What Glass Types Should Be Used In Glass Railings
There are three types of glass used in glass railings:
- Annealed Glass
- Heat-Strengthened Glass
- Tempered Glass
Flat glass is initially manufactured as float glass. It’s manufactured by floating molten glass on a bed of molten metal – typically tin. This method gives the sheet uniform thickness and very flat surfaces.
This process was developed in the 1950s by the British glass manufacturer Pilkington which pioneered the technique. Thus, it is commonly known as the Pilkington Process.
The glass is then annealed.
Annealing is the process of slowly cooling glass to relieve internal stress after it is formed. If not cooled slowly, internal tension (stress) will remain to make it unstable and subject to breakage. The process is carried out in a temperature-controlled kiln known as a Lehr.
Glass that has not been annealed is liable to crack or shatter when subjected to a small temperature change or mechanical shock.
Annealed glass is commonly found in windows. It is not considered safety glass. When broken, it will result in shards with sharp edges and points that may result in serious injuries.
Heat-Strengthened Glass and Tempered Glass
Heat-strengthened and tempered are both produced using the same processing equipment. The glass is heated to 1,200 degrees Fahrenheit. It’s then force-cooled to create surface and edge compression.
Glass is heat-treated for two reasons
- It increases the strength to resist external stresses
- To temper glass so that it meets safety glazing requirements.
With heat-strengthened glass, the cooling process is slower. This means that the compression strength is lower than tempered glass. Heat-strengthened glass is approximately twice as strong as annealed, or untreated, glass. Tempered glass is four to five times stronger than annealed glass.
Heat-strengthened glass is not considered safety glass as the break pattern will be like annealed glass. The glass will break into dangerous shards.
With tempered glass, the cooling process is accelerated to create higher surface compression and/or edge compression in the glass.
It is the air-quench temperature, volume and other variables that create a surface compression of a least 10,000 pounds per square inch (psi). This is the process that makes tempered glass four to five times stronger than annealed or untreated glass (twice as strong as heat-strengthened glass). As a result, tempered glass is less likely to experience a thermal break.
Tempered glass is safety glass as it will break into small pieces
Spontaneous Breakage of Glass
Glass breakage may occur without any obvious cause. It could be due to:
- Glass edge damage or impact
- Surface damage from handling and glazing that then weakens the glass during high winds,
building or framing system movement
- An inclusion inside the glass
There are more than 50 types of inclusions in float glass. The most widely discussed is a nickel sulfide stone.
Nickel-sulfide stones can form during the production of float glass due to contamination. It can end up in the center tension zone of tempered glass. After that glass is installed and exposed to varying temperatures, this tiny stone may grow in size and cause the glass to shatter.
Spontaneous breakage caused by nickel-sulfide inclusions occurs only in tempered glass.
There is no known technology to completely eliminate nickel-sulfide stones in float glass. And because they are so small, there is no practical way to inspect for them.
Most North American glass manufacturers have controls in place to prevent nickel-sulfide formation. Heat soaking after fabrication may destroy some flawed glass panels. But, heat soaking cannot guarantee the 100% elimination of nickel-sulfide inclusions.
Heat soak testing is a destructive test for nickel-sulfide inclusions. Tempered Glass is put into a heat soak oven and brought to, and held at, a temperature of 555 degrees Fahrenheit for two hours.
Most glass containing nickel-sulfide will shatter during this process and be eliminated from the glass project. But, heat soaking cannot guarantee the 100% elimination of nickel-sulfide inclusions.
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