What condition makes a chisel unsuitable for use?

A chisel is unsuitable for use when it has a mushroomed head. The mushrooming occurs when the hammering action on the chisel causes the steel at the head to deform and bulge outward. This alteration not only affects the cutting edge's ability to penetrate materials but also poses a significant safety risk. A mushroomed head can lead to chips or fragments of metal breaking off during use, which could potentially injure the user or others nearby. Proper tools are essential for effective work and safety, making it crucial to address any damage to a chisel's head before attempting to use it for any tasks.

The other conditions, while potentially problematic, do not directly make the tool unsafe in the same way. An uninsulated handle may not be suitable for electrical work, but it does not inherently make the chisel ineffective for its primary purpose. A stress line in the handle signals potential weakness, but unless it is visibly broken or compromised, it may still be usable. An oval blade indicates that the chisel may not cut precisely, but it can still function adequately in many applications. In contrast, the mushroomed head can affect both usability and safety significantly.