Slumping Blank Size

When you're making a piece that you intend to slump does it need to be bigger than what you're making, and by how much?

Generally,

The general advice is to make the blank the same size/diameter as the rim of the mould.  This works best for shallow moulds with a generous draft, or shallow slope to the bottom.  

There are numerous exceptions of course.

Deep moulds

Deep is a relative term.  A small round mould of 100mm/4” with a 30mm/1.25” drop can be considered deep. But this drop would be considered shallow for a 300mm/12” diameter mould.  A drop of 100mm/4” into a 300mm/12” mould would be deep.

There are two approaches to this.  We know the blank will end up with a considerably smaller diameter than the deep mould. This is because the surface of the glass does not change its dimension much.  As a result, the diameter of the slumped piece is less than the flat blank’s diameter.  Placing a flexible measuring tape on the outside of the slumped piece will show the lengths of the flat blank and curved piece are similar.

Larger Blank

As a result, we are tempted to make the blank larger than the mould.  By how much? as the questioner asks.  The safest is avoid exceeding 6-8mm/.025 – 0.375”.  With a slow slumping rate, the edges will rise as the interior bends downwards and allow the excess glass to take up the same diameter of the mould before deforming enough to catch on the edge of the mould.  With a centimetre or 0.375 inch overhang, you begin to take greater risks with the rim beginning to slump outside the mould and hanging up. 

Smaller Blank

But size matters.  The small excess works best on larger moulds (250mm/8”) or more.  Employing this oversizing on smaller moulds has problems.  The span of the smaller moulds requires higher temperatures and/or longer soaks.  The result of this greater heat work is that the rim of the glass can begin to slump outside of the mould. In extreme cases, this will cause the glass to break.  More often, the edge does come into the mould, but with heavy stretch marking and sometimes needle points where the edge drags along the mould.

It is often best to make the blank smaller than the mould.  Small enough that it fits just below the rim of the mould.  This allows forming of the glass without the frequent drag marks and needle pointing.  Placing the glass level is most important when the glass is not supported by the rim.  If the final size of the slumped piece is important, you will need to slump in a larger mould. I do not know of a method of calculating that.  It is a matter of experimentation.

Steep Moulds

Ceramic shapes from charity shop finds that are adapted to be moulds are often steep sided as well as deep. They often have no rim on which to rest the glass before the slump shape takes over.  Both these elements result in the glass being much smaller than the mould when complete.

Collar

You can counteract the effect of deep, steep moulds by placing a collar of fibre board around the mould.  

Make a cut out from the fibre board by placing the mould upside down and tracing the outline. Cut the board slightly inside the line scribed.  Then fashion a bevel to meet the angle of the outside of the mould.  Support the board at the appropriate height for the mould.  Fill any gap between board and mould with kiln wash powder or a paste of the powder, depending on the nature of the gap.  This supports the glass during the slumping while allowing it to slump into the mould.  It increases the possibility of getting a steep side on the glass.  It also allows you to put a rim on the shape if you want.

Staged Slumping

Sometimes the collar or rim is not sufficient to allow the glass to move as desired in a single slumping stage.  Then multiple slumping stages are required.  The common approach has been to purchase a three-stage slumping set.  This can limit your approach.

·        The general approach is to measure the inside surface of the final steep mould. 

·        This gives you the starting diameter for the blank. 

·        Then measure the diameter of the mould at the outside of the rim. 

·        This gives you the diameter of a slumped piece to fit into the final mould.   

Compare the largest diameter blank to existing moulds you have. Will the glass have slumped enough to reduce the diameter to fit into the final mould?  In some cases, where the answer to that question is yes, only two-stage slumps are required. 

Most times a three-stage slump is needed. You need to find an intermediate mould that will deliver a slump with the required diameter.  This will be a moderately deep mould, usually with steeper sides than the first, but less steep than the final mould.

Angular Moulds

Angular moulds are those with sudden short drops to the foot (flat part of the mould). These are often ogee curves. 

These require more time or heat to form completely to the bottom of the mould.  The glass is curving in two directions during the slump.  The glass should be only slightly larger than the mould at most.  To counteract the tendency to slide down the mould, low temperatures and long soaks are needed.

 

Rectangular moulds

The main problem of rectangular moulds is the dog boning of the straight edges of the glass during the slumping. There are some suggestions.

Cut the blank with slightly outward curves.  This will help to compensate for the tendency to dog bone.  It will require some experimentation.  Slumping a standard square or rectangular mould will give an idea of how much the glass deforms during the slumping.  That amount of curve can be added to the edge when cutting out the blank.

Round the corners of the blank.  This relies on the principle that there is more glass at the corners to slump than at the sides.  A 10mm/0.375” radius should be enough.  

There is less glass to compress along the sides than at the corners.

Another element is to provide the rectangular shapes with rims.  This forces any dog boning to the rim rather than the sides of the slump.  It can be combined with either of the previous possibilities.

These do not always work and are sometimes difficult to reconcile with the design. The additional possibility to counteract the dog boning of the shape is to use slower rates, lower temperatures, and longer soaks. This is not always successful.  Rectangular shapes remain the most difficult to get the glass to conform faithfully to the mould.

 

There are ways to get the slumping blanks to conform to the moulds, and they all involve modifications to the glass, mould, or schedules.

Square Drapes

Two pyramidical moulds. One stepped and the other smooth.

 This kind of draping mould with flat sides will never work very well as a draping mould.  The draping sides have to compress. This takes a long time and is likely to cause folds in the glass.

 The common experience is that two opposite sides drape first and conform to the mould. This displaces the compression necessity to the other two sides. This "taco" style initial drape is common in all drapes. It is usually observed in handkerchief drapes.  In the early stage of draping two sides of the glass fall, creating a taco shape. With continued heating, those long sides fall and spread the initial draped sides to become almost equal. 

 This taco formation also occurs on the pyramid style mould, giving two flat sides.  The glass on the other sides then fall. As the glass area is now larger on these sides than the mould area, a drape or fold is formed.  Imagine the drapes a square piece of cloth place on a pyramid would create. The cloth has more area than the sides of the pyramid.  The excess cloth creates folds at each corner.  The same happens with the glass.

 This draping fold can be minimised by using low temperatures and long (multiples of hours) soaks.  This allows all the sides of the glass to begin forming at more or less the same time.  I am not sure the folds can ever be completely eliminated.  With extremely long soaks, the drapes will flatten to the rest of the glass. 

 Annealing difficulties are caused by this folding.  It will create thick overlaps.  This in turn will cause the annealing difficulties. There are areas that are much thicker than others.  If you started with 6mm glass, the folds will create areas that are 18mm thick. 

 Making sure this glass - with such large differences - is all of the same temperature will require long annealing soaks.  It will also require very slow cooling segments.

 Square drape moulds are rarely successful. Folds are created at the corners, rather than fully conforming to the mould.

Evaluating Top Temperature Effects

Fire for Your Kiln and Objectives

Credit: FusedGlass.org

 Often temperatures to achieve given effects are shared on the internet to be helpful to others.  Those who receive these need to evaluate the schedules used to achieve the profile at the stated temperature.

 The same effect can be achieved at different temperatures by using different rates and times in getting to the given temperature.  This is summarised as the effect of heatwork.  The longer taken to achieve the top temperature, the lower the temperature can be used.  The amount of time the holds/soaks occupy in the schedule will also affect the profile achieved.  This means that a simple top temperature can only be a point from which to begin exploration.

 It is important to evaluate each segment. What is the apparent purpose? How does it fit with the principles of applying heat and the characteristics of glass?

 When asking for help with a firing temperature, ask for the full schedule.  This will help you evaluate the suitability of the temperature given.

 The variations in schedules and kilns mean you should fire by results not by numbers. Use the rates, temperatures and soaks that will achieve what you want in your kiln.

Making Test Tiles

Creating samples or tests provides both references on firing profiles and knowledge of the characteristics of the kiln. 

General samples

 Sample tiles are normally a series of tiles with the same lay up but fired at different temperatures.  These are likely to be intervals of temperature from a sharp tack to a full fuse.  A suitable interval might be 10°C as it is easy to interpolate between these for a slightly different profile than the tiles show. This is the basic arrangement.

 You can make this more informative by including tiles in the same basic lay up but with hot and cool colours, opalescent and transparent, black and white, strikers, etc.  The addition of these will give a richer bank of information.

 Of course, these tiles must be labelled with glass types and code numbers and the temperature used.  This is not all the information required though.

 Many sample tile sets do not include the firing rate.  The heat work required to attain a specific profile is dependent on time, temperature and hold.  These are the time to get to the working temperature, the temperature, and the soak time.  If you do not record the ramp rate used, you will have incomplete information.  It is not that you have to record the entire schedule.  But the rates and any soaks on the way to the top temperature need to be recorded. This means you can take account of any slower rate of heating, any additional holds on the way up, and the length of the soak at top temperature.  Then when contemplating something more complicated than the conditions under which the tiles were made you have better information.

 It is a good idea to maintain a photographic record of the sample tiles to avoid storage problems.  These can be made from the individual tiles and photographed from several angles. 

 Another way of keeping records - without making tiles for each temperature - is to photograph the tiles through peep holes as the set temperatures are achieved.  This means the tiles need to be placed in the kiln so they can be seen from the peep hole. You will only have a physical sample for the top temperature. The other profiles will have a photographic record.  The firing conditions for these need to be recorded just as for the series of physical tiles.

 This photographic record may not be suitable for your way of working and so require making the sets of multiple tiles.  Both these methods provide a generalised record of heat work to achieve given profiles.  Note that you will need to prepare sample tiles for each kiln, as each has different characteristics. 

Specific samples

 However, there will be cases where the general conditions exemplified by the reference tiles are to be exceeded.  In this case you will need to make a sample specific to the piece you are planning.  This can be a general representation of the piece, or a scaled mock-up. 

 The general test tile may be small scale or relatively large.  It will contain only the components in terms of height and shape that will be in the planned, but more complicated piece.

 A more rigorous method is to make a full scale – or nearly so – mock-up of the piece. This is usually done in clear. Fire it to the proposed schedule to determine the exact effect.

  

Sample making gives confidence in preparing work for the kiln and scheduling to get the desired profile.

Achieving Multiple Profiles in One Piece

People ask about whether it is possible to tack fuse additional elements without affecting the profile of the existing piece.

It is as though glass has a memory of the heat it has been subjected to.  For example, a sharp tack will become a slightly rounded tack, even though refired to a sharp tack again.  So, it is impossible to refire a piece to the same temperature or higher without affecting the existing profile.  But it is possible to fire a piece with differing profiles if you plan the sequence of firings.

 

Tack fuse onto existing profile

 

It is possible to add pieces to be tack fused with little distortion to the existing piece through careful scheduling and observation.  There are several requirements.

 •     A moderate rate of advance to the working temperature is required, rather than a fast one. This is because the piece is a single thicker piece with uneven thicknesses.  Also, a slow rise in temperature allows completion of the work to at a lower temperature.  This means there will be less change to the existing profile.

•     A minimal bubble squeeze - or none at all - is required on this second firing.  The added pieces generally will be small, so if possible, eliminate the bubble squeeze.  The requirement is to add as little heat work as possible.

 •     The working temperature should be to a low tack fuse temperature with a long soak. 

 •     Observation is required from the time the working temperature is achieved.  Peeking at 5-minute intervals is needed.  This to be certain that the current tack fuse can be achieved without much affecting the existing profile.  It will be a compromise that you will be able to choose during the firing.  The decision will be whether to retain existing profile and have a sharp tack.  Or a slightly rounded tack and more rounded profile on the original piece.

Planning for multiple levels of tack

It is possible to design a piece with multiple profiles within the completed piece.  You need to plan out the levels and degrees of tack you want before you start firing. 

To do this planning, you need to remember that all heat work is cumulative. In simple terms it means that on a second firing you will start where you left off with the first one. The texture in the first firing will become softer, rounded, or flatter than the second or even the third firing.

Three degrees of tack can be achieved with a little planning.  It works similarly to paint firings.  Some paints fire higher than enamels, and enamels hotter than stain.  You have to plan to fire all the tracing and shading first.  Then you add the opaque enamels, followed by the transparent enamels.  Finally, you add the silver stain.  This is unlike painting on canvas where you build up the image all together.

The same principle is true of a multiple level tack fuse piece.  When creating various profiles in glass, you proceed from firing the areas that will be the flattest first. Then proceed to the areas which will have the least tack last.  This is a consequence of the cumulative effect of heat on re-fired glass.

Plan out the areas that you want to have the least profile.  Assemble the glass for those areas. I suggest that a 6mm base is the initial requirement for anything that is going to be fired multiple times.  Add the initial pieces that will become a contour fuse or a very rounded tack. 

First firing

Put this assembly in the kiln and schedule.  Do not fire to the contour profile temperature.  Instead, you will be scheduling for a sinter or sharp tack. This depends on how many textures you plan to incorporate.  Start with a sharp tack.  Fire at the appropriate rate with a bubble squeeze to about 740°C for 10 minutes and proceed to the anneal cool.   Different kilns will need other temperatures to achieve a sharp tack.

You do not fire to the contour fuse temperature, because the base will be subject to more firings.  Each of these firings will soften the base layers more than the previous one.  This is the application of the principle of cumulative heat work.  When you fire a piece for a second time, there will be little effect until the softening point of the glass is reached. Once there, the glass further softens, giving the effect of a contour fuse.

Any glass that had already achieved contour profile from the first firing will flatten further.  This can be used in cases where the working temperature was not high enough.  Just fire again to the original schedule’s temperature.  Take account of the need for a slower ramp rate to the softening point.

Second firing

Once cool and cleaned, you can add your next profile level of tack fusing to the base.  Note that “level of tack” does not refer to thickness being built up.  It is about the amount of roundness you want to impart to the pieces.  You may be placing this second - sharper – level of tack in the spaces left during the first firing.  Again, schedule to the original approximate 740°C. But remember the base is now a single piece.  You need to slow the ramp rate to the softening point, after which the speed can be increased.  You will not need to retain the bubble squeeze unless you are adding large pieces, or into low areas. 

The second firing will show the pieces added for the second firing to have the profile of the original pieces.  Those pieces having their first firing will have a sharper appearance.

 

credit: vitreus-art.co.uk 

This is a piece where the flower petals and leaves could have been placed for the second firing to give a softer background with less rounded flower details.

 

Third firing

Clean well and add the pieces for the final level of tack.  Schedule the initial rate of advance a little slower than the second firing.  The piece is growing in thickness and complexity.  Once the softening point is reached, the original rate of advance can once again be used up to original temperature. 

Final firing

Clean well and add the pieces for the final level of tack.  Schedule the initial rate of advance a little slower than the second firing.  The piece is growing in thickness and complexity.  Once the softening point is reached, the original rate of advance can once again be used up to original temperature. 

Further notes on multiple firings

It is a good idea to observe the firing, once the working temperature is achieved.  This is to ensure enough roundness is being given to the final pieces being tacked to the whole.  Be prepared to extend the soak if the final pieces are not rounded enough.   Although you should have a good idea of the degree of tack for the final pieces from the previous two firings.

You may need to experiment a little with the temperature and length of soaks at the working temperature.  For example, if the degree of tack is too sharp in the first firing, you can extend the soak or increase the temperature for the next ones. 

If you are firing at 740°C, you may feel you can afford to extend the soak for the subsequent firings, because you are in the lower part of the devitrification range. Consider the risk of devitrification increases with the number of firings of the glass.  The preference is to increase the temperature a bit for subsequent firings to ensure you are not spending a cumulatively long time in the devitrification range but still be able to get the final tack level desired. 

The preference is to increase the temperature a bit for subsequent firings to ensure you are not spending a cumulatively long time in the devitrification range but still be able to get the final tack level desired. 

Because most of your heat work is happening in the low end of the devitrification range, the cleaning regime must be very thorough.  Any chemicals or soaps used must be completely washed off with clean water.  The piece must be polished dry to ensure there are no water marks left on the glass.

You can, of course, have more levels of tack.  One approach would be to start with a sinter, or tack to stick, firing. And repeat that four or more times.  Another is to increase the working temperature and reduce the length of time soaked there.  The shorter time means there is less rounding of each level, allowing the build-up of many levels of tack.  All of these require some experimentation. 

More information is available in the ebook Low Temperature Kilnforming.

Three firings to the same sharp tack profile will give multiple profiles in the finished piece. 

Softening the Tack Profile

Often people want a particular profile not provided by the schedules in the controller or the ones they normally use for tack fusing. The question arises as to whether to increase the temperature or extend the soak on a previously fired piece.

You can do either.

You can extend the time or increase the temperature. There are benefits and drawbacks with each.

Increasing the temperature is the choice for a quicker firing. But you have less control.  By increasing the temperature, you will certainly get a softer edge to the glass. You do not know until the firing is finished how much the glass has changed.

Extending the time means that you know a softer profile will be created simply by more heat work being put into the glass.  If you combine the extended soak time with peeking at intervals, you have much more control over the exact profile achieved.  Observation at 5- or 10-minute intervals after the target temperature is achieved, will enable you to get exactly the profile you want. Just advance to the next segment when that profile is achieved. 

The drawback is that the firing takes a little longer and you have to be present at the time the working temperature is reached.  You can schedule that by using the delay feature on your controller.

Note that on any re-firing of a piece you need to be aware that you are firing a single thicker piece rather than the original multiple layers.  This will require a more cautious rate of advance up to the softening point of the glass – generally around 540°C.  After that, the original rate(s), soaks and annealing can be used.

Of course, the considerations of temperature versus time can be applied to an initial firing as much as to a re-friing of a piece.

Observation is the best way to have precise control over the profile of your tack fusing.

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

Single Layer Circle with Decorative Rim

A question arose:

If you fuse a single 20cm diameter sheet of 3mm glass to full fuse, [with a decorative rim] what happens? … Would the lack of two layers in the centre be a problem for the 6mm rule?

This layup risks trapped air and a large central bubble.  The explanation involves the combination of volume control and weight.

Volume control

The volume control relates to the single 3mm layer in the centre.  The glass will thin in the centre and thicken at the perimeter.  This leads to the risk of thinning to the degree that bubbles are created in the centre.  The edges will also draw in as the viscosity - surface tension - of the glass pulls the glass toward a 6mm thickness.

Weight

The explanation is also about weight.  The decorative rim adds weight to the outside of the piece.  This weight will “seal” the rim of the glass to the shelf, reducing the possibility of air escaping from under the central portion of the piece.  This weight effect on the rim increases the risk of a large central bubble.

Profile

Another influence on the result of the fuse is the degree of fuse.  At full fuse the viscosity of the glass is less and so resists the force of expanding air much less than when cooler. Even at rounded tack fuse, the glass will be unable to resist the formation of bubbles. As the glass thins and viscosity decreases, any air at all will cause a bubble.

Changes for the future

Avoidance of bubbles in this piece relate to design, scheduling and technique.

Design

It is possible to design a piece of this nature to avoid the volume control issue.  The base piece could have a smaller circle or rectangle centralised on top inside the proposed perimeter.  The rim can then have the decorative elements placed.  If they are spaced widely, frit can be used to fill significant gaps.  The piece can then be placed in the kiln for a full fuse.

Scheduling

You can also fire the piece as originally described very slowly to a low temperature.  This uses the concept of heat work. By applying the heat over a long period, you can achieve the same effect as would be achieved by a faster rate of advance to a higher temperature. 

There are at least two ways to increase the heat work.  You can use a very slow rate of advance to a point slightly above the softening point of the glass.  This will be the lower end of the slumping temperature range of your glass.  The soak may be for hours.  You will need to observe when the effect you want is achieved.

You also can choose the same lower slumping temperature and reach it in your standard fashion.  This will require an even longer soak time to achieve the same result.

In both these low firing approaches, you will need to observe to determine when the piece is finished.

Technique

The “flip and fire” technique may also work on the single layer with an added rim.  To do this you build the piece upside down on the shelf.  It helps to draw an outline of design on Thinfire, or Papyros.  Place the decorative elements and cap them with the clear.  Take the whole to a rounded tack fuse.  When cool, clean well and fire to a tack fuse again.  This will give something less than a full fuse, but it will be more than a tack, as the heat work is cumulative.

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

Summary

A single layer piece with a decorative rim is most likely to produce bubbles in the centre.  There are some ways to overcome this: design, scheduling, and technique. Design is the most likely to be successful.

Digest of Principles for kiln forming

Some time ago people on a Facebook group were asked to give their top tips for kiln forming.  Looking through them showed a lot of detailed suggestions, but nothing which indicated that understanding the principles of fusing would be of high importance.  This digest is an attempt to remind people of the principles of kiln forming.

Understanding the principles and concepts of kilnforming assists with thinking about how to achieve your vision of the piece.  It helps with thinking about why failures have occurred.

Physical properties affecting kiln work

 Heat

Heat is not just temperature. It includes time and speed.

 Time

       The time it takes to get to working temperatures is important.  The length of soaks is significant in producing the desired results.

 Gravity

       Gravity affects all kiln work.  The glass will move toward the lowest points, requiring level surfaces, and works to form glass to moulds.

 Viscosity

       Viscosity works toward an equilibrium thickness of glass. It varies according to temperature.

 Expansion

       As with all materials, glass changes dimensions with the input of heat.  Different compositions of glass expand at different rates from one another, and with increases in temperature.

 Devitrification

       Glass is constantly tending toward crystallisation. Kiln working attempts to maintain the amorphous nature of the molecules.

 Glass Properties

·        Glass is mechanically strong,

·        it is hard, but partially elastic,

·        resistant to chemicals and corrosion,

·        it is resistant to thermal shock except within defined limits,

·        it absorbs and retains heat,

·        has well recognised optical properties, and

·        it is an electrical insulator. 

These properties can be used to our favour when kiln working, although they are often seen as limitations.



Concepts of Kiln Forming

 Heat work

       Heat work is a combination of temperature and the time taken to reach the temperature.

 Volume control

       The viscosity of glass at fusing temperatures tends to equalise the glass thickness at 6-7mm. 

 Compatibility

       Balancing the major forces of expansion and viscosity creates glass which will combine with colours in its range without significant stress in the cooled piece.

 Annealing

       Annealing is the process of relieving the stresses within the glass to maintain an amorphous solid which has the characteristics we associate with glass.

 Degree of forming

       The degree of forming is determined by viscosity, heat work and gravity.  These determine the common levels of sintering, tack, contour, and full fusing, as well as casting and melting.

 Separators

       Once glass reaches its softening point, it sticks to almost everything.  Separators between glass and supporting surfaces are required.

 Supporting materials

       These are of a wide variety and often called kiln furniture.  They include posts, dams, moulds, and other materials to shape the glass during kilnforming.

 Inclusions

       Inclusions are non-glass materials that can be encased within the glass without causing excessive stress.  They can be organic, metallic or mineral. They are most often successful when thin, soft or flexible.

A full description of these principles can be found in the publication Principles for Kilnforming

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.