Avoiding Slumping Breaks

Most slumping breaks are due to scheduling.  The piece to be slumped has survived the fuse, and with good practice will have been tested for stress. It has passed all the compatibility and annealing complications, so it is sound. 

There are things you should think about when determining the schedule for slumping. General considerations are thickness, and degree of fuse. There are many other factors to be considered – such as depth, mould detail, span, colour contrasts, etc. These will affect the scheduling in detail rather than the general approach.

Ramp Rates

In general, the scheduling for the first ramp rate is done by taking note of profile (degree of fuse), and so, its effective thickness.

Each profile of fused glass has its own considerations.  Full fused pieces can be fired at the rate recommended by the many schedules for slumping fused items. Tack fused and other glass configurations need further precautions.

The ramp rate for slumping should be no faster than a rate to ensure the glass is evenly heated throughout the rise to the slumping temperature. I recommend that this rate of advance should be a steady single rate all the way to the slumping temperature.  There is no need for soaking at any point during this temperature rise. 

But as much of the breaking of glass occurs below 300°C (573°F), a precaution can be added. An additional slower first ramp can be inserted with a 20-minute soak at 260°C/500°F before proceeding. This also helps protect ceramic moulds which have a cristobalite inversion at that temperature. 

The rates for moulds that are large relative to kiln size, that are heavy, or may be damp, should be considerably slower than for other glass. 

Force of Breaks

If the glass has broken during the forming process, take note of the distance between the pieces.  The amount of space between the broken pieces shows the relative force that caused the break.  Greater space is related to more stress; lesser space or only partial cracks indicate lower levels of stress.  The separation distance indicates the degree of change required in scheduling. A small parting of the glass requires only a little reduction in the rate.  Large spaces indicate that much slower rates are required, and possibly a complete rethink of the schedule.

This approach can be used for breaks on the heat up or the cool down.  Whether the glass is rounded or sharp, the force of the break will still be an indicator of the degree of change required.  On a rounded edge break, it is the heating rate that needs to be slowed.  Sharp-edged breaks indicate that the anneal soak needs to be lengthened and the anneal cool slowed.  The rounded versus sharp edges are more difficult to establish at these low temperatures and need to be combined with how well the formed pieces match.  Of course, there will be some experimentation required to determine the exact amount of change needed. 

“It hasn’t happened before” Scenario.

Often people experience breaks even though the set up was similar and the schedule was the same for successful pieces in the past.  There are two responses to this – “what did you change for the setup and firing of this piece from others?”, and “You have been skating on the edge of disaster for a while”.  Glass behaviour is predictable. Since the break occurred when the setup was similar, and the schedule was the same, something else has changed.

Consider what was different.  Review the differences in set up of the piece – colours, arrangement, thickness, volume of material used – everything that might be different at each stage of the layup.  Note these differences and review them one by one. 

• ·   Could have any one element been sufficient to make the firing conditions different? 
• ·   Could a combination of these differences have been significant? 
• ·   Are there any differences in the firing schedule? 
• ·   Have you made any little tweaks in the schedule? 
• ·   What is different? Different times of the day, different power supply, plugs in or out, venting, peeking, different shelves (or none) – any small thing that could have introduced a variable in the firing conditions. 

For each of these differences consider what needs to be altered, if anything, for a successful firing.  Combine these small tweaks into a full schedule and run it as an experiment.

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

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.

 

 

Firing Small Pieces

 Do you run small pieces of glass through the whole cycle or just bring it up to your degree posted and cool down?

 

Picture credit: Eva Glass Design

It would appear easy to ignore the need to anneal small pieces.  They can anneal with short heat soaks.  In industry the anneal of sheet glass is 15 minutes for 4mm/0.019” glass.   In kilnforming the 30ºC - 40ºC/54ºF – 72ºF below the annealing point is where annealing is effective.  If you are certain that the natural cooling rate of your kiln is more than 15 minutes for that temperature range, you can simply turn off after top temperature.

However, it is not a good practice unless you intend to confine you kilnforming to small pieces.  All glass needs to be annealed to be sound.  Small pieces may need only 15 minutes and often that can be achieved with the natural cooling rate of your kiln.  But pieces of 6mm/0.25” thick and over 100mm/4” in any direction need to be annealed with longer soaks and slower cools.  This is done with a hold of the amount of time appropriate to your glass and layup.  There is an excellent table from Bullseye that gives the hold times and rates for cooling glass of different calculated thickness. 

Using an annealing soak and a cooling cycle for every firing is a good practice.  This gets you into a habit, so that you do not skimp on the anneal and cool for larger, thicker, or tack fused pieces.  If your kiln cools more slowly than you have scheduled, that's ok.  The kiln does not use any electricity to heat the elements.  No additional electricity cost or wear on the kiln occurs.

Fire Polishing of Frit Castings

Image credit: Obsession Glass Studio

 Fire polishing castings is relatively difficult.  Even though people may suggest temperatures for this kind of fire polish for castings from frit:

• ·        They are relevant to particular kilns. 
• ·        They are also dependent on the ramp rate. 
• ·        The presence or absence of a bubble squeeze is important. 
• ·        The size of the casting is relevant.

 The objective is to get a fire polish without distorting the shape of the piece.  The general procedure is to fire slowly to the softening point. This is to ensure the casting is of similar temperature throughout. The softening point for fusing glass is around 540°C/1000°F. You should soak at that point for a time to ensure the glass is all at that boundary between brittle and plastic.

 You may prefer to use a bubble squeeze soak to achieve the same thing.  This has a slightly higher risk of distorting the piece.  If you do use the bubble squeeze, it should be done at the lower end of the bubble squeeze after a slow rise.  The casting will not be subject to much change at 600°C to 620°C/1110°F to 1150°F, if the soak is short.

 The rates to be used are dependent on the size and thickness of the piece.  Larger and thicker pieces need slower rates than thin ones.  Fire at an initial ramp rate for twice the thickness to be sure of heating thoroughly.

 When the softening point is reached, or the slump soak is complete, proceed at a rapid rate to the tack fusing temperature. To get the result you want you will need to observe.  Peek at frequent intervals. Be prepared to advance to the next segment when the gloss appears on the surface.  Your controller manual will tell you how this is done.

 

Square Drapes

Two pyramidical moulds. One stepped and the other smooth.

 This kind of draping mould with flat sides will never work very well as a draping mould.  The draping sides have to compress. This takes a long time and is likely to cause folds in the glass.

 The common experience is that two opposite sides drape first and conform to the mould. This displaces the compression necessity to the other two sides. This "taco" style initial drape is common in all drapes. It is usually observed in handkerchief drapes.  In the early stage of draping two sides of the glass fall, creating a taco shape. With continued heating, those long sides fall and spread the initial draped sides to become almost equal. 

 This taco formation also occurs on the pyramid style mould, giving two flat sides.  The glass on the other sides then fall. As the glass area is now larger on these sides than the mould area, a drape or fold is formed.  Imagine the drapes a square piece of cloth place on a pyramid would create. The cloth has more area than the sides of the pyramid.  The excess cloth creates folds at each corner.  The same happens with the glass.

 This draping fold can be minimised by using low temperatures and long (multiples of hours) soaks.  This allows all the sides of the glass to begin forming at more or less the same time.  I am not sure the folds can ever be completely eliminated.  With extremely long soaks, the drapes will flatten to the rest of the glass. 

 Annealing difficulties are caused by this folding.  It will create thick overlaps.  This in turn will cause the annealing difficulties. There are areas that are much thicker than others.  If you started with 6mm glass, the folds will create areas that are 18mm thick. 

 Making sure this glass - with such large differences - is all of the same temperature will require long annealing soaks.  It will also require very slow cooling segments.

 Square drape moulds are rarely successful. Folds are created at the corners, rather than fully conforming to the mould.

Effect of AFAP Rates

 

 

This graph illustrates the effect of a rapid increase (500C/hr) in temperature on the glass.  The blue line represents the air temperature measured in the kiln.  The orange line represents the temperature between the glass and the shelf.  At an air temperature of 815°C, the temperature of the glass at its bottom is around 750°C.  This is a large difference, even though the glass is in the plastic range.  It means that the potential for stress induced by the firing rate is large.  The graph shows the temperature difference evens out during the annealing soak.

 The fast rise in temperature at the initial part of the firing where the glass is still brittle risks breakage.  The difference in temperature between the top and bottom of a 6mm piece of glass is shown to be 100°C plus throughout this initial phase up to 500°C.  Most breaks due to thermal shock occur before 300°C. This large temperature difference that occurs with rapid rates of advance risks breakage early in the firing.

 As an example, I took a piece out at 68°C to put another in.  During the time the kiln was open, the air temperature dropped to 21°C.  I filled the kiln and closed the lid and idly watched the temperature climb before switching the kiln on for another firing.  It took a bit more than two minutes for the thermocouple to reach 54°C with the eventual stable temperature being 58°C.  I had not been aware how long it takes the thermocouple to react to the change in temperature.  Yes, it takes a little time for the air temperature in the kiln to equalise with the mass of the kiln, but not two minutes.

 With a two-minute delay the recorded temperature can be significantly behind the actual air temperature.  For example, a rate of 500°C per hour is equal to 8.3°C (15°F) per minute or 16.6°C (30°F) overshoot of the programmed temperature. Even at 300°C it is a 10°C (18°F) overshoot.  This effect, added to the way the controller samples the temperatures, means the actual overshoot can be significant for the resulting glass appearance.

 This is just another small element in why moderate ramp rates can be helpful in providing consistent results for the glass.

 More importantly at top temperature, the surface will be fully formed while the bottom is only at a tack fuse temperature. This does have implications for the strength of the piece.  There will be an only tack fused structure through much of the piece, but a full fused structure at the surface.  The potential for breaking in further kilnforming or during use is high.

 In addition to the effects on the glass, there will be effects on the operation of the controller.  Controllers operate by comparing the instructions on firing rate with the air temperature recorded by the pyrometer.  In doing this the variances become smaller with time.  An AFAP firing does not give a lot of time for the controller to “learn” the firing curve.  So, the controller tends to overshoot the top temperature by some (variable) degree.  This makes it difficult to precisely control the outcome of the firing.

 There is some concern that the structure of the kiln will be affected by AFAP firings. This is a small risk.  The expansion and contraction of the kiln materials will occur whether quickly or more slowly.  It is not a major concern.  It is a concern for the glass, though.

 AFAP firings have potentially harmful effects on the structure of the fired glass leading to thermal shock and fragile completed pieces.

Evaluating Top Temperature Effects

Fire for Your Kiln and Objectives

Credit: FusedGlass.org

 Often temperatures to achieve given effects are shared on the internet to be helpful to others.  Those who receive these need to evaluate the schedules used to achieve the profile at the stated temperature.

 The same effect can be achieved at different temperatures by using different rates and times in getting to the given temperature.  This is summarised as the effect of heatwork.  The longer taken to achieve the top temperature, the lower the temperature can be used.  The amount of time the holds/soaks occupy in the schedule will also affect the profile achieved.  This means that a simple top temperature can only be a point from which to begin exploration.

 It is important to evaluate each segment. What is the apparent purpose? How does it fit with the principles of applying heat and the characteristics of glass?

 When asking for help with a firing temperature, ask for the full schedule.  This will help you evaluate the suitability of the temperature given.

 The variations in schedules and kilns mean you should fire by results not by numbers. Use the rates, temperatures and soaks that will achieve what you want in your kiln.

Achieving Multiple Profiles in One Piece

People ask about whether it is possible to tack fuse additional elements without affecting the profile of the existing piece.

It is as though glass has a memory of the heat it has been subjected to.  For example, a sharp tack will become a slightly rounded tack, even though refired to a sharp tack again.  So, it is impossible to refire a piece to the same temperature or higher without affecting the existing profile.  But it is possible to fire a piece with differing profiles if you plan the sequence of firings.

 

Tack fuse onto existing profile

 

It is possible to add pieces to be tack fused with little distortion to the existing piece through careful scheduling and observation.  There are several requirements.

 •     A moderate rate of advance to the working temperature is required, rather than a fast one. This is because the piece is a single thicker piece with uneven thicknesses.  Also, a slow rise in temperature allows completion of the work to at a lower temperature.  This means there will be less change to the existing profile.

•     A minimal bubble squeeze - or none at all - is required on this second firing.  The added pieces generally will be small, so if possible, eliminate the bubble squeeze.  The requirement is to add as little heat work as possible.

 •     The working temperature should be to a low tack fuse temperature with a long soak. 

 •     Observation is required from the time the working temperature is achieved.  Peeking at 5-minute intervals is needed.  This to be certain that the current tack fuse can be achieved without much affecting the existing profile.  It will be a compromise that you will be able to choose during the firing.  The decision will be whether to retain existing profile and have a sharp tack.  Or a slightly rounded tack and more rounded profile on the original piece.

Planning for multiple levels of tack

It is possible to design a piece with multiple profiles within the completed piece.  You need to plan out the levels and degrees of tack you want before you start firing. 

To do this planning, you need to remember that all heat work is cumulative. In simple terms it means that on a second firing you will start where you left off with the first one. The texture in the first firing will become softer, rounded, or flatter than the second or even the third firing.

Three degrees of tack can be achieved with a little planning.  It works similarly to paint firings.  Some paints fire higher than enamels, and enamels hotter than stain.  You have to plan to fire all the tracing and shading first.  Then you add the opaque enamels, followed by the transparent enamels.  Finally, you add the silver stain.  This is unlike painting on canvas where you build up the image all together.

The same principle is true of a multiple level tack fuse piece.  When creating various profiles in glass, you proceed from firing the areas that will be the flattest first. Then proceed to the areas which will have the least tack last.  This is a consequence of the cumulative effect of heat on re-fired glass.

Plan out the areas that you want to have the least profile.  Assemble the glass for those areas. I suggest that a 6mm base is the initial requirement for anything that is going to be fired multiple times.  Add the initial pieces that will become a contour fuse or a very rounded tack. 

First firing

Put this assembly in the kiln and schedule.  Do not fire to the contour profile temperature.  Instead, you will be scheduling for a sinter or sharp tack. This depends on how many textures you plan to incorporate.  Start with a sharp tack.  Fire at the appropriate rate with a bubble squeeze to about 740°C for 10 minutes and proceed to the anneal cool.   Different kilns will need other temperatures to achieve a sharp tack.

You do not fire to the contour fuse temperature, because the base will be subject to more firings.  Each of these firings will soften the base layers more than the previous one.  This is the application of the principle of cumulative heat work.  When you fire a piece for a second time, there will be little effect until the softening point of the glass is reached. Once there, the glass further softens, giving the effect of a contour fuse.

Any glass that had already achieved contour profile from the first firing will flatten further.  This can be used in cases where the working temperature was not high enough.  Just fire again to the original schedule’s temperature.  Take account of the need for a slower ramp rate to the softening point.

Second firing

Once cool and cleaned, you can add your next profile level of tack fusing to the base.  Note that “level of tack” does not refer to thickness being built up.  It is about the amount of roundness you want to impart to the pieces.  You may be placing this second - sharper – level of tack in the spaces left during the first firing.  Again, schedule to the original approximate 740°C. But remember the base is now a single piece.  You need to slow the ramp rate to the softening point, after which the speed can be increased.  You will not need to retain the bubble squeeze unless you are adding large pieces, or into low areas. 

The second firing will show the pieces added for the second firing to have the profile of the original pieces.  Those pieces having their first firing will have a sharper appearance.

 

credit: vitreus-art.co.uk 

This is a piece where the flower petals and leaves could have been placed for the second firing to give a softer background with less rounded flower details.

 

Third firing

Clean well and add the pieces for the final level of tack.  Schedule the initial rate of advance a little slower than the second firing.  The piece is growing in thickness and complexity.  Once the softening point is reached, the original rate of advance can once again be used up to original temperature. 

Final firing

Clean well and add the pieces for the final level of tack.  Schedule the initial rate of advance a little slower than the second firing.  The piece is growing in thickness and complexity.  Once the softening point is reached, the original rate of advance can once again be used up to original temperature. 

Further notes on multiple firings

It is a good idea to observe the firing, once the working temperature is achieved.  This is to ensure enough roundness is being given to the final pieces being tacked to the whole.  Be prepared to extend the soak if the final pieces are not rounded enough.   Although you should have a good idea of the degree of tack for the final pieces from the previous two firings.

You may need to experiment a little with the temperature and length of soaks at the working temperature.  For example, if the degree of tack is too sharp in the first firing, you can extend the soak or increase the temperature for the next ones. 

If you are firing at 740°C, you may feel you can afford to extend the soak for the subsequent firings, because you are in the lower part of the devitrification range. Consider the risk of devitrification increases with the number of firings of the glass.  The preference is to increase the temperature a bit for subsequent firings to ensure you are not spending a cumulatively long time in the devitrification range but still be able to get the final tack level desired. 

The preference is to increase the temperature a bit for subsequent firings to ensure you are not spending a cumulatively long time in the devitrification range but still be able to get the final tack level desired. 

Because most of your heat work is happening in the low end of the devitrification range, the cleaning regime must be very thorough.  Any chemicals or soaps used must be completely washed off with clean water.  The piece must be polished dry to ensure there are no water marks left on the glass.

You can, of course, have more levels of tack.  One approach would be to start with a sinter, or tack to stick, firing. And repeat that four or more times.  Another is to increase the working temperature and reduce the length of time soaked there.  The shorter time means there is less rounding of each level, allowing the build-up of many levels of tack.  All of these require some experimentation. 

More information is available in the ebook Low Temperature Kilnforming.

Three firings to the same sharp tack profile will give multiple profiles in the finished piece. 

Slump Shrinkage

Glass on rectangular moulds often does not maintain a straight edge.  It pulls in and tends toward the “dog boning” of fused single layer glass even if not so dramatic.

Explanation

The reasons for the pull-in on rectangular moulds are similar to those for dog boning. You should note that squares are special cases of the general class of rectangles. The discussion here applies squares just as much as to rectangles.

If you grid the rectangular glass, it illustrates that the glass in the corners is moving in two directions.  It is moving and slightly stretching into the mould.  At the same time, it is trying to compress into the corner of the mould.  The glass along the sides are moving in only one direction – stretching only slightly and moving toward the bottom of the mould.

There is more compression than stretching in the corners. The sides have only to move in one direction and experience no compression and so move toward the bottom more easily.

Such is my explanation of the experience. 

Avoidance

The real question then is how to prevent this pull-in that is so commonly experienced on rectangular moulds with no rims.  One way would be to avoid such moulds altogether.  This of course, is not practical, so some approaches to compensate or avoid the problem are needed.

It is possible to compensate for this pull-in by slumping a rectangle with slightly bulging sides.  Rather than a regular rectangle, you create one with slightly outwardly curved sides.  Getting the exact amount of curve will be difficult and achieved only after a number of experiments.

The opposite compensation would be to round the corners of the glass, so there will not be so much glass to fit into the corners of the mould.  This again will require experimentation to achieve a predictable result.  And it often would interfere with the appearance of the final piece.

The easiest, but not always successful, way to prevent the pull-in is to alter the scheduling for slumps on such moulds.  It is a well-known property of glass that it does not have a single softening point, but progressively softens with temperature and time.  You can take advantage of this by using four elements in combination. 

·        Use a slow rate of advance to the slump temperature, to allow the glass to evenly absorb a lot of heat on the way to slumping. 

·        Use a low slumping temperature  This may be as much as 30°C less than your usual temperature.

·        Use a long soak at the slumping temperature.  This may be hours.  You need to allow the glass to slump into the mould without stretching.  To avoid stretching, you need a low temperature.  At low temperatures, the glass requires a lot of time to conform to the mould.

·        Observe at 10- to 15-minute intervals once the slumping temperature is achieved.

These processes are outlined in a blog post on dog boning.  Further information is available in the ebook: Low Temperature Kiln Forming.

Avoidance of pull-in of the glass on rectangular moulds is related to scheduling and observance.  There are some compensations that can be tried, but require considerable experimentation to be successful.

Ramp and Anneal Rates for Tack Fusing

Tack fusing is more difficult than most realise.  Many failures – usually breakages – occur because the complexity of tack fusing is not fully acknowledged.

Ramp Rate 

Calculations

One of the effects is the slower rate of advance that needs to be used.  The rate of advance needs to be slowed to that applicable to 1.5 to 2.5 times the actual total thickness of the assembled piece. 

Reasons

The reason for this firing for apparently excess thickness is the shading effect of the overlying pieces upon the glass below.  Glass is affected by radiated heat, whether the heat comes from above or the sides.  The parts of the base glass that have glass on top cannot receive the radiated heat.  This means the shaded base glass needs time for the heat to be conducted through the overlying glass to it. 

Beginning of heat input

Progress of heat input showing some parts of the base are compeletly heated while others are not

Glass is a good insulator, resisting any heat transmission through overlying glass. Slowing the rate of advance allows the convection of heat to the lower levels to be adequate to avoid heat stress.  The reason for the 1.5 to 2 factors is that experience has shown a simple applied arrangement will be safe with a factor of 1.5 as the calculated thickness.  If you have stacks or lots of difference in thicknesses, you need a slower rate to allow for the conduction of heat.  This is where the 2 times actual thickness factor is useful.

Finding the Ramp Rate

The information on the rate of advance for evenly thick pieces of 6mm to 9mm is widely available.  Determining the rate of advance for thicker items is more obscure.  You can get some guidance from the manufacturers’ websites.  But where the guidance is for thinner pieces or it is unclear, you need to find another reliable source. 

One very reliable source is the Bullseye annealing chart for thick slabs.    Yes, this chart tells you about the annealing of thick items, not about the ramp rate to the working temperature.  But you can infer the initial rate of advance from the final cooling rate.  The principle is that the glass can survive the indicated cooling, so it should also survive that rate of advance from cold to working temperature.

This means that a set up of a 6mm base with two layers of glass pieces on top distributed around the base, is a total of 12mm.  This should be fired as though 18mm (1.5 times actual) or up to 30mm (2.5 times actual).  In the first case the chart indicates the final cool rate is 150°C per hour.  This can be used as the initial rate of advance to at least 540°C (above the annealing range).  If you choose to use the 2.5 times factor, the initial rate will be 65°C per hour.

This approach gives you a reasonable degree of certainty about how fast you can fire your glass from cold.  Note that you still need to have a conservative bubble squeeze segment in your schedule, especially if the lay up includes areas where air might be trapped.

Annealing rates

Annealing times and rates are normally dependent on the thickness of the fired glass.  But published annealing rates are based on both even thickness across the piece and on cooling from two sides – i.e., not on the floor of the kiln.

Calculating for even thickness

If you have taken your stacked piece to a full fuse, you can anneal for the final thickness.  I would be a little more cautious with a contour fuse and anneal as though it were three to six millimetres thicker than when completely flat because you cannot be certain that the piece is evenly flat unless you obeserve.

Calculating for tack fused

If, however, you are firing to a tack fuse you need to look to schedules for thicker pieces.

Reasons

Glass remains an insulator as it cools.  As glass cools, it must conduct the heat through the thick parts at the same rate as through the thinner parts to avoid inducing stress.  Remember the principle of annealing is to keep all the glass with 5°C or less difference in temperature.  The thinner glass gives its heat up quicker than the thick.  This will induce stress and it can be enough to break the glass in the kiln or, more usually, some long time after the glass is cool.  This means you need to control the cooling to a rate that would be suitable for thicker glass. 

At the beginning of the cool the heat loss is from the surface and to a lesser extent through the shelf.

Further heat loss shows the exposed base layer is giving up its heat throughout, although other areas are only beginning to cool.  It will take some time for the three layer stack to cool.  The uneven cooling leads to the introduction of stress.

Determining the rate

The annealing soak length and the rate of the annealing cool are directly related to the thickness calculated for your piece.  You have already chosen a calculated thickness for the rate of advance to avoid breaking the glass.  Use the rates given in the chart for that thickness for your soak and anneal cool.  Any annealing with a shorter soak and a faster cool risks inducing stress and possible breakage.

Rates of advance and annealing are intimately connected.  A tack fused piece must be annealed as though it were 1.5 and up to 2.5 times the actual total thickness.  Annealing of tack fused pieces cannot be skimped.

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