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.

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.

Anomalous Annealing Schedules

Why do schedules vary so widely?

I frequently observe strange annealing schedules promoted on the internet. These include:

• extremely long anneal times with rapid cools risking thermal shock.

• single slow cool rates, using more time and electricity than necessary

  • to 371°C/700°F.  There are better cool arrangements that fit the needs of cooling glass.

  • to room temperature.  Again using more time and electricity than needed.

  • somewhere between.  These still do not adhere to the requirements of the glass.

• Cool rates with staged soaks. There is no use for soaks below the strain point (about 470°C/800°F) as any annealing stress is locked in by that time.

• Cool rates unrelated anneal soak lengths.  Adequate cool rates are directly (but inversely – longer soak = slower cool rates) related to the length of the anneal soak.

• Cool rates ending at 371°C/700°F which are inadequate for thick glass.

They are strange, because there is a well researched document giving annealing times and cooling rates published by Bullseye for Fahrenheit and Celsius, 

It may be understandable that users of fusing glasses other than Bullseye are cautious about the use of this table.  But it is a well researched table supported by independent academic research, which is applicable to all soda lime glass. [McLellan and Shand (1984), Glass Engineering Handbook, 3rd Edition, New York, McGraw Hill]  Only the annealing temperature needs to be changed, as all the rates and times apply equally to all fusing glasses, and even float glass.

Yes, sometimes changes are required for:

• Profile

  • Contour fuse needs times and rates for 1.5 times the thickest part

  • Tack fuse needs times and rates for 2 times the thickest part

  • Sharp tack or sinter needs times and rates for 2.5 times the thickest part

• Contrasting colours, and differing viscosities need times and rates for at least 3mm thicker than the profile indicates

• Extreme forming such as drops and deep slumps also need times and rates for at least 3mm thicker than the profile indicates

These matters are explained in greater detail in my e-book Annealing Concepts, Principle and Practice available from Bullseye and Etsy

What are the Effects of Firing on the Kiln Floor?

Credit: The Pottery Wheel

There will be differences when firing on the floor of a kiln - the two important ones are working temperature and annealing.

Differential temperatures 

When firing on the floor of the kiln, expect the effective temperature to be a little lower than when on an elevated shelf. The temperature is always lower at the floor of the kiln and hotter higher in the kiln. This effect is often experienced with glass nearer the elements than usual.

This differential temperature between the floor and the top of the kiln is alleviated to some extent by the infrared heating, if the glass is exposed to it. If the glass is shaded, there will be a distinct difference.

Annealing

Annealing and cooling will be affected most. On the floor the glass can only cool from the top surface, as the bottom of the glass can only cool as fast as the cooling of the kiln. The annealing soak needs to be longer and the cooling needs to be slower than with an elevated shelf.

I suggest that firing for one layer thicker than calculated for the profile will cope with both these conditions.

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.

What is the Annealing and Cooling Relationship?

 Annealing Includes Cooling

Often people recommend a long anneal soak for potentially difficult pieces followed by an arbitrary 55C/100F cool rate to 371C/700F or 319C/600F. It is arbitrary because the same rate is frequently recommended regardless of the length of the anneal soak.

It does not have to be guesswork. Bullseye has provided us with the science of the anneal/cool in an accessible form: Annealing Thick Slabs (which covers thicknesses of 6mm/.025” to 200mm/8”). This document provides the annealing time for the chosen thickness and the directly linked cooling rates based on scientific principles.

The anneal soak is determined by the profile and thickness of the piece. Work for the e-book Low Temperature Kilnforming showed a relationship between the profile and the annealing time. Annealing for the profiles of sintering, tack, contour and full fuses requires calculation of the thickness to be applied to various profiles:

• Sinter or lamination – 2.5 times the thickest part
• Tack fuse – 2 times the thickest part
• Contour fuse – 1.5 times the thickest part
• Full/flat fuse – 1 times the thickest part

The cool stages are not random either. They have an intimate but inverse relationship to the anneal soak. They are to keep temperature differentials within the glass to acceptable levels. The anneal soak determined by the profile and thickness is to attain and keep the internal temperature within a range of 5°C throughout the glass. This is often referred to as ∆T=5°C.

The cool rates are to maintain acceptable temperature differences within the glass. The first cool rate is to maintain that temperature differential of ∆T=5°C. The second cool rate allows a wider range of temperature differential of 10°C, or ∆T=10°C. This is possible because the glass has become viscous enough to withstand this greater range of different temperature. The final cool needs to maintained at a differential of 20°C, or ∆T=20°C. Again, this is possible because the viscosity is high enough to withstand this amount of differential.

This information about cool rates and an allowable ∆T spread also indicate that turning off the kiln at 371°C/700°F is not always safe. It is almost always safe to do this for anything calculated to be annealed as for 12mm thick, and it may be safe for a piece up to 15mm thick, but remember that a tack fused piece of two base layers and a further decorative layer needs annealing and cooling as for 19mm/0.75”. The Bullseye research shows that the cooling rate for this is less than the unpowered cooling rate of many kilns. If there are additional complicating factors such as strongly contrasting colours, the annealing and cooling needs to be longer and slower than a simple multiplication of thickness.

There seems to be a practice of a single annealing rate to 371°C/700°F or 319C/600F. So the question will arise “Why is it necessary to have multiple cooling stages.” The response is that it will use unnecessary time and power. Attempting to maintain the ∆T=5°C over extended temperature ranges will not provide extra sound annealing. As the glass can withstand a ∆T=10°C from 427°C/800°F to 371°C/700°F, there is less power required at the faster rate than the slow one. This is even more so for the cool to 319C/600F and lower temperatures.

Knowing the safe temperature to turn the kiln off, requires knowing the cooling rate of the unpowered kiln. This blog shows how to determine the natural cooling rate of your kiln.  Knowing this is as important as knowing what effect different fusing temperatures have on the glass in your kiln.

The object of this blog post is to demonstrate that cooling is part of annealing. Just as much attention must be paid to the cooling rates as the length of the annealing soak. They are inseparable for sound kilnforming practices.

Some work that may be of assistance in understanding the importance of knowing the relationship between the annealing soak and the annealing cool are:

Annealing Concepts, Principles, and Practice

Available from: Bullseye and Etsy

Kilnforming Principles and Practice

Available from: Bullseye  and Warm Glass

Low Temperature Kilnforming

Available from: Bullseye and Etsy

Is White a Difficult Glass?

Description of the Project

A white 3mm base with 3mm and 6mm decorations made up of mosaic pieces from previously fused glass (all the same CoE). At the end of the firing three corners had broken and their edges rounded. The fourth corner had sharp edges. The tentative conclusion was that there was incompatibility between the white and the previously fired pieces. There were no other cracks visible on the white or between the mosaic pieces. The author did not indicate what the schedule was for either firing, nor what the profile of the last firing was, but asserts white is a particularly difficult glass which does not work well with a wide variety of colours.

My observations are: 

• Compatibility is not an issue on the heat up. It is only a problem at annealing and cooling.
• Breaks on the ramp up (showing rounded edges at the conclusion of the firing) are normally the results of too fast rates.
• Breaks during cooling (showing sharp edges) are due to annealing, compatibility, cooling rates, or some combination of these.
• Previously fired glass can show some shift in compatibility and so needs slower up ramp rates than normal for the profile and thickness.
• Incompatibility between the base and the mosaic pieces would show up as breaks in the white glass under each top mosaic piece.
• Not all glass of the same CoE from different manufacturers is compatible.

Could this have been from incompatibility?

On the way to top temperature the pieces have not yet combined. The incompatibility will only show up during the cooling, as it is the imbalance of  viscosity and contraction between the fused pieces that cause the breaks.

Only one of the broken corners has those sharp edges, making incompatibility an improbable cause of the breaks. Further, incompatibility between the base and upper layers present either a crazed appearance at the connections, or simple breaks around the base of each decorative piece. Incompatibility would have multiple breaks all over the base, if not the top too. Finally, if the fired mosaic pieces were incompatible with the white glass, there would have been breaks throughout the whole piece, not just at the corners.

A further possibility is that the corners were very close to the sides of the kiln, because only the corners broke away from the piece,. If it was side fired, much slower rates are required. And all kilns tend to be cooler near the sides on the heat up than toward the centre, even if top fired.

My guess, based on the description, is that the up ramps were too fast, and the anneal was too short and the cool too fast. Unless the previously fused pieces were tested for stress it is not possible to know whether those were stressed before the final firing, which could have caused the break off of the three of the corners. The fourth corner break was on the cool down and is most likely to be too short an anneal and/or too quick a cool.

Is white glass especially difficult?

There is nothing in this piece to identify white glass as an extraordinarily difficult glass, or that a multiplicity of colours added to white would provoke breaks. The problems exhibited are most likely related to fast heat up ramp rates, and inadequate annealing and cooling.