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.

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

Frit Additions to Shaped Pieces

It is possible to stick frit to slumped pieces. But soaking for a long time – several hours - at 650°C/1200°F is required to stick the frit.  The added pieces will remain relatively sharp. You need to observe frequently from 600°C/1111°F to make sure that the form of the glass is not distorting. 

Credit: Pyramid Gallery, Smyth and Zebrak

Although it is possible, adding pieces to already shaped objects is not best practice, nor will it frequently give satisfactory results.  If the slump is shallow, it is more possible to do this successfully than steep or highly shaped forms.  But the most suitable practice is to flatten the piece, then tack fuse the pieces onto it. Follow this fusing with the new slump or drape.  This flattening process will not be possible with all shapes. 

The best results will be achieved by accepting what you have and make a new piece with the planned additions from the start.

This process will not be suitable for draped glass as the glass will drape further during this low temperature soak.

 

I've a book that gives more detail. Low Temperature Kilnforming, an evidence based approach to scheduling  or at Bullseye

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.

Annealing Requirements for Shaped Pieces.

 Experiments related to slumping show that shaped items such as slumped, textured and kiln carved glass need annealing for at least one layer thicker than they are. The annealing for one layer thicker than the calculated thickness provides the most stress-free result for the finished product. Annealing for the calculated thickness does not always produce a stress-free result.  

Full Fuse

 This indicates that an evenly thick 6mm thick piece will get the best result from an anneal as for 9mm.

Texture Moulds

 A piece of glass on a texture mould with 3mm or more differences in height requires careful annealing. The more defined/sharper the texture, the greater care will be required. A 6mm blank on a mould with 3mm variation taken to a well-defined texture needs to be annealed as though it were 18mm thick. A sharp tack requires annealing as for a piece 2.5 times its actual thickness plus another 3mm.  This gives the 18mm/0.75” thickness annealing requirement for the 6mm thick piece.

Kiln Carvings

 The same kind of calculation applies to kiln carved items as for sharply textured pieces. Pieces with less sharply defined profiles can be treated as one of the more common tack fused profiles.

Credit: Vitreus-art.co.uk

Tack Fusings

 A rounded tack fused piece of a 6mm base with 3mm tack elements that is being slumped will need annealing as for 21mm.  Twice the actual thickness plus 3mm giving the annealing requirement as for 21mm/0.827”.

 A contour tack of the same dimensions as given in the first example will require annealing as for 19mm/0.75”. The annealing requirement when slumping is for 1.5 times the thickness plus another 3mm.

In General

 The general approach to annealing shaped pieces is to calculate the thickness for the anneal and add one layer more to get a good anneal for slumped and other formed pieces. 

 The research and the reasoning behind this approach is given in LowTemperature Kilnforming, An Evidenced-Based Guide to Scheduling available from the Etsy shop VerrierStudio and from Bullseye. 

Simultaneous Fusing and Slumping

“I sometimes slump at the same time as I do a tack fuse. Is slumping at this higher heat bad for the mould? “

Image credit: Creative Glass

Mould

 It is possibly not bad for the mould, but it does depend on your temperature and heat work.  Ceramic moulds are typically fired to 1200° or 1300°C so higher kilnforming temperatures are unlikely to affect the moulds.  The speed at which the target temperature is reached is of concern though.  Ceramics have what is called quartz inversions.

 Two of the constituents of ceramics – cristobalite and quartz – have significantly large expansions at 226°C and 570°C / 440°F and 1060°F.  Rapid rises through these two temperatures risks breaking the ceramic mould.  This is not the case with steel moulds, of course.

Glass

 There may also be effects on the glass.  Slumping typically ranges between 620°C to 677°C (1150°F to 1250°F).  Tack fusing typically is done in the 740°C to 790°C (1365°F to 1455°F)range.  This is a significant difference even at the higher end of the slumping range and the lower end of the tack fusing range. 

 Some of the effects are:

·        The marking of the slumped glass will be greater at tack fusing. 
·        The glass will slip down the mould more. 
·        Any pieces applied to the base are likely to slide during the slumping process.
·        There is a risk of creating an uprising or bubble at the bottom as the glass slips down the side of the mould. 
·        There is more risk of creating needle points at the edges.

 Performing two processes at the same time risks difficulties.  Inevitably, compromises will need to be made between slumping and tack fusing.  Eventually, it will come to a time when the two process won't work together.

  

A slump taken to tack fusing temperatures is at risk from uprisings at the bottom, needling at the edges, excessive marking on the back, slipping down the mould and thickening

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.

Large Uprisings on Slumps

Help!  Looks like my shallow bowl wanted a boob.  [16” diameter pot melt slumped into shallow 20” mould, 4” above floor, fired at 175°F to 1100°F for 20 minutes,  and 75°F to 1250°F for 15 minutes]

What happened?

Sometimes a slump results in an uprising at the bottom of the slump. In this case, and many others, we know the vent at the bottom of the mould was open and the piece was supported above the shelf.  This indicates that everything should be set up for a good result.  Still, this uprising occurred.  It is not a bubble, as the glass is apparently evenly thick throughout the “boob”. 

The usual, and mostly unexpected, cause is too long or too hot a slump.  The firing, if allowed to continue, would result in a larger uprising and eventually a thickening of the piece at the bottom of the mould associated with a related reduction in the dimensions of the final piece.

What has happened is that the glass has become soft enough for it to slip down the sides of the mould. But it has not been hot enough for long enough to allow the glass to thicken.  The glass at the bottom is pushed up to compensate for the slightly hotter glass on the sides of the mould sliding down on the glass at the bottom.

In this case there is a moderate (97°C) rise from a soak of 20 minutes at 593°C to 676°C, soaking for 15 minutes.  This is a lot of heat work at a relatively high temperature for a shallow mould. 

What to do in the future?

Rate of Advance

Consider what you are trying to achieve at each stage of the scheduling of the process.  In this slumping there really is no need to soak at 593°C. It is a nowhere temperature.  The glass is no longer brittle.  It is at the lower end of the temperature range where the glass is softening anyway.  A simple, steady rise in temperature, as at the beginning, of at or below 100°C will be sufficient to bring the whole substance of the glass to the slumping temperature. If the piece is really thick, consider an even slower but steady rate of advance without any soaks.

Soak Temperature

Also, if the slump can be achieved in 15 minutes, it is too hot.  If the slump is complete in such a short time, it will be marked much more than needed by contact with the mould. 

The cooler the glass at the conclusion of the slump,  the less marking there will be.  Yes, the soak time needed to complete the slump will be longer, but the bottom of the glass will be cooler than a hot fast slump.  You should always be trying to achieve the effect you need at the lowest practical temperature.  The slow rate of advance will assist in completing the slump at a lower temperature, as the amount of heat work put into the piece will be greater.

Observe the progress of the firing

Observation is necessary when doing something different.  Some argue that it is necessary in every slump.  I admit that I do not always observe every slump, but this case again illustrates the need to observe each slump. 

https://glasstips.blogspot.com/2019/03/observation.html

By observing at 10- or 15-minute intervals, you will see when the slump is complete.  You may feel you do not have the time to wait for an hour or so for the slump to be complete, or that it does not fit with your activities.  The answer is to arrange the kiln’s schedule so that when the critical part of the process is reached it will fit with a space in your other activities. https://glasstips.blogspot.com/2016/12/diurnal-firing-practices.html

Can this piece be fixed?

You could put the piece on a shelf and take it to a high temperature slump with a significant soak to flatten it.  You will need to observe when the uprising is flat again, and then proceed to anneal.  However, the pattern placed in or on the glass will be distorted to some extent.  The uprising will flatten with a thicker rim around the base of the rise in the glass.  This may be visible. 

My view of these things is to learn as much as you can about causes and prevention and move on.  You advance your practice more quickly by understanding what went wrong and why than by trying (unsatisfactorily) to rectify a failed piece.  Often you can cut the glass up and use the pieces in other projects.

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

Uprisings at the bottom of slumps are often the result of too much heat work (rate, temperature and time).  Slow rates if advance to low temperatures with long soaks backed by observation prevent the occurrence of these bubble-like uprisings.

Uneven slumps

Credit: Lara Duncan

Uneven slumps – where the glass does not slide down the sides evenly, leaving one side higher than the other – are common in moulds with steep sides.  Another common cause is uneven weight on the blank – where there are more layers on one side than another.  Yet another common cause of uneven slumping is the blank having large areas of glass with different viscosities.

Things I can think of to avoid the problem.

While the glass is firing

Use a moderate rate of advance to the target temperature. Once that is reached, peek every 10 minutes to observe how the slump is proceeding.  When the slump begins to go off centre, reach in with protective gear and adjust it back to even.  The kind of protective gear you need is shown in this post.

An alternative to moving the glass is to tip the mould.  If the mould is relatively shallow with a flat bottom,  there is not much you can achieve by this action.  On deeper moulds, you can elevate one side of the mould.  This puts the elevated side closer to the top and so into a hotter part of the kiln.  This means that you elevate the side that is not slumping as quickly.  You do this because the slowly slumping glass needs more heat in relation to the faster slumping side.  It seems counter intuitive, until you realise you are putting the slow side into greater heat.  You will need to continue observing at intervals to know when the glass is slumping evenly. At that point you can return it to level.

I admit that moving the glass is my choice almost all the time.  It works well on moderately deep moulds.  Elevating one side of the mould while firing requires more time in the kiln that I want to give.  Tipping the mould works best on very deep moulds and so I view it as a special case.

Before the slumping begins

Most of the time we make our blanks the same diameter/dimension or slightly larger than the mould. This allows the glass to rest on the rim and be certain it is as level as the rim of the mould is.  However, this also creates an edge which the glass needs to slide over as it slumps.  Especially with steep sided moulds.

A fix for this is to make the blank fit just inside the rim.  Then it does not have to slide over the rim, and avoids the risk of hanging up on one part rather than another.  You will need to ensure the glass is level within the mould as well as the mould itself, in this case.  If you take this approach of internal placing and you want a piece with a particular final dimension, you should buy a mould larger than the final size needed.

You can combine this placing of the glass internally with another preventative for uneven slumps in deep moulds.  You can grind a small bevel on the underside of the edge to help the glass have greater contact with the mould, so resisting uneven movement.  This can be done separately from fitting the glass inside a steep sided mould, but is most likely to be successful if performed on a blank smaller than the mould dimensions.

Uneven slumps in kilnforming can be corrected during the firing or by preparation of the blank in relation to the mould before the firing.

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