TOUGHENED GLASS

What is Tempered Glassware?

Greater strength and impact resistance

Toughened glass (usually referred to as 'tempered glass' for the dining room and kitchen) is stronger and has much greater impact resistance than normal 'annealed' glass. It is from two up to six times as strong as annealed glass, depending on the manufacturing process and how this extra strength is measured. When used in the production of glassware, tempering normally results in a longer lasting as well as a safer product.

Highly durable though it is, toughened glass is not unbreakable. It should last a lifetime but tempered glassware must still be cared for and examined for chips and scratches from time to time.

Making toughened glassware

The toughening effect of 'tempering' glass has been known for centuries, though the underlying mechanism only became understood in modern times. Annealed glass objects are subjected to a thermal tempering process by being passed on a roller through a furnace which heats them to above their annealing point of 600°C. The glass is then rapidly cooled by air, taking the surface to below its annealing point and causing it to harden and contract while the core remains free to flow for a short time. The core finally cools into multiple layers under tensile stress, balanced by compressive stresses in the surface.

These compressive surface stresses are the key to the increased strength of tempered glassware. Whilst this produces much more resistance to pressure or impact, a small surface chip or scratch can weaken the surface layer, occasionally causing the glass to shatter into small pellets, sometimes for no apparent reason and well after the original damage occurred.

Toughened glassware is also highly resistant to thermal shock, but extremes are best avoided, such as pouring boiling water into a frozen glass straight out of the refrigerator. More on this at Chowhound »

Tempered glass actually has a softer surface than annealed glass and is slightly more susceptible to scratching. For this reason some glassware is tempered only at the rim - the most vulnerable part to knocks - leaving the rest of the glass more scratch resistant, but unlike a fully-tempered glass a rim-tempered one is unlikely to survive a drop from the table onto a hard ceramic or concrete floor.

Toughened for improved safety

Some authorities argue that although toughened glass is not unbreakable, it is inherently much safer than annealed glass because when it is made to shatter, it forms small cube-like and nearly blunt pellets (like a shattered car windscreen) instead of the sharp, jagged shards normally associated with glass breakage.

Duralex tempered glassware

Duralex France International, one of the world's leading glassware brands and maker of the famous "ver trempé", has been producing high quality fully-tempered toughened glassware for well over sixty years.