Tempered glass and standard glass differ in the way they are processed. To manufacture any glass, sand, soda ash, and lime are mixed together and melted at very high temperatures. This hot liquid is formed into glass though blowing, pressing, or drawing. Once the glass is formed, it goes through an annealing process where it is reheated and cooled. This keeps it from shattering and restores its strength.
The annealing, or cooling process, is what determines whether glass will be tempered or standard. Tempered glass is forced to cool very rapidly, while standard glass is allowed to cool slowly. Cooling the glass quickly makes it stronger, so it can withstand at least four times the pressure of standard glass. It also reacts to breakage differently.
One of the most obvious differences between the two types of glass is how they shatter or break. Standard glass breaks into large, irregularly shaped, sharp shards. Tempered glass, on the other hand, shatters into small, evenly shaped pieces that pose much less risk of injury to those coming in contact with them.
Standard glass will break in the specific area of contact, resulting in cracks or a hole in one location, but leaving the rest of the pane intact. Tempered panes are more impact resistant, but shatter completely, leaving no intact areas. Because of this, they are preferred in applications where safety is important. Standard glass provides greater security, as parts of a pane can remain undamaged even after another section is broken.
Tempered glass is more heat-resistant and scratch-resistant than non-treated glass. Outwardly, however, it does not appear any different than standard glass. Both types of glass are made in varying sizes and thicknesses and can be colored or tinted.
Standard glass can be cut to size or pressed into shape after it is processed. If an application requires, its edges can be polished or holes drilled into it. Tempered glass cannot be reworked after it is tempered. Attempts to cut or drill into a pane would result in it shattering completely.
Because it is stronger and its fracture pattern safer than ordinary glass, tempered glass was once the standard for car windows and glass doors. In applications requiring the highest degrees of safety, laminate glass is now more commonly used. Laminate glass is formed by two layers of glass fused together with a layer of plastic in the center, producing a stronger glass. In situations of extreme force, laminate glass will crack, but it will not shatter.