The Importance of Three-Stage Cooling

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.

 

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.

 

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.

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.