What does 'mushroom head' on a chisel mean?

The opposite end from the cutting edge of a chisel is the striking head. With use or abuse, the steel of the striking head can become deformed, pounded out of shape. Striking with a mallet or hammer can compress and distort the flat surface. Some of the flattened metal may begin to overhang the sides, flaring out. read more

A “mushroom head” on a chisel is where you hit the head of a chisel so much that the head (opposite the point) gets deformed so that it looks like a mushroom. read more

Impact tools including wedges, chisels,and drafts are unsafe to use if they have mushroomed heads. This is because the head of the tool might shatter on impact into sharp fragments that are dangerous to any worker in the vicinity. read more