The exchange of small alkali ions in glass with larger ions from a molten salt bath below Tg produces surface compression in the glass surface, which results in glass strengthening. Two competing processes occur: (1) generation of stress due to "ion stuffing" and (2) relaxation of the same stress due to viscous flow and network ductility. The economics of superior protection of glass products under common challenging environments generally require a compromise between obtaining greater depths of compression versus aiming for higher compression magnitudes. Control of process parameters such as bath temperature and compositions essential. Methods of quality control include measuring the modulus of rupture, surface compression and case depth. The newly instituted ASTM standard C1422-99 classifies chemically strengthened flat glass products.
