Annealing Includes Cooling
Often people recommend a long anneal soak for potentially difficult pieces followed by an arbitrary 55C/100F cool rate to 371C/700F or 319C/600F. It is arbitrary because the same rate is frequently recommended regardless of the length of the anneal soak.
It does not have to be guesswork. Bullseye has provided us with the science of the anneal/cool in an accessible form: Annealing Thick Slabs (which covers thicknesses of 6mm/.025” to 200mm/8”). This document provides the annealing time for the chosen thickness and the directly linked cooling rates based on scientific principles.
The anneal soak is determined by the profile and thickness of the piece. Work for the e-book Low Temperature Kilnforming showed a relationship between the profile and the annealing time. Annealing for the profiles of sintering, tack, contour and full fuses requires calculation of the thickness to be applied to various profiles:
- Sinter or lamination – 2.5 times the thickest part
- Tack fuse – 2 times the thickest part
- Contour fuse – 1.5 times the thickest part
- Full/flat fuse – 1 times the thickest part
The cool stages are not random either. They have an intimate but inverse relationship to the anneal soak. They are to keep temperature differentials within the glass to acceptable levels. The anneal soak determined by the profile and thickness is to attain and keep the internal temperature within a range of 5°C throughout the glass. This is often referred to as ∆T=5°C.
The cool rates are to maintain acceptable temperature differences within the glass. The first cool rate is to maintain that temperature differential of ∆T=5°C. The second cool rate allows a wider range of temperature differential of 10°C, or ∆T=10°C. This is possible because the glass has become viscous enough to withstand this greater range of different temperature. The final cool needs to maintained at a differential of 20°C, or ∆T=20°C. Again, this is possible because the viscosity is high enough to withstand this amount of differential.
This information about cool rates and an allowable ∆T spread also indicate that turning off the kiln at 371°C/700°F is not always safe. It is almost always safe to do this for anything calculated to be annealed as for 12mm thick, and it may be safe for a piece up to 15mm thick, but remember that a tack fused piece of two base layers and a further decorative layer needs annealing and cooling as for 19mm/0.75”. The Bullseye research shows that the cooling rate for this is less than the unpowered cooling rate of many kilns. If there are additional complicating factors such as strongly contrasting colours, the annealing and cooling needs to be longer and slower than a simple multiplication of thickness.
There seems to be a practice of a single annealing rate to 371°C/700°F or 319C/600F. So the question will arise “Why is it necessary to have multiple cooling stages.” The response is that it will use unnecessary time and power. Attempting to maintain the ∆T=5°C over extended temperature ranges will not provide extra sound annealing. As the glass can withstand a ∆T=10°C from 427°C/800°F to 371°C/700°F, there is less power required at the faster rate than the slow one. This is even more so for the cool to 319C/600F and lower temperatures.
Knowing the safe temperature to turn the kiln off, requires knowing the cooling rate of the unpowered kiln. This blog shows how to determine the natural cooling rate of your kiln. Knowing this is as important as knowing what effect different fusing temperatures have on the glass in your kiln.
The object of this blog post is to demonstrate that cooling is part of annealing. Just as much attention must be paid to the cooling rates as the length of the annealing soak. They are inseparable for sound kilnforming practices.
Some work that may be of assistance in understanding the importance of knowing the relationship between the annealing soak and the annealing cool are:
Annealing Concepts, Principles, and Practice
Kilnforming Principles and Practice
Available from: Bullseye and Warm Glass
Low Temperature Kilnforming
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