The (fluted) plugging chisel is designed for removing old mortar from brickwork during a process known as “repointing”.
The tool is shaped to facilitate this, and the“flute” (grooved part) of the chisel is designed to allow any remains to be removed.
How much mortar do you need to remove?
Exactly how much mortar needs to be removed will depend on the job at hand. As a general rule, you should remove the mortar until you have a space double the width of the gap between bricks (known as the joint).
Some cold chisels are made from steel which is simply not suitable for cutting through all metals. Instead, these may be useful for removing old mortar.