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Chemical strengthening is the name given to glass products that have been strengthened by means of an ion-exchange process. It is a surface treatment which occurs at a temperature lower than glass melting temperature. This process increases the thermal and mechanical strengths of common annealed soda lime silicate glass without affecting its optical properties. The process is particularly useful for thin glass, tiny glass and shape glass which cannot be tempered by ordinary physical tempering.

The glass to be treated is dipped into a bath of dissolved potassium salts at a temperature about 380oC for duration from 4 to 30 hours, producing an ionic exchange between the superficial sodium ions in the glass and potassium ions inside the bath. The cycle time would be greatly reduced if the glass is made of certain elements such as lithium or magnesium because ion mobility between potassium and these elements is a lot faster. The process parameters such as ion exchanging time and temperature would be modified according to the type of glass to be treated and the required strengthen specification.

The introduction of potassium ions which are larger in size than the sodium ions results in the establishment of a system of residual stress characterized by compression stretches on the surface counterbalanced by traction stretches within the glass