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, which relieves internal stress.

The piece is allowed to heat-soak until the temperature is consistent throughout the piece.

This is a very important step in your fusing process.

If you want your items to last over the ages, then take the extra time to assure that this step is taken!

As glass cools down, the outside will cool down much faster than the inside of the glass.

Think of a piece of molten wax that has dripped off and puddled from a burning candle.

As it puddles the outside shell cools off quickly, but if you poke a finger into the shell, you will find that the inside is still hot enough to burn your finger.

Glass contracts as it is cooling down.

Much like the candle example, the outside cools and contracts faster than the inside and this causes stress in the glass.

If there is too much stress, the glass will break from this unreleased stress.

Therefore slowing down the cooling process will enable less of a temperature difference throughout the entire piece and that will cause less stress.

The 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.

This is based on the dimensions and mass of the individual piece of glass.

Thickness is a very important factor in this process.

The insulating effect of the thick glass is more of a factor than the size, although the size is still important.

A thicker say one inch thick piece of glass needs more annealing that a piece that might only be a half an inch, even if they both have the same mass.

The larger and thicker the piece of glass, the more time it takes to anneal the piece properly.

You are trying to achieve even heat in the entire glass and holding it at that temperature.

This is to remove the stress caused from the heating process and cooling phase.

Although stress can't be viewed, it is there and can cause the piece to break at some point in time.

The molecules inside glass are disturbed because of the heating and cooling.

When the piece is annealed the molecules are brought back into line.

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 the temperature can be safely brought down to room temperature.

No peeking until it has cooled down below 100 degrees Fahrenheit.

Soaking is keeping glass at a constant temperature for a period of time.

Soaking the glass at a high temperature could cause the glass to distort out of shape, but it also requires a shorter soak time for any stress to settle down and even out throughout the piece.

However, soaking at a lower temperature might only require a shorter cool down time, but also requires a longer soaking time to remove any built up stress, and might not 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 900º 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 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.

Return to any of the following pages:

Technical Terms

From this page to Glass Fusing Made Easy

E-mail Address
First Name (optional)

I keep this private.

Site Affiliate Links

Delphi Glass