What makes a hand tool like a chisel unsuitable for use?

A chisel becomes unsuitable for use when it has a mushroomed head due to the impact from striking it with a mallet or hammer. A mushroomed head means that the striking surface of the chisel has deformed, creating a widened area that can lead to improper force distribution when in use. This deformation can also increase the risk of the chisel slipping during use, making it less safe and effective.

Furthermore, a mushroomed head compromises the precision required for tasks like cutting or carving, as it can affect how accurately the chisel penetrates the material being worked on. Therefore, ensuring the chisel maintains a proper shape with a well-defined edge is critical for both performance and safety in hand tool use.