Bubbles on Drop-out Rims

Sometimes people doing dropouts get bubbles or unevenness on the rims of their pieces.  This means that it is not suitable to leave the rim on the piece.  Most times, this does not matter, as you intend to cut the rim away. But if you do want to have a rim these uneven surfaces are unsightly and not suitable for high quality pieces.

One person has indicated that they used a schedule of 250°C per hour to 520°C for a 30-minute soak and then proceeded at 330°C to the top temperature of 710°C.  This is probably too fast a heat up at the second segment.  Slower rates of advance are advisable.

One of the advantageous methods of scheduling for dropouts is to put the heat into the glass steadily.  I suggest there are two problems with the rates of advance and soaks in the above (partial) schedule.

The soak at 520°C would be more useful if it were at around 600°C.  This would allow the heat to be distributed throughout the glass before it begins to droop significantly.

The rapid advance of 330°C is much faster than needed, or desirable.  This rapid rate of advance allows the glass to move into the aperture, before the rim is plastic enough to stay on the supporting ring.

These rough drawings show how the rim initially rises from the ring, pivoting on the edge of the aperture.  This happens on all moulds (drops or slumps) where there is a rim.

With a rapid rise in temperature the raised rim edge gets more heat than the depressed middle, as it is closer to the elements.  This additional heat allows the edge of the rim to curve downwards forming air pockets as the edge touches back to the supporting ring.

Some people use fibre paper between the ring and the glass to prevent bubbles. This addition allows a passage of air from under the glass and reduces bubble formation.

Others have developed sloped drop out rings that eliminate the rising of the glass from a flat ring.  The glass is suspended above the aperture and only touches the edge of it as the glass softens. These crude drawings show the process.

To be certain of avoiding air bubbles under the rim of dropouts whatever the style of ring, you should use moderate rates of advance, with a possible soak at around 600°C which is before the glass begins any significant movement. The moderate rate of heating should be continued after this soak, rather than increased.

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.

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.

Large Bubbles

As you move up from smaller pieces to pieces that occupy most of the shelf, you sometimes begin to get large rounded bubbles at tack fuse and burst ones at full fuse.

Image from B Stiverson

You have to go back to basics to discover the cause.

Schedule

It is not likely to be the schedule. It has worked for smaller items. But it is important to review the schedule.  Is it like others you have seen? Is it similar to what the glass manufacturer recommends?  Both these will reassure you that the schedule is OK, if not perfect, or to revise it.

Cleanliness

Going back to the basics relates to the cleanliness of your kiln, among other things.  Even a small speck of material under the glass can result in a bubble. Although the grit lifts the glass off the shelf only a fraction, as it heats up the glass slumps around that and creates an air pocket.  That grows as the glass heats up and creates a large diameter bubble. If there is no grit in evidence, you need to check another element of your kilnforming practice.

Shelf

The large bubble might often occur in the same relative place in the kiln, although different places on the glass pieces, depending where they are placed.  This is an indication that you may have a hollow in the shelf. It may not have been obvious with smaller pieces.  You need to check the shelf with a straight edge. If any light is seen between shelf and edge, you have a depression in the shelf.  It may only be a sliver of light, but that indicates a depression which is enough to create a large bubble. That must be fixed.

Image from Suze

There are temporary and permanent fixes for avoiding bubbles due to depressions in the shelf. 

The temporary fix is to use 1mm fibre paper on the shelf, to allow air out from under the glass.  This can be topped with Thinfire or Papyros. Alternatively, a thin layer of powdered kiln wash can be smoothed over the fibre paper to give the smoothest back possible in the circumstances. You can use a plasterer’s float, or simply a piece of float glass.

The permanent fix is to sand the shelf smooth and level.  A method for doing this is here.

Single Layer Bases

If you are firing with single layer bases, there may be nothing wrong with the shelf.  It is typical in tack fusing to use single layers with glass placed decoratively around the surface of the base.  This leaves gaps where the base glass is exposed.  Even though the whole piece may survive the differential heat up of the exposed base glass and the covered parts, there is the possibility of creating an air pocket under the exposed base.  This comes from the weight of the stacked glass pressing any air out to the side.  If the design is unable to provide a route out for the air, the possibility of creating an air bubble increases.

It is possible to create conditions to reduce the possibility of these large bubbles developing. 

One solution is to use a layer of fibre paper as for a shelf with slight depressions.  This allows air out from under the glass, even with a single layer layup.

The other solution is to change the rate and temperature of the firing.  By using the low and slow principle, you can reduce the risk of bubbles.  Use a much slower rate of advance to a lower temperature with a longer soak you can achieve the look you want without bubbles.  This utilises the concept of heat work.  It does require observation to determine when the effect you desire is achieved and then advance to the next segment.

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

Factory Installed Firing Schedules

Factory installed schedules are a quick starting point for the novice kilnformer.  

Many kiln manufacturers install schedules in the controllers of entry level kilns.  Some install them in larger kilns too.  They will work for for gaining basic experience of kiln operations.

However, these schedules are not universal.  Each maker programmes schedules according to their understanding of a mid-range firing schedule for various processes. 

An example of some installed programmes from Scutt

This means that when referring to an installed programme on your controller, you need to give the full schedule so others can understand.

Why?

Not only because a tack fuse schedule may be to a different temperature, but also a "fast" schedule as programmed into one kiln might be quite different to one in another.

This matters, because how fast you get to the top temperature affects what temperature you need to use. You will probably experience the difference in final effect between a fast and a slow fuse to the same temperature.  If you haven’t seen it yet, try both schedules on the same layup of glass.

You will see that a fast rate of advance to a tack fuse will give a much more angular appearance, while a slow rate of advance will give a much more rounded appearance.  This is the effect of heat work, which is essentially the effect of the combination of temperature and time.

The longer it takes the glass to reach a given temperature, the greater the heat work.  Longer times to the top allow the use of lower temperatures. 

The consequence of accounting for heat work is that a simple top temperature cannot be given.  It is not just that kilns are different, but that the amount of heat work put into the glass will change the top temperature required for a given look.

Heat Work

“Heat work” is a term applied to help understand how the glass reacts to various ways of applying of heat to the glass. In its simple form, it is the amount of heat the glass has absorbed during the kiln forming heat up process.

There is an relationship between how heat is applied and the temperature required to achieve the wanted result.  Heat can be put into the glass quickly, but to achieve the desired result, it will need a relatively higher temperature. If you put the heat into the glass more slowly, it will require a relatively lower temperature.

For example, you may be able to achieve your desired result at 814C with a 400C/hr rise and 10min soak. But you may also be able to achieve the same result by using 790C with a 250C/hr rise and 10min soak. The same amount of heat has gone into the glass, as evidenced by the same result, but with different kinds of schedules. This can be important with thick glass, or with slumps where you want the minimum of mould marks. Of course, you can also achieve the same results with the fast rise with a longer soak at the lower temperature, e.g. a 400C/hr to 790C with a 30 min soak.

In short, this means that heat work is a combination of time and temperature.  The same effect can be achieved in two ways: 
- fast rates of advance and high temperatures
- slow rates of advance and low temperatures.

You obtain greater control over the processes when firing at slower rates with lower temperatures.  There is less marking of the back of the piece.  There is less sticking of the separators to the back and so less cleanup.  There is less needling with the lower temperature.  

The adage “slow and low” comes from this concept of heat work. The best results come from lower temperature processing, rather than fast processing of the kiln forming.

Separators sticking to Opalescent glass

It is worth thinking about how fast you fire pieces, especially where your current working temperature and rates of advance are giving difficulties.  One common difficulty is where opalescent glass picks up kiln wash or fibre paper and partially incorporates it, requiring a lot of work to remove it. 

At higher temperatures opalescent glass seems to incorporate some of the separator, especially near the edges.  It does not seem to matter whether kiln wash or fibre papers are used – there is frequently a little pick up.

The difficulty is achieving the profile you want without the higher temperatures.  This is where heat work concepts can assist.  Glass reacts to the heat applied, rather than simply the temperature.  Heat is a combination of time and temperature.  Rapid rates of advance require higher temperatures than slow rates of advance to achieve the same effect.

These facts should make you consider slower rates of advance to achieve the work at a lower temperature and so pick up less of the separators.  Perhaps you could consider a rate of advance of 150°C or 200°C instead of 330°C once you have passed the bubble squeeze temperature.  This would allow you to have a full fuse at ca. 800°C or even a little lower instead of 816°C (for Bullseye).  You will need to observe to find what is the appropriate temperature for the effect you want.  This will apply both with different rates of advance and with different lay-ups.

Slow and Low

Low and Slow Approach to Kilnforming

We are often impatient in firing our pieces and fire much more quickly than we need. After all, our computerised controllers will look after the firing overnight. So there is no need to hurry more than that.

The concept of heat work is essential to understanding why the slow and low method of firing works. Glass is a poor conductor of heat which leads to many of our problems with quick firings. The main one is stressing the glass so much by the temperature differential between the top and the bottom that the glass breaks. We need to get heat into the whole mass of the glass as evenly and with as smooth a temperature gradient as possible. If we can do that, the kiln forming processes work much better. If you add the heat to the glass quickly, you need to go to a higher temperature to achieve the desired result than if you add the heat more slowly to allow the heat to permeate the whole thickness of the piece.

Graphs of the difference (blue line) between upper and lower surfaces of glass of different thicknesses against cooling time

However, this slower heating means that the glass at the bottom has absorbed the required heat at a lower temperature than in a fast heat. This in turn means that you do not need to go to such a high heat. This has a significant advantage in forming the glass, as the lower temperature required to achieve the shape means that the bottom of the glass is less marked. The glass will have less chance of stress at the annealing stage of the kiln forming process as it will be of a more equal temperature even before the temperature equalisation process begins at the annealing soak temperature.

Applying the principles of low and slow means:

• heat is added evenly to the whole thickness of the piece
• there is a reduction in risk of thermal shock
• the glass will achieve the desired effect at a reduced temperature

The alternative - quick ramps with soaks – leads to a range of difficulties:

• The introduction of heat differentials within the glass. Bullseye research shows that on cooling, a heat difference of greater than 5ºC between the internal and external parts of glass lead to stresses that cannot be resolved without re-heating to above the annealing point with a significant soak to once again equalise the heat throughout the piece.
• It does not save much if any time, As the glass reacts better to a steady introduction of heat. Merely slowing the rate to occupy the same amount of time as the ramp and soak together occupy, will lead to fewer problems.
• It can soften some parts more quickly than others, e.g., edges soften and stick trapping air.
• Quick heating, with “catch up” soaks, of a piece with different types and colours of glass is more likely to cause problems of shock, bubbles, and uneven forming.
• Pieces with uneven thicknesses, such as those intended for tack fusing, will have significant differences in temperature at the bottom.
• Rapid heating with soaks during slumping and draping processes can cause uneven slumps through colour or thickness differences, or even a tear in the bottom because the top is so much more plastic than the bottom.

However there are occasions where soaks during the initial advance in heat are useful:

• for really thick glass,
• For multiple - 3 or more - layers of glass,
• for glass on difficult moulds,
• for glass supported at a single internal point with other glass free from contact with mould as on many drapes.

Of course, if you are doing small or jewellery scale work, then you can ignore these principles as the heat is gained relatively easily. It is only when you increase the scale that these principles will have an obvious effect.

Slow, gradual input of heat to glass leads to the ability to fire at lower temperatures to achieve the desired results, with less marking and less risk of breaking.

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

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.

Equalising Effects on Both Sides of the Glass in the Same Firing

The desire is to have the same degree of fusing on both sides of the glass.  An example is where a person wants to have their colourline paints equally matured on both sides of the glass in one firing.  This is difficult and requires a different strategy than normal fusing.

Background

A bit of background first. Glass is a very good insulator. This means that heat travels slowly through the glass. Its practical effect is that we have wavy lines on the top and very crisp lines on the bottom.  This results from the temperature differential between the two surfaces.  This can be many degrees different during the plastic phase of the glass.  It is dependent on how fast the temperature rise is.  The faster the rise in temperature, the greater the difference as the glass transmits the heat from top to bottom so slowly.  The problem is how to keep the temperature differential as small as possible.

Heat Work

The concept of heat work relates to the way heat is put into the glass.  It can be done quickly to a high temperature, or slowly to a low temperature and still get the same effect.  This shows glass reacts to the combination of temperature and time. Putting heat into the glass slowly allows lower temperatures to be used to achieve the desired effect, than fast rises in temperature.

The insulating properties of glass means that the heat work needs to be applied slowly to achieve similar temperatures on both sides of the glass.  The thicker the glass the longer it will take to temperature equalisation.

The mass of materials also needs to be considered.  The glass will normally be on a ceramic shelf of 15mm to 19mm.  This mass also needs to heat up to the temperature of the top of the glass.  Until it does, it will draw heat from the glass.  This also points to the need for slow heat input.

The question that prompted this note was how to get glass strainers paints to have the same degree of maturation on both sides at the same time.  The maturation temperature of Reusche tracing paints is around 650°C.  If you use a normal rate of advance – say, 200°C – the bottom of the glass will be considerably cooler than the top.  This is both because of the insulating properties of the glass and the mass of the shelf.

Methods to achieve the effect.

Some methods are worthy of consideration separately or in combination.

Use refractory fibre board as shelf.  This dramatically reduces the mass of the shelf to be heated up.  This kind of shelf requires more care to avoid damage than a ceramic shelf.  It would be possible to place smaller fibre shelves on top of the standard ceramic shelf rather than having one large fibre board shelf.  This will not be so efficient an insulating mass as fibre board on its own.  Also, it will not be sufficient on its own to obtain equal temperatures on both sides of the glass.

Use 3-6mm refractory fibre paper between shelf and glass.  This again reduces the heat sink effect of the ceramic shelf, but not as much as a fibre shelf on its own.  Again, the fibre paper on its own is not enough. The scheduling is important.

Use very slow rates of advance.  A slow rate of advance in temperature is important to achieving equal temperatures throughout the glass.  Even using 3mm glass, the rate of advance might need to be as slow as 50°C per hour.  The corollary of this is that you will not need to use as high a temperature to achieve the effect.  Heat work means that it is not an absolute temperature that will achieve the effect.  The slower you put the heat into the glass the lower temperature required.  The understanding of this relationship will require experimentation to establish the relationship to the rate of advance and the top temperature required.  For example, a satin polish of a sandblasted surface can occur at 650°C, if held there for 90 minutes.

In this case, a 50°C rate of advance will probably not require more than 600°C – and probably less - to achieve the shiny surface normally achieved at 660°C with a 200°C rate of advance.  At 50°C per hour, it will take 12 hours to reach 600°C, although a little more than 3.25 hours at an advance of 200°C to reach 660°C.  The input of heat acts upon the glass throughout the process, making lower working temperatures possible.  The reduction in temperature required is not directly related to the reduction in the rate of advance.  You will have to observe during the experimental phase of this kind of process.

If it was desired to fire enamels that mature at 520°C to 550°C you could put the sheets in vertical racks to allow the heat to get to both sides equally as Jeff Zimmer does.  But this will only work for very low temperatures and for quick firings, otherwise the glass will begin to bend.

There are limits to this strategy of getting upper and lower surfaces to the same temperature, both in terms of physics and practicality.  There are temperatures below which no amount of slow heat input will have a practical effect, for example,  due to the brittle nature of the glass.  Even where it is possible, it can take too long to be practical.  For example, I can bend float glass at 590°C in 20 minutes into a 1/3 cylinder.  I could also bend it at 550°C (just 10°C above the annealing point), but it would take more than 10 hours – not practical.

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. Glass bending 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 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 at 590C in 20mins

A 1/8 sphere requiring bending in two directions was achieved at 570 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.

Bas Relief Moulds

Bas relief moulds that have an image carved into the surface are popular at the moment. They are most often called texture moulds.  The image is “carved” into the back of the glass, creating uneven thicknesses of glass that refract the light to show the image through the smooth plane of the front.

One of the problems with these kinds of moulds is that lots of bubbles are created, often very large ones.  This results from the many places where the air cannot escape from under the glass during the forming process.

Solutions

There are some strategies that can help avoid these bubbles.

Use the 6mm rule

Fuse the glass into a six-millimetre thickness first.  Two layers of glass give more weight to help the glass conform to the texture of the mould.  It also resists bubble formation more than a single layer.

Use the Low and Slow approach

It more important to have low and long bubble squeezes.  The most successful strategy will have a slow rise in temperature to put as much heat work into the glass as you can before the bubble squeeze.  The bubble squeeze is the most important part of firing these texture moulds.  It will start at about 600°C rising at only about 25°C/hr to around 680°C – that is, taking three to four hours. 

Use slow rates of advance

A third element is to rise slowly toward the forming temperature.  Possibly nothing faster than 75°C.  This enables you to keep the forming temperature much lower than a fast rise will.  The usual temperature recommended is about 780°C.

By using a slow rate of advance you can probably reduce the forming temperature by about 20°C.  You will need to peek at intervals to be sure the glass has taken up the required texture. Again, it is about putting as much heat into the glass at as low a temperature as possible.

Use Long soaks

An alternative to the slow rate of advance is to use a long soak at as low temperature as seems suitable.  You will need to peek at intervals to determine when the texture is achieved.  When the appropriate texture is imparted to the glass, you need to advance to the next segment.  This means that you need to know how to get your controller to skip the following segment.  Or, if the texture is not achieved before the end of the scheduled soak, how to extend the soak time.  If you are using 760°C as you target temperature with a rise of 150°C, you may wish to soak for about an hour or more.  Remember that this is in the devitrification range.

Alternative - Frit

A completely different approach is to use fine frit and powder to give a patè de verre appearance by sintering the frit.  This eliminates the bubble problem entirely.

You will need a lot of frit if you are trying to make a sheet of 6mm from the frit.  You could just take the sheets of glass cut to the size of the mould and smash them up to get the required amount of glass.  Or you can use your cullet, by weighing and smashing up enough glass. 

The calculations for weight are best done in the metric system (in cm) as there are easy conversions between volume and weight.  Assume your mould is 20cm square.  The area is 400cm².  The volume is that times 0.6cm or 240cm³.  The specific gravity of glass is approximately 2.5, so you multiply the volume by that and get 600gms of glass required to get a 6mm thick sheet. 

You could full fuse this into a clear sheet, although this would take a much higher temperature and longer soak that would be good for the mould. Better is to sinter the glass.

To sinter the glass, you need slow rises in temperature and long soaks.  A rise of about 75°C to the softening point of the glass (around 600°C) followed by a very slow rise (ca. 25°C per hour) to about 660°C is needed to allow the small grains of glass to settle together.   At the upper end of the bubble squeeze you need a three- to four-hour soak to sinter the glass. The thicker the layer of glass frit, the longer soak needed to ensure all the particles are heated.  The densest glass will be formed by a 50/50 combination of powder and fine frit.

Much better is to have a much thinner sheet formed from the frit.  This will be about two to three millimetres thick.  The weight of powder and or frit can be determined by the formula above, substituting 0.2 or 0.3 for the thickness.  This frit mixture needs to be evenly spread over the mould, with as much on the high points of the mould as the low ones.

If the mould has a lot of variation in height, you can sinter the frit mixture as a flat sheet first.  Then place it over the texture mould and give it a slow rate of advance to the to the top end of the bubble squeeze and soak for an hour or more, as required.  This will ensure you get the same thickness across the whole piece even though there differences in height.

The resulting piece will be very light and translucent.  It will have a fine granular feel to the touch.  It will have the same shape on both sides of the piece, with the upper surface having a slightly more shiny appearance than the bottom.

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