Deep Slumps with Bubbles

 

Photo Credit: Rachel Meadows-Ibrahim

The main causes of the large thin bubble is most probably  too high a temperature combined with a long soak.

Elevation of the Mould

The poster indicated there are eight holes total – four on the sides and four under the glass. This means any air has an exit out from under the glass and from the inside of the mould. So, in this case it does not need to be elevated for exit of air.  In my practice l have never, except in tests, elevated my slumping moulds. I have not had failures. My experiments involved in writing the eBook Low Temperature Kilnforming  showed no significant temperature differences between elevating, or not, below the mould.

Effect of Fast Rates

Slow rates to low temperatures with long soaks avoid sealing the glass to the mould. This means air can move out from under the glass during the slump. 

• Fast rates, and elevated temperatures can restrict air movement from under the slumping glass.  
• Fast rates and high slump temperatures can each cause uprisings because the glass slides down the mould during the soak, and that weight pushes the bottom upwards.

Temperature and Uprisings

This uprising is different from the bubble at the bottom on this piece. It is possible to see the glass bubble is thinner than the surrounding glass. As there were holes for air to escape, it seems the temperature and or speed was great enough to allow the glass to form to the mould at the bottom.  This covered the air holes and allowed the remaining air to push upwards on the glass.  A lower top temperature may have avoided this bubble formation.  Certainly, a combination of a slower rate and a lower temperature would have avoided the formation of the bubble.

Observation

Further, observation during the firing would have caught this bubble formation early enough to skip to the annealing and result in a piece with only a slight uprising, and before it became a bubble. Peeking should start at the beginning of the slumping soak and be repeated at 5 to 10 minute intervals.

Longer Soak or Higher Temperature?

 ‘Is it better to extend the soak or add more firing time when the firing program isn’t quite enough? What are the meanings of “soak,” “hold,” “ramp,” “working temperature” and “top temperature”?’  

Let’s start with some of the terms.

“Soak” and “hold” have the same meaning in scheduling.  Schedules are made up of a series of linked segments.  Each segment contains a rate, temperature, and time.  The time is often called a “hold” in the schedules.  That time can have several effects.  It can allow enough time for a process, such as slumping, to be accomplished.

Although “soak” is entered into the schedules in the same way as a hold, it has a different concept behind it.  The hold when used as a soak allows the set temperature to permeate the whole thickness of the glass.  An example is in annealing. An annealing hold/soak is set.  This is to allow the glass to become the same temperature throughout. 

The ramp is the rate at which the controller is set to increase/decrease the temperature.  This is normally the first element in the segment.

“Top” and “Working” temperature are the same thing.  It is the temperature at which the desired effect is achieved.  They have slightly different nuances.  Top temperature is normally considered as a point where the desired profile will be achieved in a few minutes.  The working temperature is also that, but includes the idea that it will take time for the effect to be achieved.

Which should you alter first – soak time or temperature?

Most important is that you alter only one at a time.  If you alter the two elements at the same time, you do not know which was the cause of the result.

In general, you lengthen the soak if the effect is not achieved at the temperature and in the time set.  There are two reasons for this.  Glass has fewer problems at lower temperatures.  Secondly, the controllers are set up in such a way that it is easy to extend the time. Check your manual for the key sequence to extend the time.  It is more difficult to alter the temperature during a firing. 

To determine if you need more time, you peek into the kiln as the kiln approaches the top temperature.  If the profile has not been achieved by the time set at your working temperature, you enter the combination of keys to keep the kiln at the top temperature until you see the effect you want.  Then enter the combination of keys to skip to the next segment.

Whether you alter time or temperature, depends on what you are doing.  Soak plus temperature equal heat work.  With heat work you can accomplish the same effect at lower temperatures.  It may be that taking more time (usually slower ramp rates) to get to the same or lower temperature, will give the results desired.

For slumping, draping and other low temperature processes extending the hold/soak is appropriate. It reduces the amount of marking that is created by the mould or surface supporting the glass.

When tack, contour, or full fusing, you should be aiming to finish the work in about 10 minutes. Soaking/holding significantly longer increases the risk of devitrification.

For high temperature processes such as pot and screen melts and some flows, increasing the temperature is probably the right thing to do, to avoid the devitrification possibilities of long holds of open face high temperature work.

These can only be guidelines.  Your instincts and experience will help you determine which is the right thing to do in the circumstances.

 

Diagnosing Slump fractures

Once you have an initial idea of the source of the problem, think about it.  Test it against the evidence.  Is there enough evidence to make a call?  Make sure you have considered alternative explanations.  It is just too easy to make a snap decision about causes in low temperature processes.  The source of breaks in slumping are most often complex and stem from interrelated factors.

I give you an example of the difficulties of diagnosing a slumping break.

On a Facebook group a person showed the break of a single layer on a cyclone mould.  Others commented the same had happened to them.

Picture credit: Esther Mulvihill Pickens

Possible causes suggested on Facebook included:

• Thermal shock on the way up
• Thermal shock on the way down
• Too large on the mould and broke due to differential contraction
• Too many holds on the way up
• Too hot
• Too thin
• Follow the CPI programme
• Glass extending over the sides

Some of these suggestions were of general applicability, some in relation to the state of the broken glass.

The suggestions did not include:

• Cause of the rounded dots at the bottom of the mould.
• A cause for the state of the flat piece off the mould (it appears sharp edged.  Does it show some forming already?).
• The cause for the location of the fully formed remaining glass.
• The effect of the location of the mould and glass in the kiln.
• The consequences of a short soak at top temperature. 
• Is the kiln running hotter than most (1290ºF/698ºC for 10 minutes at top temperature was used)?

Of course, it is difficult to diagnose a problem from just one picture. It is difficult even with many pictures. And so, without handling the object, only suggestions can be made.

But….

You must spend enough time examining the piece with whatever other information is available to make specific suggestions.  The first thought may not consider all the factors.  Consider what kinds of causes there are for breaks during or after slumping.

More close inspection reveals the rounded edges of the break.  That supports the idea that the temperature was too high. It also supports the diagnosis that the break occurred on the heat up.

The edges of the piece that has fallen off the mould, and now rests on the shelf, seem to be square or sharp. This shows the extent of the difference of temperature between shelf and top of the mould – less than 100mm/4 inches.  Also, how small the differences in temperature are between slump and tack.  The extent of difference in fusing does depend on how high in the kiln the mould is placed.  That is demonstrated here by the different elevation of the two pieces. 

The conformation of the glass to the mould is complete.  This supports the diagnosis of the break occurring early in the firing, and certainly before the slump was complete.  These pieces will not fit together.  So, even if the edges were sharp the fact they will not fit together shows they conformed independently to the mould surface.  Therefore, the break was before forming temperature was reached.

The glass hangs over the mould edges on only three sides and at an angle.  This indicates the cause of the overhang was the break.  Not the reverse. An overhang at the beginning of the slump is likely to be even.

The piece on the floor of the kiln combined with the movement of the glass toward the back gives an indication that the origin of the break is at the front.  This relates to uneven temperatures and to the placement of the mould.

No one mentioned the placement of the mould and glass at the back of the kiln.  This will have an effect on scheduling.  The mould and glass are very large in relation to the kiln.  There is little space between the glass on the mould and the walls of the kiln.  Also, the mould is placed asymmetrically in the kiln – very close on three sides.  This will cause uneven heating in any kiln.  To have a successful firing of glass on this mould in this kiln will require radically different schedules to that for a centrally placed mould that is moderate for the size of the kiln.

The large size (relative to the kiln) and the asymmetrical placing are the causes of the break, in my opinion.  I admit that it took me several looks to realise the placement was a key cause of the break.

So, the generalised comments about thermal shock are correct, but not as to the cause of that shock.  The kiln will be hotter in the central part and cooler at the corners.  This is true of all rectangular kilns.  The important thing is to learn how to cope with these temperature differences.

Slow firings to low temperatures with long soaks are the three important elements.  These make up the heat work of the kiln. Applying this to a schedule means:

• slow ramp up rates – as little as one half the recommended rates for centrally placed moulds that are moderately sized in relation to the kiln.
• Low temperatures present lesser risks to the control of the outcome of the firing.  Determining the lower temperature possible requires peeking into the kiln to monitor the progress of the firing.
• Long soaks combined with low temperatures get the kilnforming done with minimal marking of the underside.  Low temperature soaks - in excess of 30 minutes - are required to minimise the marking.  Observation of the slump will be necessary to determine when it is complete.

My suggestions for the causes of other elements are:

·        Cause of the rounded dots at the bottom of the mould.

The temperature was too high. 698ºC/1290ºF is much hotter than needed for a slump. It was hot enough to round edges and small shards of glass.  Which shows excessive heat was received by the glass.

·        A cause for the state of the flat piece off the mould (it appears sharp edged. Does it show some forming already?)

The soak of 10 minutes was too short for the temperature in the kiln to equalise from top to bottom.  The glass on the shelf may not have reached 650ºC/1200ºF with such a short soak.

·        The cause for the location of the fully formed remaining glass.

The glass broke and was forced apart by the size of the expansion differences within the glass.  The movement of a piece at the front of the mould combined with the rearward and side movement of the glass indicate the origin of the break was at the front.  The distance apart shows the amount of force, and so the degree of reduction in the ramp rate required to fire this successfully.

·        The effect of the location of the mould and glass in the back of the kiln has already been discussed.

·         The consequences of a short soak at top temperature.

A high temperature is often considered necessary to pick up all the detail in moulds, whether slump or texture moulds.  The same effect can be achieved at lower temperatures with longer soaks.  The results of this strategy are fewer mould marks on the bottom of the work.

·        Is the kiln running hotter than most (Used 1290F/698C for 10 minutes at top temperature)?

This is one that cannot be answered other than by experiments carried out by the owner of the kiln.  Look at the Bullseye Tech Note #1 Knowing your Kiln for methods of testing temperatures. 

In short:

Diagnosis of slumping breaks is more complex than it appears at first.

More information is available in the eBook Low Temperature Kilnforming, an Evidence Based Approach to Scheduling.

This is available from Bullseye or Etsy

Draping Different Thicknesses and Sizes

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

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

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

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

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

Different thickness and sizes of glass require different firing conditions.

Slumping Strategy

A schedule was presented for a slumping problem of a 6mm/0.25” blank.  It consisted of three segments each of a rate of 277C/500F with short holds up to 399C/750F and then a rapid rise to 745C/1375F.  The cool was done with two long holds at 537C/1000F and 482C/900F followed by cooling rates for 12mm/0.5”

My response was that, yes it was fired too high.  Not only that, but the firing strategy, as shown by the schedule, is odd. 

Strategy

The general strategy for slumping follows these ideas.

·        Glass is slow to absorb heat, and in one sense, this schedule accepts that by having short soaks at intervals.  As glass is slow to absorb heat, it is necessary to use slow ramp rates and without pauses and changes in rates.  This should be applied all the way to the slumping temperature.

·        Holds of short durations are not effective at any stage in a slumping firing.  The objective is to allow the glass time to form to the mould with as little marking as possible.  This implies slow rates to low temperatures with significant holds at appropriate stages.  This about putting enough heat work into the glass that higher temperatures are not needed.

·        This kind of firing requires observation for new moulds and new arrangements of glass to ensure the slump is complete.  Once you know the mould requirements and are repeating the layup of the glass, the firing records will tell you what rates and times to use to get a complete slump with minimum marking.

·        The hold at annealing temperature is to equalise the temperature throughout the glass to produce a stress-free result.  Any soaks above are negated or repeated by the necessary soak at the annealing temperature.  The hold there must be long enough to complete the temperature equalisation that is the annealing.

·        My work has shown that annealing for one (3mm/0.125”) layer thicker produces a piece with less stress.  This indicates that a 6mm/0.25” piece should be annealed as for 9mm/0.35” to get the best result.

The summary of the firing strategy for slumping is:

• ·        A single ramp of a slow rate to the slumping temperature.
• ·        Observation of the progress of the slump to determine the lowest practical temperature and hold time.
• ·        Annealing for one layer thicker that being slumped.
• ·        Three stage cooling of the piece at rates related to the annealing hold.

Critique

This is a critique of the schedule. For comparison, my schedule for a full fused 6mm blank would be different.

• ·        140ºC/250ºF to 677º/1250ºF for 30 to 45 minutes.
• ·        9999 to 482ºC/900ºF for 1.5 hours
• ·        69ºC/124ºF to 427ºC/800ºF, no hold
• ·        125ºC/225ºF to 371ºC/700ºF, no hold
• ·        330ºC/600ºF to room temperature, off.

The rate of the published schedule is fast for a full fused blank and extremely fast for a tack fused blank. This needs to be slowed.  The schedule provides a single (fast) rate of heating, but with unnecessary holds.  The holds are so short as to be ineffective, anyway. There is no need for the holds on the way up to the slumping temperature.  In general slumping schedules are of fewer segments.   This is because glass behaves well with steady slow inputs of heat.

Then strangely, the schedule increases the rate to top temperature.  It does so with a brief soak at 593ºC/1100ºF.  This fast rate of 333ºC/ 600ºF begins at 400ºC/750ºF.  This is still in the brittle phase of the glass and risks breaking the glass.  The brittle stage ends around 540ºC/ 1005ºF.

This rapid rate softens the surface and edges of the glass without allowing time for the underside to catch up.  This explains uneven edges.  It also risks breaking the glass from too great expansion of the top before the bottom.

Additionally, the schedule uses a temperature more than 55ºC/100ºF above what is a reasonable highest slumping temperature.  The top temperature of this schedule is in the tack fusing range.

There is no need for a hold 55ºC/100ºF above annealing soak. It is the annealing soak that equalises the temperature before the cool begins.  The higher temperature equalisation is negated by the cooler soak at annealing temperature. So, the hold at the higher temperature and slow cool to the annealing temperature only delays the firing by about two hours.  It does not have any effect on the final piece.

The schedule is cooling for a piece of 12mm/0.5”.  This is slower than necessary.  As noted above, cooling for one layer thicker than the piece is advisable to get the most stress free result.  The annealing soak could be 1.5 hours following this idea.  Cooling with a three stage schedule reduces the risk of inducing temporary stresses that might break the glass.  Although the initial cooling rate I recommend is very similar to this schedule, it safely reduces the total cooling time.

• ·        69ºC/124ºF to 427ºC/800ºF, no hold
• ·        125ºC/225ºF to 371ºC/700ºF, no hold
• ·        330ºC/600ºF to room temperature, off.

Using my kind of schedule for the first time will require peeking once top temperature is reached to determine when the slump is complete. It may take as much as an hour. Be prepared to either extend the hold, or to skip to the next segment if complete earlier. The controller manual will explain how.

 More information is given in Low Temperature Kilnforming, An Evidence-based guide to scheduling.  Available from Etsy and Bullseye

Texture Moulds

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

 

Example of wave form texture mould

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

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

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

 

Example of textured area surrounded by a rim

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

 

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

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

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

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

 

An example of some nearly vertical elements and a rim

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

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

 

Other information is available:

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

Bas relief

Layups promoting bubbles

 

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.

Softening the Tack Profile

Often people want a particular profile not provided by the schedules in the controller or the ones they normally use for tack fusing. The question arises as to whether to increase the temperature or extend the soak on a previously fired piece.

You can do either.

You can extend the time or increase the temperature. There are benefits and drawbacks with each.

Increasing the temperature is the choice for a quicker firing. But you have less control.  By increasing the temperature, you will certainly get a softer edge to the glass. You do not know until the firing is finished how much the glass has changed.

Extending the time means that you know a softer profile will be created simply by more heat work being put into the glass.  If you combine the extended soak time with peeking at intervals, you have much more control over the exact profile achieved.  Observation at 5- or 10-minute intervals after the target temperature is achieved, will enable you to get exactly the profile you want. Just advance to the next segment when that profile is achieved. 

The drawback is that the firing takes a little longer and you have to be present at the time the working temperature is reached.  You can schedule that by using the delay feature on your controller.

Note that on any re-firing of a piece you need to be aware that you are firing a single thicker piece rather than the original multiple layers.  This will require a more cautious rate of advance up to the softening point of the glass – generally around 540°C.  After that, the original rate(s), soaks and annealing can be used.

Of course, the considerations of temperature versus time can be applied to an initial firing as much as to a re-friing of a piece.

Observation is the best way to have precise control over the profile of your tack fusing.

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