Elevation of Moulds

Putting slump and drape moulds on kiln posts is not necessary.  If you think you need air circulation under the mould, pieces of thick fibre paper are all that is needed.

The widely accepted notion that all moulds need to be elevated from the shelf is not backed up by evidence.  There is a lot of anecdotal discussion which supports the idea that it is necessary, though.  The idea is that air circulation under the mould will keep the lower part of the mould the same temperature as the upper part. That may or may not be the case. But it is the glass that needs attention.

Heating glass on moulds

Slumping

The glass is suspended either at the edges in slumps or at the centre in drops. The suspended glass will heat independently of the mould. Any difference in temperature between the glass and slumping mould will only become important at the end of the slump when it touches down onto the mould. If slow ramp rates and long soaks are used, the mould will be heated through by the time the glass touches down to the mould. But there could be an argument, based on the use of cold moulds in glass blowing, that a cool mould will impart less texture on the glass than a hot one.

Draping

In draping, the glass is supported in the centre on the mould and the differential temperature between mould and glass in the early stages of the firing may be a cause of a break. The supporting surface of the mould is being heated through the glass and, to a lesser extent, by conduction from the lower part of the mould. This requires slower heating than a slump mould does, to ensure the heat is not drained from glass by the mould which would allow the temperature differential between the supported and the unsupported parts of the glass to become great enough to break the glass.

Air Circulation

The idea that air circulating under the mould will even out the temperature between the bottom and top of the mould is not supported by two pieces of evidence.

The main effect to heat glass is infrared radiation. This is often described as the glass “seeing” the heat. Glass is only secondarily heated by the ambient temperature of the kiln. How heating the mould in the slumping process will protect from breaks or uneven slumping is not clear, and is supported only by analogies.

The idea that convection will help to heat the glass as well as the mould is countered by the general agreement that there is very little movement of air in the closed kiln. What convection of air there may be from the differential air temperature between the bottom and the top of the kiln is minor. If it were major, there would not be the large air temperature differential experienced – in deep kilns in particular, and to a lesser extent in shallow kilns – between the top and bottom of the chamber.

Temperature Differences

We all agree that there are temperature differentials from top to bottom in kilns.

The evidence is that the temperature difference is much greater between air and glass, than between mould and glass. The average temperature difference (∆T) between raised moulds and on-the-shelf moulds in tests ranged from about 50°C to 40°C (90°F to 72°F) on the rise to top temperature, depending on the ramp rate.  (a ramp rate of 150°C/ 270°F averaged a 50°C/90°F differential vs a 120°C/216°F ramp rate which gave the average 40°C/72°F differntial.)  It is also clear that by the time the slump temperature is reached, the ∆T between the two mould arrangements is about 25% less at the end of a 30 minute slump than at the beginning. So, yes, there is a difference in the temperatures under the mould versus an on-the-shelf mould.   Is it significant?

The ∆T between the raised mould and the air is approximately three times any difference between the on-the-shelf and raised moulds. This is consistent at ramp rates of 120°C, 150°C and 240°C (216°F, 270°F and 432°F). This indicates there is a greater risk to the glass with rapid ramp rates than there is from the difference in mould elevation. Slower ramp rates produce smaller differences between air temperature and mould temperatures. They also produce smaller differences between the mould bottom and top temperatures, making the argument for slow ramp rates up to the slump temperature, regardless of whether the mould is elevated or not.

Ramp Rates

The more rapid ramp rates create significant ∆T between the air and the mould. This much larger ∆T than between moulds raised or not, has the greater potential for causing breaks. The slower the ramp rate the less difference. These two elements on their own make the argument for slower ramp rates clear.

Distance from Elements

The distance between glass and heating elements is important in slumping.
The closer the glass is to the heat source, the more uneven the heat reaching the glass will be. This blog post indicates the distance required to even out the temperature. 

Where the elements are located is significant too. Side fired kilns need to use slower rates in going up in temperature, because the heat is concentrated on the edges of the glass unless baffles are in place. The uneven heating risks breaks, and this can be reduced by using rates for one to two layers thicker than actual thickness.

Moulds and Shelves

It can be argued that care in heating up ceramic moulds is required because of the quartz inversion.  If the ramp rate is fast enough to cause a mould to break, it is also too fast for the safety of the glass. The same caution in relation to the shelf needs to be employed for the glass.  Any large heat differential across the shelf will cause a break. 

Heavy, wet, damp, or large casting moulds must be elevated to keep the shelf from cracking due to a large ∆T between mould and shelf which has the real potential to break the shelf. The remedy for these kind of moulds is to elevate them on short kiln posts to maintain a relatively even temperature across the whole of the shelf.

Caution – the temperature differentials are averages and individual moulds will differ. But the principle remains that slow rises to top temperature are less risky than fast ones.

Data for these figures is available in table form and in graph form

Breaks after Slumping

 

Picture credits: Heidi Woj

I”m assuming this was an anneal problem. All four corners broke/cracked.

Steps: full fuse 2 layers of Tekta with 2mm leaves. Then contour-fused on the flower centers and petals (1-2 layers). [A total of 4 layers at the thickest]

Finally, slumped onto GM97 tray mold. All was well until a couple days later. Thought I used very conservative schedules, but maybe the last anneal wasn’t long enough? Also, I erroneously annealed at 950 instead of 900 on the contour fuse. Any suggestions?

Reactions

My view of the photos show the sunflower centre to be 4 layers top to bottom. A contour fuse needs to be fired as for 1.5 times the thickest part, so 19mm/0.75” at least. However, the picture shows a tack fuse was used which requires firing as for twice the thickest part so 25mm/1”.  Anyway, the piece held together during the first firing and any stress in the flat piece will have been relieved during the rise in temperature to above the strain point during the slump firing.  Annealing Bullseye at 510°C/950°F can still provide a good anneal, although it takes longer than an anneal at 482°C/900°F.  So, although not ideal, the contour/tack firing anneal is not part of the present problem.

Annealing

However, the anneal on the slump is critical.

I have suggested that the anneal for the tack fuse should be as for 25mm/1”, so I recommend the minimum allowable soak and cool is:

• Anneal soak for 4 hours,
• 15°C/27°F to 427°C/800°F, 0”
• 35°C/85°F to 371°C/700°F, 0”
• 120°C/216°F to room temperature

In addition, I have found that annealing a slump for one layer thicker than for the fuse leads to a less stressed piece. So I would anneal the slump as for 28mm/1.125”. This makes for an annealing of:

• Anneal soak for for 4.5 hours
• 11°C/20°F to 427°C/800°F, 0 time
• 24°C/43°F to 371°C/700°F, 0 time
• 65°C/117°F to room temperature.

The cool rates are intimately related to the anneal soak. The first rate is to maintain the small temperature differences at the anneal soak to below the strain point. The next two rates are to avoid introducing large contraction differences within the glass.

Analysis

It is the fast cool that has caused stress sufficient to break the piece days later. The corners broke away/cracked, because the stress in a piece is concentrated at corners and points.

Suggested reading

I have written an e-book Annealing, Concepts, Principles, Practice, which explains the whole annealing process in greater detail. It may be of use.  It is available from Bullseye and Warm Glass or me, of course.

Breaks in Slumping

Despite not opening the kiln until it was 120 degrees, a ¾" crack formed in the middle of a 15" x 15" flat fused 1/4” slumped platter, fused, then slumped. The bottom layer was four squares, topped by a single layer of Tekta. How can I avoid crack at the meeting of the squares?

The fuse schedule was

• 300°F to 1110°F, 30’

• 200°F to 1240°F, 60’

• 600°F to 1475°F, 17’

• 9999 to 900°F, 60’

• 150°F to 700°F, 1‘

The slump schedule was

• 250°F to 1225°F, 60’

• 9999 to 900°F, 90’

• 100°F to 700°F, 1’.

Any ideas why the platter cracked? Thanks.

Hannah Gulick adds: “We see this phenomenon happen a lot with this kind of checker board pattern. I find it occurs more on the underside than the topside, but it's always at the point where the four corners are meeting. It's a perfect storm situation to do with the design, colors, mold, etc. Bullseye recommends doing checker board patterns in general at a slower initial rate of heat for slumping, usually 100°F/hour.”

This piece shows extreme stress in several places.

Stress generally is most concentrated at corners and points in any piece. It is logical to treat pieces with meeting corners and points more cautiously than normal. This will include a stress test of the full fused piece to determine what stress there may be at that point in the process. I would have annealed as for 9mm, resulting in an annealing schedule of:

• 1.5 hour anneal soak

• 125°F – 800°F, 0’

• 225°F to 700°F, 0’

• 600°F to room temperature

If the stress test shows significant stress, it needs to be fired again and annealed for at least one more layer thick, depending on the amount of stress showing, and checked again before slumping.

I have found that slumping a full fused piece needs to be fired for at least one layer thicker than usual. In this case, where known stress points are at corners, even if one rather than multiple points, more caution is needed. I would anneal as for two layers thicker, or at least for12mm, resulting in:

• anneal for 2 hours

• 100°F to 800°F, 0’

• 180°F to 700°F, 0’

• 330°F to room temperature

Relieving Existing Stress - How?

Why is the stress not relieved after the strain point when slumping?

The answer relates to whether it is on the cool or on the heat up.

Cooling

The annealing occurs at a higher temperature than the strain point. The aim of the annealing soak is to even out the temperature within the glass to be equal to or less than 5°C/10°F (∆T=5C). When this small differential in temperature is achieved, there is little stress in the glass. In an adequate anneal, stress will be relieved during the soak. This differential needs to be maintained through the first cool, taking the glass temperature to below the strain point.

Relieving stress occurs between the glass transition point to just above the strain point. The viscosity of the glass is so high below the strain point (brittle phase) of the glass that no stress can be relieved.

The more rapid cooling during the brittle phase of the glass needs to be slow enough to avoid creating large contraction differentials within the glass. The reason for progressive cooling stages during the brittle phase of the glass is that it can withstand greater temperature differentials and so the cooling rates can be increased.

Heat up

Any stress on the way up for an already fused piece is induced by uneven heating. This can be across the piece, which is most evident in side fired kilns. The source of the infrared heating is nearest the edge of the glass, so it heats first leaving the centre cooler – sometimes the difference in expansion is great enough to break the glass.

In top fired kilns the differential is usually between top and bottom surfaces. Glass transmits heat slowly so the difference in temperature between the top and the bottom can be enough to cause a break from unequal expansions.

Both these conditions are caused by rapid ramp rates and short anneals on the cool.

Ramp Rates

Breaking of a flat piece on the kiln shelf is from too short a soak or too fast a cool, or both (unless there is an incompatibility). Breaking in a slump most often is a result of too rapid an initial ramp rate. A fused piece needs slower ramp up rates in a slump than in the initial fuse. It is now a single thicker piece, rather than multiple pieces as at the beginning of a fuse. While you might fire a flat 6mm/0.25” piece at 200°C/360°F for the fuse, the ramp rate for the slump needs to be no more than about 100°C/180°F. Tack fused pieces need much slower heat up rates during the slump, usually only half of the rate used to fuse the piece.

Tests have shown that even though the anneal soak for both firings can be the same, a more stress-free piece can be achieved by annealing as for one layer thicker. I do not know why, but I speculate that it is more difficult to achieve the ∆T=5C in the curved piece, than in a flat one.

More information is available in the ebook Annealing Concepts Principles and Practice.

The Relationship of Stress and Slumping

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

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

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

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

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

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

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

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

Elevation of Moulds

Is it necessary to elevate slumping moulds above the shelf? 

I first heard of the need to elevate moulds from a Bhole representative about 2007. I ignored it, but didn't get around to testing until working on my e-book Low Temperature Kilnforming.

That work showed there is a larger difference in air temperature above and below the unsupported mould than the supported one. But that difference is much smaller than between the air temperature and the glass.

At 150°C/270°F per hour the maximum difference in the temperature under the mould between the elevated and on-the-shelf mould at top temperature was 41°C/74°F while the air temperature difference was 126°C/227°F higher than under the elevated mould.  Many of the tests showed less difference than the maximums given here.

By reducing the ramp rate from 150°C/270°F per hour to 120°C/216°F, the under mould to above mould differential was reduced by a quarter. I didn't test beyond that. But it would appear that slower rates of 100°C/180°F and less will reduce that differential.

The graph also shows that there is a large difference between what the pyrometer reads than the mould temperature of the slump. Slower ramp rates produce an air temperature much closer to the mould temperatures.

Shortly into the rapid cool towards anneal soak and cool only minor temperature difference showed between elevated and on-the-shelf moulds throughout the anneal soak and anneal cool.

These details make it clear to me that elevating moulds is completely unnecessary with slow ramp rates. This of course, fits with the low and slow mantra that many of us promote. However elevating the mould will not harm the slump.

One caution, though. Damp. Wet, or heavy moulds must be supported to avoid breaking the shelf. So I advocate placing these moulds on the floor of the kiln with 2cm posts, rather than on the shelf. I don't know if it is necessary. I haven't tested it. But I do know that moulds in this condition will break the shelf without significant separation between the two.

Low Temperature Kilnforming e-book is available from Bullseye  and Etsy and is applicable to all fusing glasses.

Devising Slumping Schedules

A while ago Bob Leatherbarrow gave a presentation to Lunch With A Glass Artist (LWAGA) on slumping schedules. You can follow a recording of the Zoom meeting after joining the Facebook group: Lunch With A Glass Artist – Larry Pile.

The most important point for thinking about the process he brought up is his order of consideration of factors. They are:

• span
• thickness
• viscosity

How big is the unsupported part of the glass. Glass on larger span moulds will begin slumping at lower temperatures.

The thickness has an effect. With the same ramp rate thicker glass will need higher temperature.

The viscosity of the glass also affects the temperature of the slump. Low viscosity glass will slump at lower temperatures than higher ones, e.g., black vs. white.

Then you can begin to think about temperature and time. The objective is to use the lowest temperature to get the slump done in 30 minutes, so there is no marking of the glass touching the mould.

There is a lot more in the presentation to LWAGA. Join the Facebook page by answering two questions to get access.

There is even more information about fusing principles and practices in his book FiringSchedules for Kilnforming, Just Another Day at the Office.   This inexpensive eBook is worth much more than the purchase price!

A lot of information is also contained in my e-book Low Temperature Kilnforming, available from Bullseye, Etsy and from Stephen.richard43@gmail.com

Slumping Research

The following is an abstract of an article investigating the behaviour of glass in slumping (or as they term it - sagging).  They are using optical glass which has a much higher viscosity at its slumping temperature than fusing glass, but the principles are transferable.  They indicate quick firing to the strain point (which is about 500C/930F) is possible, but that best results come from progressing slowly to the slump soak temperature.  

The article is as follows|: 

Numerical and Experimental Investigation of the Heating Process of Glass Thermal Slumping

• Author: Zhao Dachun, Liu Peng, He Lingping, Chen Bo

• Publish: Journal of the Optical Society of Korea Volume 20, Issue2, p314~320, 25 Apr 2016

https://oak.go.kr/central/journallist/journaldetail.do?article_seq=20778

The abstract of the article follows.

“Simulations performed for different heating rates resulted in different sag variation between glass and mold; higher heating rate caused larger sag variation. Therefore, to save time in manufacturing, the furnace should be heated to about 500℃ quickly, but then to the soaking temperature slowly. According to the simulation, the maximum sag variation decreased with higher soaking temperature. Considering the temperature-dependent viscosity of glass, the optimal soaking temperature was about 800℃.

Based on the optimized experimental conditions, glass samples of thickness 0.5 mm were formed in a furnace with a concave parabolic mold made of steel. Slumped glass was produced and tested. Comparison between surface profiles of the formed glass, theoretical data, and mold suggested that the quality of the slumped glass can be significantly improved by optimizing the shape of the mold. The RMS and PV sag deviations between formed glass and mold were 2.3 and 4.7 μm respectively, along the axial direction.

On the other hand, with a soaking temperature of 800℃ the formed glass was still not fully slumped at the edges, according to the radial deviation seen in experimental results, which meant that the glass was not making full contact with the mold. The experimental results agreed with the numerical predictions. Therefore, trimming the edges is necessary to achieve a better surface profile. Based on the simulated and measured results, improvements can be made in future research, and new mold materials or manufacturing processes should be adopted to improve the shape accuracy of the mold.”

Devising Slumping Schedules

A while ago Bob Leatherbarrow gave a presentation to Lunch With A Glass Artist (LWAGA) on slumping schedules. You can follow a recording of the Zoom meeting after joining the facebook group Lunch With A Glass Artist – Larry Pile.

The most important point for thinking about the process he follows is the order of slumping factors. They are:

• span

• thickness

• viscosity

How big is the unsupported part of the glass?   Glass on larger span moulds will begin slumping at lower temperatures than on smaller spans.

The thickness has an effect. With the same ramp rate thicker glass will need higher temperature or longer time.

The viscosity of the glass also affects the temperature of the slump. Low viscosity glass will slump at lower temperatures than higher ones, e.g., black vs. white.

Then you can begin to think about temperature and time. The objective is to use the lowest temperature to get the slump done in 30 minutes to avoid marking of the glass touching the mould, leaving a smooth shiny back.

There is a lot more in the presentation to LAWAGA. Join the facebook page to get access.

There is even more information about fusing principles and practices in his book Firing Schedules for Kilnforming, Just Another Day at the Office.

https://www.leatherbarrowglass.com/purchase/firing-schedules-for-kilnformed-glass

This inexpensive eBook is worth much more than the purchase price!

Schedules for Specific Mould Shapes

A suggested temperature for a slump can only be a starting point and not all moulds will have slumping or draping temperatures given. The main factors to be considered are mould shape, span and depth, and glass thickness and profile.

Fortunately, Bullseye have a mould tips document on suggested slumping temperatures (in Celsius and Fahrenheit) for a variety of moulds.

The mould shapes are of course are those that Bullseye sell. If the shape is not one of theirs, a similar shape can be chosen and the suggested temperature and soak time can be used as a starting point.

Study of this tip sheet can give you a lot of information on various aspects of time and temperature for variations:

• Angularity – Moulds with sharper angles require longer times and sometimes higher temperatures.

• Span – The smaller the span of the mould, the longer the time required to slump into the same shape.

• Depth – It is not surprising that greater depth requires longer soaks

• Ramp rate – The rate up can affect the temperature and time of the slump.

What the Bullseye table does not give is information on the effect of the thickness of the piece. The Mould Tips document assumes a 6mm/0.25” flat fused blank. Thicker pieces will need a combination of slower ramp rates and longer soaks to fully form than given in this document. They will also need longer anneal soaks and slower cools.

It does not provide information on the effects of the profile either. Tack fused pieces need slower ramp rates and longer annealing than full fused blanks. The additional rates and times can be worked out by applying the following multipliers and firing for the resulting thickness:

• Flat fuse – the actual thickness

• Contour fuse – 1.5 times the thickest part

• Rounded tack fuse – 2 times the thickest part

• Sharp tack fuse – 2.5 times the thickest part

The key to getting the right temperature and soak length for a successful slump or drape is to observe the progress of the firing with quick peeks from the start of the top temperature. Slumping temperatures are effected by all the factors above, so that no one temperature can be given for even one mould.

My ebook Low Temperature Kilnforming gives more information on the factors in writing schedules for slumping. Available here and here.

Dog Boning in Slumps

I have done a few experiments on rectangular moulds with 3mm and 6mm thickness. I could not eliminate dog boning with larger rims, slower rates, or lower temperatures in any combination - although they did reduce the effect.

Square single layers dog boned even with increased rim width, and reduction of slumping depth made little difference in the amount of dog boning. 

Rectangular single layers shapes persisted in dog boning on the long side regardless of the rim dimension, and exhibited more dog boning on the long side than in the equivalent single layer square.  Two layer slumping had a decrease in dog boning with increased rim width, but with less effect on the long side.

In general, glass slumped in rectangular moulds is more sensitive the shape of the rectangle than the size of the rim, and very sensitive to symmetrical placing on the mould.  The depth of the mould has less influence than the size of the rim, especially for single layers.  The wider the rim, the less dog boning, in general terms.

Deeper moulds, higher temperatures, longer holds, narrower rims, all increased the dog boning. I conclude slumped square glass looks better because the dog boning is symmetrical.

My solution is to make bigger rims and cut the piece square after slumping. This approach needs cold work to the edges, of course.

The reason rectangular slumps dog bone is because the glass at the sides is drawn into the mould more easily than the corners, because there is more glass to draw in, just as in flat dog boning.

An alternative to the cold working is to round the corners of the rectangles to reduce the amount to draw-in.  A 1cm/0.375” radius curve will reduce the extent of the dog boning, but does not eliminate the effect.

Time and Temperature

credit: timeanddate.com

Credit: Shutterstock

“What are the pros and cons on turning up the max temperature slightly Vs. a longer hold time”? Lea Madsen

This is a difficult question to answer, because there are variables such as

the temperature range,

the ramp rates, and soaks,

the forces acting upon the glass at a given temperature, 

the process,

the desired outcome of the firing,

etc. 

When talking about temperature vs. time, it is heat work that we are considering.  In many processes time and temperature are interchangeable, although the temperature range is important too.  This is a brief discussion of heat work in various processes.

Slumps

Slumping temperature is generally in the range of 620˚C-680˚C/1150˚F -1255˚F *, which is below the devitrification range.  This allows the exchange of time for temperature without risk, allowing more time rather than more temperature.  Higher temperatures cause more marking from the mould since the bottom of the glass is softer than at lower ones.  Lower temperatures give higher viscosity, so the glass is stiffer, resisting marks.

Low temperature fuses

Sharp tack fusing, freeze and fuse, some pate de verre processes, and sintering occur in the 650˚C -720˚C /1150˚F - 1320˚F range, risking devitrification only at the upper end of this range.  Extending the time rather than the temperature is important to maintain detail in these processes.  Higher temperatures will smooth the surface, risking loss of detail.  

Rounded tack processes (720˚C – 760˚C /1320˚F - 1400˚F)

These are within the devitrification range making the choice between time and temperature a balance of risks.  In my experience, it takes about an hour for visible devitrification to develop.  This means that you can extend the time, if the total time between the end of the bubble squeeze and the working temperature, including the hold time, is less than an hour.  It has the advantage of a more secure attachment between the pieces of glass, without altering the surface much. 

But if extending the soak time increases the time in the devitrification zone to be more than an hour, it is advisable to increase the temperature, rather than time.  Devitrification develops in the presence of air, so reducing the time in that range reduces the risk of devitrification developing.  The glass is moving during rapid ramp rates, reducing the chance of devitrification.

Drops

This includes drapes, and other free forming processes.  Kilnformers will be observing the progress of these firings, making it easier to balance temperature and time.  There are already long holds scheduled for the processes, so it is a matter of getting the right temperature.  If, after half an hour at the scheduled top temperature, the glass has not moved much, it is time to increase the temperature by, say 10˚C/18˚F and observe after another half hour, repeating the temperature increase if necessary.   Be aware of thinning the glass at the shoulder by setting a high temperature.  Free drops may take as much as 6 – 8 hours, so patience and observation are important to get good results.

Full fuse

At full fuse try to get the work done in 10 minutes to avoid complications with devitrification.  So, increasing the temperature rather than the length of the soak seems best.

Flows

Whether frit stretching, making pattern bars, pressing, etc., low viscosity is important.  Viscosity is closely related to temperature, so increasing the temperature is the better choice.  Increasing time without increasing temperature does not change viscosity much.

Casting

Extending time at top temperature seems best for open face casting, as the temperature is already high.  A slow ramp rate to that top temperature may make adding time unnecessary, because the heat work will be increased by the slow rise.  Experience has shown that a rate of 200˚C/360˚F is enough to avoid devitrification.  With enclosed castings devitrification is not such a risk, so time can be added without concern.

 

Observation

In all these processes it is advisable to observe the progress of the firing by quick peeks to determine the effective combination of temperature and time.  Also note that heat work is cumulative, making for changes in profile with repeated firings. 

 

* The softening point of float glass is around 720°C/1328°F, so the slumping range is about 700°C/1292° to 750°C/1382°F.

Break Diagnosis in Slumping

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

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

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

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

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

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

 

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

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

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

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

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

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

Slumping Splits

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

Credit: Maureen Nolan

Observe the piece.

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

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

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

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

Threads and particles of glass are connecting across the split. 

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

History of the Piece

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

The schedule in essence was:

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

83ºC/150ºF to 688ºC/1270ºF for 10’

222ºC/400ºF to 516ºC /960ºF for 60’

111ºC/200ºF to 427ºC/800ºF for 10’

167ºC/300ºF to 38ºC/100ºF, off

 

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

 

Diagnosis

Too fast

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

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

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

20ºC/36ºF to 427ºC/800ºF, 0

36ºC/65ºF to 371ºC/700ºF,0

120ºC/216ºF to room temperature

 
Commentary on the proposed schedule:

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

 

Suspect Tack Fuse

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

 

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

 

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

 

Slump Split

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

 

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

 

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

 

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

 

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

 

Rates

 

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

 

Conclusion

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

 

Remedy

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

 

 

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

Bullseye

and

Etsy