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.

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.

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.

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.

What to do to Tweak Schedules?

What do you alter first – time or temperature – when tweaking schedules?

The choice is largely dependent on what the objective is and the process being used.

Since I am always trying to get the work done at the lowest temperature, I usually alter time first. This allows a greater amount of heat work which it the important element in getting the work done with the least risk.  I alter this general approach in relation to the process being conducted.

In casting, the objective is to get all the glass in the mould flowing and fully incorporated, so if past experience has shown too little flow at a relatively high temperature, my choice would be to extend the soak time. This avoids high temperature damage to the mould, and can help to avoid sucklers, or indents, in the casting.

In a full fuse, I attempt to get the work done in around 10 minutes. This implies that I would increase the temperature first and then the time if necessary.

At firings from contour to sharp tack and sinter fuses I tend to increase the soak time. The amount of time is determined by using peeks to know when the desired shape is achieved.

In general, peek at or near the top temperature. If the piece has not formed as desired, the time can be extended, with no extra firing needed.  This saves a lot of the extra firings that would be required.  And multiple firings still do not give you the time or temperature required for a single firing.

If, when you peek, the work has already been done, you can skip to the anneal. And on the next similar firing, use a slightly lower temperature, again peeking near the top temperature.

More detailed information about scheduling options can be found in the e-book Kilnforming Principles and Practices, available from Bullseye and Warm Glass

How do I Evaluate Some Suggestions about Annealing?

There are writings from a teacher attempting to make glass fusing simple.  Unfortunately, glass physics and chemistry are very complicated.  Attempting to avoid these complications leads to failures and other difficulties as the practitioner progresses. 

Proper annealing is one of the fundamentals to achieving sound kilnforming results.  Some suggestions have been made by a widely followed person to “simplify” the understanding of the annealing process.  Discussion of the meaning and importance of annealing can be found in many places, including here.  

Annealing temperatures

It has been suggested that the annealing temperatures can be inferred from the CoE of the glass that is being used. Discussion of what CoE is and is not can be found here and here.

Annealing temperatures are not directly related to the expansion coefficient (CoE) of the glass.  This can be shown from the published annealing temperatures for different glasses organised by presumed CoE:

·        “CoE96”: Wisssmach 96 - anneal at 482°C;  Oceanside - anneal at 515°C

·        “COE94”: Artista - anneal at 535°C

·        “CoE 93”: Kokomo - anneal between 507°C and 477°C – average 492°C

·        “CoE 90”: Bullseye - anneal at 482°C; Wissmach90 - anneal at 482°C; Uroboros FX90 - anneal at 525°C

·        “CoE 83”:

o   Pilkington (UK) float - anneal at 540°C;

o   typical USA float - anneal at 548°C;

o   Typical Australian float - anneal between 505°C and 525°C, average 515°C

This shows there is no direct relationship between CoE and annealing temperature.  Do not be tempted to use a CoE number to indicate an annealing temperature.  Go to the manufacturer’s web site to get the correct information.

Temperature equalisation soak

Annealing for any glass can occur over a range of temperatures.  The annealing point is the temperature at which the glass can most quickly be annealed.  However, the glass cannot be annealed if it is not all at the same temperature throughout the substance of the glass.  It has been shown through research done at the Bullseye Glass Company that a temperature difference of more than 5°C will leave stress within the glass piece. To ensure good annealing, adequate time must be given to the temperature equalisation process (annealing). 

From the Bullseye research the following times are required for an adequate anneal soak:

6mm /   1/4"            60 minutes

[9mm /  3/8"           90 minutes]

12mm  / 1/2"          120 minutes

[15mm  /   5/8"       150 minutes]

19mm   / 3/4"         180 minutes

[ ] = interpolated from the Bullseye chart for annealing thick slabs

Anneal Cooling

There are suggestions that a “second anneal” can be used on important pieces.  Other than observing that all pieces are important to the maker, the suggestion should be investigated.  On looking into the idea, it is essentially a second soak at 425°C, which is slightly below the strain point, rather than controlled cool from the anneal soak temperature.

It is reported that the Corning Museum of Glass considers 450°C as the lower strain point – the temperature below which no further relief of strain is possible.  This means that any secondary soak must occur above 450°C rather than the suggested 425°C. Such a soak is unnecessary if the appropriate cooling rates are used. 

Cooling Rate

Except in special circumstances, the cooling rate needs to be controlled as part of the annealing process.  Soaking the glass at the anneal is not the completion of the annealing.  Most practitioners follow the practice of choosing a slow rate of cooling from the annealing soak to some point below the strain point rather than a rapid one with a soak at the strain point temperature.

Annealing is not just the soak time (which is there to equalise the temperature), it is about the rate of the annealing cool too. The rate at which you cool is dependent on the thickness of the glass piece and whether it is all of one thickness or of variable thicknesses.

Even thickness

                                         Cooling rate

Dimension      time (mins)     to 427°C to 371°C

6mm              60                 83°C       150°C

9mm              90                 69°C       125°C

12mm            120                55°C       99°C

15mm            150                37°C       63°C

19mm            180                25°C       45°C

                                        Cooling rate

Dimension      time (mins)     to 800°F   to 700°F

0.25"              60                 150°F       270°F

0.375"            90                 124°F       225°F

0.5"               120                100°F       178°F

0.675"           150                67°F         114°F

0.75"             180                45°F         81°F

Tack fused/ uneven thickness

If your piece is tack fused, you need to treat the annealing rate and soak as though it were twice the actual total thickness. This gives the following times and rates:

Tack fused

Dimension (mm)                                Cooling rate

Actual     Calculated       time (mins)    to 427°C   to 371°C

6            12                 120                55°C       99°C

9            18                 150                25°C       45°C

12          25                 180                15°C       27°C

15          30                 300                9°C         18°C

18          38                 360                6.7°C       12°C

Dimension (inches)                                Cooling rate

Actual     Calculated       time (mins)    to 800°F   to 700°F

0.25          0.5                 120                100°F       180°F

0.375        0.75               150                45°F         81°F

0.5            1.0                180                27°F          497°F

0.675        1.25               300                16°F         36°F

0.75          1.5                360                12°F          22°F

Contour fusing requires firing as though the piece is 1.5 times thicker.  Sharp tack or laminating requires 2.5 times the the actual thickness.

Fusing on the floor of the kiln

There is a further possible complication if you are doing your fusing on the kiln floor, or a shelf resting on the floor of the kiln.  In this case you need to use the times and rates for glass that is at least 3mm thicker than the piece actually is. 

Thus, a flat 6mm piece on a shelf on the floor would use the times and rates for 9mm: anneal soak for 90 minutes, anneal cool at 69°C to 427°C and then at 124°C to 371°C.  It would be safest if you continued to control the cooling to room temperature at no more than 400°C per hour.

But if it were a tack fused piece of a total of 6mm you would use the times and rates for 18mm.  This is using the rates for twice the total thickness plus the additional 3mm for being on the base of the kiln.  This gives the times and rates as being an anneal soak of 360 minutes and cooling rates of 7°C to 427°C and 12°C to 370, followed by 40°C per hour to room temperature.  Any quicker rates should be tested for residual stress before use.

Source for the annealing and cooling of fused glass

These times and rates are based on the table derived from Bullseye research, which is published and available on the Bullseye site.   It is applicable to all fusing glass with adjustments for differing annealing soak temperatures.

Annealing over multiple firings

It has been recommended by a widely followed person that the annealing soak should be extended each time subsequent to the first firing.  I am uncertain about the reasoning behind this suggestion. But the reasons for discounting it are related to adequate annealing and what is done between firings.

If the annealing is adequate for the first firing, it will be adequate for subsequent firings unless you have made significant alterations to the piece.  If you have added another layer to a full fused piece, for example and are using a tack fuse, you will need to anneal for longer, because the style and thickness have been altered.  Not because it is a second firing.  If you are slumping a fired piece, the annealing does not need to be any different than the original firing.

The only time the annealing needs to be altered is when you have significantly changed the thickness of the piece, or the style of fusing (mainly tacking additional items to the full fused piece).  This is when you need to look at the schedules you are planning to use to ensure your heat up is slow enough, that your annealing soak is long enough, and the cool slow enough for the altered conditions.

Determining the annealing point of unknown glass

You don’t have to guess at the annealing temperature for an unknown glass.  You can test for it.  It is known as the slump point test.

This test gives the softening point of the glass and from that the annealing point can be calculated.  This test removes the guess work from choosing a temperature at which to perform the anneal soak. The anneal temperature is important to the result of the firing.  This alone makes this test to give certainty about the annealing temperature worthwhile.

You can anneal soak at the calculated temperature, or at 30°C below it to reduce the anneal cool time.  This is because the annealing can occur over a range of temperatures.  The annealing occurs slowly at the top and bottom of the range. But is at least risk of "fixing in" the stress of an uneven distribution of temperature during the cool when the annealing is done at the lower end of the range.

Do not be fooled into thinking that CoE determines annealing temperatures.  Use published tables, especially the Bullseye table Annealing for Thick Slabs to determine soak times and cooling rates.  Use the standard test for determining the softening and annealing points of unknown glasses.

Further information is available in the ebook Low Temperature Kiln Forming and in Annealing Concepts Principles and Practice 

Revised 14.10.25