Annealing, << uh NEEL ihng, >> is a controlled process of heating followed by slow cooling. It is used on metal, glass, or other materials. Annealing produces various desirable changes in the properties of these materials. For example, it makes metals softer. As a result, they can be machined more easily. Annealing can also reduce internal pressures called stresses in both metals and glass. Stresses are often introduced when materials are cooled from their molten state or manufactured into products.
Glass is annealed to remove internal stresses that might later cause sudden cracking or breaking. Glass objects are annealed by placing them on a metal belt that travels slowly through a long, heated enclosure called a lehr. Thin objects can be annealed in 30 minutes or less. But it took more than a year to anneal the huge 200-inch (508-centimeter) telescope lens for the Palomar Observatory in California.
