Toughened or tempered glass has properties which are of particular interest to the Forensic scientist. Toughened glass is created by taking a piece of glass that has been fashioned into the final shape and then heating it until the temperature reaches a stress-relief point (called the annealing point). At this annealing temperature, the glass is still too hard to deform but soft enough for the internal stresses within the glass to relax. The surface of the glass is then cooled quickly so that the internal stresses are retained. This creates a very strong glass which, if broken, will fracture into small pieces.
This type of glass cannot have holes drilled in it, or be cut, as it would shatter. When the glass shatters, most of the glass pieces will have both surfaces of the original glass pane. This allows the forensic scientist to measure the thickness of the glass. The further a piece of glass was from the point of impact, the larger the piece will be. This effect can be used to estimate the position of the point of impact.