Annealing Fused Glass

Annealing fused glass is the process of slowly cooling glass so that the internal temperature matches the external temperature. This allows the glass to reach a stress-relief point. The piece is allowed to heat-soak until the temperature is consistent throughout the piece.

As glass cools, the outside will always cool faster than the inside of the glass. Glass contracts as it is cooling. If the outside of the glass cools and contracts faster than the inside, this causes stress in the glass. Too much stress and the glass will break. The slower the glass is cooled, the less the amount of temperature difference throughout the glass and less the amount of stress that will develop.

The anneal point reflects the viscosity of the glass, and not the COE. This temperature and the holding time really depend on the size of the piece and the COE of the glass. Smaller pieces don’t require as much time to achieve this process as large bodies of glass. A higher COE of the glass will require a lower annealing temperature than glass with a lower COE.

You are trying to achieve even heat in the entire glass and holding it at that temperature. It is held steady and for a period of time to remove the stress caused from the heating process. This also includes a cooling down phase. This is done slowly so as to not permit too much stress to build up in the glass.

Although invisible to the naked eye, the strain is present and can be a possible point of failure in the glass piece unless relieved. You don’t want your piece to crack or break at some point in time. Any time glass is heated and cooled, it messes with the molecules. When you anneal the piece, it brings the molecules back in line. If the molecules don’t get lined up again, the glass will break. Resist the temptation to peek into the kiln before the process is complete. Any rapid temperature changes will obstruct the process.

The glass is cooled slowly at a rate until the temperature is below a critical point when the internal stress is balanced with the surface tension on the outside of the glass. At this point it can be safely dropped to room temperature.

Soaking is when you hold glass at a steady temperature over any span of time. Soaking the glass at a high temperature runs the risk of being so hot that it will distort. But this process has the advantage of requiring a shorter soak time for the built up stress to even out through the piece. Your piece will also need to cool down through a longer temperature range. Soaking at a lower temperature has the benefit of a shorter cooling time, but also involves a longer soaking time to remove any stress, and may not even remove all of the stress.

After the glass has soaked for the required time, all of the stress will dissolve, but stress can reappear during the cooling phase. The quicker the glass is cooled down, the more the amount of stress that can be built up in the glass.

This temperature to anneal any glass is a range, and is located at approximately the center of this range. It is not important to reach an exact temperature. Keeping the glass temperature steady for an amount of time before cooling slowly to room temperature is important.

The temperature recommended to achieve this process for most glass, is around 940º F. If you are using borosilicate glass, use around 1050º F. If you don't know the COE of you glass, try the shotgun method.

The cooling down of the glass is important between the anneal temperature and the strain point. Glass can develop stress throughout the entire cooling process. Any stress that develops below the strain point is only temporary. If stress develops in the glass above the strain point it can't be removed. You can speed up the cooling time below the strain point temperature, because the stress that is caused during that process will not cause the glass to break any time in the future.

You want to avoid thermal shock by not cooling the glass too quickly though. Don't open the lid of your kiln during this process. If you use a temperature below the strain point to cool slowly down to before then increasing the cooling rate, you don't need to know the actual strain point temperature of every glass. Use approximately 800º F.

Once your glass has reached the anneal temperature:

Soak the glass for a period long enough to remove any stress.

For a smaller piece about the size of a tiny bead, use about twenty minutes. 600º F per hour (10º F per minute).

For larger pieces about the size of large beads, use about one hour. This size may require a rate as slow as 50º F per hour (less than 1º F per minute). For an even larger paperweight size, it can take about half of a day.

When fusing or slumping glass, for each thickness from ½ inch to 1 inch, it should be annealed for 30 minutes. If you are having a hard time determining the annealing time, play it safe and anneal longer. You can’t over anneal glass.

After you soak the glass, cool it down past the strain point temperature slow enough so it does not allow too much stress to build up. Once the temperature has gone below the strain point temperature, you can increase the cooling rate without producing permanent stress. But cooling the glass too quickly below the strain point temperature can still cause the glass to break due to thermal shock.

You can control the temperature by using a temperature controller that is programmable. If you are doing this process manually, use an infinite control switch and a pyrometer. First you must soak the glass. This has been explained above.

After the soak time has finished, there are several options you have, depending upon the size of the glass.

For small glass items just turn the infinite control switch to off. The hot brick walls will have soaked up enough heat to keep the kiln from cooling very fast.

For medium to large pieces turn the infinite control switch to a lower number. Once the temperature drops to about half way to the strain point, set the dial to low. This setting the dial to a lower number will allow the temperature to cool down at a slower rate. Check the kiln in about fifteen minutes and by then the oven temperature should drop to below the strain point temperature. You can now turn off the kiln and allow it to cool to room temperature.

Breaks that occur during the process are typically "S" shaped. If a break runs along the line of two different glasses, it is usually due to the glasses not being compatible. Thermal shock will cause the broken glass to move apart and separate.

This process is more easily achieved with a uniform mass, such as a round bead. As the number of edges and depths increase, so does the annealing time. So you need to anneal even tack-fused and slumped projects.

If you are extending the time, and it doesn’t seem to work, you can try insulating the edges of your work with fiber paper, fiber blanket or fiber board. These items will help to stabilize the cooling rate throughout the entire glass piece.

Always remember when you can’t over soak or cool your pieces too slowly. So if you are soaking and cooling a variety of sizes of glass, use the rate that is recommended for the larger pieces.

For a visual of why annealing is an important part of your process, watch the following video by Corning Museum of Glass. This video can be located at annealing.

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