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.

To Repair or Not to Repair

 Breaks during slumping sometimes occur. What can be done?

Cause of Break

The first element in assessing the piece is to determine why it broke. 

Should it be Repaired?

The second element is whether it should be repaired or re-used. Is it worth the effort of repairing? This will be about the importance and the time and effort you have already put into the piece.

Can it be repaired?

This is a third element of assessment. If the break resulted from incompatibility, any attempt at refusing will also break for the same reason. If inadequate annealing caused the break, it may be possible.

It is sometimes suggested that those pieces which fit together exactly, should be fused together flat and re-slumped. This ignores the fact that the glass will have stretched or deformed from the flat piece it once was.

• ·   This re-fusing may be successful for shallow and simple slumps. But the piece will not be corrected by fusing the broken pieces from deep or complex slumps as a result of the stretching and thinning or thickening in the slumping process.
• ·   The glass pieces will have an imperfect join when flattened because of deformations from the changes during the slumping.
• ·   If the base is a single layer, the separate pieces will pull apart during the re-fusing process due to the lack of volume.
• ·   The fusing process will make a tack fuse much flatter than originally intended. A contour fuse - at minimum - will be required to join the pieces.

For all these reasons, any flattening, fusing and then attempting a slump again is unlikely to be successful.

Fusing in the mould

In recognition of these problems about flattening, re-fusing, and slumping again some people suggest mending by firing in the mould. This would get over the difficulty of changes of shape. However, the required contour or full fuse will leave marking on the back and may lead to thickening at the bottom. It is also hard on your ceramic moulds if you fire quickly.

Changing the Shape

If it is desired to flatten an unbroken slumped piece for use in a mould of a different shape without much change in tack profile dimensions, there are two things to do. The maximum temperature to be used to get the glass flat and retain the degree of tack is the sharp tack - or lamination - range. It will require a significantly long soak at top temperature - hours.

This long soak time is a consequence of the effects of weight and span. The effective weight is less at the unsupported edges than at an unsupported centre. The slumped piece has most of its weight on the shelf now. This makes the flattening have to use a higher temperature or a longer soak. The effective span and weight at the edge is almost zero. This requires long soaks and frequent observation to know when the flattening is complete. Both these effects make the flattening of a piece without altering the profile a lengthy process.

 

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

Repairing a broken slumped piece of glass requires knowing why it broke, can it be repaired, is it worth repairing. Difficulties related to the changed shape, temperature to fuse, and changes in tack profile.

Elevation of Moulds in the Kiln

The placing of the mould may have a significant effect on the outcome of a slump. The ideal placing is in the centre of the kiln to ensure it has the most even temperature. This avoids any uneven temperature that may exist within the kiln.

Hot and Cool Spots

Sometimes this is not a practical use of kiln time or space, but if the heat distribution in the kiln is uneven, the placing may be critical. If the cool areas are known, avoid them in the placing of the larger moulds. Simpler moulds, or those which do not require as much heat can go in the cooler areas of the kiln. A good and simple method to test for the heat distribution within your kiln is given in Bullseye’s Tech Note no.1.

Effect of Elevation of Mould

Elevation of the mould by a centimetre or two is often recommended to help evenly distribute the heat under the bottom of the mould as well as the top. This is viewed as a way of avoiding breakage or uneven slumps. There are differences between moulds on the shelf and those elevated. Recordings show differences up to 49°C/88°F. The differences on the cool down ramps are minimal and do not interfere with annealing. These differences appear to have no effect on breakages in the mould.

ΔT Shelf vs. Elevated Moulds (Celsius)

	Max. ΔT	Average ΔT
Rate / hour	on Rise	Start of slump	End of 30 min slump	On cool
150	49	41	30	8
120	39	31	24	5

 

These differences should be put in context. The air temperature is approximately three times any difference between the two arrangements of moulds. Much more important in breakage is the ramp rate, as it creates significant differences in expansion between the top and the bottom of the suspended glass. This much larger difference has the potential for greater effects than whether the moderately sized mould is elevated or not. This table demonstrates the air and mould temperature differences.

ΔT Difference Between Air and Elevated Mould (Celsius)

Ramp Rate	Air Minus Mould Temperature (ave)
240	138
150	112
120	97

 

Large, Heavy, Wet Moulds

The elevation of large, heavy, or wet moulds is very important. It is needed to protect the supporting shelf from breaking. The amount of shading of heat from the shelf that these kinds of moulds can do is large. Wet moulds, especially, can cause large temperature differences in the shelf. Always elevate moulds that are large relative to the kiln, contain thick glass, are heavy, or are damp to avoid difficulties with the shelf.

 

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

Spider Web Cracks

 

Credit ASTM

 The nature of the cracks - and spider web describes it perfectly - shows an adhesion problem. It is not an annealing problem as that shows a single sinuous line with a hook at each end. It is not a compatibility problem, as that shows as cracks or breaks along the edges of the combined glasses. It is not a thermal break, as those show as breaks where the glass has separated to some amount.

 

Glaze crazed in a ceramic vessel

 

The cracks are exactly like crazed glazes on ceramic objects. And for the same reason. The glass is trying to contract more than the underlying ceramic. It is stuck to the pores of the ceramic and creates a crack where there is a slightly weaker part of the glass. These cracks in ceramic glaze propagate across the surface as it wears, or in the kilnforming case as it cools.

 

Glass puddled in ceramic

 Most usually it results from a lack of separator in that area of the shelf, or uncoated kiln furniture. It indicates either the glass has adhered to the shelf or mould, or (rarely with fusing glass) that the glass has suffered severe devitrification.

 

 

 Occasionally there will be the appearance of shards of glass. This will be where the glass has stuck to some particle on the shelf. Sometimes it can be a speck of something resistant to the temperatures we use in kilnforming that “grabs” the glass and breaks it into shards from that point as the glass cools.

 It is not the schedule that causes the breaks. It is in the shelf preparation.

 The shelf should be cleaned of all the kiln wash and lightly sanded down to smooth. It should then be coated with four thin layers of kiln wash painted in a different direction for each layer. No drying is necessary or even advisable. All kiln furniture must be completely coated with kiln wash.

 If you are re-using a shelf, it must be swept clean before any glass is laid on it.

 Crazing results from the glass sticking to the surface it is resting on.

 

Some additional information:

https://glasstips.blogspot.com/2019/05/kiln-cleanliness.html

https://glasstips.blogspot.com/2020/07/crazing.html

 

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

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.

Problems when Slumping

A range of problems appear in slumping.  These include bubbles, splits, puddling and more. Several causes are possible.  This blog looks at the problems, possible causes and remedies.

Bubbles

Blocked Vent Holes

 Absence of, or blocked holes at the bottom of the mould to allow air out into the kiln on all but shallow or cylindrical moulds can be a cause of bubbles. Prop the mould up on stilts if the hole does not go directly from under the glass and out of the side of the mould. Alternatively, drill a hole in the side to allow the air to escape from under the mould.

Wet moulds

In kiln forming, the moisture resulting from recently applied kiln wash is considered by some to be a cause of bubbles. The water in the mould will be evaporated by around 250°C/482°F in any sensible slumping schedule. At this temperature, the glass will not have begun to move, so the moisture can move out of the mould through any vent holes at the bottom of the mould, or past the glass as it rests on the edge of the mould.

The circumstance when a damp slumping mould could cause difficulties is when using an extremely fast rise of temperature. This is detrimental to the mould also, as the rapid formation of steam is more likely to break the mould rather than the glass. It is also unlikely to result in a good slump conforming to the mould without significant marking.

In casting with wet plaster/silica moulds water vapour can move toward the glass. Casting practice has alleviated some of the problem, by having an extended steam out before 200°C/395°F, or pouring the glass into the hot dry mould from a reservoir.

In pate de verre, the mould is most often packed while wet. The small particles normally allow any steaming of moisture to pass through, and so be dry at forming temperatures without blowing any bubbles.

Top Temperature

Bubbles at the bottom of the glass are much more likely to be the result of too high a process temperature if the previous two conditions are met. This high temperature allows the glass to slide down the mould.  The glass is not plastic enough to thicken and form a puddle at the bottom at most slumping temperatures. Instead, it begins to be pushed up from the lowest point due to the weight of the glass sliding down the sides.

 

Avoiding uprisings on the bottom of bowls.

Vent Holes

Make sure the holes are clear before placing the glass.

Wet Moulds

Ensure that the moulds are no more than damp before placing in the kiln.

Top Temperature

Firing for too long or at too high a temperature will cause the glass to continue sliding down. Having nowhere else to go, the bottom begins rising. This is the result of the weight of glass pressing down onto the bottom, especially on steep-sided moulds. This is a consistent experience across several kilns and with multiple users.

Low Slumping Temperatures.

Glass at low temperatures is affected largely by its weight and viscosity.

Viscosity Effects

Thick glass will fall more slowly than thin, when using the same schedule. Thick glass takes longer to equalise the upper and lower surface temperatures. Since the lower surface is stiffer (has a higher viscosity) it will move less using the same heat up rate. This means slower rates should be used, or a significant soak just above the strain point will be required. This softening of the glass evenly throughout the rise to the top temperature is critical in obtaining even slumps.

Splits in slumps

Without the slow progress to top temperature there can be problems. Sometimes the upper surface of the slump appears fine. It is the bottom that exhibits a split or tear that does not go all the way to the upper surface of the glass. It indicates the rate of advance was too - but only just - too fast to achieve the desired result.

 The ramp rate has been quick enough to get the top heated and become plastic. But the lower surface is still cold enough that it is brittle. The weight of the upper softened glass begins to push down before the bottom has become hot enough to be fully plastic. The force of the weight on the bottom can be enough to cause the glass to separate, rather than move as the surface does. This split on the bottom but not the top indicates a slower rate for that thickness is required. This shows the interaction between viscosity and weight.

 Sometimes the split is evident from the top. The cause of this kind of split is the same as a split on the bottom. But the ramp rate has been much faster in relation to the thickness or profile of the piece.

Weight

It is possible to have glass slightly overhang slumping moulds if you use low temperatures. The glass has the appearance of behaving differently at these low temperatures than at fusing temperatures.  

 

At low temperatures it cannot form exactly to the mould. It falls first in the middle. Because the glass is not very plastic, the edges rise up from the mould at first, because the weight there is not great enough to allow the unsupported glass to bend. The edges stay in line with the beginning of the bend in the middle.  

 

At the beginning of the slump the glass is not soft enough to stretch. It maintains its dimensions as it falls. For deep moulds, the glass moves progressively to move over the lip of the mould and begins to fall into the mould.

As the slump proceeds, the glass stretches very little and so the edges move further down the mould. The glass continues to slide down at the edges until the centre settles down onto the mould bottom. 

During this slide into place, the glass can become marked. This is usually most evident on back of the upper portions of the glass where most sliding is happening.

 With higher than necessary temperatures, the glass can continue to slide down the mould. Since the glass is still not fully plastic, the weight pushes the glass at the bottom upwards. This gives the appearance of a bubble, but is an uprising due to the pressure of the glass at the sides of the mould.

 

During the sliding of the glass along the mould, it becomes more marked. The marks often look like stretch marks. And in many senses, it is exactly that.

At higher temperatures or longer holds, the glass softens more. At this point the uprising collapses and the glass begins to thicken at the bottom. It also thins slightly at the top.

Remedies

Ramp Rates

The ramp rates should be slow.

• ·        This allows the glass to heat evenly throughout. This is important to get even slumps. 
•          Contrasting colours or a combination of opalescent and transparent glasses heat evenly with slow rates.
• ·        Slow rates allow glass with tack profiles to heat evenly.
• ·        It helps avoid splits in the bottom of slumped glass.
• ·        It allows lower slump temperature to be used.

Low Temperatures

Using the lowest practical slumping temperature gives the best results.

• ·        It allows glass with small overhangs of the mould to be successfully slumped.
• ·        Low temperature reduces the mould marks on the back of the glass.
• ·        Fewer stretch marks are in evidence.
• ·        Low slumping temperatures with long soaks reduce the uneven slump that is sometimes in evidence with deeper moulds.
• ·        Low temperatures allow different colours to heat more evenly.
• ·        Low temperatures reduce the thinning or thickening of glass in a high temperature slump.

More information is available here.

This information shows you need to keep the slumping temperature to the minimum required. To find out what that temperature is, watch the slumping in stages in brief peeks (do not stare!). Look at the piece for a second or two every five minutes before you reach your desired temperature and at intervals throughout the hold.

If it has slumped completely at the beginning of the hold, you are firing too high. Reduce your temperature in subsequent firings and watch in the same way to find what the required temperature and time is. There is absolutely no substitute in slumping but to watch by peeking to learn what your mould and glass require. 

What Temperature?

To determine the temperature needed for your piece, use slow ramp rates – between 100°C to 150°C/ 180°F to 270°F. Set your top temperature around 630°C/1170°F for a simple slump of fusing glass. For bottle or window glass you will need a temperature closer to 720°C/1330°F.

It is necessary to observe the progress of the slump as you do not know the best slumping temperature. Start watching the glass at about 10-minute intervals from about 600°C/1110°F. There is not much light in the kiln at this temperature, so an external light is useful. You can also observe the reflections of the elements on the glass. When the image of the elements begins to curve, you know the glass is beginning to bend. You then know that is the lowest possible slumping temperature when using that ramp rate.

Hold for at least 30 mins at the temperature when the glass begins to visibly drop. This may or may not be long enough. Continue checking at 5-10 minute intervals to know when the slump is complete. If the glass is completely slumped before the soak time is finished, advance to the next segment. If not fully slumped, you need to extend the soak time. These operations mean you need to know how to alter your schedule while firing. Consult your controller manual to learn how to do these things. Stop the hold when complete and advance to the anneal.

In some cases, you may need to increase temperature you set by 5-10°C. You can do this by scheduling a couple of segments with 10°C/18°F higher temperature each and 30 minute soaks each.  If you do not need them, you can skip them. If you do need the extra temperature, you have it scheduled already.  You will know if you need the extra segments by whether the glass has begun to curve at the start of the first of the soaks.  If it has not after 10 minutes, skip to the next segment. Once the new temperature has been reached, check for a curve in the glass. Again, if after 10 minutes there is no curve, skip to the next (higher temperature) segment.

A low temperature slump will allow the glass to conform to the shape of the mould without softening so much that it takes up all the markings of the mould. That in turn means there are spaces for the air to escape from under the glass all the way to the slumping temperature as well as through the air holes at the bottom. It also gives the most mark-free slump possible for your shape.

If you are slumping at such a temperature that the glass has sealed to the mould, you are firing too hot anyway. Or put more positively, use a low temperature slump, that is, a slump at the lowest temperature to achieve the desired result over an extended period of your choice.

More information is available in the eBook Low Temperature Kilnforming available through Etsy or Bullseye.

Fixing a Broken Piece

 This conversation is reproduced by permission (with some editing out of extraneous information). It is presented as an example of how conducting a critique of your schedule can have dramatic effects on the results of your firing.

 

This is the piece as it came out of the kiln

Picture credit: Ike Garson

 You may have seen the photo I posted of a large copper blue streaky piece that has cracked right across. …  I’m wondering if it would be better trying to bring the 2 pieces together instead of opening up the 2 pieces and inserting frit. I was thinking of firing it with a tack or contour schedule.

This is the crack that developed later through the frit and single layer centre.

Picture credit: Ike Garson

 

I have 4 questions:

A.   Even if I manage to fix it, do you think that fissure line will always be too weak and liable to break off at any point?

Response: The strength of the joint will be dependent on the firing conditions.  To make it strong, the temperature should go to full fuse.  Tack fusing will leave the joint more visible and weaker.  To stop the joint rounding during heat up, you will need to dam the piece tightly to stop the normal expansion of the glass and ensure the glass is forced together during the higher temperatures.

B.     I have some large pieces of clear confetti. Would it benefit using them to bridge the 2 sections from below?

Response: Anything you put on the bottom will have distinct outlines and visibility.  The temperature on the bottom can be 10C or more different from the top surface, which is why you can get crisp lines with the flip and fire technique.

C.    Would clear powder hide the crack or would it always be visible after firing?

Response: Any additions to the top may be less visible, but adding clear powder makes the join more obvious.  You need to use powder of the same colour as the sheet glass.  Since you are using a streaky glass, you can’t use coloured power either as it is very difficult to imitate the steaks even with powders of the same colours. 

More information was given indicating the first contour fuse schedule in Celsisus:

1.  260 730 00.20
2.  FULL 515 00.60
3.  260 150 End

This is the contour schedule I have used many times successfully but never for a piece during this week. 

My critique of the schedule. 

Segment 1.

·        It is too fast for the small distance to the side of the kiln. 

·        It is too fast for a piece of varying thicknesses. Most expansion breaks occur below 300˚C, so a soak at ca.260˚C will help ensure the glass maintains an even temperature, especially with large differences in thickness. Then you can advance more quickly. 

·        There is no bubble squeeze.

·        The top temperature seems low for a good tack, or the soak is a bit short.  Long soaks allow the glass molecules to bind at the atomic level firmly. This is the principle used in pate de verre.

·        It definitely needs to be on fibre paper covered with thinfire to allow air out.

Segment 2.

·        The soak at 515˚C is better done at 482˚C for Bullseye.

·        My tests have shown that contour firing a piece like this at rates and holds for 1.5 times the height of the piece is necessary for good results.

Segment 3.

·        Also, my tests have shown that a three-stage cooling provides the best result.  Slow cooling keeps the glass within the 5°C difference required for avoiding stress.

·        Annealing at the bottom end of the range combined with an appropriate length of soak and slow cooling gives a denser glass than soaking at the middle of the annealing range. 

·        The best cooling comes from a three-stage cooling process.  This involves a slow rate for the first 55C, a rate of 1.8 times this for the second 55C, and a rate of 3 times this for the final cool to room temperature. 

These points mean that I would recommend you fire for at least 10mm thick.  This recommendation is for a new piece, not a repair. In this repair case and for the conditions, I would choose 12mm as being more cautious. My schedule would look something like:

1. 120˚C to 260˚C, 20’
2. 300˚C to top temperature, 10’
3. Full to 482˚C, 120’
4. 20˚C to 427˚C,0’
5. 36˚C to 370˚C, 0’
6. 120˚C to room temperature, off

The anneal soak is for a piece 12mm thick.  The cool rates are for 21mm thick.  This is to compensate for the nearness of the glass to the edge of the kiln.  It will help to ensure the glass does not have excess stress locked into the piece during the cooling. 

D. Do you think this schedule would work [for a repair]? It's adapted from a standard tack schedule.

• 1. 222 677 00.30
• 2. 222 515 00.40
• 3. FULL 482 01.30
• 4. 63 371 ENDS

 

Critique of the re-firing schedule.

Segment 1. 

·        Too fast given earlier difficulties. 

·        Too low for good adhesion unless you use about 10 hours soak. 

·        Even at sintering temperature (690°C) you would need 2 hours.  But at sintering temperature you do not alter the surface 

Segment 2. 

·        Too slow a cool from top temperature and risks devitrification. Should be FULL.

·        You do not need the soak at 515˚C.  It only delays the annealing process.  It seems this idea of soaking at the upper portion of the annealing range was introduced by Spectrum over 2 decades ago. 

·        Any advantage that might be achieved by the higher soak is cancelled by the FULL rate to the annealing soak. 

·        Go straight to the anneal soak. 

Segment 3. 

·        You need a more controlled 3 stage cooling to get the best result.

 

My schedule for repair would look something like this:

1. 120˚C to 540˚C, 10’
2. 300˚C to 780˚C, 10’
3. Full to 482˚C, 210’
4. 20˚C to 427˚C,0’
5. 36˚C to 370˚C, 0’
6. 120˚C to room temperature, off

I am making the assumption that 780˚C is full fuse in your kiln.  Anything less than full fuse will certainly show the crack. 

A Look at Causes.

·     The piece is far enough away from the elements.  It is not on the floor. These are not the causes.

·     It is very near the sides of the kiln.  These are always cooler than the centre. There is always a risk of breaking in this case.  Very slow rates are needed. 

·     There is a 3.5 times difference in thickness within the piece. This also requires slow rates.

·     If the break were to have been on the heat up these elements of uneven heating, and rapid rates are a problem.  But the break occurred after the cool down. So, the annealing soak and cool is a problem. 

·     I have suggested some alterations to the schedules to address these things.

 

Fixing for Yourself

·        Dam it tightly to avoid expansion within the glass as it heats.  This holds the join together and causes the glass to gain a little height during the firing. 

·        Place the piece on 1mm or thicker fibre paper topped with thinfire.  This will help avoid a bubble forming in the clear.

·        I have suggested a schedule which is slower to ensure no further breaks.  It is slow to the strain point and fast after that. 

·        It needs to be a full fuse to fully join the two pieces and ensure it is sound.

·        The cool to annealing should be FULL.  Eliminate the soak in the upper annealing range. The effects of the time spent there is nullified by the rapid rate to the main annealing soak. 

·        Anneal as for 12mm, but with slower cool rates (for 21mm) to ensure there are no stresses built into the piece by the nearness of the glass to the edge of the kiln.

·        These methods and schedules will make it a strong whole.  But the join will still show on the bottom. 

·        After fixing, if you are still not satisfied, break it up for incorporation in other projects.

Finally, and unfortunately, I do not think it can be satisfactorily repaired for a client.  The crack will show on the back. You will know it is a repair, rather than a whole. And that will reflect on your feeling about the piece, and possibly your reputation. 

Conclusion

The commission was successfully re-made from scratch by the artist using some of my suggestions on scheduling. This is the resulting piece.

 

Picture credit: Ike Garson

 

Careful analysis of the conditions around a break are important to making a successful piece in the future. Many factors were considered, but the focus became the schedule. Analysis of each step of the schedule led to changes that resulted in a successful piece with the original vision and new materials.