Strengthening of Glasses by Partial Leaching

A technique for strengthening certain alkali borosilicate glasses is described. Porous surface layers of a desired thickness are created on an article, taking advantage of the phase separation and leaching phenomena in such glasses. The semiporous composite is then heated to sinter the high‐silica clad about the borosilicate core. On cooling, residual compressive stresses develop within the cladding. The mechanical properties of cylindrical specimens prestressed by this method were characterized by photoelastic measurements and moduius‐of‐rupture tests. Residual stresses as high as —410 MPa were observed and found to be in reasonable agreement with the form of a previously developed thermal‐stress model. The roles played by thermal tempering and stress‐relaxation effects are also discussed.