All glass is not created equally, and tempered glass is the perfect example of that. While regular glass shatters into fragments, including the occasional spearlike shard, tempered glass is designed to be much safer, breaking into small, interconnected chunks rather than shattering into a plethora of sharp pieces. Though it's easiest to tell that glass is tempered after it cracks or shatters, a few telltale signs can help you figure things out when the glass is completely intact and unscathed.
1. Check the Edge of the Glass
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If you're dealing with a sheet of glass, such as a glass topper for a side table, inspect the edges. If they're smoothed and rounded and not one area looks like it could slice your skin or a piece of paper, there's a chance it's tempered glass. On the other hand, if the edges look too sharp or chipped in some places, as may be the case with a piece of glass that goes over a photo in a frame, it is not tempered glass. If the glass appears smooth on the edges, run your hands along the edges. If the glass has imperfections or rough areas, it's not tempered glass.
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Though items such as drinking glasses have smoothed edges, they are not tempered glass; they're simply smoothed around the rim for safety purposes. The average, relatively inexpensive household glass items are not made of tempered glass, as tempered glass costs much more to manufacture. Tempered glass is usually reserved for items where safety could be of major concern if the glass breaks. Shower doors, car windows, and sliding patio doors are often made of tempered glass.
2. Look for a Stamp
True tempered glass carries with it a stamp on the edges or marked on the surface, usually near a corner. Look for the letters CPSC along with a series of other numbers and letters. CPSC stands for Consumer Product Safety Commission, and it indicates that the glass meets the safety requirements necessary to be considered tempered glass. In some instances, the word "tempered" or the term "safety glass" might be in part of the etching or stamped mark.
Car windows and glass doors are two instances where tempered glass is common, so check one face near the edge, usually the bottom, to find the markings. Unfortunately, the tempered designation sometimes wears off, so it might not be on the glass even if it is tempered.
3. Wear Your Shades
On a sunny day, look at the glass through polarized sunglasses that filter out some ultraviolet or UV rays. Feel free to also inspect another piece of glass or even a clear drinking glass for comparison. If the glass is tempered, you'll see some darker spots, lines, and other imperfections that stand out quite well when viewed through polarized lenses. These dark marks are made by the equipment used during the tempering process. Untempered glass does not have these dark spots.
4. The Glass-Cutter Test
If you're planning to cut a pane of glass to a smaller size and that's why you need to know if the glass is tempered, a glass cutter is a good tool to use to find out in a hurry. Score the glass in a straight line using a straightedge as a guide. If it's not tempered glass, you'll get a neat, tidy score line. If it is tempered glass, tiny glass bits akin to shavings will show up where you've cut. Tempered glass cannot be cut in this manner; in fact, it's always cut to size after the initial annealing and before the tempering process. Regular annealed glass panes can be cut or snapped easily along a scored line.