Scheduling for Thick Landscapes

Thick slabs often involve numerous firings of increasingly thick work.  I am using an existing example, with their permission, of the first stages of a thick landscape.  The initial concern was with bubbles in the first layup, then the strategy for firing the thick slab.

Plans

This is the first part of a landscape with depth.  It will be fired 5-7 more times.  This first piece will be inverted for the next firing with the clear facing up, to avoid reactions between the colours.  It is similar to an open face casting. There is a Bullseye Tip Sheet on open face casting that will give a lot of information.

Layup

Picture credit: Osnat Menshes

This work has a base of clear that is mostly overlaid with one layer of 3mm pieces, although in some places another layer, and there are some pre-fired elements as well.  It is fired on Thinfire shelf paper.

Bubbles 

There is concern about the number and size of the bubbles after the firing, and how to avoid them.  Will they grow over the multiple firings?

The many small bubbles are characteristic of kilnformed glass.  The few larger bubbles may result from the frit that is under the pieces that form the top surface.  And there are some overlaps of clear over colour that may form pockets where air can collect. I advise leaving the scattering of the frit until all the decorative pieces are in place.  The bubbles will migrate toward the top during the multiple firings.  They will not grow in size unless they combine during the upward migration.  A later suggestion about reducing the number of firings will reduce the bubble migration and risk of increasing in size.

Picture credit: Osnat Menshes

Schedule

Proposed Schedule (Temperatures in degrees Celsius)

1: 180 – 560, 30’    I would go to 610 for 30'

2: 25 – 680, 120’    I would use only 30'

3: 220 – 810, 15’    I would set the top temperature at 816, 15’.

4: 9999 – 593, 30’  Eliminate this segment. 

5: 9999 – 482, 120’ I suggest one hour soak

8: 55 – 370, off      83 – 427, 0’

7: 150 – 371, 0’

8: 330 – to room temperature, off.

 

Eliminate segment number 4.  Any temperature equalisation done at this temperature, is undone by the AFAP to the  annealing.  The temperature equalisation occurs at the annealing temperature. No soak at an intermediate temperature is required.  This blog post gives some information about annealing above and below the annealing point (Tg). 

Firing Incremental Layers

The plan is for five to seven more firings.  Continuing to build up the thickness on each firing, may have some problems.

• There is increased risk of compatibility problems when firing a piece to full fuse many times.
• There is a risk of more bubbles and of the existing ones becoming larger as they move upwards and combine with other smaller ones.
• With each firing the thickness is increasing and so becoming a longer firing.  This is because the heat up, annealing, and cooling each need to be longer.  For example - 6mm needs 3hour cooling, 12mm needs 5 hours, 19mm needs 9 hours. 

Multiple Slabs

These are the main reasons that I recommend firing a series of 6mm slabs separately and combining them in one final firing.  Firing a series of 6mm slabs and then combining them in a single long and slow final firing has advantages.

• The individual pieces do not need to go through so many full fuse firings, reducing the risk of compatibility problems.
• The small bubbles in each firing will not have the chance to rise through all the layers to become larger.
• The total time in the kiln for the combined pieces will be less than adding layers to already fired layers.

Examples

It is often difficult to convince people that firing by adding incrementally to an existing slab, longer firing times are required than by firing a group of 6mm slabs and a single combined firing of all the slabs.  I give an example to illustrate the differences.

Annealing

Assume there are to be a total of eight firings (existing 6mm slab and 3mm for each of seven more firings).  Also assume that each additional firing is of 3mm. This makes a total of 28mm.  Compare annealing and cooling times for each firing:

Firing      thickness       anneal and cool (hours minimum)

1            6mm                    3

2            9mm                    4

3            12mm                   5

4            15mm                   7

5            18mm                   9

6            21mm                   11.5

7            25mm                   14

8            28mm                   17

Total                                   70.5 hours annealing time (minimum)

To fire up 5 six millimetre slabs takes less time – 3 hours annealing and cooling time for each firing cumulates to 15 hours.  Add to that the final firing of 17 hours annealing time.  A total of 32 hours.  This is half the time of adding to the existing slab at each firing.  Multiple 6mm slabs can be fired at the one time if there is space in the kiln, which would reduce the kiln time for the 6mm slabs even further. 

An additional advantage of firing 6mm slabs and combining them, is that bubbles can be squeezed out more easily in the final thick slab fring because of the combined weight of the  slabs.  You could make the individual slabs a little thicker, but that would involve damming each slab.  Not an impossible task of course.  And it would change the calculations, by reducing the number of firings.

Heat Up

Another time saving is to use the second cooling rate from the Bullseye document Annealing Thick Slabs as the first up ramp rate. Take this rate up to a minimum of 540˚C. Although, this is an arbitrary temperature above the strain point to ensure all the glass is above the brittle phase.  It is possible to maintain this initial rate to the bubble squeeze.  But with the slow rises in temperature required for thicker slabs, it is sensible to increase the rate from 540 to bubble squeeze to reduce the firing time.  Once past the bubble squeeze a more rapid rate can be used to the top temperature.  

The heat up times could be about half the minimum cooling times.

A worked example (with certain assumptions) would be:

Firing      thickness       time to top temperature total time.

1            6mm             6.3               

2            9mm             7.1

3            12mm            8.4

4            15mm            10.7

5            18mm            15.9

6            21mm            19.4

7            25mm            25.1

8            28mm            29.1              ca.122 hours

But firing five times for 6mm equals 31.5 hours plus the final firing up of 29.1 hours equals a total of 60.6 hours.  Again about one half the time of progressively building up a base slab to the final thickness.

Savings

This example shows that approximately 90 hours of firing time can be saved by making a series of six millimetre slabs and combining them in a final firing.  There is the additional advantage of reducing the occurrence of bubbles between the layers in the final firing because of the weight of the combined slabs.

Draping Different Thicknesses and Sizes

Scheduling for different sizes and thicknesses of drapes requires schedules specific to these factors in addition to observing the progress of the drape.

Bob Leatherbarrow's research shows 6mm drapes more slowly than 3mm. It seems the thicker glass takes longer to begin the slump.  Glass behaves in a similar way for a drape.  My experience of draping 6mm/0.25” and 3mm/0.125” in the same firing confirms that 6mm takes longer.  Or, it needs a higher temperature.  I know this goes against common sense, but tests and experience show it to be true.

If you try to drape 3mm/0.125” and 6mm/0.25” pieces at the same time, the 3mm will reach the desired shape before the 6mm. You then have the choice of an under draped 6mm piece or an over draped 3mm piece.  This indicates that draping different thicknesses in the same firing will be unsuccessful.  To a lesser extent, the size of the drape will influence the speed of the drop.  So, you are unlikely to achieve completely desirable results with significantly different sizes of drape in the same firing either.

Observation is essential in all draping operations. You cannot know how long it will take for a piece to drape or drop to your requirements.  To be sure of your result you need to observe the progress of the drape.  There is rarely a safety net of a form to drape onto as in slumping.  To observe, set your top temperature with a long soak/hold.  Start peeking at frequent intervals from the time top temperature is reached.  

When the glass has reached the desired shape, advance to the next segment.  Your controller manual will give you instructions on how to do that.

Different thickness and sizes of glass require different firing conditions.

Effects of Dam Materials on Scheduling

 I once made a statement about the effects of various dam materials on scheduling. This was based on my understanding of the density of three common refractory materials used in kilnforming – ceramic shelves, vermiculite board and fibre board. I decided to test these statements.  This showed I was wrong in my assumptions.

I set up a test of the heat gain and loss of the three materials. This was done without any glass involved to eliminate the influence of the glass on the behaviour of the dams. The dam materials were laid on the kiln shelf with thermocouples between. These were connected to a data logger to record the temperatures.

Test Setup

 The thicknesses of the dams may be relevant. The vermiculite and fibre boards were 25mm thick. The ceramic dam material was 13mm thick.

The schedule used was a slightly modified one for 6mm:

• 300°C/hr to 800°C for 10 minutes
• Full to 482°C for 60 minutes
• 83°C to 427, no soak
• 150°C to 370°C, no soak
• 400°C to 100°C, end

 

The data retrieved from the data recording is shown by the following graphs.

Temperature profile of the air, ceramic, fibre, and vermiculite during the firing.

Highlights:

• The dam materials all perform similarly.
• This graph shows the dams have significant differences from the air temperature – up to 190°C – during the first ramp of 300°C/hr. (in this case).
• There is the curious fall in the dams’ temperatures during the anneal soak. This was replicated in additional tests. I do not currently know the reasons for this.
• The dams remain cooler than the air temperature until midway during the second cool when (in this kiln) the natural cooling rate takes over.
• From the second cool to the finish, the dams remain hotter than the air temperature.

 Some more information is given by looking at the temperature differentials (ΔT) between the materials and the air. This graph is to assist in investigating how significantly different the materials are.

This graph is initially confusing as positive numbers indicate the temperature of the first is cooler than the material it is compared with, and hotter when in negative numbers.

 

A= air; C=ceramic; F=fibre board; V=vermiculite

Temperature variations between air and dams

 As an assistance to relating the ΔT to the air temperature some relevant data points are given. The data points relate to the numbers running along the bottom of the graph.

 Data Point       Event

• 1            Start of anneal soak.
• 30          Start of 1^st cool (482°C)
• 45          Start of 2^nd cool (427°C)
• 65          Start of final cool (370°C)
• 89          1^st 55°C of final cool (315°C)
• 306         100°C

 

At the data points:

• At the start of anneal soak the ΔT between the dams is 16°C with the ceramic shelf temperature being 18°C hotter than the air.
• At the end of the anneal soak of an hour, the air temperature is 20°C higher, although the ΔT between the dams has reduced to 12°C.
• At the end of the 1^st cool the ΔT between the dams has reduced to 9°C and the ΔT with the air is 3°C.
• At approximately 450°C the air temperature becomes less than the dams.
• At 370°C the hottest dams are approximately 17°C hotter than the air.  The ΔT between the dams is 10°C.

 More generally:

• The air temperature tends to be between 17°C hotter and 17°C cooler than the ceramic dams during the anneal soak and cool.  The difference gradually decreases to around 8°C at about 120°C.
• Ceramic and fibre dams loose heat after the annealing soak at similar rates – having a ΔT between 4°C and 1°C, with a peak difference of 9°C at the start of the second cool. This means the heat retention characteristics of ceramic strips and fibre board are very close.
• Between the annealing soak and about 300°C the vermiculite is between 12°C and 9°C hotter than the same thickness of fibre.  Vermiculite both gains and loses heat more slowly than the ceramic or fibre dams do. This means that vermiculite is the most heat retentive of the three materials.
• Vermiculite remains hotter than ceramic from the start of the second cool. This variance is up to 9°C and decreases to 3°C by 100°C.
• Fibre board is cooler than ceramic dams until the final cool starts, when there is little variance.  At the start of the second cool there is about 15°C between the two.
• Vermiculite remains cooler than fibre dams throughout the cooling process. This ranges from about 12°C at the start of the first cool to about 3°C at 100°C.

Since we cannot see more than the air temperature on our controllers it is useful to compare air and dam temperatures. The same data points apply as the graph comparing differences between materials.

 

Ceramic-Vermiculite; Ceramic-Fibre Board; Vermiculite-Fibre Board; Ceramic-Air Temperature
This graph shows the temperature differences throughout the cooling of various materials.

• During the annealing soak, the air temperature is greater than the dam temperatures. The fibre and vermiculite boards remain at similar temperatures and the ceramic dam is the coolest.
• The three dam materials even out with the air temperature at the start of the second cool.
• Through the second and final cools, vermiculite dams remain hotter than the air temperature – between about 24°C at start of the final cool and 9°C at 100°C.
• The ceramic and fibre dams are close in temperature difference to the air from the start of the final cool. Their ΔTs are 17°C at the start of the final cool and 6°C at 100°C.

Conclusions

• Dams will have little effect during the heat up of open face dammed glass.  The slight difference will be at the interface of the glass and the dams where there will be a slight cooling effect on the glass. Therefore, a slightly longer top soak or a slightly higher top temperature may be useful.
• The continued fall in the dams’ temperature during the anneal soak indicates that this soak should be extended to ensure heat is not being drained from the edges of the glass by the dams. There is the risk of creating unequal temperatures across the glass.
• The ability of ceramic and fibre dams to absorb and dissipate heat more quickly indicates that they are better materials for dams than vermiculite board. The slightly better retention of heat at the annealing soak, indicates that ceramic is a good choice when annealing is critical.        
• These tests were fired as for 6mm/0.25” glass and so show the greatest differences. Firing for thicker glass will use longer soaks and slower cool rates. These will allow the dams to perform more closely to the glass temperature during annealing and cool.

Based on these observations, I have come to some conclusions about the effect of dams on scheduling.

• There is no significant effect caused by dams during the heat up, so scheduling of the heat up can be as for the thickness of the glass.
• The lag in temperature rise of the dams indicates a slightly longer soak at the top temperature (with a minor risk of devitrification), or a higher temperature of, say 10°C, can be used.
• The (strange) continued cooling of the dams during the annealing soak indicates that extending the soak time to that for a piece 6mm thicker than actual is advisable.
• The cool rates can continue to be as for the actual thickness, as the dam temperatures follow the air temperature with little deviation below the end of the first cool.
• Ceramic dams of 13mm/ 0.5” perform better than 25mm/1.0” vermiculite and fibre board. 
• However, in further tests of 25mm/1.0” thick ceramic dams performed similarly to the same thickness of vermiculite. So, 25mm/1.0” fibre board the best when choosing between the three materials of the same thickness. But 25mm ceramic strips are not common, nor are they needed for strength or weight.
• The performance of the three dam materials tested do not show enough difference in temperature variation to have significant affects on the annealing and cooling at times and rates appropriate to the thickness of the glass.
• It is the thermal insulation properties of the dam material, rather than the density that has the greatest influence on performance as a dam material.

 

 

Tack fusing multiple layers

The question:

Full fused 6mm base, with 3mm tacked pieces. It is to be tack fused and slumped now.  Does the number of fuse firings affect the rate of advance, and how long a soak will be required to slump it? 

Multiple firings

If properly annealed each time the glass is fired, the number of firings does not affect how the glass should be fired.  This assumes the same number of layers are being fired.

Tack fusing

Tack fusing this piece will need some care.  The portion to be tack fused is 3 layers thick – overlapping white pieces surmounted by the yellow balls.  The base layer is shaded from the heat by the white, which is generally slower to transmit the heat than many other colours. Bullseye suggests doubling the total height and firing for that thickness. Bob Leatherbarrow suggests 1.5 times the total height for creating the schedule.  Firing Schedules for Kilnformed Glass,  p. 124-6

In this case, because of the amount of white, I would go with the Bullseye suggestion.  My researches for "Low Temperature Kilnforming" also indicated that a tack fuse requires a schedule for two times the thickness. Other levels of tack fusing require different calculations.  The total height of 15mm will be treated as 30mm for scheduling purposes.  This is midway between the thicknesses in the published table.  The rates and times in the table are linear. You can calculate a mid-point in the schedule to get the numbers for your piece.  Half the difference between 25 and 38 is 6.5mm giving 31.5mm.  Using the half-way point will be slightly more conservative than using exact calculations.  It is so close as to make no significant difference.

You will notice that the table gives only annealing times and rates.  There is way you can use this table for the getting initial heating rates.  Look at the final cooling rate for the thickness. If the glass can survive the cooling rate given without showing stress, it will also survive that rate of increase.  The mid-point between 90 and 45 is 67.5°C.  This gives an initial rate of advance (68°C) which can be applied for this piece that has so much shading of the base layer. It should allow the heat to transfer through the white to the base layer without great temperature differences between the covered and the uncovered base layer.

As there is a lot of work in this piece, and it is for someone else, you can be cautious.  Introduce a soak at 260°C of about 30 minutes.  This will help to ensure the heat is distributed to the bottom layer.  If you want to be even more cautious, you can introduce a second 30-minute soak at 371°C before continuing to 540°C.

At 540°C you have passed out of the brittle zone of glass and can increase the rate of advance to 167°C per hour.  The amount of heat work you have put into this piece by the slow rate of advance may enable you to complete the tack fusing with a soak at 720°C.  You will need to observe when the appropriate amount of rounding has been achieved. You will then be able to advance to the annealing portion of the firing. 

For this piece, the annealing soak will be for 5 hours with a cool of 11°C per hour for the first 55°C. Then 20°C per hour for the next 55°C and a final cool of  65°C per hour. This anneal and cool will be about 21 hours, in addition to the ca. 21 hours, in addition to about 10 hours heat up, so don’t expect a quick firing.  Plan two days for the tack fuse.

Slumping

Slumping will need care too.  The piece has uneven layers and the same care is required as for tack fusing.  Experimentation has shown me that scheduling for an additional 3mm (1/8") is needed to ensure the piece is thoroughly heated throughout its thickness.  In addition, the white is stiffer than the other colours and will not bend so easily.  This kind of slow schedule means the glass will be at the same temperature throughout as the slumping starts.

Because of the slow rates of advance, you may be able to slump this piece at 620°C with a significant soak time.  You will need to observe when the piece is fully slumped.  Be prepared to advance to the annealing and cool segments of the schedule.  Some times you need to extend the hold time.  Be prepared for this too. The annealing time and cooling rates will be the same as for the tack fusing.

Further information is available in the ebook Low Temperature Kiln Forming.

Firing multiple layers

Glass Stela
Credit: Stephen Richard

Fusing multiple layers is prone to the creation of multiple large bubbles.  It also needs a strategy to schedule for thick layers.

Avoid bubbles

A widely recommended strategy for stacks of glass is to fire in pairs of layers. Then combine the fused two-layer pieces in a final firing. 

It is easier to fire two layers of glass than 6, 8 or 10 layers. The heat up is easier and less time consuming for multiples of 6mm than multiples of 3mm. The bubble squeeze schedule is simpler.  It also allows inclusions between the initial two-layer sheets and then between the layers of 6mm sheets.

This multiple firing strategy reduces the risk of large bubbles in a stack of multiple pieces. It seems the weight of the 6mm layers forces the air out from between the thicker glass more effectively than thinner layers. 

It is also a simpler set of firings.  If you were to want to make up a 12mm thick piece from 3mm sheets, your heat up will be very long compared to firing two layers in three firings.

E.g. Stone* recommends a heat up for 2 layers of 3mm glass:

240C/hr to 250C, no soak

400C/hr to 500C, no soak (a bubble squeeze could be inserted here by raising the target temperature to 650, with a 30-minute soak)

500/hr to top temperature.

This is about 2.3 hours to top temperature without the bubble squeeze and 6.7 hours to cool.  This means that you could fire twice in one day, if organised well.  If you are planning a final tack fused layer that should be done in the last firing of the combined layers.

However, it is a much longer schedule recommended by Stone for 6 layers of 3mm glass:

• 25C/hr to 125 for 20’
• 30C/hr to 250 for 20’
• 40C/hr to 375 for 20’
• 50C/hr to 520 for 15 (a bubble squeeze could be inserted here by raising the target temperature to 650, with a 30-minute soak before continuing at the same rate to the top temperature).
• 150/hr to target temperature

This is about 18 hours to top temperature without the bubble squeeze and another 18 hours to cool.  This strategy requires 1.5 days, assuming all the layers are even.  The same amount of time is required for both strategies, but the chance of large bubbles is dramatically reduced.

He recommends for 3 layers of 6mm glass:

• 200C/hr to 250, no soak
• 340C/hr to 500, no soak
• 400C/hr to 600, no soak (a bubble squeeze could be introduced here by changing the target temperature to 650 with a 30-minute soak)
• 500C/hr to top temperature.

This is about 2.5 hours to top temperature and 18 hours to cool without the bubble squeeze.

This means that it only takes 2/3 of the time to fire 3 layers of 6mm glass than it does to fire 6 layers of 3mm glass.  Yes, you lose some time in firing the pairs of 3mm glass, but you gain in reducing the risk of creating large bubbles that will ruin your final piece.

Inclusions

If you are putting elements between the initial two-layer pieces for fusing, you need to introduce a bubble squeeze.  Putting elements between the fused pairs will also require a bubble squeeze on the final firing.

Tack fusing the final layer

Note the times indicated above are for even layers.  If you have uneven layers or are tack fusing, the times will be extended much further than the ones noted there.

For a tack fused set of top layers, you will need to add those in the last firing, or do a sharp tack firing before the last firing.  In the case of a tack fused pair for the top layers you will need to reduce the rates of advance for the last firing by about 1/3. This would mean:

• an initial rate of 135C,
• a second ramp of 230C,
• a third of 270C and
• the fourth of 335C instead of the rates for even layers. 

You will also need to reduce the top temperature.  Observation will be required to determine when the correct profile has been achieved.

Further information is available in the ebook Low Temperature Kiln Forming.

When firing multiple layers of glass, the risk of creating large bubbles can be reduced by firing pairs of 3mm sheets, and then combining the results into one stack.

*Graham Stone. Firing Schedules for Glass, the Kiln Companion, 2000, Melbourne Australia.  ISBN 0-646-39733-8

Expansion at Edges of Tack Fused Stacks

How much will my glass expand if I put glass pieces on top of 6mm base?  

I ran some tests for both 6mm and 3mm bases. These showed that the distance from the edge is important.  The amount of glass in the stack has a big influence on expansion.  So does the tack profile and the thickness of the base.

The most expansion for any thickness and at any tack profile is when the stack is placed at the edge.  The further away from the edge, the less the expansion. There is no noticeable expansion of size when the tack stacks are placed 20mm from the edge.  In most cases there is only a little expansion at 10mm from the edge.  Although not tested, it seems that 15mm is a safe distance from the edge to avoid changing the edge.

The amount of glass in the stack being tacked to the base has an effect on the amount of expansion.  This is to be expected based on the concepts behind volume control.  Two tack layers can vary from two to three times that for a single tack layer depending on the profile of the tack.

The tack profile has an effect on the amount of expansion.  At contour there is a greater expansion than at rounded or sharp tack fuse.  This is to be expected, as there is less heat work at sharper tack profiles than at contour.

The thickness of the base has an influence on the amount of expansion too.  Thicker stacks promote greater deformation of the edge at all tack levels.  Thicker stacks need to be placed further from the edge to avoid changing the perimeter.  Thicker stacks create greater change in the edge on single layers than double layers.

The setup and results are given here.

Setup for 2 layer base and 1 and 2 layer stacks at various distances from the edge.

Contour fuse test, 6mm base
1 layer placed at edge, at 10mm from edge, at 20mm from edge, and at 30mm from edge.  2 layer stacks placed in the same way.  
 

Fired results, outlined for clarity

1 layer placed at edge – expansion of 2.5mm
1 layer placed 10mm from edge – expansion of 0mm
1 layer placed 20mm from edge – expansion of 0mm
1 layer placed 30mm from edge – expansion of 0mm

2 layers place at edge – expansion of 9mm
2 layers placed 10mm from edge – expansion of 2mm
2 layers placed 20mm from edge – expansion of 0mm
2 layers placed 30mm from edge – expansion of 0mm
 

Rounded tack test, 6mm base
1 layer placed at edge, at 10mm from edge, and at 20mm from edge.
2 layer stacks placed in the same way.
 
1 layer placed at edge – expansion of 3mm
1 layer 10mm from edge – expansion of 0mm
1 layer 20mm from edge – expansion of 0mm

2 layers place at edge – expansion of 7mm
2 layers placed 10mm from edge – expansion of 1mm
2 layers placed 20mm from edge – expansion of 0mm
 

Fired result of 6mm base with 1 and 2 tack layers, rounded tack.

 
Rounded tack test, 3mm base
1 layer placed at edge, 1 at 10mm from edge, 1 at 20mm from edge, 1 at 30mm from edge.  2 layer stacks placed as above.  
 
1 layer placed at edge – expansion of 2.5mm
1 layer 10mm from edge – expansion of 1mm
1 layer 20mm from edge – expansion of 0mm
1 layer 30mm from edge – expansion of 0mm
 
2 layers placed at edge – expansion of 3mm
2 layers 10mm from edge – expansion of 1mm
2 layers 20mm from edge – expansion of 0mm
2 layers 30mm from edge – expansion of 0mm
 

Fired result of 3mm base with 1 and 2 tack layers.

Note: the single 200mm sheet contracted to 195mm in uncovered areas.  Measurements were based on the amount of expansion from the fired dimensions. Even with the greatest expansion the piece was still 2.5mm smaller after firing than at the start.
 

Sharp tack test, 6mm base
1 layer placed at edge, 1 at 10mm from edge, 1 at 20mm from edge, 1 at 30mm from edge.  2 layer stacks placed as above.  

 
1 layer placed at edge – expansion of 1mm
1 layer 10mm from edge – expansion of 0mm
1 layer 20mm from edge – expansion of 0mm
1 layer 30mm from edge – expansion of 0mm
 
2 layers placed at edge – expansion of 2mm
2 layers 10mm from edge – expansion of 0mm
2 layers 20mm from edge – expansion of 0mm
2 layers 30mm from edge – expansion of 0mm
 

More detailed information is available in the e-book: Low Temperature Kilnforming.

When to Open a Cooling Kiln

Credit: Glass House Store

Questions about when it is possible to open the kiln during the cool down to avoid thermal shock get the answer, “it depends….”

These dependent variables include:

Temperature Differentials

Thermal shock is related to how quickly a piece can cool without developing stress that cannot be contained within the piece.  So, when the temperature differential is a few tens of degrees between room and kiln air temperature it is less risky than when the difference is hundreds of degrees.

This means that there is a relation between room temperature and when you can open the kiln safely.  If the room is at sub-zero temperatures, you will need to wait for a lower temperature in the kiln, so the temperature differentials are no greater than when the room is warm.  Remember the glass can be much hotter than the air that the thermocouple measures.

Cooling rate of the kiln

The natural cooling rate of the kiln (that is, in the unpowered state) will affect when you open.  If your kiln cools very slowly from 150°C, you may feel confident to open the kiln a little to speed the cooling from that temperature.  If you kiln cools quickly - usually in smaller kilns - then you need to wait longer for a lower temperature to be achieved.

Size of the piece

The size of the piece(s) relative to the kiln size has a bearing on when it is safe to open the kiln to speed cooling.  The more space the piece takes up in the kiln the cooler the temperature reading needs to be before you open the kiln.

Placing

The placing of the glass has an affect too.  If the glass is at the front of a front opening or top hat kiln, it will cool more quickly and unevenly than one at the back. A large piece placed more to one edge than another will also require lower temperatures before opening.

Thickness

The thickness of the glass also needs consideration.  The thicker the glass, the hotter it will be in relation to the measured air temperature, and so the longer it needs to be left to cool before opening.

Type of kiln

Your kiln may cool slowly or quickly, but the style of the kiln is important too.  The kiln may be brick lined or fibre lined, or a combination.  The greater the mass of the insulation, the earlier you can open, as the dense brick will radiate heat back toward the glass.

If you have a top hat kiln it is probable that you can open earlier than if you have a top opening or front door opening kiln, as they will dump hot air slower than top and front opening kilns.

The venting method

The way you open the kiln to increase the cooling rate is important.  If you open vents, that provides a gentler flow of cooler air than opening the lid or door.  If you open lids or doors, you need to wait for a lower temperature than for opening vents.

And I am sure there are other considerations.  But these are enough to show that there is not a single answer.  The answer is in relation to the kiln and its contents.

Acceptable Cooling Rates

The speed of cooling that a glass can sustain is indicated by charts giving the rate of cooling for the final rate of decrease to room temperature.  Faster rates might be induced by turning the kiln off at 370°C and opening the door/lid at some slightly lower temperature.

This means that you need to know how fast a cooling rate is acceptable.  The bullseye research suggests that 300°C per hour for the final cooling is as fast as you would want to cool a 12mm thick piece.  This is in a closed environment.  Therefore, you will want to be slower – at least half the speed for a partially opened kiln of say 5cm. 

My predictions for acceptable cooling rates are (with a room temperature of 20°C; a piece evenly thick and 30cm square, but less than half the area of the kiln floor; and a top hat kiln):

6mm -   300°C per hour (although I never use more than 200°C per hour)

12mm - 150°C per hour

19mm - 75°C per hour

25mm – 45°C per hour

Note: Tack fused items with these total heights need to have these rates halved, or use the rate suitable for a piece twice the thickest part.

But!

You cannot open the kiln until the natural cooling rate is at the predicted acceptable rate of cooling or less, to be safe.

The natural cooling rate at various temperatures can be determined by observing temperature falls in relation to time intervals between those observations.  You can make a chart to indicate the cooling rate at different temperatures.  The kiln will naturally cool more slowly at lower temperatures. 

Schedule to room temperature

A protection against too rapid cooling is programming to room temperature.  If your kiln is cooling less rapidly than you predict is acceptable, you are using no electricity – OK, maybe a tiny fraction of a kilowatt to keep the controller operating. But there is no worry of using excess electricity.

The point of programming to room temperature is that if the air temperature in the kiln cools faster than predicted, the controller will turn the kiln on.  You will need to be present for a while after venting the kiln to hear if it turns on and you can lower the lid to a point where the kiln does not turn on, indicating the rate of cooling is less than put into the schedule.

An example:

Assume you predict that 150°C per hour is the appropriate rate of cooling from 370°C. Also assume you open the kiln at 100°C and a minute or so later you hear the kiln start.  Then you know that you have opened the kiln too far causing a more rapid cooling than 150°C per hour and you need to close the opening to less than the current state.  This probably will be a progressive thing.  You will come back, say, half an hour later and open a little more.  Everything seems fine, but 10 minutes later you hear the kiln switch on again.  Oops! You opened too much – you need to close the kiln a little.  This may repeat several times.

The real answer to when you can open your cooling kiln is dependent on many variables.  You will have to decide on how critical these are in relation to the piece(s) you have in the kiln.  Once you have decided on the appropriate rate, you should program that into your schedule for the final segment.  This means when you partially or fully open the kiln the controller will switch the kiln on when the cooling rate is faster than you wanted.