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One of the most important concepts to learn before working with glass is annealing. The process of annealing glass is to cool it slowly enough to let the entire thickness cool with very little difference in temperature from the core to the surface. Since glass insulates very well, this can take a long (sometimes very long) time.
What we call the annealing point is the temperature just below the freezing point, which is just under 1000°F for most soda-lime “soft” glass. At this temperature, the glass is cool enough that it no longer moves or slumps with gravity, and hot enough that it cannot begin to build up stress in the glass. If glass is held at this temperature long enough (about 1 hour per ¼ in. of thickness), it will relieve any stress, and cooling can begin from there.
But what is the stress? Almost everything expands when it heats up and contracts when it cools down. This is because the molecules move or vibrate more with heat, and they need more room around them to move, making the object expand. If the core of a piece of glass is still hot when the outside surface is cooling and therefore shrinking, the outside surface cannot stretch around the core. This is what is referred to as stress, which cracks glass. So stress is really the squeezing of the surface of glass around a hotter core; heating it too quickly it will stress the glass also.
To properly anneal glass, it must first sit at the annealing point long enough for the entire thickness to become even in temperature, and then the high-temperature cooling may begin. This must be the slowest rate of cooling from the annealing temperature to what is also called the high-temp strain point. After this point, the rate of cooling speeds up to the low-temp strain point. Then it can finally cool at the fastest rate to room temperature.
Every different glass batch formula has a specific rate at which it must cool. This is measured as the coefficient of expansion or (COE), which is how much the glass grows with each degree of temperature. Before figuring your annealing cycle, you need to know the COE from the manufacturer. All glass manufacturers and vendors of glass made for melting have charts listed for annealing their glass online. They provide the annealing, high-temp cooling (HTC) and low-temp heating (LTH) temperatures, and the degrees per hour it can cool between them. You must simply follow the chart for the thickness (usually rounded up tooth nearest ¼ in.) and see how many degrees per hour it can cool between those points. Because all different glass has a different COE, you cannot mix two different types of glass together. This is known as incompatibility, and no amount of annealing can allow them to “fit” together.
The annealing of glass becomes exponentially longer with thickness. If you double the thickness, you have to more than double the annealing time. For example, .5-in.-thick glass at 90 COE anneals for about 5 hours total, a 1-in.-thick piece anneals for 14 hours, 3-in. for 99 hours, and an 8-in. piece for 654 hrs (or about a month).
I want to leave you with one last interesting fact. We know that the molecular structure of glass stays random when it freezes, rather than crystallizing. This is where the theory of “glass may still be moving under the annealing point, just very slowly” comes from. Did you ever hear someone say that this is why the glass at the bottom of old windows is thicker? Well, if this were true about glass that is probably no more than 200 years old, wouldn’t glass from ancient Greece, or Egyptian glass, or especially glass from Mesopotamia (up to 5000 years ago) be a puddle by now? The reason the glass is thicker at the bottom is because it is hand-blown glass and cut square from a large roundel, or plate. Blown plates are always thinner at the edge because of centrifugal force when they are spun flat on the end of a pipe. The window maker would always place the panes of glass in the frames with the thickest part on the bottom.