Architects and designers have begun using glass to make load-bearing structures. Glass is very strong but starts to weaken the instant it is made. Even one gas molecule can break a silicon-oxygen bond, generating a tiny fissure that grows under stress. Protective coatings can help avoid new cracks but can affect transparency, the main reason for using glass in the first place.
Thus, structural glass is often strengthened by tempering, which compresses the surface so that more force is needed for the cracks to grow. Heat tempering, the process most often used, takes advantage of the fact that when glass cools slowly it shrinks and becomes denser. In this process, a sheet's exterior is cooled rapidly, keeping the surface less dense. As the hot glass inside cools slowly and shrinks to a denser structure, it pulls the surface inward and compresses it. Tempered glass can still break, but it breaks into more and smaller pieces than untempered glass, which reduces the chance of causing injury.
Tempering alone is usually not enough, however. A primary concern when building with glass is what happens if a component breaks. Unlike other materials, glass does not deform or otherwise give advance warning of failure, so if breakage occurs, maintaining the integrity of the structure is paramount. Lamination helps address this issue: glass sheets are bonded with thin interlayers made of plastic or other polymers; should a glass layer break, the interlayers keep the structure together. But lamination makes fabricating glass for structural uses very difficult. Because cutting into tempered glass causes it to break, each layer of the glass to be laminated must be polished and drilled for connecting fittings before being tempered and bonded together.
Some builders seeking to make glass structures unencumbered by metal or other materials are investigating less common kinds of glass. Conventional glass, called “soda-lime," expands when heated, so welding introduces stresses that can lead to failure. Components made of a less expandable glass could be welded together, forming, in effect, a continuous piece. Other builders are using adhesives to join glass pieces. Unfortunately, adhesives' long-term strength and reliability have not been established.
