Chemical Glass Strenghtening
Chemical strengthening is used for float glass with thickness from 0.5 to 3 mm. It increases resistance to scratching, breaking and thermal shocks. Unlike the thermal toughening, chemical strengthening do not cause sags or any other deformations. However, if glass gets broken, it will not fall into small blunt pieces. It will create sharp glass fragments. This is the reason why it is not being used as a safety glass.
Glass is submersed into potassium nitrate bath at 300°C. This causes the replacement of ions from the surface of the glass with potassium ions from the bath. Potassium ions are larger than the sodium ions. Their replacement causes the surface of the glass to be in a state of compression which may reach up to 690 MPa. This compression serves as a shield which protects the glass but if the surface of the glass is deeply scratched or broken, this area loses its additional strength.