Texture moulds

"I could use some help here please. I’ve tried this sun mould 3x and as you can see all 3x I get a hole.  If you could tell me what I’ve done wrong I would greatly appreciate. They were all full fused to 1430F (776C)."

Example of the problem

There are a range of views that have been given on how to make texture moulds work without the glass developing bubbles.

closer view of one example

These are a summary of the suggestions made to the enquirer.

Not enough glass thickness. The view is that glass needs to be 6mm thick to be used on texture moulds, as the viscosity of glass tends to draw glass to that thickness, robbing from other areas making them thin and prone to bubbles.

Glass always wants to go to 6mm.  Not always.  It depends on temperature.  The kiln forming temperatures we use results in a viscosity that tends to equalise the forces at 6 – 7 mm.  Hotter glass will flow out more thinly, until at about 1200C, the glass is 1mm or less thick.

Full fuse two sheets first.  The object is to avoid placing two separate sheets on top of the mould, creating the potential for more bubbles between the sheets, as they may slump into the mould at different rates.

Too hot. As the glass increases in temperature the viscosity is reduced and can no longer resist the air pressure underneath the glass.

Use a lower temperature. The idea is to keep the glass relatively stiff to resist bubble formation.

Bubble squeeze needed to avoid trapped air.  Another way to reduce the amount of air under the glass is to allow the glass to relax slowly at a temperature below which the glass becomes sticky.

Elevate the mould.  The idea is that hot air circulating under the mould will help equalise the temperature of the mould and the glass.

Drill holes at low points. This gives air escape routes under the mould, assuming the mould is slightly elevated.

Go lower and slower.  Use a slower rate of advance toward a lower top temperature with longer soaks to avoid reducing the viscosity, but still get the impression from the mould.

Now for a different viewpoint.

None of the views given above are wrong, but they all (except in one case) fail to consider the fundamentals of obtaining texture from such a mould.

It is apparent that the temperature used was too high because the glass had low enough viscosity to allow the air underneath to blow the bubble.  The suggestions of thicker glass, bubble squeezes, lower temperatures, drilling holes and elevation of the mould are ways of reducing the amount of air or resisting the air pressure.  They are not wrong, but miss the fundamental point.

That fundamental point is that you need to raise the temperature slowly on these texture moulds to allow the glass to fully heat throughout. By doing this most of the air has a chance to filter out from under the glass before it conforms to the edges of the mould.  It is simpler to use the slow advance rather than a quick one with a slow-down for a bubble squeeze.  The glass is more certain to be the same temperature throughout by using a slow rate of advance.  Glass with an even temperature can conform more easily to the undulations and textures of the mould.

Mostly, the recommendations given are to use two layers, or 6mm of glass that has already been fused together.  This gives greater resistance to bubble formation and reduces the dogboning and needling of the edges.

However, you can form in these moulds with single layers.  There are of course certain conditions:

• You must advance the temperature slowly.  A rate of 100C per hour will be fast enough.
• You can add a bubble squeeze soak of 30 minutes at about 630C as additional assurance of removing most of the air.  The bubble squeeze is done at a lower temperature than usual, as the glass is less viscous because the slow rate of advance has put more heat work into the glass.
• The top temperature should not go beyond 720C. Beyond that temperature the viscosity of the glass drops quickly and so becomes subject to bubble formation.

The soak at the forming temperature will need to be long and observation will be needed to determine when the glass has fully conformed to the mould. Quick peeks at intervals will show when the design is visible on the top of the glass. The time will vary by:

• Mould texture complexity 
• Type of glass (opalescent or transparent),
• Heat forming characteristics of the glass,
• Viscosity of the glass or colour,
• Etc. 

Be knowledgeable about how to extend the soak or to advance to the next segment of the schedule to take advantage of your observations.

Your observation may show that you can do the texture formation at a lower temperature in future. This will provide results with less separator pickup and better conformation to the mould without excessive marking. 

You will need a long soak in either circumstance. This will be in terms of hours not minutes.  If you do these texture moulds at slumping temperatures, you will probably need at least twice your normal soak.

You can do a lot to fool the single layer glass into doing what you want by using low temperatures and long soaks. See Bob Leatherbarrows's book on Firing Schedules.  He gives a lot of information on how to manipulate glass through heat work - the combination of temperature and time.  You might also consider obtaining my book - Low Temperature Kilnforming.

Most of the search for the right temperature, fails to note that the important element is how you get to the temperature. You can get the same result at different temperatures by using different rates of advance.

Kilnforming is more than temperature, it is also about time and the rate of getting to the temperature. By concentrating on temperature, we miss out on controlling the speed and the soak times. You can do so much more to control the behaviour of the glass at slow rates, significantly long soaks, and low temperatures.

As Fast as Possible Firings

I have long advocated that it is best to avoid as fast as possible firings because the way controllers work.  They compare the temperatures several times a minute (the number depending on the manufacturer) to determine the rate of increase.  This allows big overshoots at the top temperature with fast rises.  This was reinforced this morning by observing a different factor.
 
I took a piece out at 68°C to put another in.  During the time the kiln was open, the air temperature dropped to 21°C.  I filled the kiln and closed the lid and idly watched the temperature climb before switching the kiln on for another firing.  It took a bit more than two minutes for the thermocouple to reach 54°C with the eventual stable temperature being 58°C.  I had not been aware how long it takes the thermocouple to react to the change in temperature.  Yes, it takes a little time for the air temperature in the kiln to equalise with the mass of the kiln, but not two minutes.
 
With a two-minute delay the recorded temperature can be significantly behind the actual air temperature.  For example, a rate of 500°C per hour is equal to 8.3°C (15°F) per minute or 16.6°C (30°F) overshoot of the programmed temperature. Even at 300°C it is a 10°C (18°F) overshoot.  This effect, added to the way the controller samples the temperatures, means the actual overshoot can be significant for the resulting glass appearance.
 

This is just another small element in why moderate ramp rates can be helpful in providing consistent results for the glass.

Sintering

This is a process used in glass to stick glass together without any change in appearance of the separate pieces.  It has various names - fuse to stick and lamination are two.

General description
“Sintering or frittage is the process of compacting and forming a solid mass of material by heat or pressure without melting it…. Sintering happens naturally in mineral deposits [and] as a manufacturing process used with metals, ceramics, plastics, and other materials.

“The atoms in the materials diffuse across the boundaries of the particles, fusing the particles together and creating one solid piece. Because the sintering temperature does not have to reach the melting point of the material, sintering is often chosen as the shaping process for materials with extremely high melting points such as tungsten and molybdenum….
 
“An example of sintering can be observed when ice cubes in a glass of water adhere to each other, which is driven by the temperature difference between the water and the ice.”
https://en.wikipedia.org/wiki/Sintering
 
Applied to glass this means that you can make a solid piece out of multiple touching or overlapping pieces that do not change their shape.  This is done by using low temperatures and very long soaks. 
 
The usual process is to take the glass at a moderate rate up to the lower strain point.  The rate of advance is slowed to 50°C or less to a temperature between slumping and the bottom of the tack fuse range.  The operator must choose the temperature, largely by experimentation. 
 
The slow rate of advance allows a lot of heat work to be put into the glass.  This, combined with a long soak (hours), gives the molecules time to combine with their neighbours in other particles.
 
Sintering can be done in the range of 610°C to 700°C.  The lower limit is determined by the strain point of the glass being used and practicality.  

The upper limit is determined by the onset of devitrification. This  has been determined by the scientific studies of sintered glass as a structure for growing bone transplants.  Devitrification reduces the strength of the bonds of the particles at the molecular level.  These studies showed that the onset of devitrification is at 700°C and is visibly apparent at 750°C regardless of the glass used.  Therefore, the choice was to use 690°C as the top sintering temperature. 
 
For reasons of practicality the lowest temperature tested was 650°C.  Indications were that at least an additional two hours would need to be added to the sinter soak for each 10°C reduction below 650°C.  This would make for a 12-hour soak at 610°C.  For me this was not practical.
 
My recent testing has indicated some guidelines for the sintering process:
 
The ramp rate has significant effects on the strength of the resulting piece. 

• A moderate rate (150°C) all the way to the sintering temperature needs a two-hour soak at the top temperature. 
  
• A rapid rate (600°C) - as used in medicine – to the sintering temperature requires approximately six-hours soaking.
• A rapid rise to the strain point followed by the slow 50°C per hour rate to the sinter temperature requires a three-hour soak.

 
The temperature range of 610°C to 700°C can be used for sintering.  The effects of the temperature used have these effects:

• With the same rates and soak times, lower temperatures produce weaker glass.
• The lower the temperature, the longer the sinter soak needs to be for similar strengths.  Generally, the soak at 650°C needs to be twice that of sintering at 690°C.
• Lower temperatures produce more opaque glass.  In this picture all the glass is clear powder and fine frit in the ratio 1:2, powder:frit.

 

The annealing of sintered objects needs to be very cautious. The particles are largely independent of each other, only joined at the contact points.  The annealing soak needs to be longer and the cool slower than for simple tack fusing. 

• Testing showed that annealing as for 12mm is adequate. 
  
• There was no advantage of annealing as for 25mm as that did not increase the strength.

 
Porosity
Although the structure of the sintered glass appears granular, it is not porous except at or below 650°C.  At the lower temperatures, the glass becomes damp on the outside and weeps water.  At 670° and 690°C the outside became cool to touch but did not leak water.  This observation depends on evenly and firmly packed frits.
 

Grain structure at 650C

Grain structure at 690C

The keys to successful sintering of glass are the use of a heat work through slow ramp rates, and long soaks throughout the whole firing.

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

Annealing at the Lower End of the Range

Annealing can be done at other than the defined glass transition temperature (Tg) - also known as the annealing point. Annealing occurs over a range rather than a single magic temperature. Bullseye did not change their glass when they altered the recommended annealing temperature.  Their research has shown that good results are obtained by annealing at the lower end of the range.  

A graph of some aspects of a specific and stiff soda lime glass illustrates this.

Annealing can be between the glass transition (annealing) point and the strain point
credit: Lehigh University

Bullseye's previous annealing temperature was 516C and Spectrum's was/is 510C. These are very close. Bullseye's research is applicable to all soda lime glasses. Therefore, the same principles can be applied to Oceanside fusing compatible glass.  It has already been applied to the Wissmach fusing lines.  This means that you can anneal both glasses at the same temperature.  If you feel the need, you can increase the 482C by 6C to 488 for both, but I don't think it is necessary.

The purpose of the annealing soak is to equalise the temperature within the glass to vary less than 5°C (i.e., +/- 2.5C).  If this is done at the lower end of the annealing range, there is less difficulty of maintaining that small difference throughout the cooling stages. 

When to Open a Cooling Kiln

Credit: Glass House Store

Questions about when it is possible to open the kiln during the cool down to avoid thermal shock get the answer, “it depends….”

These dependent variables include:

Temperature Differentials

Thermal shock is related to how quickly a piece can cool without developing stress that cannot be contained within the piece.  So, when the temperature differential is a few tens of degrees between room and kiln air temperature it is less risky than when the difference is hundreds of degrees.

This means that there is a relation between room temperature and when you can open the kiln safely.  If the room is at sub-zero temperatures, you will need to wait for a lower temperature in the kiln, so the temperature differentials are no greater than when the room is warm.  Remember the glass can be much hotter than the air that the thermocouple measures.

Cooling rate of the kiln

The natural cooling rate of the kiln (that is, in the unpowered state) will affect when you open.  If your kiln cools very slowly from 150°C, you may feel confident to open the kiln a little to speed the cooling from that temperature.  If you kiln cools quickly - usually in smaller kilns - then you need to wait longer for a lower temperature to be achieved.

Size of the piece

The size of the piece(s) relative to the kiln size has a bearing on when it is safe to open the kiln to speed cooling.  The more space the piece takes up in the kiln the cooler the temperature reading needs to be before you open the kiln.

Placing

The placing of the glass has an affect too.  If the glass is at the front of a front opening or top hat kiln, it will cool more quickly and unevenly than one at the back. A large piece placed more to one edge than another will also require lower temperatures before opening.

Thickness

The thickness of the glass also needs consideration.  The thicker the glass, the hotter it will be in relation to the measured air temperature, and so the longer it needs to be left to cool before opening.

Type of kiln

Your kiln may cool slowly or quickly, but the style of the kiln is important too.  The kiln may be brick lined or fibre lined, or a combination.  The greater the mass of the insulation, the earlier you can open, as the dense brick will radiate heat back toward the glass.

If you have a top hat kiln it is probable that you can open earlier than if you have a top opening or front door opening kiln, as they will dump hot air slower than top and front opening kilns.

The venting method

The way you open the kiln to increase the cooling rate is important.  If you open vents, that provides a gentler flow of cooler air than opening the lid or door.  If you open lids or doors, you need to wait for a lower temperature than for opening vents.

And I am sure there are other considerations.  But these are enough to show that there is not a single answer.  The answer is in relation to the kiln and its contents.

Acceptable Cooling Rates

The speed of cooling that a glass can sustain is indicated by charts giving the rate of cooling for the final rate of decrease to room temperature.  Faster rates might be induced by turning the kiln off at 370°C and opening the door/lid at some slightly lower temperature.

This means that you need to know how fast a cooling rate is acceptable.  The bullseye research suggests that 300°C per hour for the final cooling is as fast as you would want to cool a 12mm thick piece.  This is in a closed environment.  Therefore, you will want to be slower – at least half the speed for a partially opened kiln of say 5cm. 

My predictions for acceptable cooling rates are (with a room temperature of 20°C; a piece evenly thick and 30cm square, but less than half the area of the kiln floor; and a top hat kiln):

6mm -   300°C per hour (although I never use more than 200°C per hour)

12mm - 150°C per hour

19mm - 75°C per hour

25mm – 45°C per hour

Note: Tack fused items with these total heights need to have these rates halved, or use the rate suitable for a piece twice the thickest part.

But!

You cannot open the kiln until the natural cooling rate is at the predicted acceptable rate of cooling or less, to be safe.

The natural cooling rate at various temperatures can be determined by observing temperature falls in relation to time intervals between those observations.  You can make a chart to indicate the cooling rate at different temperatures.  The kiln will naturally cool more slowly at lower temperatures. 

Schedule to room temperature

A protection against too rapid cooling is programming to room temperature.  If your kiln is cooling less rapidly than you predict is acceptable, you are using no electricity – OK, maybe a tiny fraction of a kilowatt to keep the controller operating. But there is no worry of using excess electricity.

The point of programming to room temperature is that if the air temperature in the kiln cools faster than predicted, the controller will turn the kiln on.  You will need to be present for a while after venting the kiln to hear if it turns on and you can lower the lid to a point where the kiln does not turn on, indicating the rate of cooling is less than put into the schedule.

An example:

Assume you predict that 150°C per hour is the appropriate rate of cooling from 370°C. Also assume you open the kiln at 100°C and a minute or so later you hear the kiln start.  Then you know that you have opened the kiln too far causing a more rapid cooling than 150°C per hour and you need to close the opening to less than the current state.  This probably will be a progressive thing.  You will come back, say, half an hour later and open a little more.  Everything seems fine, but 10 minutes later you hear the kiln switch on again.  Oops! You opened too much – you need to close the kiln a little.  This may repeat several times.

The real answer to when you can open your cooling kiln is dependent on many variables.  You will have to decide on how critical these are in relation to the piece(s) you have in the kiln.  Once you have decided on the appropriate rate, you should program that into your schedule for the final segment.  This means when you partially or fully open the kiln the controller will switch the kiln on when the cooling rate is faster than you wanted.

Uneven Slumps

A common problem in kilnforming is that the glass slumps into the mould unevenly. Several of reasons are given in this post about high temperature or fast slumps for uneven results.

There are two other things that can be done to alleviate uneven slumps.

Place the mould in the centre of the kiln to reduce any uneven heating of the glass.  Uneven heating is a common cause of off-centre slumps.  Where you have persistent uneven slumping with a mould it may be better to fire it on its own so the conditions can be best for it.  Sometimes it is more economical to fire a single item rather than a crowded kiln shelf where the firing conditions must be for an average rather than the optimal firing schedule and conditions for one mould.  Less of the resulting slumped glass is disappointing.

There is an alternative. Cut the glass so the fused piece will be slightly smaller than the mould top. This will allow the glass to sit inside the mould rather than on top. Frequently there is evidence of the glass hanging up on the side of a mould.  Sometimes there are spikes where the glass stuck and stretched. (Another reason for Low and Slow)

A third method has been suggested, but I have not tried it.  This is to lightly bevel the underside of the piece to be slumped.  The basis for this suggestion is that a bevelled edge will fit the mould better by having a slope rather than a relatively sharp edge resting on the mould surface.  I do know the other two suggestions work, but not this one, although it sounds logical.

More detailed information is available in the e-book: Low Temperature Kilnforming.

Heat Work is Cumulative

“…. the first fuse (contour) I brought it up to 1385°F and held for 5 minutes - it did not contour as much as I would like - do I re-fire at same temp and hold longer or go up in temp and hold same amount of time or something else?”

Observe

Of course, the smart answer is “Observe to get it right first time”.   Observation will enable you to determine when the piece is fully fired.  To observe you need only peek at 5-minute intervals to determine if the piece is as wanted. 

Know your Controller

In combination with this you will need to know your controller well enough to be able to advance to the next segment if the piece is done before the segment finishes; or how to stay on the same segment until it is finished and then advance to the next segment.

Of course, there are circumstance when you cannot or do not want to be present at the top temperature of the firing.  Then consider using the delay function to enable you to be present. This gives a countdown until the kiln starts.  The practice is fully described in this blog entry.

Time or Temperature

If you are experiencing an under-fired piece and want to re-fire it to get a better finish, the usual question is whether to fire for longer or at a higher temperature.

The response is – “Neither”.

Re-fire to the same temperature and time as before, unless you are looking for a radically different appearance.  Heat work is cumulative.  You have put heat into the glass to get the (under fired) result.  By firing it again, the heat will begin to work on the glass as it rises in temperature.  The piece, in this instance, is already a slight contour.  The additional heat of this second firing will begin to work just where the first firing did, and will additionally change the existing surface just as the first firing did.  The degree of contour achieved by the first firing will be added to equally in the second firing.  It is of course, a good idea to peek in near the top temperature to be sure you are getting what you want. More information on heat work is available here with its links. 

Rate of Advance

It is important to remember that on the second firing the glass is thicker, and you need to schedule a slower rate of advance until you get past the strain point – about 540°C for fusing glasses, higher for float and bottle glasses.

Future firings

At the finish of the second firing you will have soaked at the top temperature for twice the scheduled time.  You can use this extra time for the next similar firing, or increase the temperature slightly and keep the original firing’s length of soak. 

As pointed out earlier, observation for new layups, sizes, thicknesses, etc., is important to getting the effect you want the first time.

Glass Bending Temperatures

Glass bending is the process by which glass is shaped without obtaining mould marks on the glass.  It also attempts to shape the glass without changing the thickness of the glass across its length and width. It can be done as a free drop curve or into a mould. This bending is usually done at much lower temperature than slumping.

Determining the temperature at which glass should be bent is a matter of experimentation with each new shape and thickness of glass.  If the temperature is too high you find distortions are created in the glass.  Sometimes wrinkles develop.  In general, a high temperature leaves a lack of time to compress and stretch evenly into irregular shapes.  If the temperature is too low the whole process takes an impractically long time to complete.

The just right temperature is in the region of 50C/90F above the annealing point of the glass being used.  Experimentation with the shape and thickness of the glass is needed to establish a reasonable time for the bending; and for it to be achieved at a low enough temperature to get the shape required.

An example is this tapered cylinder.

 

Lantern frame for the glass

Mould shaped from the lantern into which the glass is to be bent

Flat template for cutting the glass

The bent glass

The curve was achieved in float glass at 590C/1095F in 20mins

A 1/8 sphere requiring bending in two directions was achieved in float at 570C/1059F in 45 mins to avoid ripples at edges.

The span as well as the shape affects the temperatures and times.  More information on bending glass is given in this blog entry, and is available in the eBook Low Temperature Kilnforming from Bullseye and Etsy.

Revised 5.1.25

Getting the Right Firing Temperature

“what temperature should I use to get a tack fuse that is just less than a contour fuse?”

This is the kind of question that appears on the internet often.  Unfortunately, no one can answer the question accurately, because it depends on some interrelated variables.

Kiln characteristics

Top or side elements, size of kiln, relative size of piece, all have an effect. Also no two kilns even of the same model have exactly the same characteristics.

Ramp Rate 

How quickly or slowly you fire has a big effect on the temperature and soak needed to achieve the desired result. This is the effect of heat work.

Temperature

There are no absolute temperatures for a given effect, given the above two variables.

Soaks

The length of time and the number of soaks will affect the temperature required to achieve your effect.

OK. So, what can I do?

Observation

The only certain way to get the effect you want is to observe.

Set a schedule, guessing the top temperature and length of soak.  Know your controller well enough that you can extend the soak or end the segment by advancing to the next.  Your manual will tell you how to do this.

Peek at intervals from 10-15C below the selected target temperature. Peek at 5min intervals until the effect is achieved.  Advance to the next (cooling) segment.  Record the temperature and length of soak at which the effect was achieved.  On subsequent firings you can experiment with reducing the temperature by 5C – 10C with a 10-minute soak.  Observe and record the temperature and effect as before.

The reason for going for a 10-minute soak rather than longer is to avoid holding at the target temperature for a long time, as that can help induce devitrification.  The reason for a soak at all is to achieve the minimum of marking on the reverse or picking up kiln wash or kiln paper on the back.

If effect is not achieved by the end of the soak, extend it by using the appropriate key or combination of keys.  Keep observing at five-minute intervals until the effect is achieved.  Advance to the next segment and record both the temperature and time.  The objective is to get the heat work done with a 10-minute soak, so you will need to increase the temperature on the next firing.  The amount of increase will depend on the length of soak required to get the desired surface on the previous firing.  The longer the soak, the more temperature you need to add.  You will need to repeat the observations and recording until you find a temperature that will achieve the effect with a 10-minute soak.

Use the lessons from the observations to lower temperature, extend soak, raise temperature, reduce ramp speed, or reduce soak as required.  It will also help you judge on other pieces the approximate temperature and time required for the new layups or new moulds.

Hot Spots in the Kiln

You may suspect you have hot spots in your kiln because of bubbles or one side of pieces being more fully fused than another. A good method for determining the temperature distribution across the kiln is given on the Bullseye site.  It does not require any sophisticated equipment – just supports equal distances apart and strips of glass equally wide and long – to be witnesses for the hotter and cooler parts of the kiln.  You fire slowly to a very low slump temperature – ca. 620C - for only 5 minutes.  Go as fast as possible to the annealing point and soak for 15mins. Then you can turn the kiln off, and let it cool as fast as the kiln can.

This test will show where the hotter areas are.  You will see from the test results that there is a gradual change of temperature across the shelf, rather than small hot areas that would be required for localised large bubbles originating from under the glass.  It will tell you where the cooler areas are, so you can avoid placing pieces in that area when you need precise profiles on the finished piece.

There is little to no relation between hotter areas of the kiln and localised bubbles.  Do not think hot spots are the cause of large bubbles.

Bubbles more often relate to:

Bubble squeeze

Alack of an adequate bubble squeeze, as described here.

Avoidance is the best approach.

Do not be lead into the idea that mistakes are automatically art, or that all of them can be rescued.

Rapid firing rates

Firing rates need to be adjusted to the materials you are firing.

As fast as possible firing rates can cause problems.

High temperature rapid firings can also cause problems.

Rapid firings are more likely to harm the glass than the kiln.

Damaged shelves

Distortions or damage to shelves can trap air and so cause bubbles to form between the shelf and the bottom of the glass.

Checking for flat shelves and fixing.

There are some remedies.

Volume control

Varying volumes within the piece can give problems.

There are a variety of related things that can cause large bubbles.

Glues

Glues and adhesives have a variety of effects and dangers, especially if generous amounts are used:

There are a variety of glues each with their own characteristics.

Uneven layers/layup

You must think of ways for the air to escape from the interior of the glass and from under the glass.  Most often we set up things in a way that creates bubbles. There are two main ways that we do this.

Encased items

When we put glass or other materials between an upper and lower sheet of glass we are creating conditions for bubbles to form.  The encased items hold the upper glass above the lower glass by an amount related to the thickness of the inclusion.  Routes for the air to escape must be planned. 

One of the ways to reduce the height of the space taken up by the enclosures, is to fire upside down with the inclusions on the shelf. This allows the glass -which will be the bottom layer - to form around the materials, reducing the air space between the bottom and capping layer.  This is known as flip and fire.

You then clean the face which will be capped very thoroughly.  Place the capped piece on fiber paper – which can have Thinfire placed over it, or coat with kiln wash.  This is to allow the air in the uneven bottom surface to escape from underneath through the fibre paper.

Weight

Even when there is no encased material, the weight of the glass pieces on top can create areas where the air can be trapped.  On a single layer the arrangement of pieces can create areas where the glass cannot resist the air pressure that cannot disperse from the pockets caused by the glass on top.  Very clear and generous exits for the air are required.

This can happen with two layers as well, although usually a higher temperature is required.  A means of avoiding large bubbles when there is glass – powders, frits or pieces of glass – placed on top is a two-stage firing of the piece.  First fire the base layers together at full fuse so they become one whole.  Then add the decorative elements on top and fire.  Remember to fire more slowly than for two unfired layers.  The main piece is now 6mm thick and needs a slower rise in temperature.  The additional heat work this entails may mean that a lower top temperature, or a shorter soak will be required than normal.  You will need to peek at intervals to check on the progress of the firing.

There is a multiplicity of ways that bubbles large and small can be created.  Careful layups, bubble squeezes, slower rates of advance and lower top temperatures can minimise, but not always eliminate, bubbles.

Revised28.12.24