Deep Slumps with Bubbles

 

Photo Credit: Rachel Meadows-Ibrahim

The main causes of the large thin bubble is most probably  too high a temperature combined with a long soak.

Elevation of the Mould

The poster indicated there are eight holes total – four on the sides and four under the glass. This means any air has an exit out from under the glass and from the inside of the mould. So, in this case it does not need to be elevated for exit of air.  In my practice l have never, except in tests, elevated my slumping moulds. I have not had failures. My experiments involved in writing the eBook Low Temperature Kilnforming  showed no significant temperature differences between elevating, or not, below the mould.

Effect of Fast Rates

Slow rates to low temperatures with long soaks avoid sealing the glass to the mould. This means air can move out from under the glass during the slump. 

• Fast rates, and elevated temperatures can restrict air movement from under the slumping glass.  
• Fast rates and high slump temperatures can each cause uprisings because the glass slides down the mould during the soak, and that weight pushes the bottom upwards.

Temperature and Uprisings

This uprising is different from the bubble at the bottom on this piece. It is possible to see the glass bubble is thinner than the surrounding glass. As there were holes for air to escape, it seems the temperature and or speed was great enough to allow the glass to form to the mould at the bottom.  This covered the air holes and allowed the remaining air to push upwards on the glass.  A lower top temperature may have avoided this bubble formation.  Certainly, a combination of a slower rate and a lower temperature would have avoided the formation of the bubble.

Observation

Further, observation during the firing would have caught this bubble formation early enough to skip to the annealing and result in a piece with only a slight uprising, and before it became a bubble. Peeking should start at the beginning of the slumping soak and be repeated at 5 to 10 minute intervals.

Visible Devitrification

"Why does devitrification appear on slumped pieces?"

A brief explanation 

Scientific research in developing a glass matrix to support bone grafts gives some information.  This kind of glass matrix requires to be strong.  Development showed that devitrification weakens the matrix.  The crystals in a matrix are not as strong as the amorphous glassy state.  So, devitrification needs to be avoided.

The research to avoid devitrification showed that it begins at about 600˚C/1110˚F.  It only begins to become visible above 700˚C/1300˚F.  The process developed was to introduce a “foaming” agent.  The process fired slowly to 600˚C/1110 ˚F and then quickly to 830˚C/ 1530˚F.  It left a strong open matrix around which bone can grow. Although the research used float glass, it is also a soda lime glass, just as fusing glasses are.  The formation of devitrification begins at the same temperature for fusing glasses as for float.

The result of this medical research shows that devitrification begins on glass before it is visible. Devitrification is cumulative. A little becomes greater with another firing.  This is so even with good cleaning between firings. The new devitrification builds on the previous.  It does this from 600˚C/1110 ˚F.

A subsequent firing can continue this devitrification to the point where it is visible. This can happen, although the temperature at which we can see it after one firing has not been reached.  This continued devitrification at low temperatures can become great enough to be visible at the end of one or multiple slumps.

Credit: Bullseye Glass Co.

What can we do?

Clean all the glass before every firing very well.

·         Avoid mineralised water.

·         Final clean with isopropyl alcohol.

·         Polish dry at each stage with white absorbent paper.

 

Soak longer at lower temperatures.

·         Use longer soaks to achieve the slump.

·         Keep the temperature low.

·         Observe the progress of the firing with quick peeks.

 

Use slower ramp rates.

·         Slower rates enable the heat to permeate the glass.

·         Enables a lower slump temperature.

 

If there is any hint of devitrification after the first firing,

·      use a devitrification spray, or

·      provide a new surface.

• o   remove the surface by abrasion on sandblasting,
• o   cap with clear, or
• o   cover whole surface with a thin layer of clear powder.

·      Fire to contour fuse to give a new smooth surface.

·      Clean very well and proceed to slump.

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.

Bowl Split Analysis

The visual evidence relating to this enquiry is a sharp-edged break through the middle of the slumped piece. The two parts have slumped separately, and seem attached at the rim, leaving the middle opened.  A moderate slumping temperature was used to fire the piece at the bottom of a stacked kiln load.

This is used as an example of the kind of thinking required when investigating breaks in slumping.

The split occurred before the slump was complete. We know this because the pieces no longer fit exactly together.  This means the crack opened as the slump continued.  There is other evidence.

The opening of the crack cannot have happened at or after the annealing. It would have already formed to the mould in a whole state. It would break completely across, because it would be in a brittle state.  And the pieces would fit exactly together.  But they do not.

This piece was at the bottom of a stack of shelves in a deep kiln.  At the bottom, there is no radiant heat, only side heat.  This could be a major cause the kind of break described.

It is possible that the split was not across the whole piece.  At the bottom of the kiln, the glass is not receiving any radiant heat from the top.  It is getting radiant heat only from the sides of the kiln.  That means the edges were considerably hotter than the centre.  The edge may be in a plastic state while the centre is still in the brittle state.  The contrasts in expansions are often great enough to break a piece.

From the evidence we have, it can only be said the ramp rate was too fast for the conditions. 

This little exercise shows that a lot of information about layup, schedule, place in the kiln, and any other relevant variation on the usual, must be detailed when asking why something has not turned out as expected. 

 

 

Slumping Breaks on “go-to” Schedules

 An "It has always worked for me before" schedule implies a single approach to slumping regardless of differing conditions.  Layup alterations, thickness variations, colour contrasts, mould variations all affect the scheduling.  The schedule for each piece needs to be altered when there are changes from the schedule for the “standard” piece, or mould.

Photo credit: Emma Lee

In the example shown, we are not told the schedule, but it shows that the rate was a little too fast. If it had been faster the glass would have separated further apart. The heat was enough to appear to recombine at the edges where it was not slumping so much. 

Review your "go to" schedules whenever something changes. It may still be a good base from which to work. But you need to assess the layup, thickness, and any other variations to help adjust the schedule to fire each piece.

Some of the variations from the “standard” to be considered are:

Single layer slumping 

Weight

Mould sizes

Relative Slumping Depth

Mould shape and detail

Diagnosing Slump fractures

Once you have an initial idea of the source of the problem, think about it.  Test it against the evidence.  Is there enough evidence to make a call?  Make sure you have considered alternative explanations.  It is just too easy to make a snap decision about causes in low temperature processes.  The source of breaks in slumping are most often complex and stem from interrelated factors.

I give you an example of the difficulties of diagnosing a slumping break.

On a Facebook group a person showed the break of a single layer on a cyclone mould.  Others commented the same had happened to them.

Picture credit: Esther Mulvihill Pickens

Possible causes suggested on Facebook included:

• Thermal shock on the way up
• Thermal shock on the way down
• Too large on the mould and broke due to differential contraction
• Too many holds on the way up
• Too hot
• Too thin
• Follow the CPI programme
• Glass extending over the sides

Some of these suggestions were of general applicability, some in relation to the state of the broken glass.

The suggestions did not include:

• Cause of the rounded dots at the bottom of the mould.
• A cause for the state of the flat piece off the mould (it appears sharp edged.  Does it show some forming already?).
• The cause for the location of the fully formed remaining glass.
• The effect of the location of the mould and glass in the kiln.
• The consequences of a short soak at top temperature. 
• Is the kiln running hotter than most (1290ºF/698ºC for 10 minutes at top temperature was used)?

Of course, it is difficult to diagnose a problem from just one picture. It is difficult even with many pictures. And so, without handling the object, only suggestions can be made.

But….

You must spend enough time examining the piece with whatever other information is available to make specific suggestions.  The first thought may not consider all the factors.  Consider what kinds of causes there are for breaks during or after slumping.

More close inspection reveals the rounded edges of the break.  That supports the idea that the temperature was too high. It also supports the diagnosis that the break occurred on the heat up.

The edges of the piece that has fallen off the mould, and now rests on the shelf, seem to be square or sharp. This shows the extent of the difference of temperature between shelf and top of the mould – less than 100mm/4 inches.  Also, how small the differences in temperature are between slump and tack.  The extent of difference in fusing does depend on how high in the kiln the mould is placed.  That is demonstrated here by the different elevation of the two pieces. 

The conformation of the glass to the mould is complete.  This supports the diagnosis of the break occurring early in the firing, and certainly before the slump was complete.  These pieces will not fit together.  So, even if the edges were sharp the fact they will not fit together shows they conformed independently to the mould surface.  Therefore, the break was before forming temperature was reached.

The glass hangs over the mould edges on only three sides and at an angle.  This indicates the cause of the overhang was the break.  Not the reverse. An overhang at the beginning of the slump is likely to be even.

The piece on the floor of the kiln combined with the movement of the glass toward the back gives an indication that the origin of the break is at the front.  This relates to uneven temperatures and to the placement of the mould.

No one mentioned the placement of the mould and glass at the back of the kiln.  This will have an effect on scheduling.  The mould and glass are very large in relation to the kiln.  There is little space between the glass on the mould and the walls of the kiln.  Also, the mould is placed asymmetrically in the kiln – very close on three sides.  This will cause uneven heating in any kiln.  To have a successful firing of glass on this mould in this kiln will require radically different schedules to that for a centrally placed mould that is moderate for the size of the kiln.

The large size (relative to the kiln) and the asymmetrical placing are the causes of the break, in my opinion.  I admit that it took me several looks to realise the placement was a key cause of the break.

So, the generalised comments about thermal shock are correct, but not as to the cause of that shock.  The kiln will be hotter in the central part and cooler at the corners.  This is true of all rectangular kilns.  The important thing is to learn how to cope with these temperature differences.

Slow firings to low temperatures with long soaks are the three important elements.  These make up the heat work of the kiln. Applying this to a schedule means:

• slow ramp up rates – as little as one half the recommended rates for centrally placed moulds that are moderately sized in relation to the kiln.
• Low temperatures present lesser risks to the control of the outcome of the firing.  Determining the lower temperature possible requires peeking into the kiln to monitor the progress of the firing.
• Long soaks combined with low temperatures get the kilnforming done with minimal marking of the underside.  Low temperature soaks - in excess of 30 minutes - are required to minimise the marking.  Observation of the slump will be necessary to determine when it is complete.

My suggestions for the causes of other elements are:

·        Cause of the rounded dots at the bottom of the mould.

The temperature was too high. 698ºC/1290ºF is much hotter than needed for a slump. It was hot enough to round edges and small shards of glass.  Which shows excessive heat was received by the glass.

·        A cause for the state of the flat piece off the mould (it appears sharp edged. Does it show some forming already?)

The soak of 10 minutes was too short for the temperature in the kiln to equalise from top to bottom.  The glass on the shelf may not have reached 650ºC/1200ºF with such a short soak.

·        The cause for the location of the fully formed remaining glass.

The glass broke and was forced apart by the size of the expansion differences within the glass.  The movement of a piece at the front of the mould combined with the rearward and side movement of the glass indicate the origin of the break was at the front.  The distance apart shows the amount of force, and so the degree of reduction in the ramp rate required to fire this successfully.

·        The effect of the location of the mould and glass in the back of the kiln has already been discussed.

·         The consequences of a short soak at top temperature.

A high temperature is often considered necessary to pick up all the detail in moulds, whether slump or texture moulds.  The same effect can be achieved at lower temperatures with longer soaks.  The results of this strategy are fewer mould marks on the bottom of the work.

·        Is the kiln running hotter than most (Used 1290F/698C for 10 minutes at top temperature)?

This is one that cannot be answered other than by experiments carried out by the owner of the kiln.  Look at the Bullseye Tech Note #1 Knowing your Kiln for methods of testing temperatures. 

In short:

Diagnosis of slumping breaks is more complex than it appears at first.

More information is available in the eBook Low Temperature Kilnforming, an Evidence Based Approach to Scheduling.

This is available from Bullseye or Etsy

Slumping and Annealing bottles

"Can a tack fuse schedule for fusing glass can be used to slump bottles?"

It may be that this person does not have the confidence to write a new schedule.  They may wish to use an existing schedule for another purpose. The short answer is “Although a Bullseye or Oceanside tack fuse temperature will be high enough to slump bottles, they are not suitable for annealing”.  There are reasons for this. 

The softening point of float glass, which is similar to bottle glass, is 720ºC/1330ºF.  Slumping would normally be done at about 20ºC/36ºF above this. You also need a slumping hold at this temperature much longer than a tack fuse schedule would use.

if you use a tack fuse schedule for a fusing glass, your annealing will be inadequate. Bottle and float glass tend to have an annealing point of around 540ºC/1005ºF. An annealing for fusing glass will be between 515ºC/960ºF and 482ºC/900ºF.  This is likely to be too low an annealing point for bottles.  Also, the annealing soak is likely to be too short. Slumped bottles are very thick at the base where it folds over the cylinder of the bottle.  This requires a longer anneal soak and slower cool than a schedule for a tack fuse of fusing glass.

Checking for stress in the completed work is normal.  It is essential for your finished bottle if you use a tack fuse to fire it.

 

Schedules should be devised for the glass and layup of each piece. Transferring a schedule for fusing to bottle glass is unlikely to be successful.

Slumping contrasting colours and styles

 A question about why a tack fused 6mm/0.25” piece of combined dense white and black in a slump firing broke has been raised.  Other pieces of black and other whites also tack fused in the same firing did not break.

"Living in the Grey" Stephen Richard

Contrasting colours

Combining the most viscous and the least viscous of bullseye glasses - dense white and black - is a challenge.  The survival of other pieces in the firing with slightly less viscous white give an indication.  Their survival shows that the anneal and cooling conditions were too short and fast for the broken piece. 

It may be worth checking how much stress is in the surviving pieces.  It may not be possible directly on these fired pieces. There is a way.  Mock up the black and white in the same way as the surviving pieces.  Put this on a larger clear piece and fire in the normal way. This enables you to see stress in opalescent layups. If there is any, it is revealed on the clear by using polarising filters. 

The usual recommendation is to anneal and cool as for twice the thickness was followed in this firing.  It is important to anneal and cool more conservatively in cases of contrasting colours. Strongly contrasting colours and styles (low viscosity transparent and high viscosity white opalescent) require more time at annealing and need slower cooling.  I do that by using a schedule for one layer thicker than calculated.  In this case, as for 15mm/0.61” (two tack layers needs firing as for four tack layers, plus one extra for the high and low viscosity combination).

Viscosity

The reasons for this are viscosity:  

·        Annealing is done at a temperature that achieves a viscosity of around 10^13.4 poise. It can be done in a range from there toward the strain point of 10^14.5 poise.  Below the strain point temperature (which is determined by the viscosity), no annealing can occur.  The glass is too stiff.  The closer to the strain point that the annealing is done, the more time is required at the annealing temperature.

·        The annealing of Bullseye is already being done in the lower range of viscosities. It is possible the viscosity of the white is so high as to be difficult to anneal with the usual length of soak.

·        Although I do not know the exact viscosities of dense white and soft black at the annealing temperature, it is known white has a higher viscosity than the black.  The means to achieve less stress in the glass is to hold at the annealing temperature longer than normal.  A cooling schedule related to the length of the anneal hold is needed.  This information can be obtained from the Bullseye chart for annealing thick glass.  The rates and times apply to all soda lime glass, which is what fusing glass is. Only the temperatures need to be changed to suit the characteristics of your glass.

Slumping

The slumping of this combination of high and low viscosity glasses requires more care too.  My research has shown that the most stress-free result in slumping is achieved by firing as for one layer thicker than that used for the fuse firing.  For a tack fuse, this means firing for twice the thickness, plus one more layer for contrasting colour and style.  Then schedule the slump by adding another layer to the thickness.  This means scheduling as for 19mm/0.75"instead of as for 12mm/0.5”.  This is to account for profile, contrasting colours, and stress from slumping.  This is about three times the actual height of the piece.  

Slumping tack fused pieces of contrasting colours requires very cautious firing schedules.  These longer schedules need to have a justification.  It is not enough to add more time or slow the cooling just in case.  Excessively long anneal soaks, and slow cools can create another set of problems. 

More viscous glass needs more time at the annealing soak to an even distribution of temperature between the more and less viscous glasses.

More information about other low temperature processes can be found in my eBook Low Temperature Kilnforming.  Available from Bullseye and Etsy  

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

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.