Tempering and annealing are manufacturing methods for adding strength to glass. The cycle of heating and cooling used during the manufacturing process is the difference between tempered and annealed glass. A third type of glass -- heat-treated -- lies somewhere between the other two in terms of strength. 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.
Manufacturers mix silica sand, soda ash and limestone -- the basic raw ingredients for making glass. Sometimes, the mix also contains cullet -- ground pieces of recycled glass. They then subject this mixture to temperatures around 2,800 degrees Fahrenheit. This extreme heat melts the raw materials, and the slurry is poured onto a sheet of molten tin in a process called the float method. Tin cools perfectly flat, an ideal bed for glass production. It also cools more quickly than glass, so the bed is solid even while glass remains in a molten stage. During the float bath, glass cools to about 2,000 degrees F.
Cooling It All Down
Annealed glass cools as it rides along a conveyor and enters an annealing oven, which is called a lehr. Once inside, manufacturers control the speed at which the glass cools, slowing the process down in order to build strength into the product. During cooling, the glass drops from about 1,100 degrees F to room temperature. Annealed glass is then ready to cut to order. Tempered glass gets cut to size when it is solid enough to do so. It then is washed and reheated back to a molten stage at around 1,100 degrees F. Forced cold air quenches the molten glass, bringing the temperature down rapidly. The outside cools more quickly, creating a compression zone of glass that surrounds a tension zone in the middle. This rapid cooling is what sets tempered glass apart from annealed glass.
Strength and Durability
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. As a result, it's required in this type of application. Tempered glass may also be used in large architectural windows, where added strength is a requirement. 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.
Advantages and Disadvantages
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. While the cost of glass varies according to size and geographical area, tempered glass is typically more expensive than annealed glass. The strength limitations of annealed glass limits the size of usable pieces. Tempered glass sometimes exhibits undesirable optical qualities, such as minor distortion and waviness.