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. 

Firing different glass at the same time

Can I fire COE 90 and 96 in the same firing using the same schedule?

 In one sense this is the wrong question. The more general question is:

“Can I fire different glasses in the kiln at the same time?”

 Not all “CoE90” or “CoE 96” glass from different manufacturers have the same firing characteristics. This blog post compares the key temperatures for various glasses.

 For example, two glasses that are presumed to be “CoE90” have different published full fuse temperatures. Wissmach states its full fuse temperature is 777°C/1432°F and Bullseye states theirs is 804°C/1481°F. Both have an annealing temperature of 482°C/900°F.

 Wissmach 96 anneals at 482°C/900°F and Oceanside at 510°C/951°F. Wissmach96 has a full fuse temperature of 777°C/1432°F and Oceanside Compatible full fuses at 796°C/1466°F.

 Since annealing occurs over a range it is possible to anneal Wissmach96 and Oceanside Compatible together even though there is a published difference of annealing temperature of 28°C/50°F.  You could shotgun anneal – go very slowly from the soak at 510°C/951°F to 482°C/900°F with another soak.

 Fusing different manufacturers’ glasses – even if they are the same supposed CoE – is more difficult, unless you do not mind significantly different results.  Bullseye will full fuse at 804°C/1481°F. But Wissmach90 fuses at 777°C/1432°F. This will provide significantly different results.  The same for Oceanside and Wissmach96. The 21°C/38°F difference in full fuse temperature will provide lesser difference than the Bullseye Wissmach90, but will still be noticeable.

 This indicates that the full fuse effect of even the supposed same CoE will not be the same when fired together.

  

Finding the slumping temperature is determined less from the manufacturer than by observation. This post tells you how to find the slumping temperature.

 If these characteristics are similar, you can slump them at the same time. Expect some significant variation.

 There are some – possibly many – who will say firing different glass at the same time is both possible and successful. The manufacturers’ recommended temperatures show some wide variations. This makes it unlikely you will get the same desirable results for any but one of the glasses.

 My recommendation is do not try to fire different glass at the same time.  And why are you using two incompatible glasses anyway.

Digest of Principles for kiln forming

Some time ago people on a Facebook group were asked to give their top tips for kiln forming.  Looking through them showed a lot of detailed suggestions, but nothing which indicated that understanding the principles of fusing would be of high importance.  This digest is an attempt to remind people of the principles of kiln forming.

Understanding the principles and concepts of kilnforming assists with thinking about how to achieve your vision of the piece.  It helps with thinking about why failures have occurred.

Physical properties affecting kiln work

 Heat

Heat is not just temperature. It includes time and speed.

 Time

       The time it takes to get to working temperatures is important.  The length of soaks is significant in producing the desired results.

 Gravity

       Gravity affects all kiln work.  The glass will move toward the lowest points, requiring level surfaces, and works to form glass to moulds.

 Viscosity

       Viscosity works toward an equilibrium thickness of glass. It varies according to temperature.

 Expansion

       As with all materials, glass changes dimensions with the input of heat.  Different compositions of glass expand at different rates from one another, and with increases in temperature.

 Devitrification

       Glass is constantly tending toward crystallisation. Kiln working attempts to maintain the amorphous nature of the molecules.

 Glass Properties

·        Glass is mechanically strong,

·        it is hard, but partially elastic,

·        resistant to chemicals and corrosion,

·        it is resistant to thermal shock except within defined limits,

·        it absorbs and retains heat,

·        has well recognised optical properties, and

·        it is an electrical insulator. 

These properties can be used to our favour when kiln working, although they are often seen as limitations.



Concepts of Kiln Forming

 Heat work

       Heat work is a combination of temperature and the time taken to reach the temperature.

 Volume control

       The viscosity of glass at fusing temperatures tends to equalise the glass thickness at 6-7mm. 

 Compatibility

       Balancing the major forces of expansion and viscosity creates glass which will combine with colours in its range without significant stress in the cooled piece.

 Annealing

       Annealing is the process of relieving the stresses within the glass to maintain an amorphous solid which has the characteristics we associate with glass.

 Degree of forming

       The degree of forming is determined by viscosity, heat work and gravity.  These determine the common levels of sintering, tack, contour, and full fusing, as well as casting and melting.

 Separators

       Once glass reaches its softening point, it sticks to almost everything.  Separators between glass and supporting surfaces are required.

 Supporting materials

       These are of a wide variety and often called kiln furniture.  They include posts, dams, moulds, and other materials to shape the glass during kilnforming.

 Inclusions

       Inclusions are non-glass materials that can be encased within the glass without causing excessive stress.  They can be organic, metallic or mineral. They are most often successful when thin, soft or flexible.

A full description of these principles can be found in the publication Principles for Kilnforming

Achieving the Striking Colour

"Is there anything special I have to do to fire striker glass?  Can I mix striker and non-striker in the same kiln or piece?"

Strikers generally need a two-hour soak at slumping temperatures, about 660C.  This heat soak helps ensure full development of the colour. If the soak is not long enough, the colour may not achieve the target colour at all, or be paler than anticipated.

The rate of advance to the heat soak is not critical.  But it does need to be the appropriate rate for the thickness and nature of the assembly of glass being fired.

If you were to have too short a heat soak, you can fire again to help mature underdeveloped colours.  This will, of course, change the profile of the finished piece.

Strikers are compatible within their manufacturer’s own range. So, they can be combined in the same piece as any other of the glass in the fusing compatible range.  That means that they can be fired in the same kiln load as non strikers.

The two-hour soak at slumping temperature will not harm the later stages of firing, but it might lead to use of a slightly lower temperature tack fusing than without the long heat soak.  That is because of the heat work put into the glass at the lower temperature.   Only observation will tell you how much less temperature is required.  It may be possible that only a little less time at the forming temperature is required.  Again, only observation will tell you that.

Strikers require a heat soak to mature the final colour.  These striking glasses are compatible with the rest of the fusing range from a single manufacturer. Glass from different manufacturers must be tested for compatibility before combined into a project.

Including Incompatible Glass

The question on whether incompatible glass can be included in a piece gets a range of positive and negative responses.

The real answer, as indicated by the diversity of responses, depends on where you start, and what assumptions are being made.  However, responses such as "Less than 10% of area is ok" are not helpful because they take no account of the conditions.

Degree of compatibility

How incompatible are the two glasses?  The greater the difference, the less can be used. If you have two test pieces of glass that show a little stress upon viewing with a set of polarised filters, 

you can attempt to combine a greater area than if the test pieces show significant stress.

Mass

The relative mass of the two glasses are important.  Thin Bullseye confetti placed sparingly across an Oceanside glass of 6mm thickness and 300mm diameter will usually survive, although there will be some stress visible through polarising filters.  If you are placing a large or thick piece on the disc, you will have much more trouble.

Placing

The placing of the incompatible glass has an effect too.  The nearer the incompatible glass pieces are to the edge, the more likely a fracture is to develop.

Shapes

The fourth consideration is the shape of both the base and the added incompatible glass.  A circular base can contain more stress than a rectangular one.  An angular incompatible inclusion will show greater stress than a circular one.

With included incompatible glass you are asking the main piece of glass to contain the stresses.  The factors affecting the ability of the base glass to contain the stress are:

The degree of difference in stress between the pieces

the mass of glass applied to the base

the shapes of the base and the inclusions

where the incompatible glass is placed.

These all affect how well the main or base glass can contain the stress.  If the piece is at all important to you, do not include incompatible glass at all.  If it is really important, test all the glass you will be using.

Crazing

Crazing appears as the multiple cracks similar to what is seen on ceramic glazes.  These occur when there is a great deal of incompatibility between the glaze and the clay body.  This can also be seen in glass.

Crazing as seen on a ceramic object

I have see crazing of glass in two circumstances.  It happens with severe devitrification, to a maximum extent of crumbling under light pressure.  This usually happens with glass not formulated for fusing, and especially on opalescent glass.

The more common occurrence is where the glass has stuck to the supporting structure.  This is frequently the case where the separator has not been sufficient to keep the glass from sticking to the shelf.  This will happen on kiln washed shelves when the coating of the separator has not been even, leaving areas with bare or very thin areas.

The standard of mixing kiln wash in the ratio of 1:5 parts by volume of powder to water is important.  The application should be with a wide soft brush such as a hake brush.  The kiln wash should be painted on in four coats, one in each direction of up, down, and the two diagonals.  A well coated shelf should have an even appearance of the coating.  Only an even film of separator is required to keep the glass from sticking to the shelf, mould or other kiln furniture.

Containing Stress

People frequently report success in combining incompatible glass pieces with a larger, different base.

Questions arise.

Have the resulting pieces been tested for evidence of stress with polarised light filters?

Other destructive methods such as hot water, or placing in the freezer are not adequate measures of the long-term effects of incompatibility stress.  When you are doing something outside the accepted norms, then you must test for stress to be certain what you are producing remains sound before announcing success.

Why does glass with incompatible pieces survive?

Incompatible glass will show some stress when viewed through polarised filters. You will need to decide when it is excessive.  When viewed between polarised light filters high stress will be shown by a rainbow effect in the halo of light.  Lesser stress will be shown by pale light. The degree of stress will be shown by the amount of light.

Survivability

There are some circumstances where the glass can contain the stress, and others where it cannot.

Generally, large mass pieces can contain the stress from small incompatible pieces of glass. 

Spherical objects can contain a lot of stress over a long period, which is why glass blowers and lamp workers are generally less concerned about incompatibility than kilnformers are.

Flat glass pieces behave a little differently.

Circular forms can contain stress more easily than other shapes.  Rectangular  shapes generally show the most stress at the corners.  Narrow or wedge-shaped pieces have the most difficulty in containing stress.  The stress is concentrated at the points.

The placing of the incompatible glass is also important to the survivability of the glass.  The further from the edge of the piece, the less likely there will be breaks. 

The smaller the pieces of incompatible glass in relation to the whole, the less risk of breaking. 

The more spread apart the pieces are, the greater the chances of survival for a while or long term.

The most essential piece of equipment for people starting out and those who are investigating new setups or working at the edges of accepted norms is a pair of light polarising filters to test for stress.

When combining incompatible glasses the general case is that the greater the mass of the whole object in relation to the incompatible glass, the greater the chance of survival. 

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.

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.