What do you alter first – time or temperature – when tweaking schedules?
The choice is largely dependent on what the objective is and the process being used.
Since I am always trying to get the work done at the lowest temperature, I usually alter time first. This allows a greater amount of heat work which it the important element in getting the work done with the least risk. I alter this general approach in relation to the process being conducted.
In casting, the objective is to get all the glass in the mould flowing and fully incorporated, so if past experience has shown too little flow at a relatively high temperature, my choice would be to extend the soak time. This avoids high temperature damage to the mould, and can help to avoid sucklers, or indents, in the casting.
In a full fuse, I attempt to get the work done in around 10 minutes. This implies that I would increase the temperature first and then the time if necessary.
At firings from contour to sharp tack and sinter fuses I tend to increase the soak time. The amount of time is determined by using peeks to know when the desired shape is achieved.
In general, peek at or near the top temperature. If the piece has not formed as desired, the time can be extended, with no extra firing needed. This saves a lot of the extra firings that would be required. And multiple firings still do not give you the time or temperature required for a single firing.
If, when you peek, the work has already been done, you can skip to the anneal. And on the next similar firing, use a slightly lower temperature, again peeking near the top temperature.
More detailed information about scheduling options can be found in the e-book Kilnforming Principles and Practices, available from Bullseye and Warm Glass
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