The Difference Between Annealed and Tempered Glass

broken tempered glass

There is a lot of confusion knowing the difference between annealed and tempered glass. What makes them unique, their pro’s and con’s, and what applications they’re used for.

I was going to call this “Plate Glass vs. Tempered Glass“, because most people refer to glass that way. Especially large panels of glass. But I figured instead, I’d flex my knowledge  on you and put you on the right path.

First, let’s clear something up. There is no plate glass manufactured today. Also, the term plate glass doesn’t refer to size, shape, or any physical characteristic. Usually, people still refer to large panels of glass as plate glass. In actuality, plate glass is an outdated manufacturing process and term. When people use the term plate glass, they are referring to a glass type called annealed glass.

What is (or was) Plate Glass?

Plate glass was made by placing molten glass on tables. Since the glass was in a liquid form, it would spread out into a sheet. Machines rolled over the top to smooth the glass.

What is Annealed Glass?

 

Today manufacturers use the float glass process. A sheet of liquid glass is spread out and floats on top of melted tin, making the floating glass smooth. The glass is then placed in a temperature-controlled space where it is cooled at a very slow rate, called annealing.

That cooling process is known as annealing. It reduces the internal stress in the glass and makes it stronger than plate glass.

What is Tempered Glass?

Tempered glass starts its journey as annealed glass. From there, it is heated up to around 1300 degrees Fahrenheit (700c). This is achieved in a large furnace comprised of 3 parts. First, the glass rolls through the furnace on heated rollers to heat the glass as it passes over.

The heated coils in the furnace generate radiant heat. Air jets placed inside the tempering oven distribute the hot air evenly across the glass. When the glass reaches the desired temperature, the temperature is quickly changed in the furnace, and the jets blow cold air on both the top and bottom of the glass.
This process makes the glass panel four times stronger than untempered annealed glass.

Comparing Annealed Glass and Tempered Glass

Annealed Glass Summary

Annealed glass is generally great for general applications. One popular use is windows since annealed glass can withstand temperature fluctuations very well. Annealed glass can also be readily cut, making it great for projects.
A significant downside to annealed glass is that when it breaks, it does so in large, sharp shards that can seriously injure if not kill. So, annealed glass usually isn’t up to code in high-traffic areas or in use with most architectural glass outside of windows in houses.

breakage comparison

Tempered Glass Summary

Tempered glass has a lot of great uses but is generally more expensive and has a few pretty big cons, especially for DIY’ers.

First, tempered glass is a safety glass, is very strong and difficult to break. But, if it does break, it breaks into little pebbles of glass that, at most, cause superficial injuries. So, tempered glass is the glass of choice for builders of high traffic areas, commercial buildings, glass shower doors, glass barn doors, stair railings, partitions, skylights, patio doors, and glass tables tops, to name a few…

With all the great perks and ability with tempered glass, it is just that. Tempered glass. Once it’s tempered, there’s no going back. You can’t drill it, cut it, shape it, or alter it physically at all. Any cutting or drilling should be done to the glass before tempering. However, you can etch, laminate, among various other decorative glass techniques just like annealed glass. One other downside is tempered glass has this amazing ability to randomly shatter.

Why Does Tempered Glass Shatter?

Tempered glass is under a massive amount of stress and with the enough of an impact in strategic areas can break or damage the glass. Most of the time, although highly rare, it’s one of two things: edge damage or internal stress from inclusions.

There are not many ways to break tempered glass, but one sure way is to bang the edges just right with a hard object. The glass can suddenly burst if the edges are already chipped, damaged, or have manufacturing flaws. The already weak, damaged edges will give way, and the glass will shatter.

Another common reason to an uncommon occurrence is internal stress breakage due to nickel sulfide inclusions. Say that five times fast. Sound like a bunch of nerd talk, but, it’s really good to know. Although highly rare, nickel sulfide stones can be formed during the glass making process. These stones change in structure with time and cause internal stresses in the glass. Once the glass reaches its peak stress point, it will shatter.

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