I use a mallet and chisels and occasionally pneumatic tools for rougher work on larger pieces. This is in part as a result of my training, but also because hand tools are better suited to fine detail work. Using compressed air is also noisy, dust producing, and leaves your hands numb, while carving with a mallet and chisel, although more time consuming, I find to be a more intimate and enjoyable way of working with stone.
Once I am satisfied with the image that I have chosen to carve, I trace the image onto the stone using transfer paper. As these transferred lines can be easily lost, I lightly go over the prominent lines with the mallet and chisel. Next I bring the general shape of the figure out in relief, keeping in mind what is intended to stand forward and carving out what is brought back. As other transferred lines are lost at this point, I then cut in other important lines referring to the original image as needed. The rest I leave to my own interpretation and carve as I go.
LEARN TO CARVE
Among its many joys, stone carving concentrates and quiets the mind and leaves an enduring expression of a creative process. It does not require much to get started and with a little patience you can learn on your own.
The technique is quite simple, but like anything, it takes time before it comes naturally. As the chisel is struck it is constantly adjusted up and down to keep from burying too deep into the stone or from slipping and not catching enough of it. The angle of the point of the chisel itself and the hardness of the stone also determine the angle at which the chisel is hammered. With a firm grip and some determination and patience these adjustments eventually become intuitive. The mallet is tapped with a soft repetition while cutting fine lines, or with harder strokes (and sometimes with a larger mallet) to remove stone more quickly.
The stone you choose to carve will determine which tools to use. For harder stones carbide tipped chisels are preferable, otherwise tempered steel chisels are well suited to softer stones such as slate.
Primarily I use carbide tipped chisels which I bought in India. With their delicate size (about that of a pencil) and fine but sturdy points, they are best suited for detailed work.
Larger chisels are used for deeper relief work or for sculpting. I often use American made Trow and Holden carbide chisels which are heavier in the hand and can remove stone with greater ease and speed (and can also be used with a pneumatic hammer). TrowandHolden.com
It is essential to have a good sharpening stone (diamond grit for carbide tools) and to keep chisels sharp. Detailed work requires frequent sharpening of tools as a dulled chisel will leave a broken edge to the line it cuts. Proper sharpening also takes time to learn. The chisel is held at about 30 degrees and it is important that it is pressed evenly against the stone. Besides a sharpening stone, a grinding wheel is useful while cutting harder stone due to the heavier wear on the chisels.