i.e. steam pressure, overpowered
the surface tension of the rapidly cooling metal and blew up the