a piece of broken pottery to scrape himself

The scraping scratches the skin to lessen the itch.

sat down in the middle of ashes

This probably refers to a place where trash and garbage were dumped and perhaps burned. Sitting in such a place was a sign of deep mourning. AT: "sat on the trash heap" (See: rc://en/ta/man/translate/figs-euphemism and rc://en/ta/man/translate/translate-symaction)