Domed drapes

 

Apologies.  I failed to keep the name of the maker.

A draped bowl didn't end up with a flat bottom. What schedule would flatten it out again and what to fix to get a flat bottom?

Drape schedule:

• 167°C/300°F – 527°C/980°F, 30’
• 111°C/200°F – 710°C/1310°F, 10’
• AFAP – 472/880°F, 1:00

Scheduling Factors

Ten minutes soak time at this top temperature is not enough time to allow the glass over the top to relax and become flat. To understand the dome formation on the top of the bowl drape mould, visualise the way the glass moves during the drape. I will attempt to describe it, but peeking at intervals from 55°C/100°F below the top temperature will show better than I can explain, of course. At the beginning of the drape, the weight of the glass at the edge bends the glass into a domed shape because it is not yet at a low enough viscosity to conform to the flat base. It is not until it has absorbed enough heat that it can begin to stretch and sink towards the mould surface at what will be the base of the glass piece.

At the rate used, the glass needs more time to absorb the heat to allow the glass to relax to the central surface. The extra time may lead to distorted edges, of course. So, a slower rate up to the top temperature may be the best solution. It is a curious fact that clear glass does not react as coloured glass does. It seems the infrared radiation goes through the clear in a way that it does not with coloured glass. This lower viscosity of the clear than coloured glass may be an additional factor in in allowing the dome to form.

The possibility of trapped air holding the glass up is not likely. At 710°C/1310°F the glass will not have made a seal with the upper edge of the mould. 

Drilling a hole in the supporting mould will not solve the problem, but it won’t create another either. Just as in draping a votive, a hole is not necessary, but of course the draping is done at a higher temperature. The amount of heat work the glass receives is important to the glass being able to relax to the surface of the mould.

The drape schedule

• 167°C/300°F – 527°C/980°F, 30’
• 111°C/200°F – 710°C/1310°F, 10’
• AFAP – 472°C/880°F, 1:00

This schedule is suspect for two main reasons. The first segment might be best maintained to 621°C/1150°F. The second segment ramp rate might be reduced to 33°C/50°F to 740°C/1364°F for up to 30 minutes to allow the glass to relax to the base part of the mould while the rest of the glass continues to drape. Observation, by peeking at intervals, from 720°C/1328°F until the glass has fully formed flat at the base and is fully draped, will indicate when the segment should be skipped to the anneal. 

Anneal

And, although the annealing temperature is not the cause of the dome, the anneal temperature used is not one normal for either Oceanside or Bullseye and could be responsible for a later break due to inadequate annealing.  

If not a typo, 472°C is below the accepted annealing temperature for any of the fusing glasses, and is so near the strain point that it will be difficult to anneal at all. If the glass is Oceanside, the anneal temperature is 510°C/960°F; if Bullseye the anneal temperature is 482°C/900°F. The soak time indicates the glass is being annealed as for two layers. The associated cool rate for 6mm is:

• 83°C/150°F to 427°C/800°F, 0 minutes, 

• 150°C/270°F to 371°C/700°F, or to room temperature.

However, this is a tack fused piece of a single layer plus 3 to 6 mm of decorative elements. So, it should be fired as though twice the 9mm thickest part, or 19mm. This profile of glass needs three hours anneal soak, and cooling rates of:

• 25°C/45°F to 427°C/800°F

• 45°C/81°F to 

• 371°C/700°F

• 180°C/270°F to room temperature

Note that this tack fuse profile needs cooling rates all the way to room temperature. If your kiln cools at a quicker rate from 371°C/700°F than 180°C/270°F, the schedule should include that final segment to room temperature.

Anneal Cooling

In the original schedule, there were no cooling rates given, as though the kiln will cool slowly enough (the natural cooling rate)to achieve a good anneal. This is rarely the case for any thing being fired for more than 3mm. The purpose of controlled cooling is to initially maintain the small difference in temperature across the whole piece to avoid it increasing. As the glass temperature falls below the strain point, it can withstand greater temperature differentials, but the temperature fall still needs to be controlled enough to avoid large contraction differentials. For thicker glass, the rate to room temperature also needs to be controlled.    

This piece is being fired as for 19mm and does need controlled cooling to avoid inadequate annealing and possible fractures from too quick cooling.

Layup

One thing that may have helped the formation of a flat bottom would be to have a tack fused a disc of clear, the same size as the base of the former in the first fuse. This would give additional weight to the glass and help to flatten the base sooner. The bending of the outer edges of the draping glass, would not pull the glass on the base into such a large dome, and possibly not at all. Think about weight when draping, as the heaviest will tend to fall first.

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.

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.

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.

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.

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