I'll take a guess that they are chunks of
"Slag" from a foundry. Slag is the impurities that are skimmed off the the top of molten metal (using a ladle) from the melting pot.
Those pieces might be about the size of the contents of the ladle. Perhaps in this case the foundry practice was to toss the slag from the ladle into a bucket of water to cool it off before being disposed of.
NB
PS: Anyone who has been to Sudbury, Ontario will remember the huge slag heaps with railroad dump cars dumping the still hot slag into piles on either side of the tracks. Sudbury is the site of the HUGE "International Nickel" mine. A local sign entering town says "Welcome To The Big Nickel".
http://en.wikipedia.org/wiki/Big_Nickel
http://en.wikipedia.org/wiki/Inco_Superstack