Annealing - What is it, and Why do it?

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Annealing - What is it, and Why do it?

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. The glass will also need to cool down 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 temperature 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.

We recommend the following annealing temperatures:

· Effetre (Moretti), Bullseye and Lauscha glasses - around 940º F.
· Borosilicate glass - around 1050º F for.
· Satake - around 890º F

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 the 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 temperature below which any stress that develops in the glass is only temporary, and above which it 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.

Select a temperature well below the strain point for all the glasses to be annealed, and then slowly cool to this point. Once the selected temperature is reached you can safely increase the cooling rate. You will not need to know the exact strain point temperature of each individual glass.

Suggested strain point temperatures:

· Satake - Use 750º F
· Everything Else - use 800º F

The process to anneal glass once it is at the annealing temperature is as follows:

1. 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, use one hour. For a large paperweight, it can take half a day. Very large glass castings weighing hundreds of pounds can even take months.

2. 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 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 procedure described above is very easily accomplished using a properly programmed temperature controller. 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 to quickly.

For medium to large beads, or small hollow sculptures, instead turn the infinite control switch to Low. This setting will slow down the temperature loss. After about fifteen minutes the oven temperature will have dropped to below the strain point temperature. As the temperature in an oven gets lower, its heat loss slows. Once the temperature is below the strain point (for the size of the items described) you can turn off the oven and let it cool to room temperature.

For larger items, you should slowly cool to a lower temperature before turning off the oven. To cool even more slowly, put the infinite control switch to a setting of 2, then after the temperature drops 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 distortion temperature for any of these glasses. 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, 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.


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[paintingwithglass]

Thanks for the tips. Wow!

[Hranilovich]

Thank you for the most concise and understandable approach to annealing I've seen. I've been too nervous to approach it, but now feel I can.

[hyperT]

Technically 10 minutes per millimeter thickness soaked at the annealing point. Smaller pieces of glass such as small beads may require less. Once the oven comes down to 500 F
the kiln can be vented a bit to speed the cooling down. The Mt. Palomar telescope lens is 26 inches thick and over 16 feet across and weighed 20 tons. It was held at the annealing point for over a year.
That was the second attempt. The first one shattered do to cooling to fast.