Chemical tempering replaces atoms with bigger ones that have the same valence. The one I'm familiar with involves sodium and lithium ions. The increased side of the replaced ions results in the compression. This is very good; glass treated like this can be bent almost like plastic. However, if memory serves, the compression is only to the depth of about 5 microns.
Note that chemical tempering is similar to case-hardening, in which carbon monoxide is allowed to seep into a metal. I once case-hardened some aluminum rabbit in a smoky flame. It worked great.