Volume 36, Issue 6, June 2001

As Good As It Gets

Examining Tempered Glass

by Dez Farnady

From the time of its birth in a tempering furnace until the end of its life in the bottom of a landfill, tempered glass is temperamental. Unlike most inert objects that always act the same under the same or similar conditions, tempered glass responds as it pleases.
The birth of tempered glass does not occur exactly under laboratory conditions. A lot of little variables may be introduced to aid its temperamental behavior. After all, tempering is an art and not, contrary to public perception, a science.

It all starts in the tempering furnace. Commodity tempered safety glass is heated to approximately 1,100 degrees Fahrenheit, then cooled. Each furnace charge is loaded with the same color and same thickness glass. But, the glass sizes and distribution in the furnace load (called the charge) vary a lot. As a result, the way the glass absorbs the heat also varies somewhat. The cooling, while under controlled air pressure, is at atmospheric temperature. During a normal production cycle in California, from the day shift to the swing shift, you can have air temperature variations of as much as 30 to 60 degrees Fahrenheit. 

This introduces another uncontrolled variable: the heat in the furnace may not be uniform either. There may be cold spots or hot spots causing slightly uneven heating. The operator may not have set the correct temperature in all of the heating zones, or the time cycle may be off a few seconds, and so on and so on. 

Remember when your high school metal shop teacher showed you how to temper a steel chisel by sticking the tip of the white-hot metal into a bucket of cold water? The quick-cooling of the steel tempers the metal just as quick-cooling tempers glass. The teacher claimed he could tell if the steel was tempered by looking at the color of the steel at the tip of that homemade chisel. Well I never could see what he was talking about. It looked like the thing was all the same color to me. When tempered glass comes out of the furnace it doesnít look any different from when it went in either. If you really want to know if itís tempered, you have to break it. 

Once the glass is broken, the density of the break pattern is the only thing that reveals the true nature of the temper. A pattern thatís too tight may indicate a temper that is susceptible to spontaneous breakage. If the break pattern is too big, the glass has not taken a full temper and will not meet safety requirements. The correct pattern is someplace in between. Unfortunately, these simple truths can only be determined by breaking the glass, and you canít sell too much glass that way. 

Temper, Temper
So how temperamental is tempered glass? I have taken a full swing at a piece of 1/4-inch tempered with a baseball bat only to have the bat pop back at nearly the same speed at which I swung it. I have taken a set of keys and thrown them full force at the center of a piece of 8-inch tempered window glass, and spent a half hour trying to find where the keys flew off to, as they bounced off glass that wasnít even scratched. While the blow from the baseball bat couldnít break the glass, a carefully-placed tap on the exposed edge shattered it to a thousand pieces.

Annealed glass may only be one quarter as strong as tempered, but you have never heard of annealed glass spontaneously breaking. Glaziers frequently experience the temperamental nature of tempered glass when it breaks in their hands. A glazier carrying a patio door may suddenly find himself with only a handful of cullet and a pile of little, broken pieces at his feet. 

Tempered glass is also unique in its ability to retain a visible strain pattern. These strange shadow patterns show up most frequently on the back windows of sports cars, particularly when youíre wearing polarized sunglasses. The distinct pattern of the shadows results from the uneven air pressure of the cooling process. 

The wrinkled, reflective glass on large office buildings is another manifestation of the tempering process.
It may be warped or bowed, it may blow up ďspontaneously,Ē it may show a strained pattern and look ugly and may be unpredictable, but it is safe and strong and we canít come up with anything cheaper and better. So relax and live with itóitís as good as it gets. 



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