In spite of its brittle nature, glass can be strengthened by a
process called tempering. Traditionally this has been done by cooling
the surface of the glass more rapidly than the interior. The surface
becomes rigid first, and when the interior cools and contracts it
pulls on the surface, causing a residual compressive stress. This can
be done relatively easily and inexpensively during processing, but it
is hard to control and often results in nonuniform surface stress. It
is commonly used for strengthening glass windows and doors.
Chemical tempering is another way to strengthen glass by developing a compressive stress in its surface. In this process smaller sodium ions in the glass are replaced by larger potassium ions. This "stuffing" of a larger ion into the glass causes a compressive stress in the glass surface, and is accomplished by treating the glass in a fused potassium salt, such as potassium nitrate, at about 400 deg C (752 deg F). This process is more expensive than ordinary tempering by rapid surface cooling, but is more easily controlled and leads to a more uniform stress. It is now used for strengthening eyeglass lenses and will probably be extended to other forms of glass as its cost decreases.