Showing posts with label Slumping. Show all posts
Showing posts with label Slumping. Show all posts

Wednesday 6 March 2024

Slumping Strategy

A schedule was presented for a slumping problem of a 6mm/0.25” blank.  It consisted of three segments each of a rate of 277C/500F with short holds up to 399C/750F and then a rapid rise to 745C/1375F.  The cool was done with two long holds at 537C/1000F and 482C/900F followed by cooling rates for 12mm/0.5”



My response was that, yes it was fired too high.  Not only that, but the firing strategy, as shown by the schedule, is odd. 

Strategy

The general strategy for slumping follows these ideas.

·        Glass is slow to absorb heat, and in one sense, this schedule accepts that by having short soaks at intervals.  As glass is slow to absorb heat, it is necessary to use slow ramp rates and without pauses and changes in rates.  This should be applied all the way to the slumping temperature.

·        Holds of short durations are not effective at any stage in a slumping firing.  The objective is to allow the glass time to form to the mould with as little marking as possible.  This implies slow rates to low temperatures with significant holds at appropriate stages.  This about putting enough heat work into the glass that higher temperatures are not needed.

·        This kind of firing requires observation for new moulds and new arrangements of glass to ensure the slump is complete.  Once you know the mould requirements and are repeating the layup of the glass, the firing records will tell you what rates and times to use to get a complete slump with minimum marking.

·        The hold at annealing temperature is to equalise the temperature throughout the glass to produce a stress-free result.  Any soaks above are negated or repeated by the necessary soak at the annealing temperature.  The hold there must be long enough to complete the temperature equalisation that is the annealing.

·        My work has shown that annealing for one (3mm/0.125”) layer thicker produces a piece with less stress.  This indicates that a 6mm/0.25” piece should be annealed as for 9mm/0.35” to get the best result.

The summary of the firing strategy for slumping is:

  • ·        A single ramp of a slow rate to the slumping temperature.
  • ·        Observation of the progress of the slump to determine the lowest practical temperature and hold time.
  • ·        Annealing for one layer thicker that being slumped.
  • ·        Three stage cooling of the piece at rates related to the annealing hold.

Critique

This is a critique of the schedule. For comparison, my schedule for a full fused 6mm blank would be different.

  • ·        140ºC/250ºF to 677º/1250ºF for 30 to 45 minutes.
  • ·        9999 to 482ºC/900ºF for 1.5 hours
  • ·        69ºC/124ºF to 427ºC/800ºF, no hold
  • ·        125ºC/225ºF to 371ºC/700ºF, no hold
  • ·        330ºC/600ºF to room temperature, off.

The rate of the published schedule is fast for a full fused blank and extremely fast for a tack fused blank. This needs to be slowed.  The schedule provides a single (fast) rate of heating, but with unnecessary holds.  The holds are so short as to be ineffective, anyway. There is no need for the holds on the way up to the slumping temperature.  In general slumping schedules are of fewer segments.   This is because glass behaves well with steady slow inputs of heat.

Then strangely, the schedule increases the rate to top temperature.  It does so with a brief soak at 593ºC/1100ºF.  This fast rate of 333ºC/ 600ºF begins at 400ºC/750ºF.  This is still in the brittle phase of the glass and risks breaking the glass.  The brittle stage ends around 540ºC/ 1005ºF.

This rapid rate softens the surface and edges of the glass without allowing time for the underside to catch up.  This explains uneven edges.  It also risks breaking the glass from too great expansion of the top before the bottom.

Additionally, the schedule uses a temperature more than 55ºC/100ºF above what is a reasonable highest slumping temperature.  The top temperature of this schedule is in the tack fusing range.

There is no need for a hold 55ºC/100ºF above annealing soak. It is the annealing soak that equalises the temperature before the cool begins.  The higher temperature equalisation is negated by the cooler soak at annealing temperature. So, the hold at the higher temperature and slow cool to the annealing temperature only delays the firing by about two hours.  It does not have any effect on the final piece.

The schedule is cooling for a piece of 12mm/0.5”.  This is slower than necessary.  As noted above, cooling for one layer thicker than the piece is advisable to get the most stress free result.  The annealing soak could be 1.5 hours following this idea.  Cooling with a three stage schedule reduces the risk of inducing temporary stresses that might break the glass.  Although the initial cooling rate I recommend is very similar to this schedule, it safely reduces the total cooling time.

  • ·        69ºC/124ºF to 427ºC/800ºF, no hold
  • ·        125ºC/225ºF to 371ºC/700ºF, no hold
  • ·        330ºC/600ºF to room temperature, off.

Using my kind of schedule for the first time will require peeking once top temperature is reached to determine when the slump is complete. It may take as much as an hour. Be prepared to either extend the hold, or to skip to the next segment if complete earlier. The controller manual will explain how.

 More information is given in Low Temperature Kilnforming, An Evidence-based guide to scheduling.  Available from Etsy and Bullseye


Wednesday 6 December 2023

Dog Boning During Slumping

Does the size of the rim affect the amount of dog boning when slumping rectangular items?

This question was prompted by previous testing on the amount of distortion by adding additional elements. I found that single layer pieces stacked 15mm/0.6” or more from the edge do not affect its shape.

This led me to think: “how wide a rim would be required to avoid dog boning of rectangular pieces while slumping?” The premise was that there must be some relation to the width of the rim and the amount of dog boning.



Method

The method I chose was to make two vermiculite moulds. One with an almost square aperture and the other with a rectangular one. These were not large pieces. 

  • One was 27cm by 22cm/ 10.6” by 8.66” with an opening of 10cm by 10.5cm/4” by 4.12”. 
  • The other was 25cm by 22cm/9.84” by 25cm/8.66” with an opening of 19.5cm by 13cm/7.68” by 5.1”. 
  • Both had a drop of 25mm/1”.

The sizes of the rim were proportional to the opening of the mould. The remainder of the mould was merely a support to the rim.

The firing schedule for all pieces was kept the same.

  • Ramp 1   220˚C/396˚F to 677˚C/1252˚F     hold for 1.75 hrs
  • Ramp 2   Full to 482˚C/900˚F                     hold for 1.0 hours
  • Ramp 3   83˚C/150˚F to 427˚C/800˚F         Hold for 0 hours
  • Ramp 4   150˚C/270˚F to 371˚C/700˚F        Hold for 0 hours
  • Ramp 5   300˚C/540˚F to 50˚C/122˚F         Off

Results for single layer slumping

Various widths of single layer rim were tested from 1cm/0.4” to 3cm/1.18” at 2.5cm/1” deep. The 2cm/0.79” rim was also tested at 3cm/1.18” and 3.8cm/1.5” deep.

Square openings

The results showed there is no further reduction in dog boning with rims greater than 2cm/0.79” for square apertures of this size. The dog boning of a 1cm/0.4” rim was 1.5mm/0.6”. The amount of deflection from straight was 0.5mm/0.02” for both 2cm/.079” and 3cm/1.18” rims.

There was no effect of increasing the depth of the slump to 3.8cm/1.5” on a 2cm/0.79” rim.

Rectangular openings

The results were different for slumps into rectangular apertures. The glass on the long side of the opening had greater dog boning at all rim widths from 1.25cm/0.5” to 3cm/1.18” than the shorter side.

  • ·   A 1.25cm/0.5” rim deformed 3mm/1.18” on the long side and 2.5mm/0.98” on the short one.
  • ·   With a 2.5cm/1.0” rim the deformation on the long side was 2.5mm/0.98”. The short side of the opening was 1.5mm/0.6”.
  • ·   A rim of 3cm/1.5” deformed 1mm/0.02” on the long side. The short side of the opening deformed 0.5mm/0.02”.

Results for Two Layer Slumping

The big surprise for me was the greater amount of dog boning on the slumping of two layers. I expected less.

The two-layer slumping was done on the same moulds with the same schedule. The results of greater rim widths showed gradual reductions in the amount of dog boning. But there was significant sensitivity to the difference in the square opening.

Square Opening

The square opening is only slightly rectangular by 5mm/0.02” but the 6mm/0.25” glass reacted to that small difference. The amount of dog boning with a 2cm/0.79” rim was 4.5mm/0.18” on the long side. But 2mm/0.18” on the side only 5mm/0.02” shorter. 

This amount of dog boning reduced gradually until with a 5cm/2” rim the deflection was 3mm/0.12” on the long side. The deflection was too small to measure on the short side.

Rectangular openings

The rectangular opening was 1.5 times longer than wide. This had significant effects on the extent of dog boning. Although increasing the rim width did reduce the deformation, the long side continued to exhibit greater deformation than the short one.

  • ·   With a 3cm/1.5” rim, the long side deformed by 4.5mm/0.12”. The short side by 3.5mm/0.14”.
  • ·   A rim of 3.5cm/ reduced the deformation to 4mm/0.16 on the long side. But 2mm/0.08” on the short side.
  • ·   At 4cm/1.57” the rim deformed 2mm/0.12” on the long side and 1mm/ on the short one.
  • ·   Strangely, a 4.5cm/1.77” rim had a little larger deformation than the 4cm/1.57” rim. It was 3mm/0.12” on the long and 2mm/0.08” on the short side. It may be that the greater length of the rim contributed to increased dog boning.

 

A general reflection on the two-layer tests. 

It is possible that there was too long a hold at 677c for 6mm. I did not do a check on the time it took to reach full slump. The long soak was required to get the single layer to conform to the mould. At the time, my requirement was to keep the firing of single and double layer slumping the same for comparison. Perhaps keeping that hold constant was the wrong decision. Further testing will be required.

 

Summary

I learned some things from these (incomplete) tests that I did not expect. This is good for my learning. The things I found out are:

  • ·        In general, the wider the rim is, the less dog boning occurs.
  • ·        The extent of dog boning is more sensitive to the dimensions of the opening than to the size of the rim for both single and double layers.
  • ·        The depth of the slump of a single layer has less influence than the size of the rim. Once the rim is of sufficient size to minimise the dog boning, the increase of the depth by 20% or 50% did not affect the dog boning.
  • ·        Thicker glass with the same schedule deforms more than single layers. This does need more investigation, though.

 

More Informaton:

The basic cause of dog boning is related to volume control.

The causes of dog boning other than volume control.

More about the effects in slumping.

Much more information is available in the eBook Low Temperature Kilnforming.


Wednesday 22 November 2023

Slumping Schedules


 When slumping fired pieces, it is most often appropriate to use a slow ramp rate to avoid too rapid expansion of the glass that might lead to a break. Most glass breaks on the ramp up are below 300°C/573°F. It is in this range that there is a rapid expansion of ceramic. This means a slow rate is protective for both glass and ceramic moulds.


Slumping Schedules by Profile (Celsius)

Flat Fuse and Contour Tack

Actual thickness

Ramp 1 rate to 260°C

Soak time (min)

Ramp 2 rate

Slumping  temp. for mould *

Soak time (min)

Anneal as for contour:

6

240

20

240

 

30

9mm

Rounded Tack

Actual thickness

Ramp 1 rate to 260°C

Soak time (min)

Ramp 2 rate

Slumping  temp. for mould *

Soak time (min)

Anneal as for round tack:

6

150

20

150

 

30

9mm

Sharp Tack

Actual thickness

Ramp 1 rate to 260°C

Soak time (min)

Ramp 2 rate

Slumping  temp. for mould *

Soak time (min)

Anneal as for sharp tack:

6

120

20

120

 

30

9mm

 

Slumping Schedules by Profile (Fahrenheit)

Flat Fuse and Contour Tack

Actual thickness

Ramp 1 rate to 500°F

Soak time (min)

Ramp 2 rate

Slumping  temp. for mould *

Soak time (min)

Anneal as for:

0.250”

432

20

540

 

30

0.375”

Rounded Tack

Actual thickness

Ramp 1 rate to 500°F

Soak time (min)

Ramp 2 rate

Slumping  temp. for mould *

Soak time (min)

Anneal for:

0.250”

270

20

270

 

30

0.375”

Sharp Tack

Actual thickness

Ramp 1 rate to 500°F

Soak time (min)

Ramp 2 rate

Slumping  temp. for mould *

Soak time (min)

Anneal for:

0.250”

216

20

216

 

30

0.375”

 

*Of course, the slumping temperature will be altered for the glass according to the manufacturer’s stated range. The nature of the mould will also have a big effect on temperature and time. The soak times at the slump soak are those appropriate for the mould. The annealing soaks are related to the profile of the glass.


Rates

It is most often best to use a slow ramp rate to at least 500°C/933°F. This avoids the risk of inducing a too rapid differential expansion within the glass as it heats up. Experiments relating to the first ramp rate have shown firing as for two layers thicker than indicated by the profile schedule provides the best results. It is then possible to increase the rate as determined by the profile schedule.

The rates for the anneal soak and cool are those that are one layer thicker than determined by the schedule for the profile. This has been shown by experimentation to give the best annealing result – i.e., least stress.

Temperatures

The slumping temperature needs to be altered for two factors:

  • ·        the glass according to the manufacturer’s stated range, and
  • ·        the nature of the mould.

Many manufacturers are giving recommended temperatures and times for slumping in their moulds. An example is the Bullseye “Quick Tip” which gives suggested temperatures and times for various sizes and natures of moulds that can form the basis for scheduling of slumps. The rates are normally for flat uniformly thick pieces. This will need alteration for tack profile pieces.

Take note of the soak time in these recommendations. If it is less than 10 minutes, it is possible to reduce the temperature by about 10°C/18°F by using a 30-minute soak. This will reduce marking on the back of the glass.

Soaks / Holds

Slumping schedules tend to be more difficult to devise than many other operations in kilnforming because of variations in moulds and what is placed on them. This, consequently, makes observation of the slump more important. It is needed from a point below the target temperature – say 20°C/36°F – to ensure the slump is stopped when it is complete.  If it is not complete, the soak can be extended. The controller manual will give the information on how to do these operations. In general, you schedule slower ramp rates for thicker pieces in combination with the half hour soak. This means for each thickness greater than 6mm, the top temperature can be reduced and still achieve a full slump.

The schedules here are applicable for pieces up to 9mm actual thickness.

Slumping of thicker pieces needs to apply the underlying scheduling method:

  • ·        Apply the rate for two layers thicker for the advance to 260°C/500°F.
  • ·        Increase the rate after that to one for a single layer thicker than calculated all the way up to the slumping temperature.
  • ·        For annealing, also select the rates and times for one layer thicker than indicated by the profile.

 

For example:

  • ·        Rounded Tack of Bullseye, 12mm/0.5” thickness
  • ·        Schedule for 25mm/1” (2 times multiplier)
  • ·        Initial ramp rate for 31mm/1.25” (two thickness greater)

Celsius schedule for up to 9mm actual thickness:

Segment >

1

2

3

4

5

6

7

Rate

150

150

ASAP

15

27

90

off

Temp

260

Top

482

427

370

RT

 

Time(mins)

20

30

240

0

0

0

 

and in Fahrenheit:

Segment >

1

2

3

4

5

6

7

Rate

270

270

ASAP

27

49

162

off

Temp

500

Top

900

800

700

RT

 

Time(mins)

20

30

240

0

0

0

 

 

A further example:

  • ·        Sharp Tack of Bullseye, 0.5” thickness
  • ·        Schedule for 31mm/1.25” (2.5 times multiplier)
  • ·        Initial ramp rate for 38mm/1.5” (two thickness greater)

 Celsius schedule for up to 9mm actual thickness:

Segment >

1

2

3

4

5

6

7

Rate

78

78

ASAP

11

20

65

off

Temp

260

Top

482

427

370

RT

 

Time(mins)

20

30

300

0

0

0

 

and in Fahrenheit:

Segment >

1

2

3

4

5

6

7

Rate

140

140

ASAP

20

36

117

off

Temp

500

Top

900

800

700

RT

 

Time(mins)

20

30

300

0

0

0

 

 

These examples show that considerable differences in scheduling are needed for different tack profiles. It also shows longer annealing soaks and slower cooling rates are required for sharp than rounded tack pieces.

 

More information is given in the e-Book Low Temperature Kilnforming. and at Bullseye eBooks