Changing size in Slumping

 “I have full fused a single piece of glass with a few small pieces on top.  I thought it would shrink some as I had been told, but it maintained its size and still fit the mold for slumping.” 

I believe the enquirer is talking about a single layer circle changing size at full fuse.  Dog boning is much less evident in circles than rectangles.  The glass retreats evenly all along the edge.  This gives the appearance of retreating less than rectangles.  The absence of any big change in size may also result from thinning of the centre.  The amount of size change will be affected by the temperature of the full fuse too.  In this case there were additions which will have resisted any tendency to shrink.

Lower top temperatures, more rapid ramp rates to the top, and shorter holds will have the effect of limiting the movement of glass toward 6mm thick.

credit: Bullseye Glass Co

The viscosity of glass at full fuse is enough for it to attempt to pull up to 6mm. At casting temperatures, the viscosity is so low that 6mm of glass spreads out.  Temperature affects viscosity.

 

At slumping temperatures (620˚C - 680˚C / ca.1150˚F - 1260˚F), the viscosity high enough that the dimensions of a circle do not change. A circular piece of 3mm glass held at slumping temperatures does not change dimension.  It may, if held long enough take on a kind of satin sheen, rather than a fire polish.  But the viscosity  is low enough to allow the glass to form to the mould, given sufficient time. The resulting slumped piece will appear to be smaller than the mould. If you measure the piece around its outside curve, you will find the distance is almost the same as the diameter of the blank. 

 

Changing size on a single layer piece is dependent on the temperature and heat work applied to the piece.

Uneven Slumps on Deep Rectangular Moulds

 "Can anyone please tell me why this mould always comes out wonky and devitrifies and pulls in on the edges. I used Primo Primer; my kiln is level, and this is the slump schedule I use for 3mm base with 6mm in places [temperatures in Celsius]: 100/593/30 mins; 66/663/25 mins; 204/482/60 mins; 66/371/10 mins; END.*   12cm square."

 GlassAlisa

The suggestion has been made that having a 6mm base would lessen the irregular slump in the mould.  I am not convinced that making the base thicker will sort the problems.

 

1)  This is a very deep mould in relation to the span.  The mould sides are steep.

 

2) The glass slides down and picks up marks from sliding down the walls of the mould.  The marks are not devitrification.

 

3) Deep slumps are prone to going off centre. One fix is to watch and be prepared to reach in with wet sticks to readjust the glass placement on the mould.  

 

4) Deep moulds (deep is relative to the span of the mould) require two or more stages of slumping. Start with shallow a slump, and progress through steeper ones.

5) The sides dog bone on many rectangular moulds.  One way to reduce this is to round the corners with a 10mm radius.

 

6) Reducing the forming temperature, and extending the soak time dramatically, will go some way to alleviating the previous problems. I suggest trying a 620C slump temperature and soak for 2 - 3 hours.  Peek at intervals to see when the slump is complete, then advance to anneal and cool.

In my view, it is a mould from a maker that does not fully understand glass behaviour.

And in passing, the ramp speed from top temperature to annealing should be as fast as possible, to avoid any risk of devitrification on the way down.  

* Schedule in Fahrenheit for the Americans.  

• 198 to 1100, 30' 
• 119 to 1225, 25'
• 367 to 900, 60'  [ASAP is the recommended rate.  As it is a tack fused piece, I would anneal as though 12mm/4 layers.  This would use a 2 hour soak, cool at 100 to 800, 180 to 700, off ]
• 120 to 700, 10'
• End

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.

Kilnforming with 3mm Glass

 A power point presentation I made a few months ago to the group Lunch with a Glass Artist.

It is 33 slides long.

Kilnforming with 3mm Glass.pptx

Annealing Tack Fused Pieces

"I'd like advice about annealing. I'm about to start a series that are to be wall hangings. The outside 100mm is only 3mm thick and the centre is 6mm and occasionally 9mm thick. They are going to be A3 and A2 size. I intend to tack fuse. I'm happy to experiment with the tack fuse temperature (I think about 677°C). How long should I anneal it? That's my question."

Determining the Tack Temperature

The high end of slumping, and the low end of sintering is 677°C/1252°F. Unless your kiln fires very hot, this is not hot enough for a tack fuse. Make some small-scale mock-ups in clear. Schedule the kiln to a full fuse on a rate not more than 300°C/540°F per hour. Peek into the kiln at 10°C/18°F intervals from 677°C/1252°F upwards. When you see the profile you want, note the temperature. When scheduling the tack firing, reduce the target temperature by 5C and add a 10-15 minute soak to get approximately the same result as you observed in the test firing.

Scheduling to Avoid Dog Boning

You have a border of 100mm/4.0” that is only one layer thick. This has the risk of becoming irregular at the corners compared to the sides (dog boning). To avoid dog boning of the 3mm base, the lowest temperature you can use is important. This is the main reason for the peeking – to determine the temperature at which dog boning begins. It is not only the degree of rounding of the edges you are looking for, but also the degree of retraction of the sides of the piece. When you note the beginning of the dog boning, you have reached just beyond the temperature to avoid that.

You will of course have to set the mock-up in such a way that you can see at least one side through the peep hole. The front will not give you accurate information, but if the side is in your sight line, you will see when it begins to deform. This peeking will keep you occupied for about 3/4 hour. Make sure you have gridded paper and pencil to hand to record information in between peeking.

It may be that the glass has not begun to round when the dog boning starts. In this case you will need to make the border larger and cut the glass back to straight lines. 20-30mm/0.75-1.125” extra all around will make it easy to trim the excess cleanly.

Annealing

I do not know the degree of tack you are aiming to achieve. It is important to the scheduling of the anneal. A sharp tack profile will require annealing for longer than a contour profile for your thicknesses. These suggestions assume the total height is 9mm/0.375”. If it higher, the soak and cooling times and rates will be longer and slower.

A sharp tack profile will need:

• Annealing for 270 minutes (4.5 hours) with a cool rate of:

First 55°C/100°F cool at 13°C/23°F per hour.

Second 55°C/100°F cool at 23°C/41°F per hour.

Final cool at 78°C/140°F per hour to room temperature.

A rounded tack profile will need:

• Annealing for 180 minutes (3 hours) with a cool rate of:
• First 55°C/100°F cool at 25°C /45°F per hour
• Second 55°C/100°F cool at 45°C /81°F per hour
• Final cool at 150°C /270°F per hour to room temperature.

A contour tack profile will need:

• Annealing for 120 minutes (2 hours) with a cool rate of:
• First 55°C/120°F cool at 55°C /99°F per hour
• Second 55°C/100°F cool at 99°C /178°F per hour
• Final cool at 330°C /216°F per hour to room temperature.

More detailed information is in my eBook Low Temperature Kilnforming, An Evidence-Based approach to Scheduling. It is available from VerrierStudio on Etsy or through Bullseye. 

It is not cheap, but at 300pp worth it (in my opinion!). It discusses the three profiles of tack fusing - sharp, rounded, contour. It also deals with slumping, sintering, freeze and fuse, and bas relief or texture mould firings. The method for determining schedules is outlined and specific sample schedules are listed.

Slump Shrinkage

Glass on rectangular moulds often does not maintain a straight edge.  It pulls in and tends toward the “dog boning” of fused single layer glass even if not so dramatic.

Explanation

The reasons for the pull-in on rectangular moulds are similar to those for dog boning. You should note that squares are special cases of the general class of rectangles. The discussion here applies squares just as much as to rectangles.

If you grid the rectangular glass, it illustrates that the glass in the corners is moving in two directions.  It is moving and slightly stretching into the mould.  At the same time, it is trying to compress into the corner of the mould.  The glass along the sides are moving in only one direction – stretching only slightly and moving toward the bottom of the mould.

There is more compression than stretching in the corners. The sides have only to move in one direction and experience no compression and so move toward the bottom more easily.

Such is my explanation of the experience. 

Avoidance

The real question then is how to prevent this pull-in that is so commonly experienced on rectangular moulds with no rims.  One way would be to avoid such moulds altogether.  This of course, is not practical, so some approaches to compensate or avoid the problem are needed.

It is possible to compensate for this pull-in by slumping a rectangle with slightly bulging sides.  Rather than a regular rectangle, you create one with slightly outwardly curved sides.  Getting the exact amount of curve will be difficult and achieved only after a number of experiments.

The opposite compensation would be to round the corners of the glass, so there will not be so much glass to fit into the corners of the mould.  This again will require experimentation to achieve a predictable result.  And it often would interfere with the appearance of the final piece.

The easiest, but not always successful, way to prevent the pull-in is to alter the scheduling for slumps on such moulds.  It is a well-known property of glass that it does not have a single softening point, but progressively softens with temperature and time.  You can take advantage of this by using four elements in combination. 

·        Use a slow rate of advance to the slump temperature, to allow the glass to evenly absorb a lot of heat on the way to slumping. 

·        Use a low slumping temperature  This may be as much as 30°C less than your usual temperature.

·        Use a long soak at the slumping temperature.  This may be hours.  You need to allow the glass to slump into the mould without stretching.  To avoid stretching, you need a low temperature.  At low temperatures, the glass requires a lot of time to conform to the mould.

·        Observe at 10- to 15-minute intervals once the slumping temperature is achieved.

These processes are outlined in a blog post on dog boning.  Further information is available in the ebook: Low Temperature Kiln Forming.

Avoidance of pull-in of the glass on rectangular moulds is related to scheduling and observance.  There are some compensations that can be tried, but require considerable experimentation to be successful.

Preventing dog boning

Firing a single layer, even with decorative elements on top, is most likely to “dog bone” due to lack of volume.  With a single layer you are always going to have difficulties with volume control. 

Photo credit: Paul Tarlow

Unless you are satisfied with an angular tack fuse at the lower end of the tack fusing range, you will always run the risk of dog boning. All the other variations of tack fusing use increased temperatures causing the glass to begin to pull in along the long sides to a greater or lesser extent (more with contour fuse, less with angular tack fuse).

Dog boning occurs because as the glass softens and the edges begin to round, the viscosity takes over from the solid phase of glass as a major force.  Viscosity can be thought of as an approximation of surface tension. 

Glass is a material with a plastic range over several hundred degrees.  This means that the hotter the glass becomes, the less stiff it becomes, and the viscosity force thickens the glass toward 6-7mm in the kilnforming temperature range. The greater the temperature, the more the glass pulls into a ball shape, or in the case of sheets, thickens at the edges and thins in the middle.  Higher temperatures reduce the viscosity to the extent that it becomes as thin as one millimetre.

Trick the glass

To avoid dog boning on tack and full fusing, you have to trick the glass with some special scheduling.

The trick employs the concept of heat work.  The nature of glass allows you to put a lot of heat work into a piece by soaking for a long time at a low temperature.  You might think of it as a kind of sintering.

A description of sintering:

The atoms in the [glass] diffuse across the boundaries of the particles, fusing the particles together and creating one solid piece. [This can be done by heat at low temperatures with extended soaks.]  Examples of pressure-driven sintering are the compacting of snowfall to a glacier, or the forming of a hard snowball by pressing loose snow together.   https://en.wikipedia.org/wiki/Sintering

By sintering (sometimes called fuse to stick) or - in kilnforming terms - by the use of heat work you can achieve the result you want without dog boning.

By taking the temperature slowly to about 700°C to 720°C and soaking there for two to four hours you can achieve a rounded tack fuse without dog boning.  You will have to experiment with the exact temperature and length of soak to get exactly what you want.

The length of soak time or exact temperature is not vital.  The two in combination will achieve the effect you want.  The importance of observation of your firing is re-enforced in the cases of sintering.  You cannot be sure until you check during the firing whether the edges of the glass are rounded enough for your purpose. That observation will also tell you whether a slightly raised temperature would be useful.  You will learn the time required to achieve the effect by recording the soak time when you advance to the anneal soak and cool.

Further information is available in the ebook Low Temperature Kiln Forming.

By the use of heat work in kilnforming you can achieve tack fused pieces without dog boning.

Dog boning in Slumping

Often even in shallow rectangular moulds the sides pull in during the slump.  To know what things to try to correct this effect, you need to understand why this effect is occurring.  These two pieces show the effect in different ways.

ebay 0916_slump_01

 This slump shows that even with thick glass the sides curve inwards even on shallow slumps.

theglassundergroundnj.org

This slump shows the interesting effect that the further up the piece you look, the greater the curvature. This relates to the greater amount of movement required by the glass to conform to the mould at the outer edges.

Why

During the slump of a rectangle or square the whole shape of the glass sheet is changing.  It is slightly stretching to form into the “hollow” of the mould, but it cannot stretch evenly all over, especially at the corners.  If you think of the analogy of Draping a piece of cloth into a rectangular depression, you will find it wrinkles up at the corners if you smooth it at the sides. This indicates the material is attempting to overlap there as it does not have a dart to take up the excess cloth.

This similar to what is happening to the glass sheet.  It is relatively thicker at the corners than along the sides.  Therefore, it does not slide down the mould at the corners as on the sides. It is simply thicker and is compressed by the movement of the glass at the sides.

Prevention

The question is how to use that knowledge to avoid or minimise the dog boning during the slump.  There are probably lots of methods, but three have occurred to me and others.

Add more material along the sides.  This involves fusing a piece with shallow arcs rather than straight sides.  This gives more material to counteract the dog boning effect when slumping a rectangle.  The difficulty is getting the proportions of the arc correct in relation to the length of the sides. You also need to ensure the arcs on the sides are not so much larger than the mould that they slump over the edge.  This means the whole piece will need to be cut smaller than the mould.

Remove material at the corners.  This takes the opposite approach.  To avoid the increased amount of glass at the corners, you remove some of it.  That is, you round the corners of the pieces to be fused. How much you will need to round the corners is a matter of experience, but is a shorter learning curve than cutting the edges in an arc.

Reduce the temp and increase soak time.  This approach requires less skill in cutting a shape.  It relies on giving the glass time to relax into mould with a minimum of stretch.  You need to find the lowest practical temperature at which to slump.  This will be the temperature at which you can first see the deformation of the glass in the mould.  Hold the temperature there for as long as it takes – possibly one or two hours. It is likely that you will still need some rounding of the corners of the glass, but only your experience will determine that, and if so how much.

Cold work the edges until straight.  This can be done by hand or by machine.

Further information is available in the ebook: Low Temperature Kiln Forming.