Slumping Breaks on “go-to” Schedules

Picture credit: Emma Lee 

 

An "It has always worked for me before" schedule implies a single approach to slumping regardless of differing conditions. 

In the example shown, we are not told the rate up to the slump.  But is clear the rate was too fast for the glass layup.  It cracked on the way up. This tells that the rate was only a little too fast.  If it had been faster the glass would have separated further apart.  The heat was enough to appear to recombine at the edges where it was not slumping so much. 

Review your "go to" schedules for each firing. It may still be a good base from which to work. But you need to assess the layup, thickness, and any other variations to help adjust the schedule to fire each piece. 

Some of the variations from the “standard” to be considered are: 

 Single layer slumping 

 Weight 

 Mould sizes 

 Relative Depth 

 Mould shape and detail

Drying Kiln Washed Moulds

A question about kiln wash. Do you have to let each coat dry while applying before applying the next coat?

 There seems to be a popular notion that newly kiln washed moulds must be cured before use.  I'm not sure where the information comes from, and no reasoning is given.  It is suggested that that quickly heating newly kiln washed moulds to 550°F (290°C) is important.

 If you want to make sure the mould is dry, this may not be the best way to do it.  All ceramics have a cristobalite inversion at around 225°C/437°F.  This a very rapid increase in volume of 2.5% that often leads to cracks and breaks in ceramics when the rate of advance is quick.  The mould will react better and last longer if the rate of advance is slow until that inversion temperature is passed.  But also note there is a quartz inversion at around 570°C/1060°F that is significant.

 

 This is another reason to advance the temperature slowly when slumping or draping with a ceramic mould.  A further reason to heat slowly is to avoid steam formation within the ceramic body.  If the steam is created over a short time, the force can be great enough to break the ceramic.  To ensure the water evaporates, a soak at 95°C/203°F for a significant amount of time is a better, safer option.

 But in addition to all these precautions, it simply is not necessary to cure kiln wash on slumping and draping moulds made of ceramics.  The glass does not begin to move until after 540°C/1000°F. Therefore, the kiln wash will be dry long before the glass gets near slumping temperatures.  Any vapor caused by evaporating water will escape through the vent holes in the mould or under the glass at the rim, as it will not form a seal until higher temperatures.

 

Newly kiln washed mould beside others already fired

 If you want to be sure your kiln wash is dry before you put the mould in the kiln, you can leave it in a warm ventilated space, or even on top of your kiln while it is being fired.  Using either drying method will dry the kiln wash sufficiently before the glass is placed on the slumping mould.

 The other part of the question was about drying the kiln wash between applying coats. It is not necessary to dry between coats of kiln wash.  In fact, a better result is obtained by applying all the coats at one time. It is not like painting wood. The result of applying all coats is a smoother surface.  There is no dragging of the dry powder along with the wet kiln wash as it is being applied over the existing coats.

 Kiln drying ceramic slumping and draping moulds is not necessary. It only adds another, unnecessary step in kilnforming preparations.  There are exceptionally good reasons to avoid rapid firing of damp moulds. 

 Some extra care could be taken with texture moulds and those intended for casting.

Sintering Ramps and Soaks

Sintering (or laminating) is a special form of low temperature kilnforming that requires attention to the ramp rates and the length of soaks. The rates and soak times were determined by the strength of the resulting pieces.

Credit: Researchgate.net

Rate

 The ramp rate has a significant effect on the strength of the resulting piece.

 A moderate rate (150°C/270°F) all the way to the sintering temperature of 690°C/1080°F gives the glass particles time to settle together. It works similarly to a slow ramp rate in slumping.

 A rapid rate (600°C/1275°F) - as used in medicine – to the sintering temperature of 690°C/1080°F is used for float glass particles.

 An alternative to both these is to schedule a rapid rise to the strain point followed by a slow - 50°C/90°F per hour - rate to the sinter temperature.

Soak

The soak time is extremely important in sintering to provide strong results. It is loosely related to the ramp rate, but in an inverse manner. The quicker the ramp, the longer the soak required.

 The moderate rate of 150°C/270°F needs a two-hour soak at the top temperature for maximum strength.

 The rapid rate of 600°C/1275°F requires about six hours of soaking at the top temperature.

 The alternative of a rapid rise to the strain point followed by the slow 50°C/90°F per hour rate requires at least a three-hour soak.

 These results show the ramp rate is important to the strength of the resulting piece. Fast ramp rates require increasingly long soaks at top temperature. Even slowing the ramp rate after reaching the strain point requires longer soaking than a steady rate. This is so even though the steady rate is faster than the two-part schedule to the top temperature.

 These results indicate that heat work is put into the glass throughout the temperature rise. The heat put slowly into the structure below the strain point still has an effect on the sintering of the glass.

 This is shown by the two-part schedule that has a slow ramp rate after the strain point. And even then, the time required is only 0.3hour shorter than for the moderate steady rise and soak. 

There is no time advantage to rapid rises to the strain point followed by a very slow rise to top temperature. The six-hour soak required by fast rises to top temperature show there is a large time disadvantage with rapid rise scheduling of sintering.

More information is available in the ebook Low Temperature Kilnforming and from Bullseye.

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.