Guess what? You are already very familiar with annealed glass. From light bulbs to drinking glasses, wine bottles to windshields, you’re surrounded by annealed glass every, single day. Also, if you collect glass art like vases or sculptures, they are annealed too.
When a glass product is created, whether a glass beaker, vial or bottle, it is formed from glass that has been heated to a molten state. If the product is allowed to cool at room temperature (or below its strain point), then the exterior of the glass cools at a faster rate than the interior creating a form of internal tension referred to as “stress”. Although stressed glass looks the same to the naked eye as annealed glass, it is neither as strong nor as stable and can be subject to breakage at any point
The annealing process begin after the glass product is formed. It takes place in a lehr. A lehr is typically a long kiln with a temperature gradient from end to end, through which the newly made glass products are transported on a conveyor belt. The lehr controls the rate at which the glass cools. By controlling this rate, manufactures prevent reintroducing permanent annealing strain or fractured glass due to thermal shock. Essentially the lehr equalizes the temperature of the glass products being produced removing any strain introduced during the forming process. Basically, an annealed glass product has had its interior stress removed and is therefore much stronger and more stable than an unannealed product.
Annealing is such a standard part of glass manufacturing that we don’t even have to think or know much about it. Since glass isn’t randomly shattering or exploding all around us, we can just understand and assume that all professionally made glass has been properly annealed. Glass that is not annealed may have “thermal crack” or “thermal shock”. The result could be very dangerous.
Is all glass annealed the same way? No, different types of glass require different cooling cycles. The annealing schedule is calculated from technical information based on the type of glass, size and thickness of the product.
The video below describes the annealing process and why it is so important. The video was made by the Corning Museum of Glass which is located in upstate New York.