Glass almost always fails because of tiny surface imperfections which act as stress concentrators. One strategy for decreasing susceptibility to fracture due to these imperfections is to place the surface of the glass in a state of compression, which effectively forces these imperfections closed. This can be done by chemical means, by a process of ion exchange. Sodium ions have a calculated ionic diameter of 0.19nm, while potassium ions, with the same valence, have a diameter of 0.27nm. If K ions replace some of the Na+ ions in a soda-lime-silica glass, the larger potassium ions will produce a state of compression in the glass surface. The stresses were measured to be very high, up to 860MPa. The compressive layer in ion-exchanged glass is very thin, limited by the diffusion process to approximately 0.001cm.
