Playing in the Sandbox

How can sand pictures be produced in glass?

This process provides flowing, abstract images that can be used as autonomous pieces or formed into other objects, such as free drops, bowls, cut for jewellery or into pattern bars.  The appearance provided is unique to this combination of using frit and pressing.

In principle, this process is the same as creating sand pictures.  The process is in three stages: making the box, adding frit, and pressing.

The Sandbox

Determine the size of the box.  It should not be more than two-thirds the size of your kiln shelf depending on thickness.  Thicker glass pressed to 6mm will spread more than thinner.  As a guide, 12mm should have an allowance to spread to about 1.3 times the original size; 19mm should have an allowance to spread about 1.5 times the original dimensions.

Cut two sheets of the same size from clear fusing glass. One will be the front. The other will be the back.

Determine whether the image you are creating will be portrait, landscape, or square.  Orient the sheets in the appropriate way to have the top away from you.  Choose the top piece of the pair and cut two 6mm strips from the designated top.  This gives you a lip to be able to pour the frit into the box easily.

Box formed with bottom and sides glued to back and front.  The filling lip shows on the right.

From another piece of clear glass cut two 6mm strips for the sides.  If you cut them the same length as the side of the glass, they will stick above the back about 3mm. You can cut this off, but it really is not a worry for the construction of the box.  These strips form the spacers to allow the frit to be poured into the box.  Their thickness will determine the amount of frit needed to fill the box.

Get out the back sheet and clean and prepare it for attaching the strips. My preferred method is to glue the bottom 6mm strip on its edge with super glue.  It is advisable to wear plastic gloves when gluing the strips, to avoid sticking your fingers to the glass.  Super glue cures quickly and does not delay the construction of the box.  It burns out cleanly without any health and safety concerns.  Place a thin film of super glue on one edge of the strip.  Attach it to the bottom by placing it carefully at the edge of the sheet.  Do the same for the sides.

  

When the strips are stuck down to the back, place  a thin line of super glue on the top edge of the strips in preparation for attaching the top sheet.  Using a strip of wood placed at the bottom of the backing glass will help in placing the sheet accurately. Lower the sheet from contact with the bottom to the strips forming the sides of the box.

When the glue is cured, inspect the sides of the box for gaps. If there are gaps, use clear Sellotape to seal the gaps in the sides. It will burn off cleanly in the kiln.

Adding the Frit.

Place the box on an easel or other support so it is slightly tipped backwards.  This helps ensure the box does not fall toward you while working on it.  It also allows the frit to slide toward the bottom rather than bouncing off the other frit.

The early stages of filling with the box on a stand

The size of frit you choose to use will affect the final appearance.

·        Generally, powder will appear greyer and more opaque than frit. This is due to the multiplicity of tiny bubbles between the grains of powder.

·        Fine and medium frit provide more clarity than powder.

·        Coarse frit provides the most clarity, but with fizzy bubbles between pieces of frit.

When preparing to place the frit in the box, it is a good idea to take small amounts out of jars and place it into small cups to avoid contamination of the main source of the frit.

Pouring the frit into the sandbox

You can use a jeweller’s scoop or a teaspoon to move the frit from the cup to the box.  Tip the frit into the box above where you want the colour to be placed.  

Using a wire to poke frit through to lower layers.  This also shows  a difference between front and back.

If the frit does not land just where you want it, you can move it with stiff wire that is long enough to reach the bottom of the box.  Gently sweep the frit with the end of the wire toward the place you want the coloured frit to be.

Using a jewellers scoop to add the frit.

Continue adding colours to create the profile and shapes you wish.

You can make additional alterations to the way the frit is placed.  You can poke the frit from one layer into lower layers with a stiff wire by pushing the wire directly downward.  You cannot do this more than 2 or 3 centimetres deep, as the frits and powders become compacted.

A thick copper wire being used to poke down from an upper layer to the lower ones.

When filled to the top or to your desired level, use the fourth strip to close the box.  If full, glue the strip to the top.  If not full, cut strip to the length needed to drop into the opening of the box.  Place a couple of drops of super glue on the top of the already placed strip to keep it in place while moving to the kiln.

The Pressing

Prepare the shelves

You will need two shelves for each pressing. One is the base to hold the glass and the spacers.  The other is to provide the weight to press the glass thinner.

Clean off old kiln wash from the shelves. Experience shows that adding new kiln wash over old for this process promotes the sticking of the kiln wash to the glass.  Add new kiln wash that performs well at extended times at upper temperatures.  I find Bullseye shelf primer works very well.

Once partially dried, with the pink beginning to pale, you can smooth the surface brush marks.  Some use balled up material such as tights to rub over the surface.  I find very good results from rubbing lightly over the kiln washed surface with a sheet of paper between the palm of my hand and the shelf.  The advantage of doing this smoothing while slightly damp is that no dust is created that needs to be cleaned away.  The disadvantage is that too much pressure will pull bits of kiln wash from the shelf.

Do not use fibre papers as the separator.  The glass will be moving within the space between the shelves.  It will pick up and incorporate parts of the fibre paper, if used.

If you have shelves of different thicknesses, reserve the thickest shelf for the upper, pressing one.  If all your shelves are the same size, put a second on top for adequate weight, or add heavy bricks or a steel weight to the top shelf.  (Note: if you use bricks for weights, they need to be dried first.  A two-hour to three-hour soak at 95C should be sufficient.)

Placing

Place the sandbox centrally on the shelf.  If you are doing more than one, ensure there is plenty of space between the pieces and from the edge, so they don’t contact each other, or drip over the edge of the shelf.  The allowances given for the size of the sandbox are a guide.

Two sandboxes placed on separate shelves

Place spacers of the desired thickness around the four corners of the shelf to restrict the extent of thinning.  This also regulates the evenness of the glass across the whole surface.  Usually, 6mm is a desirable height for the pressing.  Other thicknesses can be chosen for different purposes.  The spacers can be steel washers, although they will spall in the cooling stages of the firing.  If you have pieces of ceramic of the desired height, they can be used.  Fibre paper stacked up to the appropriate height are surprisingly robust spacers.  They also provide a cleaner set of spacers than steel.

A corner of the shelf with the 6mm fibre spacer

Place the upper shelf gently down onto the glass piece. The glass at this stage is taking the whole of the weight of the pressing shelf.  The shelf must be placed both gently and evenly down onto the glass to avoid breakage.

Check that everything is in place. This may require additional, directional light such as from your mobile phone or a torch.  It is now ready to fire.

The Firing

This assembly of materials has a lot of mass.  It is 2 to 3 times the normal mass for a standard firing.  

Pressing shelf placed on top of the glass sandbox

This promotes variations in practice:

·        Even with this additional mass, you can fire quickly.  This is because the glass is in small pieces and that the mass of the shelves gains heat slowly. 

·        The greater mass does require longer soaks than a normal fuse firing. 

·        The upper temperature for a full fuse is required to get the glass to a sufficiently low viscosity to allow the glass to move.

·        The long soak at the top temperature does not promote devitrification as in normal fusing.  My speculation is that the glass is not exposed to the air, so the devitrification cannot form. 

·        A further difference in a pressing firing is that the annealing can be at the rate for the final thickness of the glass.  The mass of the shelf and weights above the glass means the glass is cooling evenly from both sides, unlike normal fusing.  The glass may be cooling more slowly than programmed, but the programmed rates limit any possibility of too rapid a cooling.

A schedule for a 12mm thick Bullseye piece with a 19mm upper shelf might look like this:

300°C/hr to    670C       for   180 minutes

300°C/hr to    816C       for   180 minutes

AFAP       to    482C       for   180 minutes

55°C/hr   to    427C       for   0 minutes

110°C/hr to    370C       for   0 minutes

200°C/hr to    50c         for   0 minutes

Off

A piece of 19mm should be slower:

150°C/hr to    670  for   240 minutes

150°C/hr to    816  for   240 minutes

AFAP       482  for   240 minutes

45°C/hr   to    427  for   0 minutes

90°C/hr   to    370 for   0 minutes

180°C/hr to    50    for   0 minutes

Off  

The schedule for glasses other than Bullseye only needs to have the top and annealing temperatures altered to the ones appropriate to the glass.

Results

The pressed glass will have the texture of the shelves on both sides.  Normally, no kiln wash will be stuck to the glass.  If there is kiln wash to be removed, you can do this by abrasive means – sandblasting, diamond pads, wet and dry sandpapers or Dremel style tools.  It is important to keep the glass damp during this process.

If the surface of the glass is without sticking kiln wash or other marks, you can use it with the matte surface.  You can also fire polish the piece, once you have thoroughly cleaned it.

Alternatives

Tape box together

After super gluing the bottom and side strips, you can bind the box together with clear Sellotape.  Pull off at least three strips of tape and set them where you can reach them easily.  Place the upper sheet on the prepared base. Move the box to the edge of the work surface so a little of the box hangs over.  The first stage is to place a strip of tape at right angles to the side to bind the top to the bottom.  Do this for each of the three sides.  When the top is securely attached to the base and sides tape along the length of each of the three sides. 

This shows on the lower left a loosened piece of sellotape on the edge of the sandbox.

This process avoids any difficulty in attaching the top.   Attempting to use only Sellotape to bind the box together is very difficult and requires at least three hands.

Spacers for the frit

Spacers do not always need to be strips on edge.  The spacers can be one or two wider strips placed on their sides to provide the needed height.  They can be coloured, forming a border; but remember the border will become curved. The strips will need to be glued to the back.  The top can be attached with super glue, or taped to the sides and back.

Pressing without a box

It is possible to use the pressing technique without a box or frit.  You can arrange clear and coloured cullet on the shelf.  The arrangement needs to be such that there are no gaps between the pieces.  This means that the glass will probably be 3 to 4 layers thick.  Be careful to avoid creating thick layers of dark colour by interfiling clear. Place the spacers at the corners of the shelf in the thickness desired and fire.  The slower rate of firing (as for 19mm) should be used.

This sandbox process is a combination of arranging frits and pressing.

What are the Effects of Firing on the Kiln Floor?

Credit: The Pottery Wheel

There will be differences when firing on the floor of a kiln - the two important ones are working temperature and annealing.

Differential temperatures 

When firing on the floor of the kiln, expect the effective temperature to be a little lower than when on an elevated shelf. The temperature is always lower at the floor of the kiln and hotter higher in the kiln. This effect is often experienced with glass nearer the elements than usual.

This differential temperature between the floor and the top of the kiln is alleviated to some extent by the infrared heating, if the glass is exposed to it. If the glass is shaded, there will be a distinct difference.

Annealing

Annealing and cooling will be affected most. On the floor the glass can only cool from the top surface, as the bottom of the glass can only cool as fast as the cooling of the kiln. The annealing soak needs to be longer and the cooling needs to be slower than with an elevated shelf.

I suggest that firing for one layer thicker than calculated for the profile will cope with both these conditions.

Annealing dots

 

"No need to anneal dots assuming they’ll be used in projects."

Because dots are small they anneal in a short time, therefore no need for an anneal *hold*, but still the dots anneal. The larger the dots become – to cabochon size – the more important it is to have a temperature equalisation soak. This is what is usually called the anneal hold.

If the dots didn't anneal, they would break. Also because they are nearly spherical, they can withstand greater amounts of stress than flat glass. Industrial guidelines state that the easiest shape to anneal is a sphere. The next easiest is a cylinder. But the most difficult to anneal is flat slabs. It is even more difficult to anneal glass with uneven thicknesses.

So, annealing is always important, even though for small or thin items a soak is not necessary, because the annealing is achieved during the time the kiln cools. If there are a lot of dots in a small kiln, it may be a good precaution to use a 15minute soak at the annealing temperature.

Relieving Existing Stress - How?

Why is the stress not relieved after the strain point when slumping?

The answer relates to whether it is on the cool or on the heat up.

Cooling

The annealing occurs at a higher temperature than the strain point. The aim of the annealing soak is to even out the temperature within the glass to be equal to or less than 5°C/10°F (∆T=5C). When this small differential in temperature is achieved, there is little stress in the glass. In an adequate anneal, stress will be relieved during the soak. This differential needs to be maintained through the first cool, taking the glass temperature to below the strain point.

Relieving stress occurs between the glass transition point to just above the strain point. The viscosity of the glass is so high below the strain point (brittle phase) of the glass that no stress can be relieved.

The more rapid cooling during the brittle phase of the glass needs to be slow enough to avoid creating large contraction differentials within the glass. The reason for progressive cooling stages during the brittle phase of the glass is that it can withstand greater temperature differentials and so the cooling rates can be increased.

Heat up

Any stress on the way up for an already fused piece is induced by uneven heating. This can be across the piece, which is most evident in side fired kilns. The source of the infrared heating is nearest the edge of the glass, so it heats first leaving the centre cooler – sometimes the difference in expansion is great enough to break the glass.

In top fired kilns the differential is usually between top and bottom surfaces. Glass transmits heat slowly so the difference in temperature between the top and the bottom can be enough to cause a break from unequal expansions.

Both these conditions are caused by rapid ramp rates and short anneals on the cool.

Ramp Rates

Breaking of a flat piece on the kiln shelf is from too short a soak or too fast a cool, or both (unless there is an incompatibility). Breaking in a slump most often is a result of too rapid an initial ramp rate. A fused piece needs slower ramp up rates in a slump than in the initial fuse. It is now a single thicker piece, rather than multiple pieces as at the beginning of a fuse. While you might fire a flat 6mm/0.25” piece at 200°C/360°F for the fuse, the ramp rate for the slump needs to be no more than about 100°C/180°F. Tack fused pieces need much slower heat up rates during the slump, usually only half of the rate used to fuse the piece.

Tests have shown that even though the anneal soak for both firings can be the same, a more stress-free piece can be achieved by annealing as for one layer thicker. I do not know why, but I speculate that it is more difficult to achieve the ∆T=5C in the curved piece, than in a flat one.

More information is available in the ebook Annealing Concepts Principles and Practice.

The Relationship of Stress and Slumping

From time to time the assertion is that a break during the heat up of a blank while slumping is the result of residual stress remaining in an under-annealed blank.  Is there a relationship between inadequate annealing and slumping breaks?

It seems to be the general consensus that it is true.

It is clear that poorly annealed glass is more likely to break.  The assumption is that the additional bending stress added to the existing stress, causes a break.  Even if the fused piece is stressed, but not enough to break, a slow heat up would avoid the build up of stress to breaking level.  After all, the means of relieving the stress of toughened/tempered glass is by a slow heat up to allow the stress between the interior and surface to be equalised.  The same principle should apply to stressed glass in a slump.  My contention is that the ramp rates for the slump have been too fast to relieve any additional stress applied by the bending of the glass.

I hear of very few people testing for stress after any firing.  I read of people asserting the existing stress is amplified on the slump firing.  I do not read of any experience of people testing their flat piece, discovering excess stress, and  re-firing to relieve the stress before slumping, but the assertion of excess stress from the first firing continues as a cause.

Without testing there is no way to know whether the first firing had excessive stress.  The use of polarising filters is such a simple, easy and quick way to determine if there are stress problems in the fired and cooled piece.  It should be the business of practitioners and teachers to assert the need for stress testing as the next task when the glass has cooled. Unless people asserting this possibility do the testing of their proposition, it can only remain among the untested elements of kilnforming.

If there were to be a lot of stress in the flat blank, it needs to be fired again, annealed longer and cooled more slowly than previously to relieve the stress before any other process is conducted.  The firing to relieve the stress needs to be only to the lower portion of the slumping range at maximum.  

Each piece that is intended for further firing, needs to be tested for stress before the next firing, and not just at the end of the firing sequence.  To get an accurate reading of the stress, the piece must be allowed to cool until the internal temperature equals the surface temperature.  This may take overnight, but at least as long as the combined anneal soak and the associated cool. The delay caused by waiting for the complete cool may encourage people to skip the stress testing.  But it is risky to avoid testing for stress because of impatience.  Another firing can be conducted while waiting for the first piece to completely cool.

Annealing sufficiently on every firing is the way to ensure that any slumping break is not the consequence of stress from the previous firing.  The Bullseye sheet Annealing Thick Slabs (Celsius and Fahrenheit) gives the annealing times and cool rates.  This document applies to all fusing glass, except the annealing temperature used. Study the table, and follow it closely.  Keep in mind that the effective thickness for other than full fuse, is between 1.5 to 2.5 times the thickest part.  After that first firing, test the piece for stress when it has cooled sufficiently.  It is also important to test the successfully slumped piece for stress before using, gifting, or selling it.

What is the Annealing and Cooling Relationship?

 Annealing Includes Cooling

Often people recommend a long anneal soak for potentially difficult pieces followed by an arbitrary 55C/100F cool rate to 371C/700F or 319C/600F. It is arbitrary because the same rate is frequently recommended regardless of the length of the anneal soak.

It does not have to be guesswork. Bullseye has provided us with the science of the anneal/cool in an accessible form: Annealing Thick Slabs (which covers thicknesses of 6mm/.025” to 200mm/8”). This document provides the annealing time for the chosen thickness and the directly linked cooling rates based on scientific principles.

The anneal soak is determined by the profile and thickness of the piece. Work for the e-book Low Temperature Kilnforming showed a relationship between the profile and the annealing time. Annealing for the profiles of sintering, tack, contour and full fuses requires calculation of the thickness to be applied to various profiles:

• Sinter or lamination – 2.5 times the thickest part
• Tack fuse – 2 times the thickest part
• Contour fuse – 1.5 times the thickest part
• Full/flat fuse – 1 times the thickest part

The cool stages are not random either. They have an intimate but inverse relationship to the anneal soak. They are to keep temperature differentials within the glass to acceptable levels. The anneal soak determined by the profile and thickness is to attain and keep the internal temperature within a range of 5°C throughout the glass. This is often referred to as ∆T=5°C.

The cool rates are to maintain acceptable temperature differences within the glass. The first cool rate is to maintain that temperature differential of ∆T=5°C. The second cool rate allows a wider range of temperature differential of 10°C, or ∆T=10°C. This is possible because the glass has become viscous enough to withstand this greater range of different temperature. The final cool needs to maintained at a differential of 20°C, or ∆T=20°C. Again, this is possible because the viscosity is high enough to withstand this amount of differential.

This information about cool rates and an allowable ∆T spread also indicate that turning off the kiln at 371°C/700°F is not always safe. It is almost always safe to do this for anything calculated to be annealed as for 12mm thick, and it may be safe for a piece up to 15mm thick, but remember that a tack fused piece of two base layers and a further decorative layer needs annealing and cooling as for 19mm/0.75”. The Bullseye research shows that the cooling rate for this is less than the unpowered cooling rate of many kilns. If there are additional complicating factors such as strongly contrasting colours, the annealing and cooling needs to be longer and slower than a simple multiplication of thickness.

There seems to be a practice of a single annealing rate to 371°C/700°F or 319C/600F. So the question will arise “Why is it necessary to have multiple cooling stages.” The response is that it will use unnecessary time and power. Attempting to maintain the ∆T=5°C over extended temperature ranges will not provide extra sound annealing. As the glass can withstand a ∆T=10°C from 427°C/800°F to 371°C/700°F, there is less power required at the faster rate than the slow one. This is even more so for the cool to 319C/600F and lower temperatures.

Knowing the safe temperature to turn the kiln off, requires knowing the cooling rate of the unpowered kiln. This blog shows how to determine the natural cooling rate of your kiln.  Knowing this is as important as knowing what effect different fusing temperatures have on the glass in your kiln.

The object of this blog post is to demonstrate that cooling is part of annealing. Just as much attention must be paid to the cooling rates as the length of the annealing soak. They are inseparable for sound kilnforming practices.

Some work that may be of assistance in understanding the importance of knowing the relationship between the annealing soak and the annealing cool are:

Annealing Concepts, Principles, and Practice

Available from: Bullseye and Etsy

Kilnforming Principles and Practice

Available from: Bullseye  and Warm Glass

Low Temperature Kilnforming

Available from: Bullseye and Etsy

How can I Release Glass Trapped in Casting Moulds?

How to get stuck glass out of a reusable mould?

The material is important to the method of removing stuck glass.

• Metal expands and contracts more than glass.

• Ceramic expands and contracts less than glass.

Mechanical methods

• Metal moulds can be hit relatively hard to break the contact between the mould and glass.

• Ceramic moulds should have only gentle taps, as they are more fragile than metal.

• If the glass is stuck, but moveable within the mould. It may be possible to wiggle the glass and mould against each other, which after a time may wear away the contact points and release the glass.

• Do not try to pry the glass from the mould. It is likely one or the other will break.

• Destructive method is to break the mould or the glass, which ever is the least important.

Contrasting temperature methods

Drape over metal –

• The metal contracts more than the glass, so placing the two in the freezer is one possible approach.

• Alternatively, heat the glass with hot water

• A third method is to place the drape upside down in the kiln and take it up to slumping temperature. Peek to determine when the glass has relaxed enough to be free from the mould. If the release temperature is above the annealing point, anneal again as before.

Drape over ceramic -

• The glass contracts more than the ceramic, so heating the glass with hot water may provide enough expansion to release from the mould.

• Or place the drape upside down in the kiln and take it up toward the slumping temperature. Peek to determine when the glass is released and skip to the anneal and cool process.

Slump into metal -

• The metal contracts more than the metal, so heat treatments will work best.

• Apply hot water to the metal until the glass is freed.

• Place the mould upside down on short posts and fire until the glass drops out. If the annealing temperature is exceeded, anneal again.

• Bang the metal mould with a rubber mallet. This risks breaking the glass, of course.

• Freezing only tightens the hold of the metal to the glass.

Slump into ceramic -

• The glass contracts more than the ceramic, so cold can work.

• Usually, glass sticking to a ceramic mould is a result of insufficient coverage of the mould with the separator.

• Placing the mould and glass in the freezer for a few hours may allow the glass to contract enough to be freed when taken out.

• Place the mould upside down supported on short posts. Set the firing to go to fusing temperature. Monitor with quick peeks from the slump temperature at regular intervals. When it drops, skip to the anneal process.

• Firing to a high temperature does not always release all of the glass.

Using adequate and appropriate separators to avoid trapping the mould or the glass need to be used to prevent the need to employ these release methods.

Why do Bubbles Appear in a Circle?

Example used with the maker’s permission

Piece in kiln ready to fire

Description of the piece.

A commissioned piece made up of Bullseye glass, 38cm/15” diameter, 3mm base with 2 and 3mm strips laid on top to a 6mm maximum depth, fired on a “standard” full fuse at 795°C/1463°F, annealed for 3.5 hours. The piece took up the whole of a newly primed shelf.

The fired piece 

The fired piece developed an off centre bubble, and when the piece was cooled it rocked. The shelf was checked and it was level.  

Previous firings of similar projects were mostly successful, but one was not, although felt to be interesting:

The piece bubbled and various sized holes were randomly drilled to determine the effect.

Other successful pieces were only slightly smaller:

The question was what to do to recover the piece, and why did it happen. Previous bowl blanks on the same program were ok, although this one was larger.

My Response:

The layup is the problem. A thinner (single?) layer in centre surrounded by radiating strips will trap air in the interior and so create bubbles.  This kind of layup needs to have 1 - 3 mm fibre paper topped with shelf paper, under this kind of piece to allow air out.

This is a commission, so a repair is not acceptable. A new one needs to be created, because there would be reputational damage by passing off a repair that will inevitably show evidence of the fault. In general, repairs are unsuccessful.  Repurposing the glass is a better solution.

Other possible causes in addition to the lay up are:

When a piece rocks on a flat surface it has become bowed, and it is evidence of stress developing during the annealing cool.

The annealing and cooling were likely to be inadequate – too short a soak, too fast a cool, or both.

In this case, the anneal soak was certainly long enough, so too fast a cool is the likely cause of the bowing.

The nearness of the glass to the kiln walls is often a cause of uneven heating, although in this case it did not become a problem.

It is also possible that the additional diameter was enough to push a barely adequate bubble squeeze beyond the possibility of full elimination of air from under the glass.

As a result of the experience of the “moonscape”, variously sized holes were randomly randomly around the centre of the bubbled piece to determine if that would have been an acceptable fix. The thicker ring around the burst large bubble remained in the attempt at a fix.

Holes drilled randomly and extra clear dots were added.

The client was contacted to explain the accident and to confirm the making of another.

Is White a Difficult Glass?

Description of the Project

A white 3mm base with 3mm and 6mm decorations made up of mosaic pieces from previously fused glass (all the same CoE). At the end of the firing three corners had broken and their edges rounded. The fourth corner had sharp edges. The tentative conclusion was that there was incompatibility between the white and the previously fired pieces. There were no other cracks visible on the white or between the mosaic pieces. The author did not indicate what the schedule was for either firing, nor what the profile of the last firing was, but asserts white is a particularly difficult glass which does not work well with a wide variety of colours.

My observations are: 

• Compatibility is not an issue on the heat up. It is only a problem at annealing and cooling.
• Breaks on the ramp up (showing rounded edges at the conclusion of the firing) are normally the results of too fast rates.
• Breaks during cooling (showing sharp edges) are due to annealing, compatibility, cooling rates, or some combination of these.
• Previously fired glass can show some shift in compatibility and so needs slower up ramp rates than normal for the profile and thickness.
• Incompatibility between the base and the mosaic pieces would show up as breaks in the white glass under each top mosaic piece.
• Not all glass of the same CoE from different manufacturers is compatible.

Could this have been from incompatibility?

On the way to top temperature the pieces have not yet combined. The incompatibility will only show up during the cooling, as it is the imbalance of  viscosity and contraction between the fused pieces that cause the breaks.

Only one of the broken corners has those sharp edges, making incompatibility an improbable cause of the breaks. Further, incompatibility between the base and upper layers present either a crazed appearance at the connections, or simple breaks around the base of each decorative piece. Incompatibility would have multiple breaks all over the base, if not the top too. Finally, if the fired mosaic pieces were incompatible with the white glass, there would have been breaks throughout the whole piece, not just at the corners.

A further possibility is that the corners were very close to the sides of the kiln, because only the corners broke away from the piece,. If it was side fired, much slower rates are required. And all kilns tend to be cooler near the sides on the heat up than toward the centre, even if top fired.

My guess, based on the description, is that the up ramps were too fast, and the anneal was too short and the cool too fast. Unless the previously fused pieces were tested for stress it is not possible to know whether those were stressed before the final firing, which could have caused the break off of the three of the corners. The fourth corner break was on the cool down and is most likely to be too short an anneal and/or too quick a cool.

Is white glass especially difficult?

There is nothing in this piece to identify white glass as an extraordinarily difficult glass, or that a multiplicity of colours added to white would provoke breaks. The problems exhibited are most likely related to fast heat up ramp rates, and inadequate annealing and cooling.