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.

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

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

"a shard big enough to scrape"

"ashes from the fireplace"