I used to do deep blasting/carving in thick glass, many years ago. I used black silicon carbide (because it can be used over and over without dulling). I never used walnut shells (too mild for deep glass carving I would imagine) but might be good for "frosting" glass. Wish I had tried that.

I stopped my glass blasting 'career' after experiencing some eye problems. Turns out I was plugging up some glands around my eyes/eyelids (13 had to be removed). Even though I THOUGHT I had a good blasting booth setup, goggles, air filter, etc. the fine particles still managed to mess me up. I do not remember what the glands were called, but they had something to do with producing tears (they were not tear ducts).