Tempering is not the only way to toughen glass. Glass can also be strengthened by placing it in mechanical compression. The most commonly used mechanically prestressed sight glass consists of a stainless steel ring encircling a borosilicate glass disc. The key to its success is that the manufacturing process introduces a prestress that causes the steel ring to apply a uniform radial compression onto the glass. During heating, the glass is melted within the metal ring as the ring expands. Then, temperature is raised to the point where the glass and the metal ring fuse together. When the unit cools, the glass hardens before the metal ring shrinks back to its original size. This places the metal ring in tension and the glass in uniform radial compression.
The compressive force is so great that if the metal ring is cut the compressive force will be released and the ring will shear from the glass. Some mistakenly believe this indicates that the glass was not fused to the ring. Actually, it proves that the compressive force was stronger than the adhesion between the glass and metal. In reality, fusion between the metal and glass is only a by-product of the manufacturing process and not where its strength lies.
Radial compression improves the performance of the glass because it offsets the effects of tensile stress. There are almost no process conditions under which external tensile stress would overcome the compressive stress. Under such strong compressive force the glass, in effect, becomes elastic.
In addition, the metal ring contributes to failure-resistance because mounting and sealing pressure are applied to it rather than the glass, virtually eliminating most of the common dangers of imposing bending loads and erosion on the glass. Exceptionally strong bending forces may extend beyond the metal ring into the glass, but the glass remains elastic and able to tolerate bending as long as the compressive force applied by the steel ring exceeds the tensile stress on the glass.
Taken to the extreme, bending force can theoretically exceed the compressive force. But even then, the fused-glass window will not shatter. Rather, it will fail safe because it retains elasticity. When the surface stretches beyond its elastic capability, small cracks usually develop near the surface. The cracks can interfere with visibility, but they do not compromise window integrity. The subsurface glass (product side) is still under compressive stress and can withstand the pressures. The same mechanism relieves the stress of over-pressurization. In addition, the compressive force on the glass can also resist the effects of thermally induced bending, also known as thermal shock.