Wednesday, 20 October 2021

Texture Moulds and Glass Sizes

I had an overhang [on a texture mould] and I heard a pop and opened kiln and saw it cracked off the mold. … [The piece] is 2 layers Bullseye irid placed face down and Tekta [on top]; the mold was sprayed 3-4 times with zyp and Thinfire; and I put mold on kiln posts. [I] fired to 1440[F].


The overhang of the glass caused the break. As the glass heats it expands. The ceramic does not expand as much as the glass.  This means even more glass will hang over the edge than at the start.  As the glass reaches slumping temperatures, it begins to drape over the edge. At the same time the glass on the interior is beginning to slump into the textures.  When the glass has fully taken up the texture, the overhanging glass will be touching the outer sides of the mould. This means at the end of the heating and soaking part of the firing, you have the ceramic mould partially and tightly encased in glass.  The glass has formed around the ceramic.


The physics of the two materials are that glass expands more than ceramic. On cooling, the glass grips the sides of the ceramic mould tightly. This is because it shrinks more than the ceramic.  In this case, the ceramic was stronger than the glass and the strain caused the glass to break.  Upon occasion the opposite can happen.

Two other notes.

The temperature of 781°C is higher than needed.  You will need to do a bit of experimentation to find the right combination of temperature and time for each mould.  You could consider 630°C as an initial temperature with a 90-minute soak.  Bob Leatherbarrow (p.161) describes a method of scheduling a sequence of slightly higher temperatures with soaks.  If the texture is not forming (as determined by observations), you can advance to the next segment with a higher temperature and see how that goes.  When the appropriate amount of texture has been achieved, advance to the cooling to anneal segment.

Iridised surfaces provide a very good separator.  With the iridisation down against the mould, it may be unnecessary to use Thinfire, especially when you already have used boron nitride as the separator.

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

Wednesday, 13 October 2021

Incomplete definition on texture moulds

An enquiry on incomplete definition in the glass from texture moulds:
I have this texture mould, but I’m not getting much definition. I’m using a single 3mm layer of 96. Do I need to go hotter for longer?
155C - 750C, 15 minutes
Full - 516C, 140 minutes
49C - 371C, 0 minutes
Full - 49C, off

My response:

You have sensibly increased the temperature at a moderately slow rate (for 3mm).  This ensures the glass is evenly heated through by the time it reaches the working temperature.  It is slow enough for you to be confident that most of the air has been squeezed out.

If you wanted to be more cautious about bubbles,  you could introduce a slow increase in temperature - maybe 50°C - from 600°C to 650°C.  You may want to soak there for 30 minutes, although it may not be necessary.  Once that segment is finished you can resume at 75°C to the top temperature.

I would not increase the temperature as you are already at risk of dog-boning the glass.  I would extend the soak time to 180 minutes at 750°C. You need to check frequently after the top temperature is achieved.  A quick peek is all that is required to see if the texture is fully reflected in the top surface.  You may find success by using a lower temperature, say 730°C, but it will require at least an hour more soak time.

The piece above conformed completely to the 12mm depth of the mould with a soak of three hours at 750°C. There was incomplete formation of another test piece at 740°C for three hours. So the 10°C made enough difference for complete formation over this depth.  With less extreme heights, a lower temperature or a shorter soak would be possible.

Once the texture is assured, you need to advance to the next segment.  Or, if it is not achieved by 10 minutes before the end, extend the soak.  Check your controller manual on how to skip to the next segment, or to extend the soak.

As an aside, your annealing soak and cool is very long and slow for 3mm.  You can regain the time used in the slow ramp rate and soak. Review the requirements for a single sheet of glass.  A 60-minute annealing soak and a cool rate of 83°C/hr. to 370°C is an adequately slow anneal cool for a 3mm piece.

You may find more success with a 6mm sheet.  The weight of glass helps it conform to the texture mould.  I have found that a slow ramp rate (about 150°C) to the strain point of ca. 540°C, followed by half that rate to top temperature allows a reduction in soak time to achieve the required definition. This reduction in soak time can be one half hour less than the time required to get good definition on a 3mm sheet.

The strategy outlined here for the scheduling is using the principle of slow and low and long firings.  It is much easier to control the results of the firing by using moderate ramp rates to lower temperatures combined with longer soaks and periodic peeking to check on progress.

If you do not have the time to devote to peeking when the schedule is at the top temperature, you should investigate the method of programming a delay to the start of the firing.  Your controller manual will give the method of using this function.

Texture moulds work well with the slow and low principle of kilnforming.  Long soaks may be required with periodic observation to determine when the process is complete.

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

Wednesday, 6 October 2021

Removing kiln wash from moulds

“How do I remove kiln wash from a mould that I have decided would work better with ZYP?”

Once coated with kiln wash, slumping or draping moulds do not need to be re-coated until the surface is damaged.  Then it is best to remove all the kiln wash to prepare a new smooth surface for the kiln wash.  You may, of course, as the enquirer above states, want to use a different kind of separator.  The cleaning of the kiln wash from the mould will be the same process whatever you want to do with the mould next.

There are many ways to get the old kiln wash off.  Some of them depend on the material from which the mould is made.


If the mould is made of stainless steel or other metal, the easiest method is to sandblast with lots of air and a minimum of grit.  You can also use sandpapers or open weave sanding screens. The methods used on ceramic moulds, as described below, can also be used on metal.


Sandblasting is not safe to use on ceramic moulds, as the sandblast medium can erode the surface very quickly and often unevenly.

Preparation for manual removal of kiln wash.

It is best to wear a mask during this process to reduce the amount of dust you inhale. Spread a cloth, newspaper or other covering to be able to easily gather the removed kiln wash and place it in the waste.  Have a vacuum sweeper at hand to remove powder rather than blowing it around the work space.  Of course, if you can do this outside, there is much smaller risk of contamination.


I suggest that removing the kiln wash while the mould is dry should be the first stage. 

Flat surfaces can be cleaned with a straight edged wooden stick, or wooden clay modelling tool.  Firmly push it along at a slight angle from the vertical to remove most of the kiln wash. 

On curved surfaces you will need a rounded tool such as a plastic burnisher or all nova tool for the coarse work.  This can be followed up by using a stiff sponge to clean up any stray kiln wash still adhered. If the kiln wash is persistently sticking to the mould, you can cut a small piece from an open weave sanding screen and use it to gently remove the most difficult remaining kiln wash.  Do not use more than light pressure, as with heavy pressure, the screen can begin to remove the surface of the ceramic mould.

Texture moulds and those with a lot of detail or right-angle corners need a bit more attention.  You can use a variety of non-metal tools to get into areas of detail.  Some of these are a rounded chopstick, a wooden skewer, a plastic knitting needle, and other similar items with rounded points.  These can be backed up with a small stiff nylon brush.  It is while working on these detailed areas that the vacuum sweeper will be most useful to clear out the debris and enable you to see how well the kiln wash is being removed.


Some people do not like the idea of the dust created from the removal of the kiln wash being in the air at all.  And sometimes, the dry removal is not complete.

My recommendation is to dampen the kiln wash that is on the surface of the mould.  This will cause some difficulties in removal, because a slurry is created along with the flaking of the baked-on kiln wash.  The same tools can be used to clean the mould as when dry.  The vacuum sweeper will not be of use though.  Once the kiln wash appears to be cleaned away, the mould needs to dry to enable removal of the remaining kiln wash.  Once dry, you can use dry sponges, or the small nylon brush to clean the remaining film of kiln wash from the mould.  This cleaning may reveal areas where the kiln wash is still adhering. These can be dealt with wet or dry, although I prefer dry.

Soaking or washing the mould does not remove the kiln wash as easily as you might think.  It is especially to be avoided where the mould has an internal hollow, as it may take days to dry sufficiently to apply other separators.  To put it in the kiln risks breaking the mould by the steam build up during the initial rise in temperature.

If you must soak the mould, I recommend that you use a 5% solution of citric acid because it has a chelating action on some of the components of kiln wash.

Remember that once the mould or shelf has been coated with boron nitride, it is almost impossible to go back to kiln wash again.  The boron nitride fills the porous parts of the ceramic making it difficult for the kiln wash to adhere to the mould.

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

Wednesday, 29 September 2021

Tacking Freeze and Fuse to Base Glass

The question has been asked:

I'm wanting to add some freeze fuse pieces on to float and just fire to a tack fuse … in one firing instead of two …[to avoid] losing the detail on the freeze fuse pieces. The top temperature on freeze and fuse is 720°C versus a … float tack temperature of 787°C. [can this be done?]

My response:

What you are doing with the freeze and fuse process is sintering the glass particles together by holding at a low temperature for a very long time.  This binds the glass together without altering the overall shape of the object. 

There is no reason why you cannot sinter the freeze and fuse piece on top of a base glass, if you pay attention to one major thing.  The freeze and fuse object will shade the heat from the base glass.  If you do not slow the rate of advance enough, you will break the base glass by creating too great a temperature differential between the part under the freeze and fuse piece and the uncovered part.

Another element to be considered, is that the frozen object is damp.  This will need to be dried by a slow ramp or it will further complicate the uneven heating problem.

Scheduling the Rate of Ramp
Choosing the rate of increase in temperature is determined by the dimensions of what is being sintered.  One widely practiced method is to double the total height and fire for that dimension.  For example, if the freeze and fuse is 8mm high, add that to the 6mm base and fire for 28mm – (6+8=14)*2 =28mm.

Another slightly less cautious approach is to multiply the total height by 1.5 and use the firing conditions for that thickness.

Determining the rate of advance for the thickness you have calculated – by either method - can be aided by using the Bullseye chart for annealing thick glass.  Look at the final cooling rate in the chart for the nearest thickness. In this case, use the one for 25mm.  The cooling rate is given as 90°C per hour.  If the glass can safely cool at that rate, it should also survive that speed of heating at the start.

If you chose the 1.5 factor, the thickness to schedule for will be 21mm.  This is between the 19mm and 25mm thicknesses given in the Bullseye chart.  The cooling rate given for 19mm is 150°C and and for 25 is 90C. As 21mm is almost the mid point between the two, you can halve the difference in rates (150 and 90) to give 120°C as the rate of advance. Although in both schedules using these rates of advance for the described circumstance, I would add a soak at 250°C for 20 minutes, to be cautious.

Remaining Parts of the Schedule

Sintering Soak
The length of soak for the sintering stage can be the same as the soak for the freeze and fuse, as you will be both sintering the glass pieces together and to the base glass too.

Anneal Cool
The annealing soak and cool should follow the rates given for the calculated thickness - in this case for 21mm or 28mm.

The Bullseye chart Annealing Thick Slabs can be used for all types of soda glass (which includes float glass) to determine the soak times and cooling rates.  You only need to make alterations for the annealing temperatures.  The annealing temperature I use for float glass is 540°C. 

The first two stages of cooling are 55C each, so simple subtraction from the annealing soak will give the temperatures for each stage of the cooling. If we use the calculated 21mm thickness, the soak time will be 3.5 hours at 540°C.  Then the Bullseye chart's displayed cooling rate of 20°C will apply from 540°C to 485°C, and the cooling rate of 36°C will apply from 485°C to 430°C. The final cooling rate of 120°C will be from 430°C to room temperature.  The chart for these adaptations is described in the post about adapting the Bullseye chart for annealing.  The reasons behind these operations are given in the ebook Low Temperature Kilnforming.

Wednesday, 22 September 2021

Firing cremains to avoid bubbles

Firing with cremation remains is very similar to firing with any organic material encapsulated into glass.

There are several possible design approaches.

Drilling holes is one method to avoid bubbles.  You can drill the base, put the remains on top and then cap.  Place the whole assembly on 1mm fibre paper to allow the air to migrate out through the hole and fibre paper under the glass.

Alternatively, you fire upside down and then fire polish the top.  Place the eventual top down onto the kiln washed shelf or Thinfire. Place the remains on the glass and cap with the glass that has the hole drilled.  Fire, then clean, turn over and fire polish the final top surface.

Design the piece and placing so there is a gap at the edge. 
This gives a route for air to escape.  If there is any gap left after fusing, it can be filled with a bit of super glue or other clear glue. 

Another method is to place pieces of frit or stringer at the very edge of the base glass to allow air out from under the centre of the piece.

If you do not need to concentrate the cremains in one area, you can disperse the material evenly across the piece to reduce the possibility of large bubbles.  The air and gasses can migrate to the edge through the particles, just as happens with powder sprinkled between layers of glass.

You can combine some of these methods as they are not mutually exclusive.

Fusing these pieces is, in principle, the same as encapsulating any organic material within the glass.  Slow advances are required with a 3 to 4-hour soak at around 600°C to burn out any residual organic material just as you might for thick vegetable matter.  You can add another bubble squeeze soak of an hour or so at around 650°C to gradually push any remaining air out from between the particles.  Then advance to the fusing temperature and anneal as usual.

Wednesday, 15 September 2021

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 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.

       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 woris 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

Wednesday, 8 September 2021

Adapting the Bullseye Annealing Chart

Adapting the Bullseye annealing chart for other glasses.

This post is about the method to adapt the bullseye annealing chart for thick slabs to any soda lime glass as used in kilnforming.

I frequently recommend that people should use the Bullseye chart for Annealing Thick Slabs.  The question returned to me is "How do I do that?"  My usual response is “Substitute the numbers for your glass into the temperatures given in the Bullseye table.”  This is inadequate for composing a full firing schedule, because there is no heat up rate, only annealing times and rates.  So, people react, saying "It’s only half a schedule."

That is so.  The heating of glass is so dependent on layup, size, style, process, and purpose of the piece.  This makes it very difficult to suggest a general firing schedule.  People generally find this out after some time using already set schedules for their kiln. What works for one layup doesn’t for another.

Devising a Schedule for the Heat Up

As there is no recommendation on heat up, you have to make up your own schedule for heating up the glass.  This is where I can give some general advice on some of the things you need to be aware of while composing your schedule without getting into the details of scheduling for specific projects.

The most important element to note is that the Bullseye chart is based on evenly thick pieces of glass.  If you are tack fusing or have different thicknesses of glass across the piece, you will need to use a thickness greater than the total height of your piece.  The calculation for this depends on the final profile.  It can be between 1.5 and 2.5 times the thickest part of the piece. 

The end cooling rate for the appropriate thickness is a guide for the first ramp rate of your schedule.  The final rate for an evenly thick piece 19mm/0.75” is 150C/270F.  This could be used as the rate for the first ramp.  However, Bob Leatherbarrow has noted that most breaks occur below 300C/ 572F, so more caution is advisable at that starting ramp rate.  My testing shows that using the final rate of cooling that is one thickness greater is suitable for the first ramp rate. In this example that would be 120C/216F per hour. 

Even though for some thinner pieces the rates given are much faster, it is not advisable to raise the temperature faster than 330C/600F per hour to care for both the glass and the kiln furniture.  This restricted ramp rate protects against glass breaking in the early stages of firing.  It also reduces the amount of overshoot in temperature at the top temperature of the schedule. 

The second ramp to the bubble squeeze can be at the final cooling rate for the calculated thickness up to a maximum of 330C/600F.  

After the bubble squeeze you can advance at up to 330C/600F rate to the top temperature.   AFAP to top is not advisable to be able to maintain control of the possible overshoot in temperature that would be created.  The top temperature should be such as to achieve the result in 10 minutes to avoid problems that can occur with extended soaks at top temperature.  

In the example given this gives a heat up full fuse schedule similar to this:

120C/216F to 260C//500F for 10 minutes

150C/270F to 566C/1052F for 0 minutes

50C/90F to 643C/1191F for 30 minutes

150C/270F to 800C/1473F for 10 minutes

Adapting the Bullseye Annealing Chart for Other Glasses 

As you have written the first part of the schedule, you can begin to apply the annealing information from the Bullseye chart to develop the schedule.  The first part of the anneal cooling starts with dropping the temperature as fast as possible to the annealing temperature.  

The question will arise “Why should the Bullseye annealing chart be used instead of some other source?”  It is researched.  It is the most thoroughly researched set of tables for fusing compatible glass that is currently available.  This means that the soak times and rates for the thicknesses can be relied upon. 

“How can it be used for glass other than Bullseye?”  The rates and times given in the chart work for any soda lime glass, even float. It is only the temperatures that need to be changed. 

The method for making the chart applicable to full and tack fusing and slumping is a matter of substitution of temperatures.  The intervals between the temperatures remain the same for all soda lime glasses – the range of glass currently used in fusing, including float glass.  The soak time for the calculated thickness of your glass piece will be the same as in the Bullseye chart.  Note again that if tack fusing, the soak time will need to be for glass 1.5 to 2.5 times (depending on the profile) the actual thickness. 

First you need to determine the annealing point of the glass. This can be done by going to the web page of the glass manufacturer.  It can also be done by using an earlier blog post giving some of the critical temperatures for a range of glasses.  This information has been taken from the manufacturers’ web sites and gathered together as they are sometimes difficult to find. 

Use the annealing temperature from your source as the target temperature in place of the Bullseye temperature.   The next target temperatures are at 55C/100F and 110C/200F below the annealing temperature. 

This means that the first cooling stage will be from annealing temperature to 55C or 100F below, depending on the temperature scale you use.  This gives you the target temperature for the first cooling segment. 

The second cooling stage is to annealing temperature minus 110C or 200F, again depending on temperature scale. This gives the target temperature for the end of the second cool. 

The third cool is from there to room temperature. 

The rates for these three stages of cooling remain the same regardless of the glass.  The temperatures are different for various glasses, but the range remains constant. 

This gives an annealing cool schedule for 19mm/0.75" Oceanside glass similar to this:

AFAP to 510C/ 951F for 3:00 hours

25C/45F to 455C/852F for 0 time

45C/ 81F to 400C/753F for 0 time

150C/270F to room temperature, off.

This demonstrates the intervals for the three stages of cooling are the same as in the Bullseye chart, it is only the temperatures that change.  The annealing soak time remains the same, even though the temperature is different. 

I will repeat, because it is so important, that the thickness to be used for the anneal soak and cooling rates for your schedule relates to the profile you desire.  A fuse with even thickness across the whole piece can use the times and rates as given in the chart as adapted for your glass.  A tack fuse or any firing of a piece with uneven thicknesses will require a schedule as for pieces between 1.5 and 2.5 times the actual thickness of the piece depending on the profile you intend to achieve. 

However, I am not going to reveal all the secrets of my research on schedules for uneven thicknesses and low temperature kilnforming.  You will need to buy my e-book for the application of the reasoning and research for low temperature fusing including tack fusing. 

I have included some tables for various fusing glasses and float glass.  The annealing temperature is given. These show the annealing soak time for various thicknesses.  You will see the rates of cooling remain the same for different glasses, regardless of the annealing temperature.  Only the temperatures change according to the annealing temperature.  If your glass is– or calculated to be - in between the sizes given in the charts, you can interpolate the required times and rates as the progression is regular.  Or, you can obtain these already calculated in the book noted above.

Soldering Iron Temperatures

Why use higher temperatures for copper foil using 60/40 than lead came using 50/50 or 40/60?

Melting temperatures

Part of this is the physical characteristics of the solder

The graph shows that all compositions of tin/lead alloy solder (above 20% tin) solidify at the same temperature - 183°C.  Pure lead melts at 327.5°C and pure tin at 232°C.  The various proportions of the two metals melt at different temperatures until at approximately 62% tin, the melting and solidification temperatures are the same.  This is ideal for running a bead in copper foiling, because there is a minimum amount of time for the liquid solder to change shape before it solidifies.

Melting temperatures of some solders
·        At 40% tin and 60% lead (40/60) the melting temperature is 238°C. 
·        At 50/50 the melting temperature is 212°C. 
·        At 60/40 the melting temperature is 188°C, just 5°C above the solidification temperature.

These figures show the 60/40 solder requires a lower temperature to melt than 50/50 does (24°C difference). 

Why should I run the iron at a hotter temperature for 60/40 then?

There are two separate elements at work here – the mass of solder being melted and the effects of the pasty range of solder compositions.

In soldering lead came you are melting small masses of solder with short pauses between each melting that allow the iron to partially recover. This means running the iron at 370°C is sufficient to maintain a melting temperature above 238°C for 40/60 solder and 212°C for 50/50.

In copper foil you are melting much greater amounts of solder, which takes heat out of the iron more quickly than in leaded glass.  The fact is that running a bead requires melting a much greater volume of solder.  The iron needs to run hot to be able to consistently melt the solder without significant periods when the iron is too cool to melt the solder quickly.  This is the reason that irons are run hotter in copper foil.

It still does not explain why it is recommended to run the iron hotter for 60/40 than for 50/50 as their melting temperatures are so close.

The explanation lies in the pasty range illustrated in the graph shown above.  You can run an iron hotter than needed to melt the solder, because the 60/40 requires fewer degrees to cool and solidify than 50/50.  This allows you to work quickly and still have a good rounded bead.

The greater pasty range of 50/50 means that you must be careful about the amount of heat you put into the solder, because the solder will continue to move for a longer time than the 60/40.  The 27°C difference between melting and solidification shows solidification is not instantaneous. This pasty range allows flow while the solder cools. This means that the bead will be less rounded, and it will show minor temperature differences in the wrinkled surface.  If you put even more heat than the 410°C that is normally used for 60/40, it will take even longer for the solder to solidify.  The surfaces effects will then be even more obvious with greater heat.

The short answer
The explanations for running hotter with 60/40 than those solders with more lead centre around the pasty range of solder.  When the pasty range is small, you can put more heat into the solder bead and so work more quickly and still get a good bead.  With wider pasty ranges you need to reduce the temperature of the iron to reduce the heat put into the solder and so keep the pasty range as small as possible.

Wednesday, 1 September 2021

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.