Home Glass Chemistry Annealing Glass

Annealing Glass


Annealing is a two-step process: (1) heat-soaking the glass for a period of time at a high temperature then (2) cooling the glass slowly to relieve internal stresses after it was formed. The process must be carried out in a temperature-controlled kiln.

The glass is heated until the temperature reaches a stress-relief point. This is a point at which the glass is still too hard to deform, but is soft enough for the stresses to relax. The piece is then allowed to heat-soak until its temperature is even throughout. The time necessary for this step varies depending on the type of glass and its maximum thickness. The glass is then slowly cooled at a predetermined rate until its temperature is below the strain point. Following this, the temperature can safely be dropped slowly to room temperature at a rate limited by the heat capacity, thickness, thermal conductivity and thermal expansion coefficient of the glass.

Glass, which has not been annealed, is liable to crack or shatter when subjected to a relatively small temperature change or mechanical shock. Annealing glass is critical to its durability. If glass is not annealed, it will retain many of the thermal stresses caused by quenching and significantly decrease the overall strength of the glass. One cannot predict when the glass will crack or shatter but it will eventually.

http://www.youtube.com/watch?v=qBlsNNPRseQ is a nice YouTube video about annealing. This video is from Corning Museum of Glass.

Arrow Springs

What Is Annealing? Why Is It Important?
And How To Do It.

The following is an excerpt from our annealer manuals.

Annealing is the process of making the entire glass item uniformly hot and holding that temperature steady long enough to remove all stress caused from the manufacturing process. The annealing cycle also includes cooling down slow enough so as to not allow too much stress to build back up.

When glass is held at a steady temperature over a length of time, it is called soaking. Soaking the glass at a higher temperature has the advantage requiring a shorter soaking time for the stress to dissipate, but also runs the danger of being so hot that it may distort under its own weight or of sticking to something. The glass will also need to cool down a through a longer temperature range, and this will take longer than if it were annealed at a lower temperature. Soaking the glass at a lower temperature has the advantage of a shorter cooling time, but requires a longer soaking time to remove the stress, and, if soaked at too low a temperature, will not even remove the stress no matter how long soaked.

After the glass has soaked for the proper length of time, all of the manufacturing stress will dissipate, but stress will reappear during cooling. The faster the glass is cooled, the more the amount of stress the glass will acquire.

The annealing temperature for any glass is actually a range. The higher end of the range is a temperature set to be safely below any possible chance of distortion. The lower end of the range is a temperature high enough for heat soaking to be effective within a reasonable amount of time. The commonly used temperatures for any particular glass is actually just a temperature chosen as a compromise between the higher and lower ends of the range. In other words, a temperature in about the middle of the range. An exact temperature is not what is important. What is important is that you keep the temperature steady for a period of time before slowly cooling the glass to room temperature.

The annealing temperature we recommend for Effetre (Moretti), Bullseye and Lauscha glasses, is around 940º F. Use around 1050º F for borosilicate glass. Around 890º F for Satake. Using a temperature controller can maintain the temperature to within a couple of degrees. Manual control using an infinite control switch cannot hold as tight a tolerance, but is adequate. This is one reason that we use annealing temperatures near the middle of the range.

As the glass cools, the outside will always cool faster than the inside. As glass cools it contracts. If the outside of the glass cools much faster than the inside, the outside glass contracts faster than the inside glass. This variance in contraction causes stress in glass. Too much stress and the glass breaks. The slower the glass is cooled, the less the amount of temperature variance throughout the glass and less the amount of stress that will develop.

The cooling of glass is most important between the annealing temperature and the strain point. As explained, glass will develop stress in itself through the cooling process. The strain point is a point in temperature at which any stress that develops below that temperature in the glass through the cooling process is only temporary. Stress that develops in the glass above the strain point is permanent. Once the glass has stabilized to room temperature, temporary stresses will disappear. Because of this fact, you can accelerate the cooling time below the strain point temperature and not worry about this strain causing the glass to break at some time in the future. However, cooling at too fast a cooling rate can still break the glass from thermal shock while still in the annealer. The strain point for glass varies between manufacturers and even between different colors from the same manufacturer. If you use a temperature well below the strain point of all the glasses you use to cool slowly down to before then increasing the cooling rate, you will not need to know the exact strain point temperature of each individual glass. Use 750º F for Satake and 800º F for every thing else.

The process to anneal glass once it is at the annealing temperature is as follows: First soak it for a period long enough to remove its stress. For a small bead, this can be as little as twenty minutes. For large beads, one hour. For a large paperweight, it can take half a day. Very large glass castings weighing hundreds of pounds can even take months. After soaking, cool the glass down past the strain point temperature slow enough so as not to allow too much damaging stress to develop. For a small bead, this can be as as fast as 600º F per hour (10º F per minute). For large beads, one half or one third that rate. A large paperweight, may require a rate as slow as 50º F per hour (less than 1º F per minute). Once the glass temperature has past below the strain point temperature, the cooling rate can be increased without causing permanent stress in the glass. However, cooling the glass too fast below the strain point temperature can still cause the glass to break during cooling due to thermal shock.

The above described procedure is very easily accomplished using a temperature controller that is programmed properly. To do it manually using an infinite control switch and a pyrometer, you first soak the glass as already described above. After the soak time has elapsed, there are several options, depending upon the size of the glass being annealed. For small glass item, such as small beads, simply turn the infinite control switch to Off. Since the annealer has been on for a while, the brick walls will have absorbed a lot of heat. This stored heat keeps the firing chamber from cooling very fast. For medium to large beads, or small hollow sculptures, instead turn the infinite control switch to Low. This setting will make it so that the temperature loss is slowed down. After about fifteen minutes the oven temperature will have dropped to below the strain point temperature. Because as the temperature in an oven gets lower, its heat loss slows, and the glass temperature is below the strain point temperature, you can turn off the oven and let it cool to room temperature for the size of the items described here. You should slowly cool to a lower temperature before turning off the oven for large items. To cool even slower, put the infinite control switch to a setting of 2, then after the temperature drops a to about half way to the strain point temperature, set it to Low.

When soaking and cooling different sizes of glass, use the rate that is best for the larger pieces. You cannot over soak or cool too slowly the smaller items.

You can anneal together borosilicate, Effetre (Moretti), Bullseye and Lauscha glasses. The temperature to use is 1000º F. It is a little high for all but the borosilicate, but at least 30º F below what any of those glasses will distort at. It is a little low for the borosilicate glass, but you can compensate for that by increasing the soaking time.

To find out more about specific soak times and cooling rates and how it relates primarily to glass thickness, consult one of the many excellent books available that cover the subject. Two very good books are: More Than You Ever Wanted To Know About Glass Beadmaking by James Kervin and Contemporary Lampworking - A Practical Guide to Shaping Glass in the Flame by Bandhu Scott Dunham.