# It will break

Here "it" refers to the part in the wall that is about to fall.

# as a potter's vessel is broken

This means that the piece of wall will break as quickly and completely as a clay jar that falls to the ground.

# potter

A potter is a person who makes pots and jars out of clay.

# there will not be found

"no one will be able to find" or "there will not be" 

# a shard with which to scrape

"a shard big enough to scrape"

# fire from the hearth

"ashes from the fireplace"