What is “Safety” Glass Glazing?

Have you decided on glass greenhouse glazing for your greenhouse but are worried about its durability? Perhaps you have a pre teenage son who loves to throw a baseball as high as he can into the air before catching it and you’re worried about an errant ball cracking the roof of your greenhouse. Or maybe hail is a concern in your area and you worry about how glass will hold up over time.

Most people who have dropped a pint glass or mirror know that it does not take much force to break glass and ruin your whole day. Normal glass has very little impact resistance. The idea of making glass stronger has been around for a long time, but it was not until the past 100 years that glass manufactures were able to design manufacturing process that allow for strengthened or safety type glass.

As with most things, this improvement in glass impact resistance was born out of a need to protect the person using the glass. Originally glass used in the automobile industry was made how glass had always been made, in large single sheets. This worked fine until the driver was involved in an accident. Rather than the glass simply cracking out into a spider web like todays glass windows, the glass would shatter; throwing several large, sharp, sections of glass into the car causing additional injury to the driver and passengers.

In order to solve his, glass manufactures figured out that if they inserted a layer of clear plastic into the glass that the glass would no longer shatter. The force would be dissipated over a larger area by the plastic film, and the glass would simply crack like a spider web. In your greenhouse glass, this layer of plastic will be sandwiched between the panes. (And we highly recommend at least double paned glass for greenhouses due to the increased heat retention capabilities.)

Tempered glass is another innovation in manufacturing that has made glass safer not only for greenhouse use, but for other every day applications like shower doors or microwave ovens. Tempering, as the name applies, uses heat during the manufacturing process in order to increase the glass’ strength.  Tempered glass is up to 4 times stronger than glass that has not been through the process.

The first step in “tempering” the glass is to cut it to it’s desired shape. Cutting the glass after it has been through the tempering process may lead to imperfections and a final product that is not as strong as it could or should be. Edges are normally sanded with fine sanding paper to ensure there are no rough edges – from there it is off to the ovens! The glass pieces are fed into an industrial furnace, which heats the glass to temperatures above 600 degrees Celsius  (that’s 1,112 degrees Fahrenheit!).  The glass then goes through what is called “quenching”, or rapid, high pressure cooling.

During quenching, high pressure nozzles blast the surface of the hot glass quickly (in a matter of seconds) with cooler air. This causes the outside of the glass to cool much quicker than the center part of the glass. This rapid cooling, and the difference in temperatures, causes the center to attempt to “pull back” from the outer surface of the glass. This means that center part of the glass in under tension while the outer surface of the glass is under compression. It is this shifting in the force profile that gives “tempered” glass it’s noteworthy strength.

Any greenhouse kit you order should come with some form of safety glass; but be sure to confirm this with your vendor. Additionally, if you are reusing glass from another application (we saw online where someone built their greenhouse from old windows) just know the limitations of the materials you are using.

Glass greenhouse glazing is a beautiful material for use on your greenhouse project, just be sure to stay as safe as possible during the process!

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