Causes of Large bubbles

 Let’s think about moisture and large bubbles from under the glass. It is not the water, but the gasses created by the decomposition of materials that can cause the bubbles. There are other causes of large bubbles too. The most common causes are discussed here.

The usual explanations are:

• ·        Uneven shelf
• ·        Heat resistant particles under the glass
• ·        Uneven heating
• ·        Glues
• ·        Organic material
• ·        Moisture
• ·        Amount of gas

 

image credit: Warm Glass

Uneven shelf

Shallow depressions in shelves can cause large bubbles. Occasionally, the shelf can be damaged in various ways causing scratches or dings in the shelf. Air can be trapped in these depressions. And it does not take much volume of trapped to be a problem. The heat of kilnforming causes the air to expand. As the glass becomes less viscous with increased temperature, the pressure from the expanding air forces the glass upwards. The amount of air and the amount of heat work combine to create bubbles from simple uprisings to large thin walled or even burst bubbles.

There are some things that can be done to detect and avoid bubbles from forming. It is possible to screed powdered kiln wash over kiln washed shelf. This gives pathways for the air to escape. It does leave a more marked bottom surface than kiln wash.

Using 1mm or 2mm fibre paper allows air from under glass. You can maintain a relatively smooth surface with Papyros or Thinfire over the fibre. Even Thinfire or Papyros on its own will allow air from under the glass.

Checking for depressions can be done by spreading kiln wash powder over shelf and drawing a straight edge over the shelf. Depressions will be shown by the presence of the powder. It can also be done with powdered glass frit.

Particles under glass

Any particle resistant to kilnforming temperatures holds the glass up while it is forming so creating an air space. It is important to ensure the shelf is clean as well as flat. Small pieces of grit or dirt that are resistant to high temperatures will hold the glass up from the shelf enough to create a bubble – small or large depending on the temperature. Vacuuming the shelf before adding anything to the surface before each firing is important to bubble free results.

Uneven heating

This is sometimes cited as a cause of bubbles. If so, the heat would need to be very localised. This is possible if the glass is very near elements. In general, the temperature is equalised at a distance equal to the width of the elements.

Glues

A wide variety of glues are used in kilnforming. Those available to enthusiasts all burn away leaving gasses between layers. These gasses - if trapped - can thin the glass below as well as above the glue’s position. This will give the impression that the bubble has come from between the shelf and the glass. Most often the bubble forms between the glass layers, pushing a bubble only into or through the top layer. The solution is to avoid using glue or minimise it and place it only at the edges.

Organic material

Organic materials can be a problem. When you are using a large or thick fibre paper sheet under a piece of glass, occasionally the gasses from burning out of the binder can be great enough to create a bubble. Although normally, this only leaves a grey to black mark on the underside of the glass. Vermiculite boards need to be fired before use, as they contain significant amounts of binder.

Inclusion of organic materials such as leaves, twigs, or bones, leads to bubbles. Very long soaks below the softening point of the glass are required to allow the organic material to burn out of the objects.  The time required increases from an hour for leaves to 24 for bones.

Moisture

Moisture is very often cited as the source of bubbles. It is possible that the steam from water may be trapped in shelf depressions, or the areas held up from the shelf. And anytime there are no precautions to allow the air from under the glass, or between sheets bubble formation can be promoted. If adequate precautions are taken (flat shelf, clean shelf, bubble squeeze) the moisture will evaporate before the glass is hot enough to form a seal around the edges and trap any steam. It is another good reason for moderate ramp rates at the beginning of a firing.

Amount of gasses

Of course, if there is a lot of moisture there can be problems. Simply applying kiln wash in four coats does not leave enough water in the shelf to be a problem.

If you have washed the kiln wash off a mullite shelf, there will be a lot of water in it even after it feels dry. Then it does need to be kiln dried before use. To avoid breaking the shelf you need to fire slowly to 99°C/210°F and soak there for a couple of hours with the vents open or lid propped up a little to allow the moisture out of the kiln.

 

 

Lustres

Lustres are metallic colourants in colloidal suspension. They provide intense reflective colour. They are most effective when used sparingly as accents. They are supplied as a dark brown viscous liquid in small bottles. They are widely available from ceramics suppliers.

A bar with gold lustre. Credit: Bath Potters

 The application of these is important, and not only because they are expensive.

 They must be applied to clean dry surfaces with a smooth brush. The gold, platinum, copper, and bronze lustres do not need dilution. The brush should not have a lot of lustre, nor too little. Too much causes burning, flaking, dullness or clouding during and after the firing. The application must be uniform in thickness. “Application of lustres is possibly the most important factor in achieving the best results. The more richly coloured lustres require a fairly thin coat while other lustres (particularly the metallic lustres) require an even thinner coat.” Bath Potters.

silver lustre brushed on.  Credit: Pottery Crafts

 The kiln should be vented until the carrier has burned off. The absence of the smell will indicate when this has been achieved. The firing of lustres can be between 586°C/1088°F and 733°C/1353°F. Metallic lustres usually fire between 586°C/1088°F and 617°C/144°F. These lower ones are in the slumping range of temperatures and can be applied to the flat blank before slumping or to the completed flat blank depending on the requirements of the lustre.

Slumping Blank Size

When you're making a piece that you intend to slump does it need to be bigger than what you're making, and by how much?

Generally,

The general advice is to make the blank the same size/diameter as the rim of the mould.  This works best for shallow moulds with a generous draft, or shallow slope to the bottom.  

There are numerous exceptions of course.

Deep moulds

Deep is a relative term.  A small round mould of 100mm/4” with a 30mm/1.25” drop can be considered deep. But this drop would be considered shallow for a 300mm/12” diameter mould.  A drop of 100mm/4” into a 300mm/12” mould would be deep.

There are two approaches to this.  We know the blank will end up with a considerably smaller diameter than the deep mould. This is because the surface of the glass does not change its dimension much.  As a result, the diameter of the slumped piece is less than the flat blank’s diameter.  Placing a flexible measuring tape on the outside of the slumped piece will show the lengths of the flat blank and curved piece are similar.

Larger Blank

As a result, we are tempted to make the blank larger than the mould.  By how much? as the questioner asks.  The safest is avoid exceeding 6-8mm/.025 – 0.375”.  With a slow slumping rate, the edges will rise as the interior bends downwards and allow the excess glass to take up the same diameter of the mould before deforming enough to catch on the edge of the mould.  With a centimetre or 0.375 inch overhang, you begin to take greater risks with the rim beginning to slump outside the mould and hanging up. 

Smaller Blank

But size matters.  The small excess works best on larger moulds (250mm/8”) or more.  Employing this oversizing on smaller moulds has problems.  The span of the smaller moulds requires higher temperatures and/or longer soaks.  The result of this greater heat work is that the rim of the glass can begin to slump outside of the mould. In extreme cases, this will cause the glass to break.  More often, the edge does come into the mould, but with heavy stretch marking and sometimes needle points where the edge drags along the mould.

It is often best to make the blank smaller than the mould.  Small enough that it fits just below the rim of the mould.  This allows forming of the glass without the frequent drag marks and needle pointing.  Placing the glass level is most important when the glass is not supported by the rim.  If the final size of the slumped piece is important, you will need to slump in a larger mould. I do not know of a method of calculating that.  It is a matter of experimentation.

Steep Moulds

Ceramic shapes from charity shop finds that are adapted to be moulds are often steep sided as well as deep. They often have no rim on which to rest the glass before the slump shape takes over.  Both these elements result in the glass being much smaller than the mould when complete.

Collar

You can counteract the effect of deep, steep moulds by placing a collar of fibre board around the mould.  

Make a cut out from the fibre board by placing the mould upside down and tracing the outline. Cut the board slightly inside the line scribed.  Then fashion a bevel to meet the angle of the outside of the mould.  Support the board at the appropriate height for the mould.  Fill any gap between board and mould with kiln wash powder or a paste of the powder, depending on the nature of the gap.  This supports the glass during the slumping while allowing it to slump into the mould.  It increases the possibility of getting a steep side on the glass.  It also allows you to put a rim on the shape if you want.

Staged Slumping

Sometimes the collar or rim is not sufficient to allow the glass to move as desired in a single slumping stage.  Then multiple slumping stages are required.  The common approach has been to purchase a three-stage slumping set.  This can limit your approach.

·        The general approach is to measure the inside surface of the final steep mould. 

·        This gives you the starting diameter for the blank. 

·        Then measure the diameter of the mould at the outside of the rim. 

·        This gives you the diameter of a slumped piece to fit into the final mould.   

Compare the largest diameter blank to existing moulds you have. Will the glass have slumped enough to reduce the diameter to fit into the final mould?  In some cases, where the answer to that question is yes, only two-stage slumps are required. 

Most times a three-stage slump is needed. You need to find an intermediate mould that will deliver a slump with the required diameter.  This will be a moderately deep mould, usually with steeper sides than the first, but less steep than the final mould.

Angular Moulds

Angular moulds are those with sudden short drops to the foot (flat part of the mould). These are often ogee curves. 

These require more time or heat to form completely to the bottom of the mould.  The glass is curving in two directions during the slump.  The glass should be only slightly larger than the mould at most.  To counteract the tendency to slide down the mould, low temperatures and long soaks are needed.

 

Rectangular moulds

The main problem of rectangular moulds is the dog boning of the straight edges of the glass during the slumping. There are some suggestions.

Cut the blank with slightly outward curves.  This will help to compensate for the tendency to dog bone.  It will require some experimentation.  Slumping a standard square or rectangular mould will give an idea of how much the glass deforms during the slumping.  That amount of curve can be added to the edge when cutting out the blank.

Round the corners of the blank.  This relies on the principle that there is more glass at the corners to slump than at the sides.  A 10mm/0.375” radius should be enough.  

There is less glass to compress along the sides than at the corners.

Another element is to provide the rectangular shapes with rims.  This forces any dog boning to the rim rather than the sides of the slump.  It can be combined with either of the previous possibilities.

These do not always work and are sometimes difficult to reconcile with the design. The additional possibility to counteract the dog boning of the shape is to use slower rates, lower temperatures, and longer soaks. This is not always successful.  Rectangular shapes remain the most difficult to get the glass to conform faithfully to the mould.

 

There are ways to get the slumping blanks to conform to the moulds, and they all involve modifications to the glass, mould, or schedules.

Longer Anneal on Each Firing

 Need for an Extension of Anneal Soak on Subsequent Firings

 Sometimes people recommend extending the length of the anneal soak each time the piece is fired. If nothing significant is added, there is no reason to extend the anneal soak.  If the piece can be fired as fast as the previous firing, the annealing will be the same, not longer.

 The physics and chemistry of annealing glass are the same for re-fired glass - without additions - as they were for the first. Extending the annealing soak seems to be more about reassurance of the kilnformer than a necessity.

 Bullseye research has shown that it IS possible to over anneal, locking in stress. If concerned about inadequate annealing, it is best to reduce the cooling rate. Especially over the first 55°C below the anneal soak temperature.  The testing and recording that I have done for a book on tack fusing shows that any differences in the glass - at the end of the anneal soak - will be relieved in that first 55°C/100°F. The remainder of the cool to 370°C/700°F can be about 1.8 times faster, and the final cool ramp can be 3 times faster than the 2nd stage cool. 

 I have observed that a three stage anneal cool is important to successfully anneal a piece. This has been reinforced by the temperature recordings of many firings. Often at the end of the anneal soak there is a little more than the desired 5°C/10°F difference in temperature across the piece. The recordings show this is relieved during the slow firs stage cool and maintained over the next two cool stages. If the kiln is cooling more slowly than the schedule, no electricity is used.  No kiln time is lost.

An example of the first cooling stage

 The first stage cool is key to a successful stress-free result.  If there are concerns about inadequate annealing, two things are important.  Be sure the right length of soak is chosen for the piece being fired.  Second, reduce the speed of cooling by the rates for a piece at least 3mm thicker.  These rates are available from the Bullseye chart for annealing thick slabs.

 The rates are applicable to other than Bullseye glass.  Only the temperatures need to be changed.

 If no significant changes (other than powder, wafer or stringers) are made to the glass before the second firing, no lengthening of the annealing is necessary.

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

Re-firing

A frequently asked question is “how many times can I re-fire my piece?”

This is difficult to answer as it relates to the kind of glass and the firing conditions.

Kind of glass

Float glass is prone to devitrification. This often begins to appear on the second firing. Some times it may be possible to get a second firing without it showing. Sandblasting the surface after getting devitrification will enable another firing at least.

Art glass is so variable that each piece needs to be tested.

Fusing glasses are formulated for at least two firings, and experience shows may be fired many of times. The number will depend on the colours and whether they are opalescent. Transparent colours on the cool side of the spectrum seem to accept more firings than the hot colours. Both of these accept more firings than opalescent glasses do.

Firing conditions

Temperature

The higher the temperature pieces are fired at, the fewer re-firings are possible. So if multiple firings are planned, you should do each firing at the lowest possible temperature to get your result. This may mean that you have relatively long soaks for each firing. The final firing can be the one where the temperature is taken to the highest point.

Annealing

You do have to be careful about the annealing of pieces which have been fired multiple times. A number of people recommend longer annealing soaks. However, I find that the standard anneal soak for the thickness is enough. What is required is cooling rates directly related to the anneal soak.  This is a three-stage cooling as described in the Bullseye chart Annealing Thick Slabs.  The slump firing can be annealed at  the standard. 

Slumping

In general, slumping is at a low enough temperature to avoid any creation of additional stress through glass changes at its plastic temperatures.  But any time you heat the glass to a temperature above the annealing point, you must anneal again at least as slowly as in the previous firing. Any thing faster puts the piece at risk of inadequate annealing.  Of course, having put all this work and kiln time into the piece, the safest is to use the cooling rate as for a piece one layer thicker.  My research has shown that this gives the least evidence of stress.

Testing

Testing for stress after each firing will be necessary to determine if there is an increase in the stress within the piece. In the early stages of multiple firings, you can slow the annealing and if that shows reduced stress, it will determine your previous annealing schedule was inadequate. When reducing the rate of annealing does not reduce the stress, it is time to stop firing this piece at fusing temperatures.

Revised 6 May 2023

Texture Moulds

 Texture moulds are a form of bas relief in reverse. The texture of the mould is the bas relief. The glass formed over the shapes is in negative relief. The light is refracted through th
e back to give an image of bas relief although the surface is smooth.

 

Example of wave form texture mould

 These moulds are prone to produce bubbles at the generally recommended tack fuse temperatures. The glass often sticks to the mould if there is not sufficient draft to the parts of the image, or if insufficient separator is used. Often the moulds are produced with a rim around the edges, which trap the glass.

 The usual temperatures are too high. These moulds are an exercise in patience. The temptation is to fire higher than slumping temperatures to get good definition in the glass. However, a number of problems, especially bubbles, can be avoided by staying at the high end of slumping temperatures. This means the top temperature would be about 680C. To compensate for this low temperature, the soak needs to be three hours or more. To be sure the definition desired has been achieved, peeking near the end of this long soak is necessary. 

 Moulds that are produced with a rim around the edges can trap air and create bubbles. The rim forms a perimeter dam to confine the glass. If the rate of rise is quick to a high temperature, the edges can be sealed against the rim before all the air has escaped. It is advisable to cut the glass for these rimmed moulds a bit smaller than the internal dimensions formed by the rim.

 

Example of textured area surrounded by a rim

 Use of single layers on texture moulds can lead to large, thin bubbles. This is most prevalent when using high temperatures. Since the single layers tend to form more slowly than an already fused two-layer piece, the temptation is to use higher temperatures. The higher temperatures soften the glass to such an extent that often bubbles form over the lower areas of the mould. Instead, low temperatures with extremely long soaks should be used to allow the glass to conform to the undulations of the texture without dog boning or developing bubbles. Of course, peeking will be required to determine when the texture is achieved. With single layers, the surface will have greater undulations than with two layers. The thinness of the single layer cannot fill the depression the way two layers can.

 

 Rapid rates to high temperatures can produce internal bubbles too. These moulds have a multiplicity of hollows and depressions. Just as people are warned about depressions in their shelves, the depressions in the texture moulds can cause bubbles too. This means there are even more possibilities for bubble creation than on apparently flat shelves. Long slow bubble squeezes are required to allow air from under the glass.

 Glass sometimes sticks to the mould. This is most often blamed on insufficient separator. Boron nitride is a good separator for these moulds especially if you go to tack fusing temperatures. At slumping temperatures, kiln wash will normally be sufficient. Both of these separators need to be applied carefully, as there are relatively steep slopes throughout the mould. Spraying needs to be done from at least four angles to ensure all the sides are covered.

 Painting on kiln wash is a little more difficult, as the solution is so liquid, it tends to run down the slopes without much sticking. One means of rectifying this is to tip the mould in a circular motion to move the still liquid kiln wash solution around the slopes.

 Less often thought about is the draft of the shapes of the mould. If the slopes (draft) in the mould are too steep, the glass will “grab” the ceramic mould, because the ceramic contracts less than the glass when cooling. If shapes of the mould are steep and deep enough, the glass may even break as a result of this compression of the mould.

 

An example of some nearly vertical elements and a rim

 Of course, if a flat front surface is required, a higher than slumping temperature must be used. This is required to allow the glass to flow to the lower portions of the mould. It still should be as low as possible, but with very long soaks.

 Avoidance of bubbles on, and sticking to, texture moulds is best achieved by avoiding high temperatures, use long soaks, use two layers, and avoid extending glass to the rim. These combined with observation of the progress of the firing will produce the best results.

 

Other information is available:

Low Temperature Kilnforming, an Evidence-Based Approach to Scheduling, an ebook

Bas relief

Layups promoting bubbles

 

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.