I’m sure I have written about this before, but a repetition will not hurt.
I have seen many schedules with initial rates of advance interrupted by soaks. These kinds of schedules that are written something like this:
250 degrees to 200, soak for 10 (or 20 or 30) minutes
250 degrees to 500, soak for 10 (or 20 or 30) minutes
300 degrees to 1100, soak for 10 (or 20 or 30) minutes
300 degrees to 1250, soak for 10 (or 20 or 30) minutes
600 degrees to working temperature (1450, 1500 etc.)
When I have asked, I’m usually told that these are catch up pauses to allow all the glass to have an even temperature. There are occasions when that may be a good idea, but I will come to those later. For normal fusing, draping and slumping these soaks are not needed.
To understand why, needs a little information on the characteristics of glass. Glass is a good insulator. It is a poor transmitter of heat. Therefore, glass behaves better with a moderate steady input of heat to ensure it is distributed evenly throughout the glass. To advance the temperature quickly during the initial heat up stages where the glass is brittle risks thermal shock.
The soaks at intervals do not protect against a too rapid increase in temperature. It is the rate of heat input that causes thermal shock. Rapid heat inputs cause uneven temperatures through and across the glass. When these temperatures are more than 5°C across the glass, stress is induced. As the temperature differential increases, so does the stress until the glass is not strong enough to contain those stresses and so it breaks. At higher temperatures these stresses do not exist as the glass is less viscous.
If, as is common and illustrated in the schedule above, you advance at the same rate on both sides of the soak, the soak really does not serve any purpose – other than to make writing schedules more complicated. If the glass survived the rate of heat input between the soaks, it will survive without the soaks.
But you may wish to be a little more careful. The same heating effect can be achieved by slowing the rate of advance. Just consider the time used in the soak and then slow the rate by the appropriate amount. Take the example above using 30-minute soaks:
250 degrees to 200, soak for 30 minutes
250 degrees to 500, soak for 30 minutes
This part of the schedule will take three hours. You can achieve the same heat work by going at 167 degrees per hour to 500 degrees. This will add the heat to the glass in a steady manner and the result will be rather like the hare and tortoise. If you have to pause periodically because you have gone too quickly, you can reach the same end point by steady but slower input of heat without the pauses.
But, you may argue, “the periodic soaks on the way up have always worked for me.” As you work with thicker than 6mm glass, this “quick heat, soak; quick heat, soak” cycle will not continue to work. Each layer insulates the lower layer from the heat above. As the number of layers increase, the greater the risk of thermal shock. Enough time needs to be given for the heat to gradually penetrate from the top to the bottom layer and across the whole area in a steady manner.
To be safest in the initial rate of advance, you should put heat into the glass in a moderate, controlled fashion. This means a steady input of heat with no quick changes in temperature. How do you calculate that rate? Contrary as it may seem, start by writing out your cooling phases of the schedule. The cooling rate to room temperature is the safe cooling rate for the final and now thicker piece. If that final cool rate is 300 degrees, the appropriate heat up rate is half of that or 150. If you are in the habit of turning off the kiln at 370°C, you can use the cooling rate that is scheduled to get you there. Normally, you would double the rate you used to get to 370°C as the rate to room temperature. So, the rate to 370°C is the same as half the final cooling rate.
This “half speed” rate of advance will allow the heat to penetrate the layers in an even manner during the brittle phase of the glass. This rate needs to be maintained until the upper end of the annealing range is passed. This is normally around 55°C (110°F) above the annealing point.
Then you can begin to write the rate of advance portion of your schedule. It could be something like:
150°C to 540°C, no soak
225°C to bubble squeeze, soak
300°C to working temperature, soak 10 minutes
Proceed to cool segments already
I like simple schedules, so I normally stick to one rate of advance all the way to the bubble squeeze. This could be at the softening point of the glass or start at 50°C below with a one hour rise to the softening point with a 30-minute soak there before proceeding more quickly to the working temperature.
I did say I would come back to an exception about soaks on the rate of advance segment of the schedules. When the glass is supported – usually in a drape – with a lot of the glass unsupported you do need to have soaks. The kind of suspension is when draping over a cylinder or doing a handkerchief drop. This is where a small portion of the glass is supported by a point or a long line while the rest of the glass is suspended in the air. It also occurs when supported by steel or thick ceramic.
The soaks are not to equalise the temperature in the glass primarily. They are to equalise the temperature between the supports and the glass. A thick ceramic form supporting glass takes longer to heat up than the glass. The steel of a cocktail shaker takes the heat away from the glass as it heats faster.
The second element in this may already be obvious. The glass in the air on a ceramic mould can heat faster than that on the mould. The glass on a steel mould can heat faster over the steel than the suspended glass. Both these cases mean that you need to be careful with the temperature rises.
Now, according to my arguments above, you should be able to slow the rate of advance enough to avoid breakage. However, my experience has shown me that periodic soaks in combination with gradual increases in the rates of advance are important, because more successful.
An example of my rates of advance for 6mm glass supported on a steel cylinder is:
100°C to 100°C, soak 20 minutes
125°C to 200°C, soak 20 minutes
150°C to 400°C, soak 20 minutes
200°C to draping temperature
Call me inconsistent, but this has proved to be more effective than dramatically slowing the rates of advance.
This exception does not apply to slumps where the glass is supported all around by the edge of a circular or oval mould, or where it is supported at the corners of a rectangular or square one.
In both these cases, these are about the materials holding or contained in the glass, rather than the glass itself.