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

Stress Build Up

Does annealing stress in one firing cause breaks in the next firing?

Stress does not cumulatively build up from one firing to the next. Once the glass has achieved the strain point temperature, the relief of stress begins and is complete by the softening point (usually the slump temperature).

So why does the glass sometimes break on the second firing?

Picture credit:  Emma Lee

There are three general causes of breaks of fused pieces on subsequent firing:

Too rapid initial ramp rates.

Inadequate annealing.

Incompatibility. If there is incompatibility, no amount of extended annealing will cure it. Some of the causes may be:

• cumulative compatibility shift due to extended time at high temperature

• long soaks during multiple firings.

• Normally, hot colours are most subject to these.

How is it possible to be sure the fired piece does not have excessive stress before the next firing?

Stress testing with polarised filters of the cooled glass will indicate the amount and sometimes the cause of stress. This test should be conducted on each piece after it has cooled and before firing again. If there is a large amount of stress, the piece must be warmed up slowly to avoid adding additional stress during the heat up for a more adequate anneal.

This heat-up needs to be a single slow ramp to the strain point.  After the strain point is reached, there is much less chance of a break and the ramp rate can be increased.   It isadvisable to go at least 55C/100F above the annealing temperature and soak for at least 10 minutes before proceeding down to the anneal. The annealing for this new firing needs to be scheduled to be much longer and the cool much slower than the first firing of the piece. The additional length may be for one layer thicker for full fused pieces, but for at least two times thicker for tack fused pieces. Guidance for calculating the thickness for the annealing time and cooling rates are available in this blog post.

Compatibility of Stained Glass

Is there a way to find out if some stained glass is compatible with fusing?

Yes, there is a way to test for compatibility. This strip test is the one I recommend. It is simple, repeatable and provides a reference for future use if labelled and stored carefully.

Determining the Coefficient of Expansion (CoE) is a testing laboratory process, not something we can do at studio level. But compatibility of several glasses can be easily established by this testing process.

Some indicators:

• Stained glass is more likely to be compatible with Oceanside or Wissmach than Bullseye.

• Transparent and streaky glasses are more likely to work in Kilnforming than wispy or opalescent, which tend to devitrify easily and have more compatibility problems.

• It is possible to find colours from one manufacturer that are compatible with each other but with none of the fusing compatible glasses, but it takes a lot of testing.

Wreaths

Can glass wreaths be made from bottles?

Last autumn a small group within the cooperative Jangling Space decided to see if we could make strip construction glass wreaths for Christmas.  This developed out of a collaborative project with Christopher Jeffree on whether bottle glass could be made to fit together at various sizes of frit.

Testing for compatibility of glass at frit levels

It was postulated that small sized frits did not allow stress to build to critical length beyond the frit size.  Work is still going on to discover the upper limits to the frit size for this postulation to become invalid.

We speculated that the same brand bottles from the same drinks manufacturer would be compatible.  We thought this possible, because there are few bottle manufacturers that can produce the number of bottles required and that they would have consistency.  After some experiments with same colour wine bottles we found very inconsistent compatibility.  But some volume wine producers use distinctive bottle colours, which seem to have consistency.

After some experimentation we settled on green Gordons gin bottles combined with blue bottles of a reueda wine bottled under the Four Lines brand, both of which had consistency between bottles and production runs and showed acceptable stress levels between them for these purposes. This was important to be able to distinguish by colour the stress between the frit particles. The microscopic investigation of these frit combinations did show incompatibilities, but at very short lengths.  The stress between less compatible frits was indeed interrupted by the connection of groups of wholly compatible ones.  This was successfully tested in various proportions (25%, 50%, 75%) and sizes from 75 microns to 1200 microns. Testing for larger frits remains to be conducted.

Producing the flat glass

Jangling Space had been producing sheet glass from bottles by analogy from hand blown glass — where the glass cylinder has the ends cut off, and then split — for several years.  After experimentation, it was found that cutting out a quarter of the bottle cylinder, the largest piece of flat glass could be made.  Of course, we tended toward brighter greens, yellow greens, pale ambers, and various shades of blue. This kept part of the glass pallet inexpensively supplied. 

Experimenting for the wreaths

A group of four gathered together to conduct the experimentation and preparing for a wreath making class.  Knowing that the Gordons green bottles were consistently compatible, whether as retail or bar sized bottles enabled progress toward using strips to combine into fused objects. 

 Strips were decided upon because:

• it is simple to provide parts for people with no fusing knowledge or experience to assemble,  

• there is less waste than from shards piled on top of one another,

• consistent widths of strips placed on edge would make annealing more accurate,

• It would provide a distinctive wreath pattern

We began experimenting with size, arrangement, paints, glues, Frits, glass dots, etc., to provide stability, texture, decoration.

The first element was to establish the principle, so began with small wreaths of about 15cm/6”.  These were quick to assemble on shelf paper and we tried sticking them together with CMC.  This was messy and required significant drying time before they could be moved.  The second assembly test was to use frit to stabilise the pieces.  We found we could use thin but relatively stiff plastic sheets to move the pieces on shelf paper to the kiln with little disturbance.

The pieces were fired at a 200°C/360°F rate to 720°C/1328°F for 10 minutes. This temperature was chosen as it is the softening temperature for float glass, which we believed would be enough to stick the glass together without loosing the textures of the radiating strips.  We did not use the double thickness for scheduling and annealing as is usual in unevenly thick projects.  Instead the pieces were annealed as for 12mm/0.5” although the strips averaged just less than 10mm, but of course there were gaps to be considered.

They came out looking well, and with acceptable levels of stress.  But they proved to be fragile when handled. Inspection showed that the wreaths were breaking where the attachment points were small.  This led to heaping fine and medium frit over the strips – rather than fitting fine frit between – for the next set of wreaths.  It was also felt that the wreaths were pretty dark, so we decided to add clear glass from Gordons gin bottles in a ratio of two green strips to one clear strip. At the same time some of the wreaths had some Bullseye red and white dots added to add more colour.  These were fired to 740°C/1364°F. There was little difference in texture of the strips – only a little more rounding – but the frits were more like sand than an enhancement to the wreath.  We also found that the clear bottle glass increased the amount of stress, so we went back to all green.  The dots showed very little stress, so they became part of the assembly.  

Again, although more robust in handling, they were still fragile.

Having learned

• frit can stabilise the unfired strips

• frits need to be medium to large for a good appearance

• higher temperatures are required

• only the green Gordons glass could be used

• Small Bullseye dots could be sparsely spread over wreaths

We went to a larger scale – 28cm/11”.

Stepping up in scale required a lot more glass – we found two bottles were needed to provide enough strips for a wreath.  That is a lot of 75mm/3“ strips! We produced a lot of quenched frit from the bottle necks and bottoms, which required a lot of pounding and sifting, to get rid of the fine and powdered glass.  At this scale drawing the limits for the strips became important to maintain the circular doughnut form.  Props were needed to keep the strips upright, as many of the cuts were not clean, due to the thickness variations caused by the embossing of the Gordons design on each bottle.

The essential "sprinkling" of coarse frit for a couple of centimetres (ca. 1”) out from from the centre of the strips was about two layers thick.  During the firing most of this sank into the voids between the ends of the strips, and left a "moraine" texture over the internal part of the wreath.  It gave an impression of the strips flowing out from the centre, or possibly as a retreating glacier.  It provided strength to the vulnerable/weak areas where the sides of the strips contact each other.

This increase in size needed radiating guide lines to keep the strips radiating from the centre, rather than bunching up side by side.   More short pieces were needed to fill outer gaps than expected from the smaller trials. The increased size required not only more glass, but a more robust method of transferring the unfired pieces to the kiln.  A couple of things were investigated until we discovered pizza shovels.  This was ideal to slide under the shelf paper and carefully carry to the kiln.  Also more storage space was needed, as not all the wreaths could be fitted into the kiln at once. The minor problems of scale!.

A contour fuse was used to

• maintain a lot of the undulations of the glass,

• allow some bending over to lower pieces, and

• make better contacts and incorporation of the pieces.

These fired well giving a much more robust, although heavy, wreath that was admired by the group and the members of Jangling Space. This reveal of what the group had been doing led to the suggestion of stringing LED lights and ribbons around the wreaths.

In preparation for the hands-on making of wreaths by members of Jangling Space, it was decided to make “statement” wreaths of 38cm/15” diameter.   A trial showed that three large Gordons bottles would be required to make them.  Also an additional 50% more frit required to be made.

Jangling Space members had to provide and clean sufficient bottles.  They were cut and flattened for them, and frit made from the necks and bottoms.  Each person had to cut their own strips after being shown that breaking thick glass is easier with cut running pliers without cushions.  They assembled and decorated their wreaths and the experimental group placed the frit around the inner part of the wreath.  The results were placed on a large pizza shovel and transferred to the kiln, and the waiting ones to storage shelves.

For the class that was subsequently offered, the bottles were provided by Jangling Space members who helped clean them.  Members of the experimenting group cut, flattened, cut strips, made frit and red and white balls so each class attender had a set of parts with which to assemble their wreaths. It went surprisingly well.

Thanks to Chris, Linda, Lyndsey and all the others who helped in this project.

Needle Points

How to avoid prickles when damming glass?

In using dams sharp needle points on the finished piece are common. This is a time when CoE has a practical effect.  The glass is expanding all the way to the top temperature. During this expansion upwards (it’s linear expansion is constricted by the dams), it takes up all the texture of the things it touches. When the glass begins to cool, it contracts and some glass is stuck in the rougher parts of the fibre paper. As the glass continues to contract, the stuck part becomes long, thin, and needle sharp as it separates from the dam materials. The length of these points indicate how much the glass expands and subsequently contracts.

Prevention

Changing the top temperature, the soak time, the bubble squeeze, etc. will not change the fundamentals of expansion and contraction. The physics of expansion and contraction are immutable.

There are several strategies to reduce the amount of needling which can be combined or used singly.

• Use clear iridised glass, iridised side to the fibre paper that is separating the dams from the glass. This should be no higher than the expected final height of the completed piece.

• Use shelf paper between the glass and the fibre paper covered dam.

• Use boron nitride (Zyp or similar) on the fibre paper.

• Use 3mm fibre paper which is 3mm shorter than the expected finish height of the glass.  This allows the glass to form a bull-nose effect without the edges touching the damming materials.

• Pile the glass in the centre of the dammed area and let it flow to the edges. But if the fibre paper is textured, the glass will still form needle points as it cools. So, one or more of the above methods also need to be used in combination.

The whole effort in preventing needling in casting is to reduce the effects of the hot glass grabbing the separating material as it cools and contracts. Provide the smoothest of surfaces or engineer the separators so that the expanding glass does not touch the damming material

Prickles in thin glass

Firing pieces less than 6mm, most often creates needle points as the glass contracts trying to reach the 6-7mm natural height at normal fusing temperatures. The cause is the same as the glass contracting in dammed glass. The glass expands when hot and conforms to any imperfections in the surface, and forms prickles upon contracting. Providing smoother or powdered surfaces minimises the needling effect.

Sometimes even 6mm thick glass forms short rough points during the firing.  The glass does not maintain its 6mm thickness throughout the firing.   As the temperature rises toward contour and full fuse, the glass spreads and slightly thins due to the reduced viscosity. On cooling the glass contracts, “beading up” toward 7mm. This sometimes leaves a rough edge at the contact points between the glass edges and the separator. It is simple to clean up these pieces by removing the rough edge with diamond hand pads, which is much quicker than grinding and fire polishing, and it has less risk and cost.

CoE vs Compatibility

CoE does not determine compatibility. Of course Coefficient of Expansion (CoE) increases the chance of compatibility, but when it is understood that it is CoE that is manipulated to give an equal but opposite force to the viscosity, you realise that CoE is a variable measurement of compatibility rather than a constant. The formulation for each colour of glass has to have a different rate of contraction to balance the force of viscosity.

Each manufacturer uses a range of CoEs to make their glass fusing compatible. Spectrum once noted that their fusing glass varied by 10 points, and since Oceanside is using the formulations from Spectrum it is most likely the same is true for their glass. So if 96 were the midpoint of the CoEs used by Oceanside, the range is 91 to 101. Bullseye has stated their CoE range is 5 points, and that when taken to the next decimal point, it is closer to 91 than 90. Again, assuming that 90.5 is the midpoint, their range is from 88 to 93. Theoretically it would be possible to find a 96 glass compatible with Bullseye.

Each manufacturer will formulate their colours differently (because they don't know the exact formulations of others' glass), and so a glass in their "96" range, for example, may vary enough from another's to be incompatible. We practitioners can't know what or which variations there are without testing the compatibility. And that's why I stick to one manufacturer, even though I may like colours from other manufacturers. Anyway, it is possible to combine colours in one manufacturers line to imitate colours in another's, and there are people who have done the work to determine combinations to provide a wide range of shades and tones. For example, Kim Brill has done it for Bullseye sheet glass and for powders.

Since CoE and Compatibility are not the same thing, it is risky to assume all glass labelled with the same CoE number will always be compatible.

More details about CoE and compatibility. 

CoE equals compatibility?

Meaningof CoE. 

Importance of CoE. 

Cautions about compatibility of glass with the same CoE number. 

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.

Sintering a Sculpture

 

The description

The objective is a sculpture 50mm x 50mm x 90mm (2” x 2” x 3.5”) composed of two layer slabs to make it 90mm high. The objective is to fuse the whole without distortion or dams.

Assumption:

My presumption is that the piece will be fired on its side rather than standing to full height in the kiln.

Proposition:

I propose that a sharp tack or sinter firing can achieve this project. Sintering is the use of a low temperature for a long time to achieve a result that a normally high temperature and short soak would achieve, but without the distortion from the crisp square edges wanted for this sculpture. It can stick pieces together with virtually no change in appearance.

Scheduling:

This is a piece of 50mm (2”) at its thinnest, so a slow first ramp rate is necessary. I suggest heating up as for 50mm will be slow enough to avoid breaks. The heat up could be;

• 22°C (40°F) - 260°C (500°F), 60’,

• 22°C (40°F) - 670°C (1238°F), 8 hrs,

Sharp tack/sintering requires annealing as for 2.5 times the thickest part, making a requirement to anneal as for 125mm/5”, which would be for 20 hours. The associated cool would be:

• cool to 427°C (800°F) at 1.2°C/hr (2°F/hr)

• cool to 371°C (700°F) at 2.2°C/hr (4°F/hr)

• cool to 21°C (70°F) at 3.05°C/hr (5.5°F/hr)

• off.

Because the rates are so slow it may be best to set the controller to use the time to reach the target temperature.

• 13.36 hours to 260°C (500°F), 60’

• 1hr to 301°C (574°F)

• 16.81 hours to 670°C (1238°F)

• 8 hrs to 671° (1240°F)

• 0 hrs to anneal temperature

• 20 hours to anneal temperature +1°

• 45.83 hours to 427°C (800°F)

• 25.45 hours to 371°C (700°F)

• 100 hours to to 21°C (70°F)

or 230 hrs minimum anneal and cool time (or 9.6 days). Casting might be quicker in the long run.

Kilns on Wheels

 

image credit: Olympic kilns

Is it acceptable to have kilns on wheels?

A number of people have small studio spaces and do not have their kiln in constant use. They sometimes move their kiln against the wall or under a bench when not in use. This is possible when the stand is on wheels, otherwise they generally are to heavy to without them. This gives more space in the studio to move around when they are stored out of the way.

But there is a disadvantage to this. Each time the kiln is moved out for use, it is important to level the kiln, the shelf and any moulds. It is surprising that put in the same place how much the levels can change. Glass will move toward the lowest point in any firing above the softening point, so to get the most predictable outcome of a firing, the levels must be checked, and adjusted if necessary, every time it is moved.

Even Frit Distribution

A light and uneven distribution of frit that provides a pleasing image.
image credit: Donna Brown

It is often difficult to get an even distribution of frit across a background with already placed glass.

It is possible to lay down a ground of frit and build the objects (in this case bees) over the frit. It is easiest to lay down an even layer of frit with no interruptions in surface levels.

Putting the whole piece on a light sheet allows you to see the dense and light areas more easily. Use a screen mesh of similar size to the frit and sift the grit through onto the piece. The brighter light shows where the thinner application is.

Another possibility is to build the objects first, add the minimum of fusing glue to the edge and wait a while for the glue to stiffen. Then sprinkle frit evenly over everything. Sweep the frit off the objects with a soft brush into the background. View the frit distribution from a shallow angle to see the thick and thin areas, which will appear as bumps and shallows.

A possible patch to an already fired piece with an uneven spread, is to add a complimentary or contrasting colour of frit to the gappy areas and refire to exploit the uneven cover.

Frit Stretch Crack

Description

A frit stretch in the flattening developed two internal cracks at 4:00 and 8:00 going through colours (but not felt on top or bottom). The flattening was to reduce the centre thickness from about 8m, to be nearer the 6mm of the edges. It was fired in a side element kiln. The maker does not know if there is stress beyond the cracks.

The question is about firing again to conceal the cracks.

Schedules first.

Generally side fired kilns need to have up ramp rates for at least one thickness greater than top fired kilns to counteract uneven heating. The heat input to glass with side fired kilns is directly to the edge of the glass and so more uneven than in top fired kilns. Slower rates for the up ramps are needed than for top firied kilns. Down ramps are not affected in the same way, so down rates can be the same as for top fired kilns.

1st firing (before the break):

277°C/500°F – 594°C/1100°F, 0’ [A reasonable first ramp for small pieces]

28°C/50°F – 691°C/1275°F, 60’ [At 691°C the glass has begun to stick together, so a lower temperature of 677°C/1250°F would continue to allow air out as the glass slumped.]

9999 – 830°C/1525°F, 60’ [This speed allowed only about 15 minutes to get to top temperature. The impossibility of the glass equalising the top to bottom temperature is alleviated by the one hour soak. The hotter top layers of the stretch begin to move toward the centre before the bottom layers. This ASAP 3^rd segment will reduce the time for the flow to the centre. Peeking at intervals will show when, or if, the centre has been filled by the flow.]

9999 – 510°C/950°F, 120’ [It is known a frit stretch will have a variation in thickness from about 9mm to 6mm. This would indicate firing for at least 1.5 times the thickest part, approximately 12mm, which is what was done]

28°C/50°F – 427°C/800°F, 0’ [A two hour soak indicates annealing for 12mm, so the cooling needs to be for that too. 55°C/100°F to 427°C/800°F is adequate. And can be followed by 99°C/180°F to 371°C/700°F. The final cool rate could be as fast as 330°C/600°F to room temperature.]

Off

If there are no more than 5 segments available in the kiln controller, use the slower first rate to 371°C/700°F. If the natural cooling rate of your kiln is more than the slow first rate at that temperature, reduce the target temperature further.

I rarely fire faster than 330C/600F to top temperature. Faster does not allow all the heat to reach the bottom and so meld the glass together. There are two blog posts that help to explain the difficulties with AFAP ramps up in temperature. This difference increases with thickness. The effects on glass temperatures, and the heat control problems

2^nd firing:

111°C/200°F – 594°C/1100°F, 0’ [because of the cracks, I suggest 55°C/100°F is fast enough as the first ramp rate. But it does not need to go to more than 500°C/932°F to be sure of being above the strain point.]

125°C/225°F – 663°C/1225°F, 30’. [I would maintain this rate to top temperature, as any bubbles are already trapped]

9999 – 830°C/1525°F, 60’ [I would use this top temperature and time but at the slower rate of the previous ramp]

9999 – 510°C/950°F, 120’ [I think this is an appropriate anneal soak time]

28°C/50°F – 427°C/800°F, 0’ [The cool rate should be as suggested for the first firing. The elimination of the bubble squeeze segment allows a final segment rate of 100°C/180°F – 371°C/700°F or lower if required – for a controller with fewer segments.]

You need to put the heat into the glass slowly when trying to flatten or thin a piece. With a slow rate it may be possible to reduce the one hour soak at top temperature and still get the same thinning result.

The break goes through colours so it is not a compatibility problem. That moves attention to annealing soak and cooling. The schedules do not set off any alarms for me as causing the crack (although I would have been more cautious). It is clearly highly stressed with the crack essentially going across the whole piece except the edges. It will continue to be delicate because of the stress unless re-fired carefully.

Testing for stress

A stress test is a means of determining how sound the fired piece is. It can also be used on smaller pieces to determine what stresses may be stored in the glass and to determine if the glasses are compatible.

This tests for both stress and compatibility.  These give notes on the use of the polarised filters.

Strain Point

The strain point is the temperature above which the viscosity of the glass is low enough that it is rarely subject to thermal shock, because it is no longer brittle. That temperature is determined by the glass’s viscosity, which is achieved at different temperatures for different glass, such as float glass.

Repair

I suggest placing a dam around the piece before firing for a repair. This can be thick fibre paper backed with kiln furniture, or some other circular material to confine the piece, in case the crack develops into a full break during the firing.

The Ramp Rate I suggested of 55°C/100°F per hour is because of the existing cracks. This is to avoid inducing additional stress to the piece during the temperature increase. Once the strain point is passed, the rate can be increased. I suggest the rest of the firing should be as for 12mm / 0.5". The anneal soak and cooling are given in the Bullseye document Annealing Thick Slabs, Celsius and  Fahrenheit,  which is applicable to "CoE 96" glasses, except for the anneal soak temperature.

Further investigation

I suggest two e-books which will be very helpful in your exploration of kilnforming:

Kilnforming Principles and Practices, by Stephen Richard

Firing Schedules for Kilnformed Glass, by Bob Leatherbarrow

Both are available from Bullseye or the authors.

Casting Hearts

DreaminColoursArt

How can veiling be reduced in heart casting moulds?

These ways of avoiding or reducing the evidence of the cullet pieces laid into a casting mould rely on analogies with other methods of glass flow.

A reservoir such as a small ceramic flower pot can be set above the mould to allow the melted glass to flow through the hole into the mould below. This removes all possibility of the edges of the glass pieces showing.  It does however, introduce the possibility of more tiny bubbles into the cast heart.  This uses the same principle as a pot melt, but into a mould. The result will not have clear definition between colours as the placement of cullet does. It may sometimes show the path of the flow into the mould.

Another possibility is to fill the mould with frit. This would eliminate the evidence of the edges of the cullet, as the small particles would present no coherent edges in the final piece. It does however risk a multiplicity of tiny bubbles throughout the piece and sometimes a greying appearance.

If you have access to a hot glass studio, you could do a pour into the mould which would completely eliminate bubbles and veiling.

Glue Amounts

Is any glue required to stabilise glass pieces in kilnforming? If so, the amount, placing and type are important. The more glue you use, the more likely glass pieces are to move in the firing.

Details here.

Glue, whether dry or wet will boil with rapid rates. Most glues have boiling points between112°C and 270°C (230°F and 520°F), although super glue has a boiling point of only about 55°C (130°F).

This fact requires slow ramp rates to above 270°C/ 520°F to promote evaporation and so reduce the risk of boiling, which will move the lighter glass pieces around.

The placement of glue is important. It has to burn out, so should be applied only at the edge of the pieces. Glue placed under the centre of of glass pieces often create large bubbles or leave unsightly black areas.

In addition to these precautions, the minimum amount of glue should be used. If the glue is water based, it can be diluted with water, such as my practice of diluting one part PVA to 5 parts water. Solvent based glues require fine tips or needles to apply the glue to the edge of the glass.

There are other ways of stabilising stringers and small pieces with frits or powders that avoid using glue altogether.

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