Wednesday, 8 October 2014

Writing a Schedule

Making your own Schedule

I've been asked about making a schedule rather than using a pre-programmed one. My response is this, but please join in with amplifications and questions.

In principle, a firing schedule for glass follows these stages:
1 – a gradual, steady heat up to a temperature above the annealing point to avoid thermal shock
2 – a soak or slow rise around the slump temperature to allow any air to escape
3 – a more rapid rise to top temperature to avoid devitrification
4 – a rapid fall in temperature to an annealing soak, saving time and avoiding devitrification. The soak at annealing temperature is to equalise the temperature throughout the glass
5 – a steady slow fall in temperature to well below the lower strain point to complete the annealing
6 – a controlled cool to near room temperature to avoid thermal shock.

The details of schedules can appear complex, but the purposes of these six stages are reasonably simple.
Segment 1 is to heat the glass evenly without causing it to break from too fast an increase in temperature. At minimum this steady increase in temperature must continue to about 40ºC above the annealing point. (This will be about 540ºC)
Segment 2. This segment can include a “bubble squeeze” to enable air to get from between sheets of glass before the edges seal, or it can be a separate segment in your schedule. The slow rise in temperature will occur from about 600ºC to 680ºC. The bubble squeeze soak occurs at around 660ºC to 680ºC. In both cases there is normally a soak of half an hour at least at the end of the range.
Segment 3 is to go through the devitrification range (say 700ºC to 760ºC) as quickly as reasonable, but usually no faster than 330C per hour.
Segment 4 is to get back through the devitrification range to the annealing soak, which will be as long as required to equalise the temperature within the glass. This soak time increases exponentially with the thickness.
Segment 5 is the annealing cool, which should be a slow steady fall in temperature to ensure the glass all cools at the same rate (to around 370C).
Segment 6 continues the cool, although faster than previously, and often is achieved by turning the kiln off and leaving it closed until room temperature.

A schedule for a 6mm piece up to 2/3 the size of your kiln could be even simpler:
Segment 1 - 220 dph to 670C for 30 minutes
Segment 2 - 330 dph to 800 (flat fuse) for 10 minutes
Segment 3 - afap to 516 for 30 minutes
Segment 4 - 80 dph to 370, no soak
Segment 5 - off

You may find a schedule that will work, but you still need to know why it works, or at least what each segment is doing. So, for example, you need to think about what a 15 minute soak at 225C will do. What is the glass doing at that temperature? What do you want to achieve in that temperature range? Is there another way to achieve your objective? These are the kinds of questions you need to think about so you can construct your independent schedule when you move outside the parameters of the pre-programmed schedules.

To make a schedule for yourself can be worrying. But you can see from this example that it does not need to be complex. The principles are simple, although the details can be confusing. It is essential to know something about how heat affects the glass and this Bullseye Tech Note is one of the best descriptions. 
Knowing what the heat up events are is useful too.