ABOUT TEMPERED GLASS
At Glasstopsdirect, we recommend and sell only tempered glass for ¼” glass table tops and 3/8” glass table tops. Here’s why…
Tempered glass – also called toughened glass – is known as safety glass. It has been specially manufactured for superior strength, and is about four to six times as strong as regular glass. It’s also been designed to withstand high temperatures, which is why you can put tempered glass cookware and dinnerware in the microwave or on the stovetop.
What’s more, in the event that the glass does become damaged, rather than breaking into dangerous, sharp shards it will shatter into small, square-shaped pieces. Most often, it is damage to the edge of a pane of tempered glass that will cause breakage, but a strong impact in the middle of the glass can also cause it to shatter. Surprisingly, if it is severely damaged, the glass doesn’t always shatter immediately, and instead a seemingly small event afterwards will cause it to literally go to pieces.
Historically, perhaps the earliest instance of “tempering” was recorded in the 17th century. Prince Rupert of Bavaria is said to have created “Prince Rupert’s Drops” by having molten drops of glass fall into a bucket of cold water. This quickly cooled them into tempered, teardrop-shaped pieces of glass. Fast-forward to America in the late 1930s, and tempered glass began to be mass-produced. By the 1960s tempered glass was being incorporated into doors, windows and other building structures.
The modern way to temper glass is with a thermal tempering process that starts with regular, or “annealed,” glass. Annealed means the glass has been heated until the high temperature is even throughout the glass – and then slowly cooled at a consistent rate. Once sufficiently cooled, the glass can then be brought all the way down to room temperature. The annealing process makes the glass more durable, so it can resist changes in air temperature or mild physical shocks. It is now ready to be cut, polished, or otherwise worked with.
Much annealed glass is flat glass. And almost all of the flat glass made these days is called “float glass,” named for the process invented in England in the 1950s by Sir Alastair Pilkington of Pilkington Glass. His breakthrough was to pour molten glass over molten tin in a mold. The glass actually floats on the tin, smoothing out to evenly fill the shape. The glass is then cooled down in a special temperature-controlled oven called a “lehr,” and thus you have everyday glass.
In the tempering process, the annealed glass is heated in a furnace to over 600 degrees Celsius (past its “annealing point”), then immediately cooled down with air jets. This forces the surface to quickly become cool and stiff, while the core of the glass, because it is still much hotter, takes longer to cool down and finally harden.
And that is what improves the strength of the tempered glass. If the surface receives any stress, it does not carry through to the core of the glass, so a crack is unable to form. In fact, if you were to look at tempered glass with a polarized light, you would be able to discern the pattern of hardening in the outer layer compared to the core. Just as importantly, if the glass does receive damage, it shatters into small cubes rather than sharp shards.
There are two different ways to temper glass in the manufacturing process, vertical tempering and horizontal tempering. In vertical tempering the glass is held by its top edge using clamps or tongs, so it passes through the furnace vertically. Conversely, in horizontal tempering the glass lies on stainless steel or ceramic rollers and passes through the furnace horizontally. Horizontal tempering is more commonly used.
Keep in mind that unlike regular annealed glass, tempered glass cannot be made into a desired size or shape. A conventional glass cutter will not work. So the glass must be cut or pressed first, then tempered. That even includes polishing, making holes for screws, etc.
As previously mentioned, we recommend tempered glass for our glass table tops. But you’ll find tempered glass in a wide range of applications, from auto safety glass in cars (but not the windshield, which is a specially laminated glass), to glass shower doors and sliding glass patio doors in the home. Also, computer monitors, LCD screens and eyeglasses, although in the case of the latter, a chemical process is used to achieve the tempering effect.
There are two kinds of tempered glass in drinking glasses and wine glasses, fully tempered and rim-tempered. As the name implies, rim tempered glasses only have the rim toughened up and not the entire glass.
In architecture, regular annealed glass is considered too dangerous for many uses and is forbidden by building codes. Instead you’ll find tempered glass in various installations – in frameless building doors, storm doors, bathrooms and skylights. Even at the gym, as racquetball court walls and basketball backboards.
Now you know why we recommend and sell tempered glass for our ¼” glass table tops and 3/8” glass table tops. Please note that the ½” glass table tops and ¾” glass table tops that we sell, per industry standards, are not tempered.