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.

Strain Points and Annealing Ranges

I received the following question a while back and thought my response might be useful, although very informal.

“Can you dumb down the concepts of 'annealing point' and 'strain point'? I understand anneal point to be a fixed point (depending on the glass) but the strain point is a range...is this correct? I understand the concept of a hold at the anneal point but I'd like to understand how to bring it down through the strain point.”


I really dislike the idea of dumbing down concepts in kiln forming glass. Glass chemistry is incredibly complicated. Glass physics is still little understood. Glass is a very complicated subject. The marketing of glass for kiln forming has led us all to think it is a simple matter of recipes. Well it's not.

Having got that rant out of my system.... Let’s go ahead.

The annealing point is roughly defined as the temperature at which the glass (if it is the same temperature throughout) will relax most quickly. In the practical kiln forming that we do, it is not possible to ensure that the glass is that temperature throughout. So it is better to think of an annealing soak at the annealing point to allow the glass to become a more even temperature throughout its thickness. As thicker glass means the heat has further to travel from the centre to the surfaces, a longer soak is needed for thicker glass.

The annealing occurs during the slow cool past the lower strain point. The annealing occurs best with a slow, but steady drop in temperature. So annealing is occurring over a range, not at a point. We all rely on a combination of the manufacturers' recommendations, various writings we read, and experience to determine that rate, although Bullseye have published a chart which is most helpful, whichever glass you use.

Strain points.

There is an upper and lower strain point, although this is disputed by some. There are mathematical definitions for these as well as observational definitions. I do not understand the mathematics of either. In lay terms, the lower strain point is that temperature below which no further annealing can take place. It is safe to assume this is 50C below the annealing point (I think it actually is 43C, but I'm not certain of this number).

So it is safest to control the cooling to at least 5C below the lower strain point. Bullseye find that cooling from the annealing soak to 370C is best - this is much more conservative than is theoretically required – 146C below the annealing soak point. This does take care of any problem of thermal shocking of the glass during the cooling.

The upper strain point might be more properly described as a softening point. This also has scientific definitions. The way I think of it is as being the temperature above which no annealing can occur. Another is to think of it as a point beyond which the molecules of the glass are in relatively free motion - which increases with temperature. This again can be considered (on the rise) as 50C above annealing. However on the way down it is safer to consider it to be not more than 30C above annealing. This is because the glass temperature lags behind the air temperature (which is what our controllers measure).

So there is no point in soaking more than 30C above annealing in an attempt to equalise the temperature throughout the glass. However, if you really need to equalise temperature at some point above the annealing point, it might be better to slow the cooling from the working/top temperature and do the final equalisation of temperature at the annealing point.

To answer directly, the strain point by definition of language cannot be a range. There are two points which form the possible annealing range, although the lower one is the critical one. The upper one I described earlier as the softening point. The softening point forms the upper part and the strain point forms the lower part of a range in which the annealing can occur. So the concepts are the opposite of what you propose. They are points which are the boundaries of the annealing range.

To complicate things further, not all glass from one manufacturer has the same annealing point. The published annealing point is a compromise that their experiments and experience have shown to be most suitable. Bullseye glass for example has three annealing points, 532C for opals, 505C for cathedrals and 472C for gold bearing glasses. NOTE: these figures may not be exact; they come from memory rather than documents. Since this list of annealing points was published, Bullseye have conducted further experimentation that shows the best annealing soak occurs at 482C which is below transparent and opalescent, but above gold bearing annealing points.

Schott recommends a range for annealing, not a point, to accommodate these variables. Bullseye, Uroboros, and Spectrum have published annealing points that are practical for people kiln forming in smaller kilns that are less well controlled than the factory lehrs.
If you look at the Bullseye site - education section, you will find a lot of useful information. Especially informative are their tech notes. Spectrum - to a lesser extent than Bullseye - gives helpful information. The information from both sites should be absorbed and the principles applied to other glasses.

Finally, kiln forming is deceptively simple. I have spent 29 years discovering how much more there is to learn. This is one of the reasons that glass is such an exciting medium - people keep discovering new things.

Reviewing the above, I realise that I have not answered your question "... how to bring {the temperature} down through the [lower] strain point". My answer is that you should look at the manufacturer's site for each glass that you use. Look at their rates for annealing for different thicknesses of glass (some also take into account the size). Consider them. Then look at some of the other sites for their published annealing rates for various thicknesses. Comparison of their rates will reveal differences. Think about what they are, how they relate, and whether they reveal that they are using the same principles with slight variations.

Also, if you can, get a copy of Graham Stone's book "Firing schedules for glass, the kiln companion". It provides a handy guide to annealing rates. But DO NOT use it as a book of recipes. Read all the commentary about the schedules, as they (combined with the introductory parts) give principles and tips about how to think about the cooling of the glass.  Bob Leatherbarrow has recently published an excellent book on kilnforming schedules, available from his website.

By the way, experience is so often lost, or misremembered, that keeping a log is essential. My first log consisted of loose leaf binder, so I could file all the same kind of firings for various glasses together (this was in the days when there was not much fusing compatible glass, and I couldn't afford Bullseye at UK prices. I was discovering lots about glass firing and using some schedules that I now wonder how I had any success. I did learn a lot from my failures and recorded them too. Now I use a log, usually an out-of-date A4 size diary, sometimes a manuscript book that is big enough to record observations and illustrations. Bullseye have a good record form on their site.

I congratulate you on your desire to understand the processes. Too many only want to put the glass in and turn the kiln on. That is the desire a number of kiln manufacturers pander to when they put pre-programmed schedules on the controllers. So, don't take any of this as criticism of you or your comments. It is meant in a constructive manner - even though I am told frequently that the manner is blunt, even rude.

Best wishes on continued successful kiln forming.

Revised 22/06/19

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

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.

Slumping Breaks

“Why does my full fused disc break when I slump it?”

There are several possibilities. The two main ones are annealing and ramp speeds.

Inadequate annealing in the fusing stage can lead to a very fragile piece when being re-heated.  If there is significant residual stress in the fused piece, it is much more sensitive to heat changes during subsequent firings whether full, tack, or slumping/draping. It is important to thoroughly anneal any piece at every firing.  If you are firing a different layup or contrasting colours and styles, you should check for stress using polarising filters.  

The slump – or drape – firing needs to be much slower in temperature rise than the fuse firing.  You now have a thicker piece which takes longer to absorb the heat evenly. 

If your piece is tack fused, it needs an even more slow rate of advance.  Sometimes this needs to be as though the piece were two to four times the actual thickness of the piece.  The more angular and pointed the tack fused elements, the greater the reduction in firing speed.  This post gives guidance on how the piece is designed and its thickness affects the rates and soaks in tack fusing. 

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

Annealing vs toughening

The statement “annealing stained glass makes it stronger” appeared on the internet some time ago.  Of course, without annealing there is no glass, it would simply crumble.  Annealing is the process of allowing the glaseous state to be achieved.

I think the statement is more about the difference between annealed and toughened/tempered glass.  In summary, it relates to the amount of stress within the glass.  Well annealed glass has less stress than inadequately annealed glass and so is more stable.  Toughening is a process that balances stress and tension in the glass.

The processes are for different purposes and follow different processes. 

Annealing

Annealing of glass is a process of slowly cooling hot glass to relieve residual internal stresses introduced during manufacture. Annealing of glass is critical to its durability. Glass that has not been properly annealed retains thermal stresses caused by rapid cooling, which decreases the strength and reliability of the product. Inadequately annealed glass is likely to crack or shatter when subjected to relatively small temperature changes or to minor mechanical shock. It even may fail spontaneously from its internal stresses.

To anneal glass, it is necessary to soak it at its annealing temperature. This is determined mathematically as a viscosity of 10¹³ Poise (Poise is a measure of viscosity). For most soda lime glass, this annealing temperature is in the range of 450–540°C, and is the so-called annealing point or temperature equalisation point of the glass. At such a viscosity, the glass is too stiff for significant change of shape without breaking, but it is soft enough to relax internal strains by microscopic flow. The piece then heat-soaks until its temperature is even throughout and the stress relaxation is adequate. The time necessary for annealing depends on its maximum thickness. The glass then is cooled at a predetermined rate until its temperature passes the strain point (viscosity = 10^14.5 Poise), below which even microscopic internal flow effectively stops and annealing stops with it. It then is safe to cool the product to room temperature at a rate limited by the thickness of the glass.

At the annealing point (viscosity = 10¹³ Poise), stresses relax within minutes, while at the strain point (viscosity = 10^14.5 Poise) stresses relax within hours.  Stresses acquired at temperatures above the strain point, and not relaxed by annealing, remain in the glass indefinitely and may cause either immediate or delayed failure. Stresses resulting from cooling too rapidly below the strain point are considered temporary, although they may be adequate to promote immediate failure.

But annealed glass, with almost no internal stress, is subject to microscopic surface cracks, and any tension gets magnified at the surface, reducing the applied tension needed to propagate the crack. Once it starts propagating, tension gets magnified even more easily, causing it at breaking point, to propagate at the speed of sound in the material.

In short, the aim of annealing is to relieve the stress to create a stable piece of glass. The above describes when and how that occurs.

Toughened/Tempered Glass

Toughening or tempering glass starts with annealed glass to form one type of safety glass.  This done through a process of controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering puts the outer surfaces into compression and the interior into tension. Such stresses cause the glass, when broken, to crumble into small granular chunks instead of splintering into jagged shards as annealed glass does. The granular chunks are less likely to cause injury – thus safety glass.

Toughened glass is stronger than normal glass.  The greater contraction of the inner layer during manufacturing induces compressive stresses in the surface of the glass balanced by tensile stresses internally. For glass to be considered toughened, the compressive stress on the surface of the glass should be a minimum of 69 megapascals (10,000 psi). For it to be considered safety glass, the surface compressive stress should exceed 100 megapascals (15,000 psi).

It is the compressive stress that gives the toughened glass increased strength. Any cutting or grinding must be done prior to tempering. Cutting, grinding, and sharp impacts after tempering will cause the glass to fracture.

Toughened glass is normally made from annealed sheet glass via a thermal tempering process. The glass is placed onto a roller table, taking it through a furnace that heats it well above its transition temperature of ca. 540°C (depending on the glass concerned) to around 620°C. The glass is then rapidly cooled with forced air drafts while the inner portion remains free to flow for a short time.

An alternative chemical toughening process involves forcing a surface layer of glass at least 0.1 mm thick into compression by ion exchange of the sodium ions in the glass surface with potassium ions (which are 30% larger), by immersion of the glass into a bath of molten potassium nitrate. Chemical toughening results in increased toughness compared with thermal toughening and can be applied to glass objects of complex shapes. 

This blog entry is largely based on Wikipedia

https://en.wikipedia.org/wiki/Toughened_glass

and other sources.

Polarising Filters

Using polarized light filters to show stress works on the principle that stressed glass rotates the polarisation direction of the light as it comes through the glass. As polarized light filters placed at right angles do not allow any light through, only unstressed glass will continue to appear dark. 

If there is stress the light is rotated slightly and becomes visible through the filters.  

You can buy stress testing kits that incorporate a light source. You can also make your own. You need polarizing lighting gels. These come in sheets and are available from theatrical lighting sources. You will need to frame these in stiff card to keep them flat.

You use them over a light source. Place one filter down above the light source. Place the piece to be tested on top. Then orient the top filter so that the minimum amount of light shows through the filters. Any stress will show up as a light source.  The amount of light rotation depends on the stress direction, magnitude and light path length. The greater the intensity of the glow, the greater the stress the glass is exhibiting.   The amount light visible through the filters is wavelength dependent, as the filter transmits light with a particular polarisation direction. If there is large stress, different colours will be visible. 

This example shows extreme stress by the rainbow effect of light rotated in multiple directions

Note that the surface through which the light comes should be rigid, as any deformation of the surface will give a false reading.  The light filters through the slight curve and gives a stress reading, which may not be true at all.  Thus a firm flat surface is required, especially if you have a large light table for your light source.

Also note that the filters are normally on plastic sheets and easily scratched, so the glass should always be lifted and placed, rather than slid, to a new position.

A description of the compatibility test can be seen here.

revised June 2018