Thermal toughening is much more widely used than chemical tougheneing. Thermal toughening is quicker and cheaper.
Thermal toughening is better for thin glasses, that can't be toughened thermaly. Please give a reply if you need further Information.

I would like to know more about chemical toughening, if anyone can help:
- does the glass break like tempered glass (small and safe)?
- does the process affect the glass color?
- are the optical properties affected as they are with thermal tempering ? (I need high flatness and parallelism of the two surfaces)
- what are the thickness limitations? I have seen this process performed on thick glass (1/2") but I do not know the results.
- can you recommend a relevant www site or book ?
Thanks

Chemical tempering follows the same concept as thermal tempering in that it puts the surface layer into compression.  In chemical tempering this is done through ion exchange.  The glass is put into a molten salt bath and the sodium ions are replaced with larger potassium ions forcing the surface into compression.  It is used quite a bit in the medical field for bottles.  One of the main benifits of chemical tempering is that it can be done on complex geometry.  It does not alter the optical properties.  A good website is saxonglass.com/