Printable PDF Tip & Tricks booklet.
Unless glasswork is very small, it needs to be annealed after it has been melted to prevent it from breaking as it cools or shortly thereafter. Annealing is accomplished only by holding a steady temperature over a period of time. The temperature and time are determined by the type of glass and its size and shape. Lower COE glasses develop less stress than higher COE glasses for a given set of circumstances. The thickness of the glass has a large influence on the length of time needed to anneal and cool the glass. However, a larger piece with the same thickness also influences annealing hold time and cooling rate. Annealing can only remove stresses that develop during the manufacturing process. Annealing cannot remove stresses due to variations in COE of the different glasses used within the item.
There are several kinds of stresses in glass that can cause it to break. If any single stress is large enough, or a combination of stresses is large enough, a critical level is reached and the glass will break. Stress caused by differences in COE within the glass item cannot be annealed away, so you need to use glasses that are compatible within the glass item. This is not always easy when that special color you want to use has a COE different enough from the other glasses that it may cause the glass item to break if not otherwise nicely annealed. Given that you properly annealed the glass to remove manufacturing stress, and not considering thermal shock and impact shock, stress developed during cooling adds to the COE stress and can be all it takes to break the item. Cooling actually puts stresses back into the glass. Cooling slowly adds less stress than cooling at a faster rate. A lesser amount of cooling stress added to the COE stress may be all it takes to keep the glass below the stress threshold that would otherwise break it. As with the annealing portion of the annealing cycle, the cooling rate is determined by the type of glass and its size and shape.
Ideally, successful annealing is done by placing just made glass items into a pre-heated annealer set at or a little cooler than annealing temperature, before the item cools much below the annealing temperature. This helps prevent the item from breaking before it has had a chance to be annealed by eliminating the possibility of thermal shock and by the fact that while glass is very hot it is not as adversely affected by stresses. Essentially, it is best to remove the stress while the glass is hot, because the stresses in glass at lower temperatures can more easily break the glass.
The annealing oven heats (Ramps up) to the annealing temperature. If cold glass is in the annealer, the ramp rate up is slow to prevent thermal shock and breakage. The glass is held at the annealing temperature long enough for the manufacturing stress within the glass to be released (Soak). The glass is then cooled (Ramps down) at a slow enough ramp rate so as not to reintroduce too much stress back into the glass.
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