Tempering and annealing are manufacturing methods for adding strength to glass. It is the cycle of heating and cooling used during the manufacturing process. Typically, the hotter the ovens during manufacturing, the stronger the final product. Both tempered and annealed glass also have other properties that set them apart from each other.
The tempering process gives glass about four times more strength than annealed glass. Tempered glass is strong enough to withstand the pressure created by slamming doors.
Annealed glass is more fragile, but is still the most widely used type of glass in the market. It is found in home windowpanes and other applications. Annealed glass tends to break into irregular, sharp pieces when broken. Tempered glass breaks into many small pieces that are harmless and not very sharp. Tempered glass once it is processed can not be modified but annealed can be. Most home windows have what is called an insulated glass unit. Either the insulated glass unit or IGU will be tempered or annealed. You tell by looking in the corners of the window and if it is tempered the insulated glass unit will have tempered etched into the glass. Take a look at your patio sliding door and see if you spot the tempered symbol.