When I did my blacksmithing course, I learned about tempering steel tools.

Briefly, you COULD use ordinary cold chisels, but

1) You should get them retempered to cut stone not metal.

2) You need different shapes and sizes from just the standard (metal cutting) cold chisels.

Tempering is not an overdifficult skill - (I was a natural at it!) - it involves watching the right colour run into the right place, and starting with the right sort of tool steel. The colour indicates the steel temperature.

If you look up the subject in a good library, you should be able with a little effort to handle it yourself, or better, try to find one of those Adult Ed blacksmithing courses if you can. If you are a sculptor (of any sort) being able to make and temper and sharpen your own tools is a good thing.

Robin