Never refuse to refuse?

“Never refuse to refuse” is a statement often seen on social media. 

I object.

If the saying were changed to “never refuse to re-use” maybe I would agree under certain circumstances.

Before you even begin to think about using the broken or disappointing glass, you need to determine what went wrong.  The difficulty may prevent you using it in certain ways, or even at all. 

You need to determine if the break is due to incompatibility.  If it is, you cannot refuse or reuse it in any way.  It will continue to break anything you combine it with.  It must be junked. This means you need to have a way to diagnose the cause to the stress that lead to the break.

If you are certain the break is from thermal shock or inadequate annealing, it is possible to combine the pieces with other glass.

It is essential to determine why the problem occurred to know whether you can re-fuse.  You also need to know, or discover, how to prevent the break for the future. Once the cause of the difficulty has been determined, it may be possible to fuse again, but consider what the appearance will be.  The nature of the difficulty will give you clues to re-usability.

A repaired piece most often shows it is repaired. To try to appropriate the Japanese art of repairing the revered but broken object, just does not work for a broken new piece.  It was a new piece, with no history of use or display.

The phrase “never refuse to refuse” – while catchy – is extremely misleading and can lead to a lot of difficulty. Learn the lessons and move on to make a whole new and sound piece, rather than a repaired piece. 

Tack Fusing Difficulties

Many novice kilnformers tend toward the use of tack rather than full fusing in their work.  This is a bit perplexing, as tack fusing is more difficult than full fusing to complete successfully.

Why is tack fusing more difficult? 

The single most important reason is that the pieces of glass on top of the base shade the heat from the area underneath. And they do that unevenly over the base glass. Additionally, the tacked pieces are not fully incorporated into the base and so tend to behave as separate pieces, especially on angular tack fusing.  Both these factors require greater thought and care in scheduling.

Evidence

The evidence for the statement that tack fusing is more difficult than full comes from several areas.

There is a lot of evidence on social media of failed tack fused projects.  It may be argued that it is natural for the difficulties to be highlighted on the self-help groups. And the successes are not so widely shared.  There are other pieces of evidence.

Breaks of base sheet while the overlaying pieces remain intact.  

 This is a result of the overlaying glass shading the heat from the lower layers.  Some writers describe the effect as glass “seeing” heat.  The glass reacts more quickly to radiant heat than to transmitted heat from the air.  As a result, the glass exposed to the radiant heat absorbs heat more easily than the shaded areas.  This leads to uneven heating during the rise in temperature and a build-up of stress which frequently causes breaks from expansion differences in the base glass.

Breaks along the borders of the thick and thin areas of pieces are common in tack fusing.  

 This usually occurs during the cooling.  Thick and thin areas take different amounts of time to release the stored heat.  As in heat up, if the temperature differential is too great, the glass will break.  Research by Bullseye has shown that significant stress can be built up by temperature differences greater than 5°C across the piece.  What temperature difference is required to develop enough stress to cause a piece to break is unknown, although it does relate to the degree of variation in thicknesses and areas of base covered.

Scheduling as for thicker pieces.

 Further evidence is given by several sources stating that tack fusing projects need to be scheduled as though between 1.5 and 2.5 times the actual thickness to be successful.  This need for more careful firing is supported by the success of this strategy which increases the heat work as applied to tack fusing.

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

Tack fusing requires more care than flat fusing because of heat shading and thickness differences.  There are some scheduling approaches that can minimise the risks of breakage.

Fusing slumped pieces together

I have a plate I made using this mold. It’s 6”x6”. … [it is broken into] 8 large pieces. Is it possible to piece it together into the mold and full fuse the plate again in the mold? … Or do I need to try to piece it together on the shelf paper and full fuse and hope for the best?

Full fusing in the mould is unlikely to be satisfactory.  The glass at full fuse will move toward the bottom of the mould, making a thick puddle. Alternatively, it will form a large thick bubble at the bottom, as I see no vent holes in the corners at the base of the mould.  It will also have significant marking from dragging along the mould and from the mould texture.  It also presents some risks to shorten the life of the mould.

Fusing a dropped and broken piece that has been slumped is unlikely to be successful, whether fused in the mould or fused flat first.

Mould repairs with ciment fondue

Ciment Fondue

Ciment fondue was a French discovery and so the French name has become common in Europe. The name ciment fondu is used for the formal name Calcium aluminate cement which is also called high alumina cement and aluminous cement.  It is composed mainly of Aluminium oxide (alumina) and calcium oxide (quicklime) with varying amounts of ferric oxide. The Aluminium Oxide varies from 40% to 80% for various applications. The calcium oxide content varies from 40% to 20% and the ferric oxide varies from 16% to none for refractory applications.  For kilnformers, the general purpose composition of 40% aluminium oxide, 40% calcium oxide and 16% ferric oxide is sufficient (the rest is made up of minor amounts of incidental minerals and metals).

It is costly in relation to Portland cement and is used mainly where quick curing strength is required and at low temperatures; in refractory concretes where strength at high temperatures is needed; and in sewer piping and other applications to provide protection against biological attack of the concrete.

It is also used in sculptural applications, both as the casting material, and as a strengthening element in a non-metallic structure.

It is mixed with water to form a paste.  The proportions are not required to be exact, as the ciment fondu separates out of the water due to its weight and very low water absorption.  Slightly different methods are needed to repair breaks, and to fill divots in the surface.

Breaks

To repair breaks or cracks in ceramic moulds the ciment fondu needs to be used on its own.  Mix the dry particles with water until a stiff slurry is formed.  Thoroughly wet the edges of the broken pieces or the cracked area.  Then apply the ciment fondu slurry to both edges.  Press the pieces together and bind them if they would otherwise separate.  This can be with elastic bands or tape or any material that will withstand moisture.

The internal surface must have all the ciment fondue cleaned from it.  It cures so hard that it is not practical to sand it smooth without damaging the ceramic surface.  This clean up can be with a lot of water and paper towels. Any tools you use need to be immediately cleaned with water.  Do not dispose of this clean up water down your drains. It will harden and narrow your drains, potentially blocking them so firmly that whole sections of the drain will need to be replaced.

When fixed together put the mould in plastic or other waterproof material for at least 24 hours to give a wet cure.  The ciment fondu is not completely cured until it is given a heat cure.  This should be above the expected operating temperature.  Although I have never fired any of my ceramic moulds above 680°C, I fire my repairs to 800°C.  The firing is smelly, so ventilate the kiln and room well.  Try to do the heat curing when the smell will not disturb you or your neighbours.

Divots

This mould had glass stuck to it and was damaged in removing the glass.

If there are scratches or divots in the mould surface, you need to add some material that will absorb water into the ciment fondue mix.  Cured ciment fondue rejects water and so does not get as well coated as the rest of the mould when kiln wash is applied.  

To prevent this rejection of water, I add finely ground vermiculite to the mix.  I use 3 parts or less vermiculite to 1 part ciment fondue (measured by volume).  This provides a firm surface that absorbs some water. Although the absorption of moisture is not as good as the ceramic, it is sufficient to get the kiln wash coverage required.

Once the mix is prepared, you need to thoroughly wet the area to be fixed. This prevents the ceramic absorbing the water from the ciment fondue too quickly. Apply the ciment fondue mix with whatever tools seem appropriate.  

You must smooth the applied mixture before it dries, as it is so hard when cured that it is not possible to sand it smooth without damaging the ceramic surrounding the repair.  Smoothing can be done with significant amounts of water and a smoothing tool such as a ceramicist’s kidney or a palette knife. 

Once smoothed to achieve the surface required, pour off the excess water.  Enclose the mould in a plastic bag for 24 hours for a wet cure. Once out of the bag and dry you can further smooth with very fine sandpaper.

Then fire to 700°C to 800°C to complete the cure.  When cool it is ready to kiln wash.  If you warm the mould to around 100°C, the kiln wash will adhere to the repaired areas a little better than the cold mould.  Once the first kiln washing of the mould is complete, further applications of kiln wash will be easier. Of course, if you use boron nitride to coat the mould, there will be no difficulty with the repaired areas.

All tools need to be cleaned immediately of the ciment fondue and the cleaning water disposed of on the garden or waste ground.  It should never be put down domestic or public drains.  It does no harm to the soil or plants, but it will certainly harm your plumbing.

Incompatibility or Annealing Stress?

It is sometimes difficult to determine what the cause of any cracks might be.  There are a variety of possibilities with pot melts and other high temperature processes.

Surface of slumped melt

Cracks only on the top of a piece indicate a stress problem. Yes, there may have been a shift in compatibility, due to long soaks at high temperature. It would be a small shift though, or the cracks would have progressed to be more obvious.

Possibilities of healing the cracks relate to the kind of stress. If the stress is from incompatibilities, there is no means of healing the cracks.  Further firing may worsen the problem. 

If the stress cracks are due to the annealing being inadequate, a very slow rise in temperature to about 40°C above the annealing point before going to a full fuse is required. To heal the crack, you will then need to go to full fuse temperature.  This may require dams to reduce the expansion of the piece, if that is critical. Then follow with an annealing that has a longer soak and slower anneal cool than previously used.

Slumping will not help. Yes, the compression may bring the open cracks together, but temperatures are not high enough to heal (if possible) any cracks or imperfections. 

The pattern of splits on the bottom of the slumped piece

In this case splits developed on the bottom during the slumping. The splits on the bottom - if not due to incompatibilities - are usually due to a too rapid rate of advance in temperature in the early stage of the heat up. 

If it is thought that the cracks occurred as a mistaken combination of, say Bullseye and Oceanside, the stress would have been great enough to break the piece completely.  There is too great a mismatch of these two glasses to co-exist in one piece.  Of course, if only one or a few pieces were mixed in, this kind of small crack could occur, but it will normally be around a particular colour.

It is possible that different manufacturers’ glasses were used in this piece. The differences in compatibility can produce mild stress within a piece that do not break immediately.  In high temperature process like this, the incompatibilities will be exaggerated more than in thinner pieces fired at lower temperatures.

More detailed information is available in the e-book: Low Temperature Kilnforming.

Tears in Slump

An enquiry was received about a tear in the slump of an intended bowl. It was reported that it was tack fused, giving a variation in thickness between 6mm and 9mm. The blank was tack fused with the following schedule: 

139°C to 560°C  30

222°C to  621°C  30

139°C to 786°C  15

9999  to  515°C  120

60°C   to 427°C  10

115°C to  350°C  10

This is a bit odd, as it had a slow rate of advance to top temperature and no bubble squeeze. But it should not have been a problem for this piece.

It was then slumped. When the kiln was opened after the completion of the slump, this split was revealed in the centre of the bowl, rather than a complete break.

The schedule was again a bit odd and high:

83°C to  148°C  15

167°C to  590°C  10

83°C   to 720°C  10

222°C  to 410°C  120

83°C    to 427°C  10

The very slow start would protect against any heat shock, but the schedule doubled the rate of advance while the glass was still in the brittle phase.  This possibly induced some stress into the glass.  There is no reason why the rate of 167°C per hour could not have been continued to about 630°C to 640°C.  There is no need to slow the rate of advance once the glass is out of the brittle phase (ca. 540C).  With this slow rate of advance, the bowl may have been slumped into this simple ball mould at 620°C or less.  

This indicates that observation is needed when trying new layups and moulds to find the appropriate temperature.  It reveals at what stage any problem occurs.

There are several possibilities to consider in diagnosing the failure of this piece.

Rate of advance 
It might have been just too quick for the thickness of the piece. The piece is reported to have varied from 6mm to 9mm. From the picture this might have meant a single layer base, or more likely, a two-layer base and then the two-layer top pieces, making it 6mm to 12mm.  The firing schedule would need be as for an 18mm piece in the first instance, and 24mm in the second.

The initial rate of advance is more than slow enough for the calculated size of the thickest. The doubling of the rate of advance in the early stage of the brittle phase of the glass is more problematic. It may have had the effect of inducing a weakness (stress) that became apparent in the next segment with a slower rise to the top temperature. It probably heated the top much more than the bottom during this second segment, leading to the tear.

Was it adequately annealed?  
If you look at the fusing schedule, the anneal soak and anneal cools were adequate for a 12mm piece, so if compatible glass was used, there should be no annealing stress in the piece.  It is important to consider this, as a stressed piece can often break during an otherwise adequate slump schedule.

The placing of the piece may have had a significant effect on the outcome.  It is placed in the corner of the kiln.  A single element shows around the side of the kiln.  This would not be enough to heat the whole kiln, so we must assume it is mainly top fired. If the kiln has significant differences in temperature (only as much as 5°C), the location in the corner may have had enough of an effect to cause the stress.

When did the split occur? 
It is possible that the split occurred on the way up and then re-attached during the slow rise to the high working (lamination) temperatures.  This would give the appearance of an incomplete split, commonly called a tear.  We don’t have any information about the state of the edges. At 720°C an early split could have re-attached. The reason for considering this possibility, is the change of curve on the left end of the split. However, this does not seem likely as the split clearly shows on the edge of one of the squares, but does not go all the way across.  Lamination temperatures are not high enough to seamlessly heal a crack.

It is most likely the split occurred during the rise in temperature.  The reason for speculating this, is because of the distortions of the squares.  If the split occurred before any significant slumping the distoritions in the squares would be explained.

Range of Considerations

This discussion shows that there is much more than just the schedule to consider in diagnosing failures. 

Yes, the schedules are odd, but not impossible to get a good result with.  The fuse and slump schedules vary to a large extent which is not ideal.  

The fuse schedule to the bubble squeeze temperature is sensible, but the slow rate of advance is continued to the top temperature, which is not usual.  This means there was no consideration of a bubble squeeze soak, although this did not prove to be a problem. The annealing is suitable for up to 12mm.

The slump schedule starts very slowly and then doubles early in the schedule.  This rapid change of rate early in the brittle phase of the glass may have induced stress that could not be relieved later in the firing. A simple single rate of 167°C to the slumping temperature would have been adequate. 

The slump temperature used is in the lamination/sharp tack range, rather than the more usual 620°C to 677°C range.  This can bring several problems with excessive marking of the bottom of the piece, shrinkage, and even uprisings at the bottom of the piece.

Are any of these criticisms of the schedules adequate to point to as causes of the break? Without handling the piece, it is difficult to tell with the information available.

General Diagnosis

You need to consider the state of the break in diagnosis.  We do not know whether the edges were sharp or slightly rounded. It is clear the split piece fits the mould as it is, so the break probably occurred on the way up in temperature.

Was the high slumping temperature an element in the break? The third rate of advance was slow, so there should be no further stress introduced in the firing.  The annealing of the fused piece seems adequate, so a stress fracture does not seem likely.

The high slumping temperature may be disguising a problem, as the glass may have re-attached at the upper temperature.  But this does not seem likely as the picture seems to show the crack enters the red piece a short distance, but not all the way through.

Was the placing of the mould problematic? It is in a corner where there may be enough difference in temperature across the glass to induce this kind of tear.

These considerations show that multiple images of a problem piece need to be taken.  Various angles are required to eliminate problems with reflections.  Pictures of the whole setup in the kiln and the piece are needed.  Multiple pictures of the top and bottom are needed. And of course, close ups of the area(s) concerned.  These are all substitutes for handling the piece itself.  

If you have a community of kilnformers or a store you can take the piece to, you are likely to be able to give responses to questions and to get the information required about the possible problems and solutions.

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

Analysis of Breaks during Fire Polishing

The analysis of breaks in fire polishing can be difficult.  The temperature and heat work are minimal, so the edges can look sharp, which would indicate that the break occurred on the cool down.

But this is where you really need to feel the edges.  If they feel very sharp, then you can be more confident that the break occurred on the cool.  But if there is even the slightest smoothness to the edge as you feel it, the break probably occurred on the heat up to fire polish.

In this picture, there appears to be an annealing break, because of the hooked ends of the break.  That is typical of a break due to inadequate annealing.  It is important to know when the break occurred, so that appropriate remedial action can be taken for future firings of similar pieces.

To determine if the annealing break occurred because the initial anneal was inadequate, it is important to do a touch test. Just looking at it will not be enough.

If the edges were even slightly smoothed, the anneal break occurred on the way up.  This would mean that the anneal of the original blank was not adequate, assuming a reasonable rate of advance was used for the thickness of the piece.

If the edges are razor sharp, the break occurred on the way down, indicating that the anneal after the fire polish was not adequate.  This would mean that in future the annealing needs to be done more carefully on fire polished pieces.

Being too quick to apply a diagnosis of a break during a fire polish can lead to the wrong conclusion, and so the incorrect alteration of future schedules.

Low temperature breaks in flat pieces

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 low temperature processes need to be considered differently to those occurring during fusing.  

The advice for diagnosing breaks 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 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.

I exclude low temperature tack fuses from the general description of when breaks occur in flat pieces as it is not applicable at low temperatures.  

Low temperature flat work includes sintering, laminating, sharp profile tack fusing, etc.  There are lots of other names used for this "fuse to stick" work.  In all these cases, the finished glass edge will be barely different than when placed in the kiln.  It stands to reason therefore that you cannot know when the break occurred, as the edge will be sharp whether it broke on the way up or the way down.  

Periodic observation during the firing is the only way to be sure when the break occurred. These observations should coincide with the move from the brittle to the plastic stage of the glass.  Therefore, about 540C.  It can be at a bit lower temperature, but not a lot.  If the glass was not broken by that time, you can be fairly certain it broke on the way down.

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

Diagnosis of Fractures

Knowing what has happened to your piece when it is broken or cracked is important to developing your skills as a kilnformer.  Most of the knowledge about diagnosis comes from looking carefully at the cracks and the shapes apparent in the flawed piece.

Breaks in the Kiln

Breaks in fusing at tack or full fusing levels in the kiln are generally of four kinds.

Breaks with hooked ends

Breaks that go across the whole piece, with a hook or significant curve at each end, usually indicate an annealing problem. The slight hook seems to result from inadequate annealing. The break will have sharp edges as it occurs as the glass is entering the brittle stage.

Multiple breaks in a crazed pattern

Crazed glass – similar to the cracks in ceramic glazes - usually indicates the glass has stuck to the supporting materials. These materials can be shelves or moulds. It is a sign there was not enough separator present between the two surfaces.

Breaks following the edge of glass pieces

Breaks that skirt around colours or pieces of glass almost always indicate a compatibility problem with the glass pieces chosen.  In severe cases the crack will be all around the incompatible pieces of glass as though it is trying to escape the base layer.  Sometimes the break will be from side to side, but skirting the incompatible glass.  These breaks will have sharp edges as the compatibility problem only becomes apparent on the cool.

Breaks from side to side following the line of glass pieces is not an infallible indicator of incompatibility, though.  Glass which has varying levels or thicknesses can break alongside the thicker areas, even though the glass is compatible. Often the break will be rounded due to temperature differentials in the glass on the heat up.  As the glass continues to get hotter, glass pieces on top - or strongly contrasting colours - can heat as such different rates that the stress overcomes the strength of the glass.

Of course, this kind of break can be sharp because the break occurred during cooling.  In effect, this appears to be an annealing problem when it really is a problem in matching the scheduling with the annealing requirements of a complex piece.  You need much longer soaks and slower cooling on tack fused pieces than on flat fused ones.

These two contrasting causes of a break means that you need to think about how the glass is layered.  One is to do with compatibility and the other to inadequate annealing due to the complexities of the layup.  They also tie up with the fourth cause of breaks.

Breaks can also follow the edges of inclusions.  This of course, indicates incompatibility.  All metals are incompatible, but if thin and not excessively large in relation to the piece, the glass is strong enough to contain the stress.  When the metal or other inclusion is too large, strong, or thick, the glass will break or show cracks around the inclusions.

Broken and separated lower layers

Sometimes people will open the kiln to find the lower layer of a multi-layer piece has broken and separated a small distance.  This is the fourth kind of break. This break will most often be a nearly straight break from edge to edge.  The broken edge will be rounded but the top layer(s) will have the expected profile.   This is an indication that the heat up was too fast not allowing the lower layer to achieve the same temperature as the top. 

This most often happens where there is an exposed lower layer (which gets hot) along with areas on top that get equally hot, but not the glass underneath.  Glass is a poor conductor of heat, so the upper layers "shade" the heat from the glass below.  The temperature difference between the two can be great enough to break the base glass apart but leave the top intact.  You know this was on the heat up because the layers of glass could move independently when the base broke and moved under the upper layers.  The glass was not hot enough to be sticky yet, so it had not reached lamination temperatures before the break.

Rounded vs. sharp edges

In addition to the location of the breaks, the condition of the edges is important in diagnosis of the cause of the problem. The accepted rule is that rounded edges mean the break occurred during the heat up.  Sharp edges occur during the cooling.  This is most often the case (but see the conditions for slumping). For flat pieces breaks that occur on the heat up will be rounded to some extent.  In a full fuse, usually the edges of the break will be rounded similar to the outside edge.


Cracks on the bottom surface

Sometimes the broken pieces will recombine either partially or all along the line.  There may even be a full recombination leaving only a crack like appearance on the bottom.  This recombination also will be the case where there was where only a partial break or crack in the early stages of firing. It leaves a smooth top surface, but a visible crack on the bottom. That means there is only a marginal reduction required in the scheduling of the initial rate of advance, as the temperature differentials were not great enough to break the piece completely across.

Force of Breaks

The 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 a lower amount of stress. The amount of space indicates the degree of change required in scheduling. A small parting of the glass requires only a little (maybe 10% - 15%) reduction in the rate of advance.  Large spaces indicate that much slower rates of advance are required, and possibly a complete rethink in the scheduling of the firing.

Slumping breaks

Breaks in slumps are usually caused by a too rapid rate of advance. But this is not always the case.  The usual check of a sharp or rounded edge to tell when the break occurred does not work well at slumping temperatures.  The edge will be sharp whether it occurred on the heat up or the cool down because the temperature is not high enough to significantly round the edges.  The test must be different on slumps than that of sharp edges.  The test is related to the shape of the pieces. Take the pieces out of the mould.  If you can fit them together exactly, the break occurred on the cool down.  This usually will mean the anneal soak was too short and the anneal cool too fast.

Most slumping breaks occur on the advance in temperature.  The means of determining when the break occurred can be tested by putting the broken pieces together.  If they do not match exactly, the break occurred during the heat up.  This is based on the observation that broken pieces separated slightly in the mould by the force of the break on the heat up, and so will slump in the mould in slightly different ways from each other due to their positions.

Remember the blank for slumping is thicker than the original un-fused pieces.  This thickness requires a slower heat up than the original blank consisting of separate pieces.  In addition, the glass is supported at the edges of the mould which can allow the central area of the glass to heat faster than the edges, so further slowing the rate of advance is required.  These two factors of thickness and supports explain most of the breaks during slumping.

Splits in slumps

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. This is similar to the cracks on the bottom of a flat piece described above. It indicates the rate of advance was too - but only just - too fast.  The rate of advance 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 plastic.  The force of the weight of the upper portion of the glass can be enough to cause the glass to separate because it is brittle, rather than move as the surface does. This split on the bottom but not the top indicates a slightly slower rate of advance for the thickness of the glass is required.

Breaks out of the Kiln

Breaks after the piece is cool

Breaks that occur days, weeks, months after a piece is cool can be impact damage, annealing or compatibility problems. 

Impacts

Impact breaks will be obvious in handling or moving other pieces near to the affected piece.  Usually there is evidence of impact by a small chip removed from the glass at the origin. The piece may or may not have been stressed to allow an easy break rather than a chip.  It is not possible to be sure of the secondary cause after the primary impact damage has occurred.

Breaks in warm glass

If the break occurs shortly after having been removed from the warm kiln, it is probable that the thermal shock to the glass has a contributory factor to incompatibility or inadequate annealing.  The diagnosis of the cause is the same as for breaks in the kiln - hooked for annealing and straight or following colours or inclusions for compatibility.

Breaks in cold glass

If the glass has been sitting undisturbed in a shaded place and suddenly breaks, the reason can be there was an incompatibility or that the annealing was inadequate.  There usually is not much difference in the breaks in a piece that has been cold for a long time.  If the break distinctly follows colours or pieces of glass, that would indicate a compatibility problem.  If the break crosses colours and thicknesses it is more likely to be an annealing issue.  But, as you can see, there is no certainty in this distinction as to the causes of breaks a considerable time after removing from the kiln.

Glass in strong light

Glass placed in strong sunlight that breaks can be incompatibility or simply contrasting colours being heated unevenly by the sunlight.  It is difficult to tell with certainty whether it is compatibility, annealing, or heat differentials that have caused the breakage.

Problem Solving

The essential purpose of problem solving is to prevent the same thing happening again. To solve the breakage problem, you need to think about the interrelationships between the various parameters – firing rates, soaks, cooling rates; and the ways in which the glass was set up.

Rounded edges

If the break is shown to be in the early stages of the firing, they most generally are caused by thermal shock.  They will generally be straight on an evenly thick piece.  If the piece is with variations in thicknesses, the line of the break may follow the thicker pieces. In both cases, you need to think about the rates of advance you are using.  If the separation of the edges is small enough that they have begun to recombine later in the firing, the rate of advance was only a little too fast.  If there is considerable space – say more than a finger width – the rate of advance was significantly too fast.

Sometimes the condition of the upper glass can give an indication of when in the firing the break occurred.  On a first firing, if the upper piece has broken together with the lower one, the break occurred after the pieces became sticky. This would mean the break occurred at or higher than laminating temperatures.  This is rare during the heat up.

If the break has moved small top pieces, it indicates the break occurred early in the heat up.  Sometimes the break will occur under the top piece.  Later it slumps and fuses into the space created by the break.  This also indicates a break early in the firing.  All these conditions indicate that the initial rate of advance needs to be slowed to avoid the thermal shock.  It does not indicate that soaks should be added at various stages up to the softening point of the glass.  Glass generally behaves better with steady, gradual inputs of heat rather than quick rises with soaks (although there are exceptions).

Sharp edged breaks

These occur generally on the cool down or after the piece is out of the kiln for a while.  If the break has occurred in the kiln, you should look at it carefully before moving it.  The relative location of the pieces can tell you some things about why.

Crazed glass normally indicates the glass has stuck to the supporting material – shelf, moulds, or other rigid materials.  This crazing may all still be in one piece, or slightly separated, sharp edged chunks.  These effects indicate there was not enough, or appropriate, separator for the process used.

The distinction between annealing and compatibility breaks is given above. 

Breaks all around a piece or pieces – looking as though they were trying to escape the base - clearly indicate an incompatibility problem.  You need to identify that glass and separate your stock of it from the rest of your fusing glass. 

Cracks that skirt pieces of glass can be incompatibility.  This is easiest to determine on flat pieces which have been full fused, or nearly so.  There is not a variation in thickness to complicate matters.  In full fusing, if the break skirts around a piece or pieces of glass along its path, it is likely caused by incompatibility between pieces and their base.

Breaks skirting pieces can also indicate problems with thickness, especially in tack fusing.  The more angular the tack fusing is, or the greater the difference in thickness, the greater the potential for an annealing break.  The annealing soak for tack fusing needs to be significantly longer than for a flat fused piece of even thickness.  Recommendations vary, but the anneal soak time needs to be at least twice the thickest part.  The anneal cool rate also needs to be half that for the the thickest area.

Breaks or cracks across the piece with hooked ends indicate inadequate annealing.  This will require some consideration to come to the appropriate length of soak and rate of the anneal cooling.  The anneal soak is about getting all the glass to the same temperature - top to bottom, side to side.  The soak is about temperature equalisation not just annealing.   This is shown by the Bullseye research on annealing thick slabs.  They discovered that a longer soak at a lower temperature can provide as good a base for the anneal cool as a higher temperature. The differences are that the soak at the annealing point can be shorter, but the annealing cool is much longer.

Annealing continues below the anneal soak - whether you chose the annealing point or a temperature below.  Bullseye uses a temperature about 30C below the annealing point.  This can apply to any glass.  Because the glass is cooler, a longer temperature equalisation soak is needed. But the anneal cooling range is shorter, making for a reduction in cooling time for thick slabs.

The point of this discussion is that when considering the solution to annealing breaks, you need to have a relation between the temperature equalisation soak and the rate of the anneal cooling.  If you have decided you need a longer soak, then you also need to reduce the rate of the anneal cool.  If you do not, you will still have annealing breaks or even thermal shock breaks, even with long soaks at or below the annealing point.

Breaks of slumped pieces

Breaks in slumping almost always appear to be sharp edged, unless you look carefully at the edge.  Fitting the pieces back together will give an indication of when the break happened.  If they fit, the break occurred upon cooling.  The anneal may have been inadequate, or the cooling too fast.  Unfortunately, in a formed piece, the curved hook of an inadequately annealed piece does not often show up.

If the break occurred early in the firing, the piece may still have sharp edges, unless you were firing at the upper end of the slumping range.  Here again the test of trying to put all the pieces back together is important.  If the pieces do not fit exactly together, the break occurred during the heat up.  This will mean that you need to slow the rate of advance for subsequent pieces.

“It hasn’t happened before” Scenario

Often people experience breaks even though the set up was very similar and the schedule was the same over several pieces.  There are two responses to this – “what did you change for the firing of this piece that broke”, 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 very similar, and the schedule was the same, something has changed.

The first thing to do is to test for stress. This means test before the piece is broken, as once the piece has broken most, if not all, the stress has been relieved.  You will need to construct another piece in the same way as the successful or the broken one – whichever you prefer.  Test the flat fired piece for stress. Remember to include an annealing test, so you can determine if the stress is compatibility or annealing related.  If there is stress in the flat piece, but not in the annealing test, you need to consider whether all the glass is compatible, or you need to slow the annealing cool for the larger test piece.

Next you need to 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.

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

Conclusion

Although breaks generally have only three causes – thermal shock, incompatibility and inadequate annealing – the diagnosis of which it is and how it was promoted is complex.  All three are forms of stress.  To problem solve, first attempt to determine the type of stress that induced the break.  Then attempt to determine the cause of that stress.

It is important in the early stages of a new kind of piece, or early in your fusing career to test for stress after each firing (although I fail in this often).  This will give you the information to progress to the next firing or to revise the conditions – glass or schedules – to remove the stress for this or subsequent pieces.