What condition makes a chisel unsuitable for use?

A chisel with a mushroomed head is unsuitable for use because this condition indicates that the metal at the top of the chisel has become deformed, often due to repeated striking with a hammer. When the head mushrooms, it expands and flattens, which can cause misalignment during use and potentially lead to injuries or damage to workpieces. A mushroomed head can catch on materials or create unsafe conditions when striking, as it may not be stable or predictable, making it hazardous for the user.

While a dull edge, a damaged handle, and a broken tip are all conditions that can affect the chisel's performance, they do not pose the same immediate safety risk as a mushroomed head. A dull edge can be sharpened, a damaged handle can often be replaced or repaired, and a broken tip can sometimes be replaced or reshaped. However, a mushroomed head is indicative of a fundamentally compromised chisel that could endanger the user during operation.