Newly made glass is very strong, but it begins to lose strength the instant it is made. Simply blowing on its surface can significantly reduce its strength. And while glass has the capacity to bear heavy loads—a necessary requirement in structural applications—it cannot easily withstand bending forces. Protective coatings are one way to give glass greater strength, although this can affect transparency, one of the main reasons for using glass. Also, altering the composition of the glass can make it harder for cracks to form, but this alteration can mean loss of other desirable properties, or make the glass more expensive to manufacture.
The manufacturing process can be modified to keep the surface of the glass clear and free of cracks. Strong, clear glass for laptop displays is made by maximizing the surface area in contact only with air as the glass cools. But with glass used for structural purposes, tempering processes are typically used. In heat tempering the surface of the glass is cooled more rapidly than the interior, a process that allows the interior of the glass to become more dense. In chemical tempering the sodium ions at the surface are replaced with potassium ions, which are larger, and this achieves much the same compression as does the heat tempering process.
Tempering makes cracks less likely to form, but breakages can still occur. When tempered glass breaks, it breaks into many smaller pieces than does ordinary glass. In structural applications this is actually preferred, since smaller pieces are generally less likely to cause injury. Use of laminated glass in a structure helps to maintain the structure's integrity in the event that the glass breaks. Glass is laminated by sandwiching thin polymer layers between layers of glass. This adds strength and keeps large broken pieces from shattering. One problem with this, however, is that it makes fabricating glass for structural purposes very difficult, since each piece must be cut, polished, and drilled for the connecting fittings before it is tempered. In fabrication, precise measurements and extremely small margins of error are needed to avoid potentially destructive stresses in the assembled structure.
