The Relationship of Stress and Slumping

From time to time the assertion is that a break during the heat up of a blank while slumping is the result of residual stress remaining in an under-annealed blank.  Is there a relationship between inadequate annealing and slumping breaks?

It seems to be the general consensus that it is true.

It is clear that poorly annealed glass is more likely to break.  The assumption is that the additional bending stress added to the existing stress, causes a break.  Even if the fused piece is stressed, but not enough to break, a slow heat up would avoid the build up of stress to breaking level.  After all, the means of relieving the stress of toughened/tempered glass is by a slow heat up to allow the stress between the interior and surface to be equalised.  The same principle should apply to stressed glass in a slump.  My contention is that the ramp rates for the slump have been too fast to relieve any additional stress applied by the bending of the glass.

I hear of very few people testing for stress after any firing.  I read of people asserting the existing stress is amplified on the slump firing.  I do not read of any experience of people testing their flat piece, discovering excess stress, and  re-firing to relieve the stress before slumping, but the assertion of excess stress from the first firing continues as a cause.

Without testing there is no way to know whether the first firing had excessive stress.  The use of polarising filters is such a simple, easy and quick way to determine if there are stress problems in the fired and cooled piece.  It should be the business of practitioners and teachers to assert the need for stress testing as the next task when the glass has cooled. Unless people asserting this possibility do the testing of their proposition, it can only remain among the untested elements of kilnforming.

If there were to be a lot of stress in the flat blank, it needs to be fired again, annealed longer and cooled more slowly than previously to relieve the stress before any other process is conducted.  The firing to relieve the stress needs to be only to the lower portion of the slumping range at maximum.  

Each piece that is intended for further firing, needs to be tested for stress before the next firing, and not just at the end of the firing sequence.  To get an accurate reading of the stress, the piece must be allowed to cool until the internal temperature equals the surface temperature.  This may take overnight, but at least as long as the combined anneal soak and the associated cool. The delay caused by waiting for the complete cool may encourage people to skip the stress testing.  But it is risky to avoid testing for stress because of impatience.  Another firing can be conducted while waiting for the first piece to completely cool.

Annealing sufficiently on every firing is the way to ensure that any slumping break is not the consequence of stress from the previous firing.  The Bullseye sheet Annealing Thick Slabs (Celsius and Fahrenheit) gives the annealing times and cool rates.  This document applies to all fusing glass, except the annealing temperature used. Study the table, and follow it closely.  Keep in mind that the effective thickness for other than full fuse, is between 1.5 to 2.5 times the thickest part.  After that first firing, test the piece for stress when it has cooled sufficiently.  It is also important to test the successfully slumped piece for stress before using, gifting, or selling it.

Stress Analysis of Broken Glass

Will stress still show with polarised filters on cracked and broken glass?

It's not a straightforward answer.

I was looking at some broken fused float glass a few years ago.  I had always subscribed to the idea that a fracture relieves the stress. Not always. The broken float glass had been slumped, and the pieces still showed stress.  This turned out to be a compatibility problem, although both layers were float.  

The stress of inadequately annealed glass is likely to remain visible through the filters, because inadequately annealed glass will have stress distributed across the whole piece.  But glass that has been cooled too quickly and suffered thermal shock, is more likely to show minimum stress because the break relieved most of it.

It is likely stress will show on the tree piece pictured because it has not completely broken a[art. And even when it does break, it may still show a residue of stress.

It is sensible when trying to diagnose the problem to perform a strip test of the glasses for compatibility of the glasses concerned to be sure what is happening. If no stress shows on the test strip, the stress showing on the cracked piece is unlikely to be from incompatible glass, and other factors need to be considered.

Photo credit:  Debi Frock-Lyons 

Break Diagnosis in Slumping

The usual advice in looking at the reasons for breaks in your pieces must be considered in relation to the process being used.  Breaks during slumping need to be considered differently to those occurring during fusing.  

The advice normally is that if the edges are sharp, the break occurred on the way down in temperature. Therefore, the glass must have an annealing fracture or a compatibility break.  It continues on to say if the edges are rounded it occurred on the heat up, as it broke while brittle and then rounded with the additional heat.

This is true, but only on rounded tack and fused pieces.

When the process is a slump, there is not enough heat to round the edges.  So, the edges will be sharp whether the break was on the heat up or the cool down.

How can you tell in a slump process when the break occurred?

The first, but not obvious, way to tell if the break is on the heat up is to peek at about 260C/500F as most heat up breaks occur around that temperature, and again at the strain point, about 540C or 1000F.  These are the two critical heat up temperatures that will give the knowledge of when the break occurred.  If at either of these peeks, the glass has broken, the firing can be abandoned.  If the break is at the higher temperature, it needs to be annealed though.

 

The other way to tell if the break occurred on the cool down is that if you can put the pieces of the slump back together and they fit perfectly, the break was on the cool down, as the piece was already fully formed at the time of the break.

If the pieces do not fit together perfectly, the break was on the heat up.  This is because the break occurred, and then the two (or more) pieces slumped independently, thus leaving slightly different shapes at the break line.

There is a special case here, of course.  Sometimes the break is only a split in the bottom, that does not come all the way to the top of the piece. This split (or splits) occur when the heat up is too fast.  The theory goes that the top became plastic while the bottom was still brittle/stiff.  The weight of the hotter, more pliable glass overcomes the strength of the cooler and heat stressed bottom, causing it to split.  

Another theory is that the layers of glass were not fully fused together, so forming a thermal break between the upper and lower pieces, again allowing the top to heat more quickly, but causing thermal shock to the lower part.

More information is given in the blog post Diagnosis of Breaks, and a full discussion in the ebook Kilnforming Principles and Practices available from Bullseye.

There is also extensive information on diagnosis of breaks in this blog entry on slumping cracks.  

Fused Glass in Dishwashers

“Can glass be put into dishwashers?”

image credit: very.co.uk

There are many recommendations to avoid placing fused glass into a dishwasher.

The main reasons given are:

·        Corrosion

·        Devitrification

·        Etching and

·        Breaking.

There are distinct differences between these effects.

Corrosion

Glass corrosion generally comes from constant contact with moisture and has a greasy feel.  As experienced by weather or washing, the wetting of glass is not constant, and it dries between wettings.  No visible corrosion is present on window glass and, although float glass is a little different from fused glass, the same effect applies.

Devitrification

Devitrification occurs at much higher temperatures than those created in a dishwasher, and therefore is not a risk.

Etching

The main risk is etching from the washing process.  This can be mechanical or chemical, and dishwashers combine both. Over time, the glass will be etched just the way lead crystal is in a dishwasher.

Breaks

Glass breaks can occur in the dishwasher because of the shock of hot water.  Most dishwashers rinse while heating the water, so the glass experiences only slow rises in temperature.  Float glass of 4mm can withstand 140˚C differentials according to manufacturers.  Full and tack fused glass is not as homogenous as float glass and will be affected by smaller temperature differentials.  So, there is a small risk of breaks in dishwashers.

Additional risks relate to the layup of the glass. 

• ·   Tack fused glass has a variety of thicknesses that make it more prone to breaks from temperature differentials.
• ·   Contrasting colours can react differently and split at the contact lines.
• ·   Large internal bubbles can cause difficulties, which may arise from the insulating element of the contained air, or simply because of thickness.

 

Slumping Splits

 This is a description of the analysis process to determine the possible causes of a split during a slump.     

Credit: Maureen Nolan

Observe the piece.

It is a tack fused piece, about 20cm (8") square, which has been slumped. 

The base layer is of clear. The piece has three additional layers, but the fourth layer is only of small glass dots and rectangles.  The central, heart, area is made of three layers.

A split has appeared during the slump. It is split irregularly through pieces rather than around them.  It is split through the thickness but only partially across the piece.

In one area the (brown) third of four layers spans the split.  Further to the left a brown second layer seems to have broken, but still spans the split.

Threads and particles of glass are connecting across the split. 

The edges are probably sharp, although only so much can be deduced from a description and one photograph.

History of the Piece

The tack fused piece has been put in a mould to form a platter and has split during the slump.

The schedule in essence was:

139ºC/250ºF to 565ºC/1050ºF for a 30’ soak (some pauses but all at a ramp rate of 139ºC/250ºF)

83ºC/150ºF to 688ºC/1270ºF for 10’

222ºC/400ºF to 516ºC /960ºF for 60’

111ºC/200ºF to 427ºC/800ºF for 10’

167ºC/300ºF to 38ºC/100ºF, off

 

The assumption is that the tack fused piece received a similar annealing soak and cool.

 

Diagnosis

Too fast

Slumping a tack fused piece of three layers plus decorative elements on top needs to be fired as for 19mm (6 layers) minimum (twice the actual).  My work for the Low Temperature Kilnforming* eBook showed best results are achieved by slumping as for one more layer (21 mm/0.825” in this case).  This gives a proposed schedule of:  

120ºC/216ºF to 630ºC/1166ºF (not 688ºC/1270ºF) but for 30 to 45 minutes

AFAP (not 400ºF) to anneal 516ºC/960ºF for 3.5 hours (not 1 hour)

20ºC/36ºF to 427ºC/800ºF, 0

36ºC/65ºF to 371ºC/700ºF,0

120ºC/216ºF to room temperature

 
Commentary on the proposed schedule:

The slump is relatively shallow, so a low temperature with a long soak is the most suitable schedule for this piece.  The drop to anneal is at a sedate rate of 222ºC/400ºF.  This is inappropriate, generally.  Just as there is a rapid rate to top temperature to avoid devitrification, so there needs to be an AFAP drop to anneal, also to avoid devitrification.  The anneal soak was not the cause of the break, but it is worthwhile noting the recommended anneal soak and cool rates are longer and slower than that used.  This is a note for the future.

 

Suspect Tack Fuse

If the tack fuse schedule was like the slump schedule, the slump was started with inadequate annealing in the previous firing.  More importantly, the evidence for an inadequate tack fuse is that the split under the brown rectangle was through the clear and red on top, but the split left the brown intact.  This means it was not securely fixed to the red below it. 

 

If the condition of the tack fuse is not sound, it is probable that difficulties will be experienced in the slump.  The poster commented “… why do [these splits] happen only when slumping – it came through tack just fine.”    It is probable the tack fuse was not “just fine”.  The way to be sure the previous firing was just fine, is to test for stress.

 

There is enough clear in this piece that an inspection for stress could be conducted by use of polarising filters before the slump.  Testing for stress is a simple viewing of the piece between two sheets of polarised light filters.  Doing this test will give information on the amount of stress, if any, in the flat tack fused blank.

 

Slump Split

During slumping the glass is subjected to more movement and therefore stress than while being fired flat.  The glass is often only barely out of the brittle zone when being slumped and that makes the stress more evident during the early part of the slump. This requires careful inspection of the failed piece.

 

Look at the glass surrounding the split.  My opinion is that the edges are sharp.  If rounded, the threads of glass from the edges of white would have melted to the edges of the split rather than spanning it. 

 

It appears the top layers were hot enough for less viscous glass on top to form stringers that span the break as the underlying layers split.  It is probable that the split was during the plastic phase of the slump for the upper glass, but  the lower layers were not as hot and suffered thermal shock. 

 

This split of lower layers, while the overlying ones are whole, is often seen in tack fuses, although the top ones do slump into the gap as the firing proceeds.  In a slump there is not enough heat, time or space, for the brown piece to slump into the gap.  Both splits appear to be a result of too rapid firing.  In the flat fusing work, the split results from too fast a ramp rate during the brittle phase of the glass.  But the slumping splits appear to occur after the brittle phase, almost as a slow tear in the glass. This may result from the differential heating of the layers if not fully combined.  It may also indicate the split developed slowly. 

 

One other observation is that these splits seem to be more frequent during the slumping of tack fused pieces.  As speculated above, it may be the inadequate tacking together of the pieces of glass during the first firing, which can form a discontinuity in transmitting heat.  And it may be that the different thicknesses across the tack fused piece allow stress to build from differential heating of the glass.

 

Rates

 

Whichever of these speculative effects may be true, it appears the ramp rates are suspect.  As mentioned elsewhere* (and in Kilnforming Principles and Practice to be published soon), the reasons for these splits are not fully known.  Even microscopic examination by Ted Sawyer has not produced a satisfactory explanation.  The only practical approach that has been successful is to slow the ramp rates.  However, the appearance of these splits is essentially random (with our current understanding), so prevention is difficult.

 

Conclusion

The probable cause of the split in the slump has been that the ramp rates were too fast.  This may have been made worse by the too short anneal soak, and the too fast cool of the tack fused blank.

 

Remedy

There is no practical rescue for this piece.  Prevention in the future is to use ramp rates that are for at least one layer thicker, if it is full fused.  If it is tack fused, firing as for twice the thickest part plus one additional layer is advisable to slow the ramp rates, allowing all the glass to heat and form at the same rate.

 

 

*Low Temperature Kilnforming; an Evidence-Based Approach to Scheduling.  Available from:

Bullseye

and

Etsy

Heat Up Soaks

Photo credit: Bullseye Glass Co.

It is often advocated that there should be a soak at the strain point to even out the temperature throughout the glass.

My question continues to be why? 

The glass has survived whatever rate has been used up to that point during its brittle phase.  So, it already has every chance of surviving a rapid rate during the plastic phase.

Instead of a soak at the strain point, Bob Leatherbarrow indicates a soak during the brittle phase will be more successful in avoiding heat up breaks.  He has observed that heat up breaks are most likely to happen around 260ºC/500ºF.  Therefore, a soak in that region is most likely to be of use in evenly distributing the heat effectively through the glass rather than at a higher temperature.  He recommends up to a half hour soak there before proceeding at the same rate to the strain point (about 540ºC/1004ºF).  The ramp rate to this heat up soak in the brittle phase should be related to the thickness of the glass and the intended profile.

The thickness to be fired for is determined by the profile.  Rates for full and contour fusing can be as for the thickness before firing.  Rounded tack fuse needs to be fired as though twice as thick, and sharp tack or laminated fuse need to be fired as though 2.5 times.  More information on initial ramp rates to the strain point can be found in Low Temperature Kilnforming available from Bullseye and from Etsy

Bowl Split Analysis

The visual evidence relating to this enquiry is a sharp-edged break through the middle of the slumped piece. The two parts have slumped separately, and seem attached at the rim, leaving the middle opened.  A moderate slumping temperature was used to fire the piece at the bottom of a stacked kiln load.

This is used as an example of the kind of thinking required when investigating breaks in slumping.

The split occurred before the slump was complete. We know this because the pieces no longer fit exactly together.  This means the crack opened as the slump continued.  There is other evidence.

The opening of the crack cannot have happened at or after the annealing. It would have already formed to the mould in a whole state. It would break completely across, because it would be in a brittle state.  And the pieces would fit exactly together.  But they do not.

This piece was at the bottom of a stack of shelves in a deep kiln.  At the bottom, there is no radiant heat, only side heat.  This could be a major cause the kind of break described.

It is possible that the split was not across the whole piece.  At the bottom of the kiln, the glass is not receiving any radiant heat from the top.  It is getting radiant heat only from the sides of the kiln.  That means the edges were considerably hotter than the centre.  The edge may be in a plastic state while the centre is still in the brittle state.  The contrasts in expansions are often great enough to break a piece.

From the evidence we have, it can only be said the ramp rate was too fast for the conditions. 

This little exercise shows that a lot of information about layup, schedule, place in the kiln, and any other relevant variation on the usual, must be detailed when asking why something has not turned out as expected. 

 

 

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

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

Go-to Schedules

 It’s a schedule I always use.

This is a frequent statement in response to a firing that has gone wrong.

You don't always fuse the same thing, or the same design, or the same thickness, etc. So why always use the same schedule?

The schedule for the firing each piece needs to be assessed individually. It may be similar to previous firings. But it may have differences. Assess what those differences mean for the firing.  Some factors to consider.

Addition of another layer to a stack in tack fusing makes a difference to the firing requirements. Even if it is only on part of the piece. It needs to have a slower ramp rate and a longer anneal soak and slower cooling.

A different design will make a difference in firing requirements too. For example, if you are adding a design to the edges of the glass, you will need different bubble squeeze schedules than when you do not have a border. It will need to be slower and longer than usual.

The placement of the piece in the kiln may require a re-think of the schedule too. If the piece is near the edge of the kiln shelf, or in a cool part of the kiln while others are more central, the same schedule is unlikely to work. You need to slow the schedule to account for the different heat work each piece will receive during the firing.

If you have introduced a strong contrast of colour or mixed transparent and opalescent glass in a different way, you may need slower heat ups and longer cools.

These are some examples of why the same schedule does not work all the time. It works for pieces that are the same. But it does not work for pieces that are different. And we should not expect it to.

There are sources to help in developing appropriate schedules. Bob Leatherbarrow’s book FiringSchedules for Kilnformed Glass is an excellent one.

Another one is especially good for lower temperature work: Low Temperature Kilnforming, anEvidence-Based Approach to Scheduling. Be aware that I have a vested interest here – I wrote it.

 

 

Mending a crack

 I had a piece crack due to an annealing oops. I put powder on it and put it back in at a higher temp with a much longer anneal time. It looks great on the front, but I can still see where the crack was on the back. Is it supposed to be like that? I didn't think to put powder on that side.

If you think about why you get crisp lines at the bottom of a strip construction and a more fluid appearance on the top, you will be near the answer of why a repair looks ok on top but shows the crack on the bottom. The temperature on the bottom of the glass is less than on the top at the working temperature. And less again than the air temperature which we measure. This means that the bottom part of the glass has less chance to fully recombine. This, combined with the resistance to movement of the glass along the shelf, results in evidence of the crack being maintained.

Credit: Clearwater Glass Studio

There are some things that can be done to minimise the evidence of the crack. Make sure you know why your piece cracked before you try to mend it. An annealing crack will need different treatment than a thermal shock crack or a compatibility crack. Simply refiring the piece may only make the problem worse.

One approach is to place a sheet underneath. Make sure the broken glass is well cleaned and firmly pushed together. Dams may be useful to keep the glass compressed together. Glass expands both horizontally and vertically during the fusing process. Confining the glass will transfer most of the expansion in a vertical direction. This additional (small) vertical movement may help in forming the glass seamlessly. The broken glass now being supported by an unbroken sheet will enable the movement required to “heal” the crack.

If you do not want to change the surface, you can fire upside down. To do this you need to have a loose bed of powdered kiln wash, or whiting (a form of chalk) that is thick enough to press the textured side fully into the separator. Make sure the glass is pressed together without any separator getting into the crack. One way to ensure the crack does not open is to use a small amount of cyanoacrylate (super) glue which will burn away during the firing.  Put a sheet of clear glass over and fire. Thoroughly clean the face after this repair firing. The ultimate top needs to be fire polished to remove the evidence of the crack, and if it has picked up any marks from the powder.

You could, of course, fire upside down in this way but without the additional sheet, to avoid making the piece any thicker. This may or may not work well. If the base layer is one layer thick, it may pull in at the sides and pull apart at the crack where it is one layer thick.  It is also possible that bubbles will develop in the thin parts of tack glass because of the uneven thicknesses.

A final note. Placing powder on the back will not improve things. The powder will not fully incorporate with the glass and so leave a rough surface without concealing the crack.

Avoiding breaks

To repair or not

The process of repairing