Stones are checked every so often to see if any have split or at worst exploded. An explosion can leave debris in the elements so the firing has to be abandoned.

(1996).  
Goldsworthy Andy; Jerry L. London: Viking.

Andy Goldsworthy OBE (born 26 July 1956) is a British sculptor photographer and environmentalist producing site-specific sculpture and land art situated in natural and urban settings.