Diagnosing Slump fractures

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

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

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

Picture credit: Esther Mulvihill Pickens

Possible causes suggested on Facebook included:

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

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

The suggestions did not include:

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

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

But….

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

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

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

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

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

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

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

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

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

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

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

My suggestions for the causes of other elements are:

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

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

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

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

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

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

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

·         The consequences of a short soak at top temperature.

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

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

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

In short:

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

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

This is available from Bullseye or Etsy

Differential Cooling of Transparent and Opalescent Glass

A statement was made on a Facebook group that transparent glass absorbs more heat than opalescent glass. And it releases more heat during cooling. The poster may have meant that the transparent heats more quickly than the opalescent, and cools more quickly.

Yes, dark transparent glass absorbs heat quicker than most opalescent (marginally), and it releases the heat more quickly (again marginally) than opalescent. The colour and degree of transparency do not absorb any more or less heat, given appropriate rates. They gain the same heat and temperature, although at slightly different rates due to differences in viscosity.

An occasional table

The rate of heating and cooling is important in maintaining an equal rate of absorption of heat. The temperature of both styles can become the same if appropriate lengths of heating, annealing, and cooling are used. The slightly different rates of heat gain can give a difference in viscosity and therefore expansion.  This slight mismatch during rapid ramp rates, might set up stresses great enough to break the glass. This can occur on the quick heat up of glass during the brittle phase (approximately up to 540ºC/1005ºF). In fact, most heat-up breaks occur below 300ºC/540ºF.

The main impact of differential heat gain/loss is during cooling. Annealing of sufficient length eliminates the problem of differential contraction through achieving and maintaining the Delta T = 5C or less (ΔT≤5C). It is during the cooling that the rates of heat loss may have an effect. The marginally quicker heat loss of many transparents over most  opalescent glass exhibits different viscosities and rates of contraction. The stresses created are temporary. But they might be great enough to cause breaks during the cooling. Slow cooling related to the thickness and nature of the glass takes care of the differential contraction rates by maintaining small temperature differentials.

Significance of Differential Heat Gain/Loss

Uneven thicknesses and the tack fusing profile both have much greater effects than the differential cooling rates of transparent and opalescent glass. It may be that strongly contrasting colours (such as purple and white) are also more important factors in heat gain and loss than transparent and opalescent combinations.  Cooling at an appropriate rate to room temperature for these factors will be sufficient to remove any risk of differential contraction between transparent and opalescent glasses.

Identification of Mechanical and Thermal Stress

The Identification of stress is important in investigating the causes of stress. We have well established clues to help us with our glass selection and alteration of our firing schedules. We can get more information about why the cold glass has broken from the scientific literature. The manufacturers of float glass and the installers of large panes investigate thoroughly the causes of breaks in glass that has been installed. 

One article - Breaking It Down, Why Did the Glass Break? by Timothy Bellovary from Vitro Architectural Glass - looks at mechanical and thermal stress and distinguishing between the two.  This post is quoted extracts from that article. [Text in square brackets are interpolations of mine].   Note that all the illustrations are from the article and are copyrighted.

Source: https://vcn.vitroglazings.com/technical-forumdiagnosing-glass-breakage

Identifying the break origin can provide hints about the following:

·         Mode of glass failure—Was it mechanical or thermally induced stress?

·         The stress or tension level at which the breakage occurred.

·         Other contributing factors—were there digs (deep, short scratches) resulting from glass-to-glass or glass-to-metal contact? Did a projectile hit the glass? Is there edge or surface damage?

 

To find the origin of a break, the first step is to assess its direction by inspecting the fracture lines… in the glass. These rib-shaped marks, distinguished by a wave-like pattern, begin at the break origin and radiate along break branches, and almost always project into the concave face of these lines.

Figure 1
Diagram of Fracture Line Direction

It’s often helpful to make a basic diagram (see Figure 1) of the fracture lines. … The origin of the break can be determined by:

·         Drawing arrows (indicating fracture line direction) pointing into the concave face of break wave markings in the glass edge.

·         Tracing point-to-tail of arrows back to the break origin.

 

Mechanical Stress

Low-stress tension breaks are experienced most frequently by residential window and IGU manufacturers. The origin of the break is typically at damaged areas of the edge or surfaces near the edge, such as digs, scratches or chips. In many cases, breakage from damaged glass occurs after the initial edge damage is incurred, such as during IGU fabrication, sashing operations, transportation, job-site handling or storage, or the installation process.

In Figure 2, the break origin is not 90 degrees to the edge of the glass, indicating a tension break caused by bending. Low-stress, mechanical tension breaks often occur from bending at less than 1,500 psi.

Figure 2

Low-Stress Mechanical Tension Break

High-stress tension breaks share one characteristic with low-stress tension breaks: The break origin is not 90 degrees to the edge of the glass, suggesting a tension break caused by bending. However, additional branching of the crack within two inches of the break origin (see Figure 3) indicates that the stress at breakage was likely higher than 1,500 psi.

…

Figure 3

High-Stress Mechanical Tension Break

 

Thermal Stress

Thermal stress breaks often originate at the edge of the glass and form virtually 90-degree angles to the edge and surface of the glass.

As with mechanical stress, there are two types of thermal stress breaks: low stress and high stress.

 

Figure 4

Low-Stress Thermal Break

Low-stress thermal breaks are often indicated by a single break line starting at the break origin point at or near the glass edge and propagating two inches or more before branching into more break lines (see Figure 4). Damaged glass edges are the most frequent cause of low-stress thermal breakage.

 

High-stress thermal breaks appear as a single break line starting at the break origin point at or near the glass edge and generally branching into additional breaks within two inches [50mm] of the origin. This indicates a breakage brought on by conditions that cause high thermal stress, such as severe outdoor shading on parts of the glazing; heating registers located between the glass and indoor shading devices; closed, light-colored drapes located close to the glass; or glazing in massive concrete, stone or similar framing.

Figure 5

High-Stress Thermal Break

Analysing the Break Origin

A reliable method for estimating the stress level of a break at failure is a mirror radius measurement. Radius dimensions are determined by crack propagation velocity characteristics.

A crack propagates itself through glass with increasing velocity as it moves further from the point of origin. If an object has sufficient energy to propagate a crack through the thickness of the glass, then a “spider web” pattern will form. ….

Near the point of origin, a smooth, mirror-like appearance on the fracture face indicates a low crack velocity. However, as velocity increases (due to higher tension stress), the fracture face takes on a frosted look; then, at the highest velocity, it assumes a ragged or hackled appearance. Mirror radii appear in various forms, depending on the stress level of the fracture.

Figure 6 shows break origins resulting from high tensile stresses, such as bending or thermal stress breaks.

Figure 6

High-Stress Mirror Radii
(R = Mirror radii)

 

Figure 7 represents the break origins of glass fracturing at low bending stresses. In this example, a smooth fracture face forms across the thickness of the substrate. When the breaking stress is low, the mirror radius is often radial and may extend deeply into the substrate.

Figure 7

Low-Stress Mirror Radii
(R = Mirror radii)

 

To identify what damaged the glass in the first place, four factors are examined during this analysis:

·         Impact

·         Inclusions

·         Thermal variance

·         Pressure differentials

Impact

Identifying the nature of the breakage pattern can determine whether a foreign object hit the glass and whether the impact was perpendicular or parallel.

Depending on the severity of the impact, the immediate area surrounding the break origin might be cracked, crushed or missing.

                 

Figure 9

High-Stress Mechanical Breakage

[This pattern of break is often exhibited when the separator fails or is insufficient to keep the glass from sticking to the ceramic support shelf.] …

Inclusions

Any undesirable material embedded in glass is considered an inclusion. ... [In general, kilnformers place inclusions within the glass and know the risks of breaks].

Thermal Variance

[This article relates to float glass installations, but the principle remains.] If the temperature difference across a [piece] of glass is great enough, the accompanying stresses can reach levels that cause breakage. … The combination of contact, surface damage and localized temperature gradients can greatly increase the likelihood of breakage.

Pressure Differentials

[This section applies mainly to Insulated Glazing Units. It points out that differences in altitude between the manufacturing and installation sites – in combination with temperature – can cause breaks. It is not of primary importance to most kilnforming, but something which should be considered when installing kilnformed glass in an IGU]

Conclusion

[Occasionally] glass breaks for no obvious reason. Whether it’s a one-off or part of a continuing pattern of incidents, glass breakage is inconvenient, potentially dangerous and costly. … Conducting “post-mortems” on glass breaks helps investigators identify the general reasons for each incident, including the type of failure that caused the break, and the potential original source of the damage. By using the techniques outlined in this article, [kilnformers] may be able to accurately identify the likely origin of such failures and … use that information to prevent future occurrences.

https://vcn.vitroglazings.com/technical-forumdiagnosing-glass-breakage

[An important element in identifying breaks in kilnforming that this article demonstrates is the difference in the angle of the break. A 90 degree angle to the surface indicates a thermal cause to the break. The more branching of the lines of breakage, the greater the stress. The branching breaks indicate there was significant temperature difference.

The breaks which are less than a right angle to the surface indicate a mechanical origin of the stress. This is usually the glass breaking at a weak point when subject to a bending stress.

If the point of origin of the stress can be identified as demonstrated in the article, it may help in determining causes. One of these causes might be hot or cold spots in the kiln.]

 

 

Soaks Below the Softening Point

There are frequent suggestions that holds in the rise of temperature for glass are required.  Various justifications are given.  A few notes before getting to the explanation of why they are uncessary.

A note is required about the softening point sometimes called the upper strain point. There is a reasonable amount of discussion about the lower strain point.  So much that it is often simply referred to as the strain point.    Below the lower strain point, the glass becomes so stiff and brittle that no further annealing can occur.  Thermal shock can happen though, so the cooling needs to be controlled.

There also is an upper point at which the behaviour of the glass is different.  Above this temperature, no annealing can occur either, because the glass has become plastic and the molecules randomly arranged.  It is only just pliable, of course, but its molecules are no longer strongly bound to one another.  This is the temperature at which much of slumping is done.

It is disputed whether such a point exists.  Still, in practical terms it is where the glass becomes so plastic that it cannot be temperature shocked.  The temperature of this “point” is approximately 45°C above the annealing point, rather than the temperature equalisation soak. 

Note that the temperature at which Bullseye recommends that the annealing soak should occur is a temperature equalisation point, which is about 33°C below the glass transition temperature - the point at which glass can be most quickly annealed.  The average glass transition point for Bullseye is 516°C.  Most other fusing glasses use the glass transition (Tg) point as the annealing temperature for the soak.  They or you could employ the Bullseye technique on thicker slabs of the glass by setting the temperature equalisation point 33°C below the annealing point, and soaking for the same kinds of time used in the Bullseye chart for annealing thick slabs.  In fact, this is what Wissmach has recently done with its W90 and W96 fusing glass ranges.  They now recommend 482C (900F) as the anneal soak temperature.

Now to the point of the post.

The soaks that are often put into schedules on the rise in temperature are justified as allowing the glass to equalise in temperature.  Glass in its brittle phase is an excellent insulator.  This means that heat does not travel quickly through the glass.  Consequently glass behaves best with steady and even rises in temperature (and correspondingly on the reduction in temperature).  Rapid rates risk breaking the glass on the temperature rise, no matter how many or how long the holds are.  

This means a slower rate of advance will accomplish the heating of the glass in the same amount of time, and in a safer manner, than rapid rises with short soaks/dwells/holds.  The slower rate of temperature increase allows the glass to absorb and distribute the heat more evenly.  This slow heating is most obviously required in tack fusing where there are different thicknesses of glass.  

This means that it is possible for thin areas of glass to heat up much more quickly than glass covered by different thicknesses of glass.  It also applies to strongly contrasting colours such as black and white, because they absorb the heat differently - black more quickly than white.

There are, of course, circumstances where soaks at intervals are required – usually because of mould characteristics, in slumping, and in pate de verre.

Sometimes people add a soak at the annealing temperature on the way up in their schedules.  This is unnecessary.  If the glass has survived up to this point without breaking, it is highly unlikely it will break with a further increase in the rate of advance unless it is very fast.  The temperature after all, is above the strain point meaning the glass is no longer in the brittle phase.

Many people add a soak at around 540°C (ca. 1000°F) into their schedule on the increase in temperature, before their rapid rate of advance to the top temperature.  The choice of this temperature relates to the lower strain point.  This also is unnecessary, except possibly for very thick pieces. By this time the glass has reached its plastic stage and if it hasn’t broken by then, it won’t with a rapid rise in temperature either.

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

Soaks at various temperatures during the advance to the upper strain points of glass are not necessary.  What is necessary is a knowledge of when the glass becomes plastic in its behaviour, and an understanding of how soaks can overcome characteristics of moulds, or how to achieve specific results and appearances of the glass.

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.

Broken base glass

Firing a piece with a partially covered base layer requires more care than two even layers to avoid the fracture of the glass during the heat up stage of a firing.  Slower rates of advance need to be used.

Glass is a poor conductor of heat and electricity. This can be good in certain circumstances but is usually one of the limitations in kilnforming.  The poor conductivity of glass means the top layer of glass will need to be heated before it begins to transmit heat to the glass below. 

A while back an example was shown that is a special case, but also illustrates the general principle (apologies to the poster, as I didn’t take down the name at the time and can’t find the original post now).

This sheet of clear glass was covered by an arrangement of stringers, with a border of clear exposed.  I don’t know positively, but I presume this was done in the knowledge that the single sheet of clear glass would become smaller, and the border would be cut down to the appropriate size.

Be that as it may, the exposure of the clear allowed the edges of the clear to heat up faster than the covered part of the sheet.  The stress of the temperature differential between the centre and the edges led to the fracture of the glass during the heat up.  This can be confirmed by the rounding of the broken edges.  It is further confirmed, by observing the relative straightness of the stringers, that the break occurred before the stringers became sticky enough to even laminate to the base glass - the clear glass broke underneath, leaving the stringers relatively undisturbed. It is also an indication that the glass broke earlier than the slumping temperature, as the stringers would have been sticky enough to break with the clear otherwise.

One speculation given for the break was that it was affected by the size.  You can see the size is relatively large for the kiln.  This may have had some influence on the fracture as well.  But it is not so much the size as the shielding of the heat from above for a large part of the base sheet. We don’t know if this was a side fired kiln, but if it was, there would be an increased exposure of the edges of the glass to the heat and so increase the likelihood of temperature differentials leading to too much stress for the base glass.

The rate of advance for partially covered sheets needs to be reduced to be slower than for evenly covered base sheets.  Even on evenly covered base sheets, there is a risk of breakage of the bottom sheet, if the rate of advance is too quick.  Slower heating reduces the temperature differentials, as the gradual rise in heat allows the glass to be closer in temperature from top to bottom.

Frit by thermal shock

Frit can be created by thermal shock.  You will still need to do some manual breaking up. The principle is that you heat the glass and then cool it rapidly, causing the glass to break into pieces.

Place the glass in a stainless steel bowl and heat as fast as possible to 300C – 400C. Turn the kiln off and pull out the bowl, using heat resistant gloves and dump the hot glass into a large bucket of water. Once the glass is cool, pour off the water and dry the glass.  When dry, you can break the crazed glass into smaller bits just as you would with other glass.  Note that pouring water over the glass has two disadvantages – one, it does not completely thermal shock the glass, and two, the large amount of steam released is very dangerous.

The advantages of this quenching method of obtaining frit are that you can create frit with less effort.  You also get less fines and powder with this method. And less effort is required to smash up the glass.

Some indicate that ice cold water to quench the glass is a good idea.  This is because warm water will not provide enough of a shock to the glass to craze it throughout.  But if you have a large bucket of water, there is no necessity, as the volume of water will cool the glass quickly enough.  Of course, if you are planning another quenching, you need to renew the water, as it will not be cold enough to thoroughly craze the glass.

You can, in part, control the size of the resulting frit.  Firing at 300C results in larger frit than firing at 400C.  However, firing at 500C does not provide even smaller frit.  The best results are between 300-400C, although frit can be made at 200C as well.  Experiment with temperatures to get the frit you want.

Once you have dried the frit, you can begin to break it up. Some can be done by hand, but the pieces are often sharp, so gloves are essential.  The other standard methods of breaking up glass to make frit are applicable. But it does not take as much effort as breaking from cullett.

The relative order of kiln forming events

When preparing for multiple firings of elements onto a prepared piece, you need to consider the order and temperatures of events so that you do not harm an earlier stage of the project.  This blog entry will not give definitive temperatures, as that varies by glass and by kiln.  Instead, it indicates what happens in progression from highest to lowest temperatures in approximate Celsius degrees.  

ca. 1300C  -  Approximate liquid temperature 

ca. 850 – 1000C  -  Glass blowing working temperature

ca. 950C  -  Raking and combing

ca. 850C  -  Casting

ca. 810C  -  Full fuse

ca. 790C  -  Large bubble formation

ca. 770C  -  High tack, low contour fuse

ca. 760C  -  Tack fuse

ca. 750C  -  Fire polish

ca. 700C – 760C  -  Devitrification range

ca. 700C  -  Lamination tack

ca. 600C – 680C  -  Slump and drape

ca. 650C  -  Vitreous paint curing temperature

ca. 600C  -  No risk of thermal shock above this temperature 

ca. 540 – 580C  -  Glass stainers enamel curing temperature

ca. 520 – 550C  -  Silver stain firing temperature

ca. 550C  -  Glass surface beginning to soften

Slow rates of advance needed from room temperature to ca. 500C

These temperatures are of course, affected by the soak times. The longer the soak time, the lower temperature required. The rate at which you achieve the temperature also affects the effective temperature.  Slower rates of advance require lower temperatures, than fast rises in temperature.  These illustrate the effect of heat work.

The table shows for example you need to do all the flat operations and firings before slumping or draping.  It also shows you can use vitreous glass paints at the same time as slumping and draping.  This emphasises that the standard practice is to plan the kind of firings you will need for the piece and do them in the order of highest temperature first, lowest last.

In general, you do need to do the highest temperature operation first and lowest last.  But there are some things you can do with heat work.  For example, if you needed to sandblast a tack fused piece, but did not want to risk reducing the differences in height there things you can do.  From the list above, you can see the glass surface begins to soften around 500C.  It is possible to soak the glass for a long time around 500C to give it a fire polish, instead of going to a much higher temperature.  You will need to experiment to find the right combination of temperature and soak length, but it can be done.

This article is to show that knowledge of what is happening to the glass at different temperatures, can help in “fooling” the glass into giving you the results you want without always following the “rules”.  This may also be what it is to be a maverick glass worker.  Use the behaviour of glass to your advantage.

Breaks in Slumping

Diagnosis of breaks during slumping processes is often difficult because the temperature is not high enough to be able to apply the usual rule.

In looking for the reasons for a break in fusing processes, sharp edges imply the break occurred on the way down in temperature, but rounded edges indicate the break happened on the way up to the top heat.

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This not a universally applicable diagnosis.

At low slump temperatures, the edges will be sharp in both the case of a break on heating up, and in the case of breaking on the way down in temperature.

The best test to determine when the break occurs is to observe periodically during the heat up.  You will be able to see if the piece breaks before the top temperature.  If it is whole at top temperature, the break occurred on the way down.

If you have been unable to observe the progress of the firing, you will need to diagnose when the break occurred. The test here is not whether the edges are rounded or sharp, because at normal slumping temperatures, the break will be sharp in both cases. 

If the break occurred before the top temperature, the pieces will shape separately. Therefore, If the pieces no longer fit together, the break was on the rise. If they do, the break was on the way down.  Place the pieces very carefully together to see if they form part of a continuous curve.  If they do, the break was on the cool down.  If they almost  match, or do not match at all, then the break was on the rise in temperature.

In general, when the break is on the cool down, there is an overhang of the glass on the mould which causes the break.  But the most common break of a slumping piece is caused by a too quick rise in temperature.

For a flat 6mm piece, the slump temperature rise should be less than 2/3 as quick as the rise for the fusing.  If you have a tack fused piece to be slumped you should reduce the rate of advance to at least half of that for a smooth, flat piece of 6mm.  Thicker glass with tack fused elements will need to be even slower.

Draping is another story.