By Gil Reynolds
This article first appeared in the Stained Glass News in December of 2001

The most common problem I encounter in my teaching is a lack of understanding about how to properly anneal or slowly cool glass in the kiln to avoid excessive internal stress.

If, for example, the stress levels are too high, the glass will not be durable and will break over a period of time…or when you try to reheat it. The worst-case scenario is when the stress levels are so high the piece is broken as soon it cools to room temperature. Now, that’s no fun!

Safe heating and annealing rates are both affected by several factors, the biggest variable being the size and shape of the piece of glass you’re heating. The smaller the pieces, the greater the tolerance to rapid temperature change.

When the Italians pull their thin rods of millefiori and lay them out in the open air to cool to room temperature, their small, pencil-sized diameters and round shapes allow for sufficient annealing with this rapid cooling rate. We often will pull a tread of glass in a torch, and just letting it cool on its own in the open air is sufficient annealing for its thin diameter.

As the size and thickness increases, the amount of time required for the glass to be safely heated or cooled also increases. Most soft brick kilns, when turned off and allowed to cool, will loose heat at a rate of about 250 F per hour through the annealing range. That is slow enough to properly anneal a 3” x 3” glass tile no more than ¼” thick.

If your piece is larger or thicker than this, you should be adding heat during the annealing process. This is where a programmable controller is darn handy. I’m not saying that controllers are an absolute necessity, but it’s difficult to manually anneal large pieces of glass. These days, buying a kiln with a built in controller is relatively inexpensive and will pay for itself by greatly minimizing the amount of broken glass and the heartaches that go with it.

There are plenty of annealing cycles published in glass working literature, but it’s important to decide if they are applicable to kiln work.

Annealing cycles are often described as one-side or three-side cooling. A blown vessel annealing in a kiln would be considered three-side cooling and would have a much quicker annealing than the same weight of glass in a thick sheet, which would be considered one-side cooling.

In kiln work we are mostly using one-side annealing cycles which are slower but safe, even for slumped or bent glass shapes. Which brings us to our first annealing axiom, ‘You can not over anneal.’ This is important because it allows us to always error on the side of ‘too long’ without the fear of damaging the glass.

Most annealing data will give you a soak temperature, which is the starting point of the annealing cycle. ‘Soaking’ means to hold the kiln at that temperature for a period of time. The data will also give you the time you should hold the glass at that temperature; an example would be to soak at 975 F for two hours. Next is the rate of descent, or how slow you should travel down through the annealing zone. This is often expressed in degrees per hour, such as 90 degrees per hour to 750 F. So your kiln would set to 975 for two hours, then slowly and evenly cool to 750 over the next two and a half hours. There may also be a token soak of one minute at the end of the cycle, which is there only for the sake of computer controllers. I find that controllers do better if they have something other than zero to process at the end of a cycle.

The key elements, then, are the annealing soak and annealing cool. The trick is to find the right amount of time for both the soak and the ramp and, most importantly, determining the right temperature to start the cycle.

Most people I talk to with annealing problems are doing two things wrong: (1) venting too long (2) annealing at the wrong temperatures. Are they to blame? No. In most cases they’re just following what others have told them to do.

Many are in the habit of venting the kiln all the way down to 1000 F. This is OK for small earring and broach-sized pieces of glass, but it can be disastrous for a 12” diameter plate, ¼” (two layers) thick. I know that’s what it says to do in one of our early books on the subject. But in the 18 years or so since that book was published, we have learned that it is better to just FLASH VENT. This is where you open the kiln for a short period of time to stop the melting process and then close the kiln to let it cool on its own.

For fusing, or if you went to 1400 F or above, you would flash for about eight seconds; below 1400, say for slumping or bending the glass, three seconds is plenty. This gives much better results because the goal of the annealing cycle is to allow the inside and the outside of the glass to cool as evenly as possible. Opening the kiln so that it cools rapidly causes the outside to lose heat much faster than the center of the glass, creating an undesirable temperature difference which we would then need to overcome with our annealing cycle. Letting the kiln cool slowly works in our favor by keeping the temperature throughout the glass fairly even.

Originally, the reason for the fast venting was to avoid devitrification, or a scum of crystal growth on the surface of the glass. However, with the new types of glass formulated for kiln work available today, most problems have been eliminated. Some examples of what you might want to consider include the new System 96 fusing products, Uroboros Fusible 90 and 96 and most of the newer Bullseye formulas.

We also have numerous overglazes on the market like Super Spray, Clear Coat, Bending Glaze, Back Magic and Spray A. Applying these to the surface of a known offender will prevent that piece of glass from devitrifying so you can fire in a manner that will produce the best results.

The temperature problem has several components. First, people use the published annealing data without realizing that most kilns are different. I can have two identical kilns that, more often that not, have different annealing points for the same piece of glass. Each kiln has it’s own personality and only time and experience will help you learn the little quirks and nuances of the one you’ve chosen for your studio. In addition, most people assume the temperature reading on their pyrometer is the temperature of the glass. Not so. Rather, such readings show the temperature of the air around the probe of the pyrometer. If the glass is sitting on a shelf several inches below the probe, it can easily be off by as much as 30 F or more.

So, how do we solve the wrong temperature problem? Well, you may have your own method…and if it works…go for it! If not, there are lots of ways to get the job done and in this fusing game there are very few absolutes. But, if you are having problems reheating pieces without them breaking, or if you find your pieces are broken when you first open the kiln to look at them, you might want to try a different annealing approach.

The complicated solution is to run some softening point tests, where you spend several hours at 940 F, then several hours at 960 F, 980 F, all the way up until all of your ½” x 9” test strips of different types of glass have bent. The strips should be lying at a 45-degree angle against the inside of the kiln and you should monitor them every hour to see when they start to bend. This will tell you the exact starting temperature for each of the different types of glass you might be using.

I highly recommend this test although most people don’t want to spend the time to do it correctly. Instead, they just heat the kiln without soaking at each temperature and the result is that they get imprecise data. A strip of glass that bends at 1075 F would actually bend closer to 980 F if it’s held there for two hours.

The easy solution is to start the soak at a slightly higher temperature than recommended. For most people that would mean starting at 1000 F. This allows the temperature of the glass to stabilize.

The cycle I would recommend programming into the controller for glass that was up to 1/8” thick x 20”diameter or smaller, or ¼” thick x 8” diameter or smaller, or even 3/8” thick x 5” diameter or smaller would be as follows: Rate 9999 DPH (Degrees Per Hour) down to 1000 F with a soak of one hour; then 90 DPH down to 970 F, soak one hour; then finally 120 DPH down to 750 F and a soak of one minute.

If the piece of glass being cooled exceeds the above dimensions, you will need to increase the soak times and decrease the rate of cooling. A ¼” thick x 18” diameter or smaller piece of glass would soak for two hours at both 1000 F and 970 F, with a cooling rate half as fast as the previous program (45 DPM and 60 DPM respectively)

As with all kiln work, the trick is to be patient and be willing to experiment.

Until Next Time,
Keep a Warm Kiln,
Gil Reynolds