It is common to think of cooling after annealing as a simple
single cool rate to an intermediate temperature between annealing and room
temperatures before turning off. This most often works well for full fused pieces up to 6mm/0.25.
But as the pieces become thicker or more complex, the need for more controlled
cooling becomes necessary.
The aim of annealing is to get the glass to be the same temperature
throughout its substance during the annealing soak. This is called the ΔT
(delta T). This difference has been shown to be 5°C to avoid high levels of stress. Therefore,
ΔT=5°C/10°F. This difference in temperature needs to be achieved during the annealing soak and maintained during the cool.
The object of controlled cooling is to maintain this small
difference in temperature. It needs to be maintained throughout the cool to
avoid inducing excessive stress in the glass, even if the stress is only temporary.
As the thickness or complexity of the piece grows, the
annealing soak needs to be longer and the cool slower. The first cool is
critical to the production of stress-free fused glass. That is the fastest rate
that can be used in a single or multiple stage cooling. If you use that rate all the way to 370°C/700°F you will need at least
1.3 times longer to get to that temperature than if you used the first two
parts of a 3-stage cool. This time saving becomes greater as complexity and
thickness demand slower cool rates. It is not only time that is saved.
The risk of breaks from rapid cooling after the anneal soak and to 370°C/700°F increases with more complex and thicker pieces. Although the stress induced by
rapid cooing below the strain point is temporary, it can be great enough momentarily to break
the glass. This is so even if the glass meets the ΔT=5°C/10° during the
annealing soak.
Examples may help understand the cooling requirements of
glass that it thicker, or tack or contour fused.
Example 1
A 12mm/0.5” full fused piece needs
a two-hour annealing soak, followed by three cooling rates of 55°C/100°F per
hour, 99°C/180°F hour and finally 300°C/540°F per hour. The first rate is for
the first 55°C/100°F, the second rate for the next 55°C/100°F, and the final
rate is to room temperature.
What happens here is instructive as to the reasons for soaks
and cool rates. In this recorded example the ΔT at the start of the anneal is 7°C/12.6°F.
During the soak, the ΔT reduces to as little as 2°C, but ends with a ΔT=3°C. The
55°C/100°F cool rate over the first 55°C/100°F enables the ΔT to remain between
3°C and 4°C. The second cool over the next
55°C/100°F maintains this ΔT of 3°C to 4°C. During the final cool the ΔT varies
from 5°C to 1°C.
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| An example of the variation in ΔT during the first 55C/100F of cooling |
Example 2
A rounded tack fuse of 1-base and 2-layer
stacks gives a total of 9mm/0.375”. Research has shown that you need to
schedule for twice the actual thickness for rounded tack fusing - so for
19mm/0.75”.
This requires an anneal soak of
150 minutes, and a first cool of 20°C/36°F. The second cool rate can be
increased to 36°C/65°F. The final rate can be at 120°C/216°F per hour to room
temperature.
The ΔT at the beginning of annealing was 7°C/12.6°F and at
the end of a 2-hour soak was a ΔT of 1°C/2°F. The first cool ramp was 20°C/36°F
per hour and gave a variance of between 2°C/3.6° and 0°. The final cool
produced variances of up to 6°C/11°F, ending at 88°C/190°F with a ΔT=2°C.
The first two stages of cooling save 1.27 hours of cooling
time over a single stage cooling of 20°C/36°F to 371°C/700°F. It still keeps
the glass within that ΔT=5°C. More importantly, the third stage cooling is able
to keep the variance to between 6°C and down to 2°C.
The natural (unpowered) cooling rate of my 50cm/19.5” kiln
at 370°C/700°F is 240°C/432°F per hour. It settles to the 120°C/216°F per hour
only at 200°C/392°F. This is a fairly typical cooling rate for medium sized
kilns. This rapid cooling at 370°C/700°F creates a greater risk of breakage
than the controlled cool.
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| An example of the ΔT during the second 55C/100F of cooling |
Example 3
A sharp tack or sintered piece
with two base layers and two tack layer stacks on top requires firing as though
30mm/1.18”.
This needs a 4-hour soak during which the ΔT varied from 8°C
to 4°C. The first cooling rate was at 7°C/12.6°F and gave a ΔT variance of 4°C
to 2°C. The second cooling rate of 12°C/22°F produced variances of 3°C to 1°C
by 370°C/700°F. The final cool of 40°C/72°F per hour gave differences ranging
from 5°C to 0° at 110°C/230°F.
Note that the test kiln’s natural cooling rate does not
achieve the third cooling rate until 140°C/284°F. This shows that turning off the kiln at 370°C/700°F
produces a high risk of breakage for thick and complicated pieces. In addition, the two stage cooling rates
saves 3.27 hours of cooling time.
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| An example of the ΔT during the final stage of cooling to Room Temperature |
The temperature differentials below the strain point can
exceed the ΔT=5. The stresses induced are temporary according to scientists. But they can be great enough to break the glass during the
cooling. It follows that the anneal soak may have been adequate, but the cool
was so fast that excess stress was induced by the differential contraction
rates. This stress being temporary, implies that testing for stress in a broken
piece may not show any. The momentary excess stress will have been relieved
upon cooling completely to room temperature. (IMI-NFG Course on Processing in Glass, by Mathieu Hubert, PhD. 2015 , p.9.)
More information on cooling is given in the book LowTemperature Kilnforming; an Evidence-Based Approach to Scheduling.
