Showing posts with label Repairs. Show all posts
Showing posts with label Repairs. Show all posts

Wednesday 17 January 2024

Mending a crack

 I had a piece crack due to an annealing oops. I put powder on it and put it back in at a higher temp with a much longer anneal time. It looks great on the front, but I can still see where the crack was on the back. Is it supposed to be like that? I didn't think to put powder on that side.

If you think about why you get crisp lines at the bottom of a strip construction and a more fluid appearance on the top, you will be near the answer of why a repair looks ok on top but shows the crack on the bottom. The temperature on the bottom of the glass is less than on the top at the working temperature. And less again than the air temperature which we measure. This means that the bottom part of the glass has less chance to fully recombine. This, combined with the resistance to movement of the glass along the shelf, results in evidence of the crack being maintained.

Credit: Clearwater Glass Studio


There are some things that can be done to minimise the evidence of the crack. Make sure you know why your piece cracked before you try to mend it. An annealing crack will need different treatment than a thermal shock crack or a compatibility crack. Simply refiring the piece may only make the problem worse.

One approach is to place a sheet underneath. Make sure the broken glass is well cleaned and firmly pushed together. Dams may be useful to keep the glass compressed together. Glass expands both horizontally and vertically during the fusing process. Confining the glass will transfer most of the expansion in a vertical direction. This additional (small) vertical movement may help in forming the glass seamlessly. The broken glass now being supported by an unbroken sheet will enable the movement required to “heal” the crack.



If you do not want to change the surface, you can fire upside down. To do this you need to have a loose bed of powdered kiln wash, or whiting (a form of chalk) that is thick enough to press the textured side fully into the separator. Make sure the glass is pressed together without any separator getting into the crack. One way to ensure the crack does not open is to use a small amount of cyanoacrylate (super) glue which will burn away during the firing.  Put a sheet of clear glass over and fire. Thoroughly clean the face after this repair firing. The ultimate top needs to be fire polished to remove the evidence of the crack, and if it has picked up any marks from the powder.

You could, of course, fire upside down in this way but without the additional sheet, to avoid making the piece any thicker. This may or may not work well. If the base layer is one layer thick, it may pull in at the sides and pull apart at the crack where it is one layer thick.  It is also possible that bubbles will develop in the thin parts of tack glass because of the uneven thicknesses.

A final note. Placing powder on the back will not improve things. The powder will not fully incorporate with the glass and so leave a rough surface without concealing the crack.

Avoiding breaks

To repair or not

The process of repairing

Wednesday 9 August 2023

Fixing a Broken Piece


This conversation is reproduced by permission (with some editing out of extraneous information). It is presented as an example of how conducting a critique of your schedule can have dramatic effects on the results of your firing. 


This is the piece as it came out of the kiln.

Picture credit: Ike Garson

You may have seen the photo I posted of a large copper blue streaky piece that has cracked right across. …  I’m wondering if it would be better trying to bring the 2 pieces together instead of opening up the 2 pieces and inserting frit. I was thinking of firing it with a tack or contour schedule.

This is the crack that developed later through the frit and single layer centre.

Picture credit: Ike Garson


I have 4 questions:

A.   Even if I manage to fix it, do you think that fissure line will always be too weak and liable to break off at any point?

The strength of the joint will be dependent on the firing conditions.  To make it strong, the temperature should go to full fuse.  Tack fusing will leave the joint more visible and weaker.  To stop the joint rounding during heat up, you will need to dam the piece tightly to stop the normal expansion of the glass and ensure the glass is forced together during the higher temperatures.

B.     I have some large pieces of clear confetti. Would it benefit using them to bridge the 2 sections from below?

Anything you put on the bottom will have distinct outlines and visibility.  The temperature on the bottom can be 10C or more different from the top surface, which is why you can get crisp lines with the flip and fire technique.

C.    Would clear powder hide the crack or would it always be visible after firing?

Any additions to the top may be less visible, but adding clear powder makes the join more obvious.  You need to use powder of the same colour as the sheet glass.  Since you are using a streaky glass, you can’t use coloured power either as it is very difficult to imitate the steaks even with powders of the same colours. 

More information was given indicating the first contour fuse schedule in Celsius:

  1. 260 730 00.20
  2. FULL 515 00.60
  3. 260 150 End

This is the contour schedule I have used many times successfully but never for a piece during this week.

My critique of the schedule. 

Segment 1.

  • ·    It is too fast for the small distance to the side of the kiln. 
  • ·    It is too fast for a piece of varying thicknesses. Most expansion breaks occur above 300˚C, so a soak at ca.260˚C will help ensure the glass maintains an even temperature, especially with large differences in thickness. Then you can advance more quickly. 
  • ·   There is no bubble squeeze.
  • ·   The top temperature seems low for a good tack, or the soak is a bit short.  Long soaks allow the glass molecules to bind at the atomic level firmly. This is the principle used in pate de verre.
  • ·   It definitely needs to be on fibre paper covered with thinfire to allow air out.

Segment 2.

  • ·   The soak at 515˚C is better done at 482˚C for Bullseye.
  • ·   My tests have shown that contour firing a piece like this at rates and holds for 1.5 times the height of the piece is necessary for good results.

Segment 3.

  • ·   Also, my tests have shown that a three-stage cooling provides the best result.  Slow cooling keeps the glass within the 5°C difference required for avoiding stress.
  • ·   Annealing at the bottom end of the range combined with an appropriate length of soak and slow cooling gives a denser glass than soaking at the middle of the annealing range. 
  • ·   The best cooling comes from a three-stage cooling process.  This involves a slow rate for the first 55C, a rate of 1.8 times this for the second 55C, and a rate of 3 times this for the final cool to room temperature.

These points mean that I would recommend you fire for at least 10mm thick.  This recommendation is for a new piece, not a repair. In this repair case and for the conditions, I would choose 12mm as being more cautious. My schedule would look something like:

  1. 120˚C to 260˚C, 20’
  2. 300˚C to top temperature, 10’
  3. Full to 482˚C, 120’
  4. 20˚C to 427˚C,0’
  5. 36˚C to 370˚C, 0’
  6. 120˚C to room temperature, off

The anneal soak is for a piece 12mm thick.  The cool rates are for 21mm thick.  This is to compensate for the nearness of the glass to the edge of the kiln.  It will help to ensure the glass does not have excess stress locked into the piece during the cooling.

D. Do you think this schedule would work [for a repair]? It's adapted from a standard tack schedule.

  1. 222 677 00.30
  2. 222 515 00.40
  3. FULL 482 01.30
  4. 63 371 ENDS

Critique of the schedule.

Segment 1. 

  • ·   Too fast given earlier difficulties. 
  • ·   Too low for good adhesion unless you use about 10 hours soak. 
  • ·   Even at sintering temperature (690°C) you would need 2 hours.  But at sintering temperature you do not alter the surface 

Segment 2. 

  • ·   Too slow a cool from top temperature and risks devitrification. Should be FULL.
  • ·   You do not need the soak at 515˚C.  It only delays the annealing process.  It seems this idea of soaking at the upper portion of the annealing range was introduced by Spectrum over 2 decades ago. 
  • ·   Any advantage that might be achieved by the higher soak is cancelled by the FULL rate to the annealing soak. 
  • ·   Go straight to the anneal soak. 

Segment 3. 

  • ·   You need a more controlled 3 stage cooling to get the best result.

My schedule for repair would look something like this:

  1. 120˚C to 540˚C, 10’
  2. 300˚C to 780˚C, 10’
  3. Full to 482˚C, 210’
  4. 20˚C to 427˚C,0’
  5. 36˚C to 370˚C, 0’
  6. 120˚C to room temperature, off

I am making the assumption that 780˚C is full fuse in your kiln.  Anything less than full fuse will certainly show the crack.

 

A Look at Causes.

  • ·  The piece is far enough away from the elements.  It is not on the floor. These are not the causes.
  • ·  It is very near the sides of the kiln.  These are always cooler than the centre. There is always a risk of breaking in this case.  Very slow rates are needed. 
  • ·  There is a 3.5 times difference in thickness within the piece. This also requires slow rates.
  • ·  If the break were to have been on the heat up these elements of uneven heating, and rapid rates are a problem.  But the break occurred after the cool down. So, the annealing soak and cool is a problem. 
  • ·  I have suggested some alterations to the schedules to address these things.

 

Fixing for Yourself

  • ·   Dam it tightly to avoid expansion within the glass as it heats.  This holds the join together and causes the glass to gain a little height. 
  • ·   Place the piece on 1mm or thicker fibre paper topped with thinfire.  This will help avoid a bubble forming in the clear.
  • ·   I have suggested a schedule which is slower to ensure no further breaks.  It is slow to the strain point and fast after that. 
  • ·   It needs to be a full fuse to fully join the two pieces and ensure it is sound.
  • ·   The cool to annealing should be FULL.  Eliminate the soak in the upper annealing range. The effects of the time spent there is nullified by the rapid rate to the main annealing soak. 
  • ·   Anneal as for 12mm, but with slower cool rates (for 21mm) to ensure there are no stresses built into the piece by the nearness of the glass to the edge of the kiln.
  • ·   These methods and schedules will make it a strong whole.  But the join will still show on the bottom. 
  • ·   After fixing, if you are still not satisfied, break it up for incorporation in other projects.

Finally, and unfortunately, I do not think it can be satisfactorily repaired for a client.  The crack will show on the back. You will know it is a repair, rather than a whole. And that will reflect on your feeling about the piece, and possibly your reputation.

 

Conclusion

The commission was successfully re-made from scratch by the artist using some of my suggestions on scheduling. This is the resulting piece.

 

Picture credit: Ike Garson

 

Careful analysis of the conditions around a break are important to making a successful piece in the future. Many factors were considered, but the focus became the schedule. Analysis of each step of the schedule led to changes that resulted in a successful piece with the original vision and new materials.

Wednesday 19 July 2023

To Repair or Not to Repair

 Breaks during slumping sometimes occur. What can be done?

Cause of Break

The first element in assessing the piece is to determine why it broke

Should it be Repaired?

The second element is whether it should be repaired or re-used. Is it worth the effort of repairing? This will be about the importance and the time and effort you have already put into the piece.

Can it be repaired?

This is a third element of assessment. If the break resulted from incompatibility, any attempt at refusing will also break for the same reason. If inadequate annealing caused the break, it may be possible.

It is sometimes suggested that those pieces which fit together exactly, should be fused together flat and re-slumped. This ignores the fact that the glass will have stretched or deformed from the flat piece it once was.

  • ·   This re-fusing may be successful for shallow and simple slumps. But the piece will not be corrected by fusing the broken pieces from deep or complex slumps as a result of the stretching and thinning or thickening in the slumping process.
  • ·   The glass pieces will have an imperfect join when flattened because of deformations from the changes during the slumping.
  • ·   If the base is a single layer, the separate pieces will pull apart during the re-fusing process due to the lack of volume.
  • ·   The fusing process will make a tack fuse much flatter than originally intended. A contour fuse - at minimum - will be required to join the pieces.

For all these reasons, any flattening, fusing and then attempting a slump again is unlikely to be successful.

Fusing in the mould

In recognition of these problems about flattening, re-fusing, and slumping again some people suggest mending by firing in the mould. This would get over the difficulty of changes of shape. However, the required contour or full fuse will leave marking on the back and may lead to thickening at the bottom. It is also hard on your ceramic moulds if you fire quickly.

Changing the Shape

If it is desired to flatten an unbroken slumped piece for use in a mould of a different shape without much change in tack profile dimensions, there are two things to do. The maximum temperature to be used to get the glass flat and retain the degree of tack is the sharp tack - or lamination - range. It will require a significantly long soak at top temperature - hours.

This long soak time is a consequence of the effects of weight and span. The effective weight is less at the unsupported edges than at an unsupported centre. The slumped piece has most of its weight on the shelf now. This makes the flattening have to use a higher temperature or a longer soak. The effective span and weight at the edge is almost zero. This requires long soaks and frequent observation to know when the flattening is complete. Both these effects make the flattening of a piece without altering the profile a lengthy process.

 

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


Repairing a broken slumped piece of glass requires knowing why it broke, can it be repaired, is it worth repairing. Difficulties related to the changed shape, temperature to fuse, and changes in tack profile.

Wednesday 18 May 2022

Large Uprisings on Slumps



Help!  Looks like my shallow bowl wanted a boob.  [16” diameter pot melt slumped into shallow 20” mould, 4” above floor, fired at 175°F to 1100°F for 20 minutes,  and 75°F to 1250°F for 15 minutes]

What happened?

Sometimes a slump results in an uprising at the bottom of the slump. In this case, and many others, we know the vent at the bottom of the mould was open and the piece was supported above the shelf.  This indicates that everything should be set up for a good result.  Still, this uprising occurred.  It is not a bubble, as the glass is apparently evenly thick throughout the “boob”. 

The usual, and mostly unexpected, cause is too long or too hot a slump.  The firing, if allowed to continue, would result in a larger uprising and eventually a thickening of the piece at the bottom of the mould associated with a related reduction in the dimensions of the final piece.

What has happened is that the glass has become soft enough for it to slip down the sides of the mould. But it has not been hot enough for long enough to allow the glass to thicken.  The glass at the bottom is pushed up to compensate for the slightly hotter glass on the sides of the mould sliding down on the glass at the bottom.

In this case there is a moderate (97°C) rise from a soak of 20 minutes at 593°C to 676°C, soaking for 15 minutes.  This is a lot of heat work at a relatively high temperature for a shallow mould. 

What to do in the future?

Rate of Advance

Consider what you are trying to achieve at each stage of the scheduling of the process.  In this slumping there really is no need to soak at 593°C. It is a nowhere temperature.  The glass is no longer brittle.  It is at the lower end of the temperature range where the glass is softening anyway.  A simple, steady rise in temperature, as at the beginning, of at or below 100°C will be sufficient to bring the whole substance of the glass to the slumping temperature. If the piece is really thick, consider an even slower but steady rate of advance without any soaks.

Soak Temperature

Also, if the slump can be achieved in 15 minutes, it is too hot.  If the slump is complete in such a short time, it will be marked much more than needed by contact with the mould. 

The cooler the glass at the conclusion of the slump,  the less marking there will be.  Yes, the soak time needed to complete the slump will be longer, but the bottom of the glass will be cooler than a hot fast slump.  You should always be trying to achieve the effect you need at the lowest practical temperature.  The slow rate of advance will assist in completing the slump at a lower temperature, as the amount of heat work put into the piece will be greater.

Observe the progress of the firing

Observation is necessary when doing something different.  Some argue that it is necessary in every slump.  I admit that I do not always observe every slump, but this case again illustrates the need to observe each slump. 
By observing at 10- or 15-minute intervals, you will see when the slump is complete.  You may feel you do not have the time to wait for an hour or so for the slump to be complete, or that it does not fit with your activities.  The answer is to arrange the kiln’s schedule so that when the critical part of the process is reached it will fit with a space in your other activities. https://glasstips.blogspot.com/2016/12/diurnal-firing-practices.html

Can this piece be fixed?


You could put the piece on a shelf and take it to a high temperature slump with a significant soak to flatten it.  You will need to observe when the uprising is flat again, and then proceed to anneal.  However, the pattern placed in or on the glass will be distorted to some extent.  The uprising will flatten with a thicker rim around the base of the rise in the glass.  This may be visible. 

My view of these things is to learn as much as you can about causes and prevention and move on.  You advance your practice more quickly by understanding what went wrong and why than by trying (unsatisfactorily) to rectify a failed piece.  Often you can cut the glass up and use the pieces in other projects.

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

Uprisings at the bottom of slumps are often the result of too much heat work (rate, temperature and time).  Slow rates if advance to low temperatures with long soaks backed by observation prevent the occurrence of these bubble-like uprisings.

Friday 13 May 2022

Fixing a Broken Piece

 This conversation is reproduced by permission (with some editing out of extraneous information). It is presented as an example of how conducting a critique of your schedule can have dramatic effects on the results of your firing.

 

This is the piece as it came out of the kiln

Picture credit: Ike Garson

 You may have seen the photo I posted of a large copper blue streaky piece that has cracked right across. …  I’m wondering if it would be better trying to bring the 2 pieces together instead of opening up the 2 pieces and inserting frit. I was thinking of firing it with a tack or contour schedule.


This is the crack that developed later through the frit and single layer centre.

Picture credit: Ike Garson

 

I have 4 questions:

A.   Even if I manage to fix it, do you think that fissure line will always be too weak and liable to break off at any point?

Response: The strength of the joint will be dependent on the firing conditions.  To make it strong, the temperature should go to full fuse.  Tack fusing will leave the joint more visible and weaker.  To stop the joint rounding during heat up, you will need to dam the piece tightly to stop the normal expansion of the glass and ensure the glass is forced together during the higher temperatures.

B.     I have some large pieces of clear confetti. Would it benefit using them to bridge the 2 sections from below?

Response: Anything you put on the bottom will have distinct outlines and visibility.  The temperature on the bottom can be 10C or more different from the top surface, which is why you can get crisp lines with the flip and fire technique.

C.    Would clear powder hide the crack or would it always be visible after firing?

Response: Any additions to the top may be less visible, but adding clear powder makes the join more obvious.  You need to use powder of the same colour as the sheet glass.  Since you are using a streaky glass, you can’t use coloured power either as it is very difficult to imitate the steaks even with powders of the same colours. 

More information was given indicating the first contour fuse schedule in Celsisus:

  1.  260 730 00.20
  2.  FULL 515 00.60
  3.  260 150 End

This is the contour schedule I have used many times successfully but never for a piece during this week. 

My critique of the schedule. 

Segment 1.

·        It is too fast for the small distance to the side of the kiln. 

·        It is too fast for a piece of varying thicknesses. Most expansion breaks occur below 300˚C, so a soak at ca.260˚C will help ensure the glass maintains an even temperature, especially with large differences in thickness. Then you can advance more quickly. 

·        There is no bubble squeeze.

·        The top temperature seems low for a good tack, or the soak is a bit short.  Long soaks allow the glass molecules to bind at the atomic level firmly. This is the principle used in pate de verre.

·        It definitely needs to be on fibre paper covered with thinfire to allow air out.

Segment 2.

·        The soak at 515˚C is better done at 482˚C for Bullseye.

·        My tests have shown that contour firing a piece like this at rates and holds for 1.5 times the height of the piece is necessary for good results.

Segment 3.

·        Also, my tests have shown that a three-stage cooling provides the best result.  Slow cooling keeps the glass within the 5°C difference required for avoiding stress.

·        Annealing at the bottom end of the range combined with an appropriate length of soak and slow cooling gives a denser glass than soaking at the middle of the annealing range. 

·        The best cooling comes from a three-stage cooling process.  This involves a slow rate for the first 55C, a rate of 1.8 times this for the second 55C, and a rate of 3 times this for the final cool to room temperature. 

These points mean that I would recommend you fire for at least 10mm thick.  This recommendation is for a new piece, not a repair. In this repair case and for the conditions, I would choose 12mm as being more cautious. My schedule would look something like:

  1. 120˚C to 260˚C, 20’
  2. 300˚C to top temperature, 10’
  3. Full to 482˚C, 120’
  4. 20˚C to 427˚C,0’
  5. 36˚C to 370˚C, 0’
  6. 120˚C to room temperature, off

The anneal soak is for a piece 12mm thick.  The cool rates are for 21mm thick.  This is to compensate for the nearness of the glass to the edge of the kiln.  It will help to ensure the glass does not have excess stress locked into the piece during the cooling. 

D. Do you think this schedule would work [for a repair]? It's adapted from a standard tack schedule.

  • 1. 222 677 00.30
  • 2. 222 515 00.40
  • 3. FULL 482 01.30
  • 4. 63 371 ENDS

 

Critique of the re-firing schedule.

Segment 1. 

·        Too fast given earlier difficulties. 

·        Too low for good adhesion unless you use about 10 hours soak. 

·        Even at sintering temperature (690°C) you would need 2 hours.  But at sintering temperature you do not alter the surface 

Segment 2. 

·        Too slow a cool from top temperature and risks devitrification. Should be FULL.

·        You do not need the soak at 515˚C.  It only delays the annealing process.  It seems this idea of soaking at the upper portion of the annealing range was introduced by Spectrum over 2 decades ago. 

·        Any advantage that might be achieved by the higher soak is cancelled by the FULL rate to the annealing soak. 

·        Go straight to the anneal soak. 

Segment 3. 

·        You need a more controlled 3 stage cooling to get the best result.

 

My schedule for repair would look something like this:

  1. 120˚C to 540˚C, 10’
  2. 300˚C to 780˚C, 10’
  3. Full to 482˚C, 210’
  4. 20˚C to 427˚C,0’
  5. 36˚C to 370˚C, 0’
  6. 120˚C to room temperature, off

I am making the assumption that 780˚C is full fuse in your kiln.  Anything less than full fuse will certainly show the crack. 

A Look at Causes.

·     The piece is far enough away from the elements.  It is not on the floor. These are not the causes.

·     It is very near the sides of the kiln.  These are always cooler than the centre. There is always a risk of breaking in this case.  Very slow rates are needed. 

·     There is a 3.5 times difference in thickness within the piece. This also requires slow rates.

·     If the break were to have been on the heat up these elements of uneven heating, and rapid rates are a problem.  But the break occurred after the cool down. So, the annealing soak and cool is a problem. 

·     I have suggested some alterations to the schedules to address these things.

 

Fixing for Yourself

·        Dam it tightly to avoid expansion within the glass as it heats.  This holds the join together and causes the glass to gain a little height during the firing. 

·        Place the piece on 1mm or thicker fibre paper topped with thinfire.  This will help avoid a bubble forming in the clear.

·        I have suggested a schedule which is slower to ensure no further breaks.  It is slow to the strain point and fast after that. 

·        It needs to be a full fuse to fully join the two pieces and ensure it is sound.

·        The cool to annealing should be FULL.  Eliminate the soak in the upper annealing range. The effects of the time spent there is nullified by the rapid rate to the main annealing soak. 

·        Anneal as for 12mm, but with slower cool rates (for 21mm) to ensure there are no stresses built into the piece by the nearness of the glass to the edge of the kiln.

·        These methods and schedules will make it a strong whole.  But the join will still show on the bottom. 

·        After fixing, if you are still not satisfied, break it up for incorporation in other projects.

Finally, and unfortunately, I do not think it can be satisfactorily repaired for a client.  The crack will show on the back. You will know it is a repair, rather than a whole. And that will reflect on your feeling about the piece, and possibly your reputation. 

Conclusion

The commission was successfully re-made from scratch by the artist using some of my suggestions on scheduling. This is the resulting piece.

 

Picture credit: Ike Garson

 

Careful analysis of the conditions around a break are important to making a successful piece in the future. Many factors were considered, but the focus became the schedule. Analysis of each step of the schedule led to changes that resulted in a successful piece with the original vision and new materials.

Wednesday 4 May 2022

Uneven slumps

Credit: Lara Duncan


Uneven slumps – where the glass does not slide down the sides evenly, leaving one side higher than the other – are common in moulds with steep sides.  Another common cause is uneven weight on the blank – where there are more layers on one side than another.  Yet another common cause of uneven slumping is the blank having large areas of glass with different viscosities.

Things I can think of to avoid the problem.

While the glass is firing

Use a moderate rate of advance to the target temperature. Once that is reached, peek every 10 minutes to observe how the slump is proceeding.  When the slump begins to go off centre, reach in with protective gear and adjust it back to even.  The kind of protective gear you need is shown in this post.

An alternative to moving the glass is to tip the mould.  If the mould is relatively shallow with a flat bottom,  there is not much you can achieve by this action.  On deeper moulds, you can elevate one side of the mould.  This puts the elevated side closer to the top and so into a hotter part of the kiln.  This means that you elevate the side that is not slumping as quickly.  You do this because the slowly slumping glass needs more heat in relation to the faster slumping side.  It seems counter intuitive, until you realise you are putting the slow side into greater heat.  You will need to continue observing at intervals to know when the glass is slumping evenly. At that point you can return it to level.

I admit that moving the glass is my choice almost all the time.  It works well on moderately deep moulds.  Elevating one side of the mould while firing requires more time in the kiln that I want to give.  Tipping the mould works best on very deep moulds and so I view it as a special case.



Before the slumping begins

Most of the time we make our blanks the same diameter/dimension or slightly larger than the mould. This allows the glass to rest on the rim and be certain it is as level as the rim of the mould is.  However, this also creates an edge which the glass needs to slide over as it slumps.  Especially with steep sided moulds.

A fix for this is to make the blank fit just inside the rim.  Then it does not have to slide over the rim, and avoids the risk of hanging up on one part rather than another.  You will need to ensure the glass is level within the mould as well as the mould itself, in this case.  If you take this approach of internal placing and you want a piece with a particular final dimension, you should buy a mould larger than the final size needed.

You can combine this placing of the glass internally with another preventative for uneven slumps in deep moulds.  You can grind a small bevel on the underside of the edge to help the glass have greater contact with the mould, so resisting uneven movement.  This can be done separately from fitting the glass inside a steep sided mould, but is most likely to be successful if performed on a blank smaller than the mould dimensions.


Uneven slumps in kilnforming can be corrected during the firing or by preparation of the blank in relation to the mould before the firing.

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