Annealed glass

Annealing is a process of slowly cooling hot glass to relieve internal stresses after it was formed. The process may be carried out in a temperature-controlled kiln known as alehr. Glass which has not been annealed is liable to crack or shatter when subjected to a relatively small temperature change or mechanical shock. Annealing glass is critical to its durability. If glass is not annealed, it will retain many of the thermal stresses caused by quenching and significantly decrease the overall strength of the glass.

The glass is heated until the temperature reaches a stress-relief point, that is, the annealing temperature (also called annealing point) at a viscosity, η, of 10¹³ Poise ("Poise" is a measure of absolute viscosity; 1 poise = 1 dyne-second/cm²), at which condition the glass is still too hard to deform, but is soft enough for the stresses to relax. The piece is then allowed to heat-soak until its temperature is even throughout (this annealing temperature is usually in the range of 850–900 °F). The time necessary for this step varies depending on the type of glass and its maximum thickness. The glass is then slowly cooled at a predetermined rate until its temperature is below the strain point (η = 10^14.5Poise). Following this, the temperature can safely be dropped to room temperature at a rate limited by the heat capacity, thickness, thermal conductivity, and thermal expansion coefficient of the glass. After the annealing process the material can be cut to size, drilled or polished.

At the annealing point (η = 10¹³ Poise) stresses relax within several minutes, while at the strain point (η = 10^14.5 Poise) stresses relax within several hours.^[2] Stresses acquired during and remaining from temperatures above the strain point are permanent unless annealed and may lead to short-term or much delayed failures. Stresses acquired during cooling below the strain point are considered temporary, although they may be adequate to cause short-term failure.

Both annealed and tempered glass are very popular types of glass that are available in the market today. They have both been used for a number of years successfully in many different applications. When trying to choose between annealed and tempered glass, there are a few different things that you will want to keep in mind. Here are the basics of annealed and tempered glass and what you should expect from each one.

Durability

One of the biggest differences between these two types of glass is the durability that they provide. Tempered glass is quite a bit more durable and strong than annealed glass. Tempered glass will hold up to quite a bit more wear and tear than other types of glass. Tempered glass is typically much thicker and harder than annealed. As a homeowner, this presents you a greater likelihood that the window will stay intact over the years. You will not have to worry about small things causing breakage like you would with an annealed window. 

Cost

Another difference between these two types of glass is the cost involved. The initial cost of buying tempered glass is usually quite a bit higher than paying for annealed glass. Therefore, annealed glass is the choice of those that are on a budget in most cases. The cheaper windows that are in the market are commonly made out of annealed glass. Tempered glass takes a longer process to make and a lot more goes into making them than with annealed glass. Therefore, in order to accommodate for the extra processes involved, they have to charge more for tempered glass.

However, if you consider the long-term costs involved with windows, tempered glass might actually be cheaper. Annealed glass has a much higher probability of breaking. Therefore, over the years, you might have to replace annealed windows more often. This means that a tempered glass window could last quite a bit longer than an annealed one and save you money in the long run.

Safety

Another key area where these two types of glass differ is in the safety concerns. With annealed glass, when the glass breaks, you will get many sharp and jagged shards of glass everywhere. This can cause significant injury to anyone that is around. With tempered glass, it will actually break into small and rounded pieces of glass. Therefore, there is a much greater likelihood that everyone in the vicinity will be safe. Therefore, if safety is a major concern for you, you will want to go ahead and spend the extra money to get tempered glass.

Application

When deciding between tempered and annealed glass you want to consider the application in which it will be used. If you are putting the glass in a very low traffic area that no one will ever touch, annealed glass might be sufficient. However, if you are putting the glass in a very high-traffic area that many people will be around, tempered glass should most likely be the choice.