) , had taught him about skidmore locks , about square-door boxes , about secondary locking devices that can permanently freeze a safe 's tumblers if the combination dial is knocked off with a hammer and chisel ; he had taught them about barrel boxes , and niggerheads , and cutting keys ; the many uses of graphite ; how you could take a key impression with a brillo pad and how to make bathtub nitroglycerine and how to peel a box from the back , one layer at a time .