Flows

 

Credit: Marcy Berman

I have not had much success [with] the Patty Gray mould despite using the recommended firing schedule. I always have holes or bubbles and the edges are not smooth.

The schedule for Oceanside was:

• 111°C/200°F per hour to 537°C1000°F for 15 minutes
• 167°C/300°F per hour to 662°C/1225°F for 30 minutes
• 195°C/350°F per hour to 798°C/1470°F for 20 minutes
• 9999 to 510°C/950°F for 120 minutes
• 55°C/100°F per hour to 371°C/700°F off

 Your picture shows a bottom view of the piece - made of cullet pieces - as fired. Two large bubbles show to have been created from the bottom rising through the glass to the top.

 Although a long bubble squeeze will not prevent this, it will help to reduce the number of bubbles, and especially large ones. Because of the number of pieces and the thickness of the glass put into the mould, a longer bubble squeeze would benefit this piece.

 The bubble squeeze can be as you have done this – at a single temperature – with a soak. In this case, I would have used 60 to 90 minutes as the soak.

 The other bubble squeeze method is to start the squeeze about 55C/100F below the top of the bubble squeeze. Most people use a soak of about 30 minutes there. They then proceed at a rate of between 30C/55F and 55C/100F to the top of the bubble squeeze and soak there for another 30 minutes. The rates and soak times will vary according to the thickness or complexity of the piece.

 I dispense with the soak at the beginning of the bubble squeeze on the grounds that at 610/1130F so little movement will be created that it is a waste of time. I would prefer to have a slower ramp rate to the top temperature and a longer soak there. I know the glass will be moving at those temperatures. Many people find the soak at the beginning of the bubble squeeze successful.

 The schedule to the top of the fuse is faster than the rest of the schedule. When I want a piece to flow, and especially, to fill gaps, I slow the rate. In this case a rate of between 100C/180F and 167C/300F would be slow enough to allow the glass to flow to fill gaps.

 I want to ensure the glass has enough time when it is flowing most freely at the top temperature to level out. This requires scheduling a longer soak at the top and observing how well the glass is levelling out. If more time is required you can add it on the “run,” and advance to the next segment when the surface is as wanted. Read up in your kiln manual how to do both these things.

 Yes, the rate is one which will enable devitrification to form on flat glass. The soak at top temperature is even more likely to promote it. However, as the glass is flowing, less devitrification has an opportunity to form. The crystallisation – which is what devitrification is - of the glass takes time to form. The movement of the glass surface is sufficient to reduce the formation of those crystals. It is of course likely there will be some devitrification, but not as much as the slow rates and long soaks would lead you to think. 

 But for these flows there always is the possibility of devitrification. You have to plan a method of removing it. Unless the surface is very flat, grinding the top is not a fast way to remove it. Sandblasting is a quick way to remove devitrification. Another way is to sift a thin layer of clear glass powder over the surface. This is an increasingly popular way to deal with devitrification for those without access to sandblasting facilities. When fired again, the powder melts and forms a new shining surface. The piece will need to be fired fire again whether sandblasted or covered in glass powder.

The summary for flows:

• Slow down to top temperature.
• Give sufficient time there to get the flow needed.
• Observe the progress as you near the top temperature.
• Extend the soak or advance to the next segment when the surface is smooth.
• Anneal soak for the calculated thickness.
• Use a three-stage cool – as outlined in the Bullseye chart for annealing thick slabs - to ensure no temporary contraction stresses are created.
• Accept there will be devitrification.

Causes of Large bubbles

 Let’s think about moisture and large bubbles from under the glass. It is not the water, but the gasses created by the decomposition of materials that can cause the bubbles. There are other causes of large bubbles too. The most common causes are discussed here.

The usual explanations are:

• ·        Uneven shelf
• ·        Heat resistant particles under the glass
• ·        Uneven heating
• ·        Glues
• ·        Organic material
• ·        Moisture
• ·        Amount of gas

 

image credit: Warm Glass

Uneven shelf

Shallow depressions in shelves can cause large bubbles. Occasionally, the shelf can be damaged in various ways causing scratches or dings in the shelf. Air can be trapped in these depressions. And it does not take much volume of trapped to be a problem. The heat of kilnforming causes the air to expand. As the glass becomes less viscous with increased temperature, the pressure from the expanding air forces the glass upwards. The amount of air and the amount of heat work combine to create bubbles from simple uprisings to large thin walled or even burst bubbles.

There are some things that can be done to detect and avoid bubbles from forming. It is possible to screed powdered kiln wash over kiln washed shelf. This gives pathways for the air to escape. It does leave a more marked bottom surface than kiln wash.

Using 1mm or 2mm fibre paper allows air from under glass. You can maintain a relatively smooth surface with Papyros or Thinfire over the fibre. Even Thinfire or Papyros on its own will allow air from under the glass.

Checking for depressions can be done by spreading kiln wash powder over shelf and drawing a straight edge over the shelf. Depressions will be shown by the presence of the powder. It can also be done with powdered glass frit.

Particles under glass

Any particle resistant to kilnforming temperatures holds the glass up while it is forming so creating an air space. It is important to ensure the shelf is clean as well as flat. Small pieces of grit or dirt that are resistant to high temperatures will hold the glass up from the shelf enough to create a bubble – small or large depending on the temperature. Vacuuming the shelf before adding anything to the surface before each firing is important to bubble free results.

Uneven heating

This is sometimes cited as a cause of bubbles. If so, the heat would need to be very localised. This is possible if the glass is very near elements. In general, the temperature is equalised at a distance equal to the width of the elements.

Glues

A wide variety of glues are used in kilnforming. Those available to enthusiasts all burn away leaving gasses between layers. These gasses - if trapped - can thin the glass below as well as above the glue’s position. This will give the impression that the bubble has come from between the shelf and the glass. Most often the bubble forms between the glass layers, pushing a bubble only into or through the top layer. The solution is to avoid using glue or minimise it and place it only at the edges.

Organic material

Organic materials can be a problem. When you are using a large or thick fibre paper sheet under a piece of glass, occasionally the gasses from burning out of the binder can be great enough to create a bubble. Although normally, this only leaves a grey to black mark on the underside of the glass. Vermiculite boards need to be fired before use, as they contain significant amounts of binder.

Inclusion of organic materials such as leaves, twigs, or bones, leads to bubbles. Very long soaks below the softening point of the glass are required to allow the organic material to burn out of the objects.  The time required increases from an hour for leaves to 24 for bones.

Moisture

Moisture is very often cited as the source of bubbles. It is possible that the steam from water may be trapped in shelf depressions, or the areas held up from the shelf. And anytime there are no precautions to allow the air from under the glass, or between sheets bubble formation can be promoted. If adequate precautions are taken (flat shelf, clean shelf, bubble squeeze) the moisture will evaporate before the glass is hot enough to form a seal around the edges and trap any steam. It is another good reason for moderate ramp rates at the beginning of a firing.

Amount of gasses

Of course, if there is a lot of moisture there can be problems. Simply applying kiln wash in four coats does not leave enough water in the shelf to be a problem.

If you have washed the kiln wash off a mullite shelf, there will be a lot of water in it even after it feels dry. Then it does need to be kiln dried before use. To avoid breaking the shelf you need to fire slowly to 99°C/210°F and soak there for a couple of hours with the vents open or lid propped up a little to allow the moisture out of the kiln.

 

 

Texture Moulds

 Texture moulds are a form of bas relief in reverse. The texture of the mould is the bas relief. The glass formed over the shapes is in negative relief. The light is refracted through th
e back to give an image of bas relief although the surface is smooth.

 

Example of wave form texture mould

 These moulds are prone to produce bubbles at the generally recommended tack fuse temperatures. The glass often sticks to the mould if there is not sufficient draft to the parts of the image, or if insufficient separator is used. Often the moulds are produced with a rim around the edges, which trap the glass.

 The usual temperatures are too high. These moulds are an exercise in patience. The temptation is to fire higher than slumping temperatures to get good definition in the glass. However, a number of problems, especially bubbles, can be avoided by staying at the high end of slumping temperatures. This means the top temperature would be about 680C. To compensate for this low temperature, the soak needs to be three hours or more. To be sure the definition desired has been achieved, peeking near the end of this long soak is necessary. 

 Moulds that are produced with a rim around the edges can trap air and create bubbles. The rim forms a perimeter dam to confine the glass. If the rate of rise is quick to a high temperature, the edges can be sealed against the rim before all the air has escaped. It is advisable to cut the glass for these rimmed moulds a bit smaller than the internal dimensions formed by the rim.

 

Example of textured area surrounded by a rim

 Use of single layers on texture moulds can lead to large, thin bubbles. This is most prevalent when using high temperatures. Since the single layers tend to form more slowly than an already fused two-layer piece, the temptation is to use higher temperatures. The higher temperatures soften the glass to such an extent that often bubbles form over the lower areas of the mould. Instead, low temperatures with extremely long soaks should be used to allow the glass to conform to the undulations of the texture without dog boning or developing bubbles. Of course, peeking will be required to determine when the texture is achieved. With single layers, the surface will have greater undulations than with two layers. The thinness of the single layer cannot fill the depression the way two layers can.

 

 Rapid rates to high temperatures can produce internal bubbles too. These moulds have a multiplicity of hollows and depressions. Just as people are warned about depressions in their shelves, the depressions in the texture moulds can cause bubbles too. This means there are even more possibilities for bubble creation than on apparently flat shelves. Long slow bubble squeezes are required to allow air from under the glass.

 Glass sometimes sticks to the mould. This is most often blamed on insufficient separator. Boron nitride is a good separator for these moulds especially if you go to tack fusing temperatures. At slumping temperatures, kiln wash will normally be sufficient. Both of these separators need to be applied carefully, as there are relatively steep slopes throughout the mould. Spraying needs to be done from at least four angles to ensure all the sides are covered.

 Painting on kiln wash is a little more difficult, as the solution is so liquid, it tends to run down the slopes without much sticking. One means of rectifying this is to tip the mould in a circular motion to move the still liquid kiln wash solution around the slopes.

 Less often thought about is the draft of the shapes of the mould. If the slopes (draft) in the mould are too steep, the glass will “grab” the ceramic mould, because the ceramic contracts less than the glass when cooling. If shapes of the mould are steep and deep enough, the glass may even break as a result of this compression of the mould.

 

An example of some nearly vertical elements and a rim

 Of course, if a flat front surface is required, a higher than slumping temperature must be used. This is required to allow the glass to flow to the lower portions of the mould. It still should be as low as possible, but with very long soaks.

 Avoidance of bubbles on, and sticking to, texture moulds is best achieved by avoiding high temperatures, use long soaks, use two layers, and avoid extending glass to the rim. These combined with observation of the progress of the firing will produce the best results.

 

Other information is available:

Low Temperature Kilnforming, an Evidence-Based Approach to Scheduling, an ebook

Bas relief

Layups promoting bubbles

 

Bubbles in Bottle Slumps

 Any suggestions on how to avoid getting the oblong bubble under the neck of the bottle? This was my first try and I’m really happy with clarity, no devitrification in these.

I used this schedule:

Fahrenheit                    Celsius

300/1150/30                167/620/30

200/1370/20                111/740/20

400/1450/20                222/787/20

AFAP/950/60                AFAP/510/60

150/800/0                    63/427/0

300/100                       167/55/off

The bubble is kind of cool but not sure what it will do when I put it in a bottle mould.

 

To minimise the bubble, you need a bubble squeeze.  There isn't one of sufficient length or at the right temperature in the schedule. The softening point of bottle glass is approximately 720C. Starting the bubble squeeze at ca. 670C/1240F and progressing slowly (ca.50/90F or less) to 720C/1340F may give a better bubble squeeze. 

Also, the anneal soak is a bit low. Bottle glass and float glass both have annealing points of about 550C. You might make use of a lower annealing soak temperature to reduce the cooling time.  It is usually possible to anneal 30C below the published annealing temperature.  In this case that would be 520C.  

There is pretty thick glass in some places due to the way the bottom and neck of the bottle form. You may want to extend your anneal soak to one for 12mm/0.5”.  The soak time for this is 2 hours.  The first cooling segment would be 55C/100F per hour to 475C/888F if you use 520C/970F as the annealing soak.  The second cool segment should be at 99C/180F per hour to 420C/790F.  And the final rate at 330C/600F to room temperature.  It is important to include all three stages of cooling.  The research for my book Low Temperature Kilnforming (Or directly from stephen.richard43@gmail.com) has shown that to get the best stress-free results  use all three stages of cooling.

Bubbles at the shoulder of the bottle are common.  The change in circumference of the bottle at the shoulder means there is a greater amount of glass to “compress”.  Bottles with tapered circumference at the top of the bottle have fewer problems with creating bubbles.  The abrupt change in size at the shoulder causes bubbles to be more common.  A long slow bubble squeeze will allow the shoulder to form more closely in line with the neck. 

There are other things you can do to help avoid the bubbles. One thing is to insert a thin kiln washed wire into the neck of the bottle. This gives a path for the air to escape and allows you to pull it out, although a mark will be left.  You could also think of drilling a hole in what will be the underside at the shoulder to allow air out to the shelf. It does not need to be a big hole.

Bubbles at the shoulder of a slumped bottle are a common problem. It results from the greater amount of glass that has to slump into the space.  This leaves a cavity.  Slower bubble squeezes can help, as well as various venting methods.

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.

Firing cremains to avoid bubbles

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

Design

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.

Firing

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.

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.

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.

Firing multiple layers

Glass Stela
Credit: Stephen Richard

Fusing multiple layers is prone to the creation of multiple large bubbles.  It also needs a strategy to schedule for thick layers.

Avoid bubbles

A widely recommended strategy for stacks of glass is to fire in pairs of layers. Then combine the fused two-layer pieces in a final firing. 

It is easier to fire two layers of glass than 6, 8 or 10 layers. The heat up is easier and less time consuming for multiples of 6mm than multiples of 3mm. The bubble squeeze schedule is simpler.  It also allows inclusions between the initial two-layer sheets and then between the layers of 6mm sheets.

This multiple firing strategy reduces the risk of large bubbles in a stack of multiple pieces. It seems the weight of the 6mm layers forces the air out from between the thicker glass more effectively than thinner layers. 

It is also a simpler set of firings.  If you were to want to make up a 12mm thick piece from 3mm sheets, your heat up will be very long compared to firing two layers in three firings.

E.g. Stone* recommends a heat up for 2 layers of 3mm glass:

240C/hr to 250C, no soak

400C/hr to 500C, no soak (a bubble squeeze could be inserted here by raising the target temperature to 650, with a 30-minute soak)

500/hr to top temperature.

This is about 2.3 hours to top temperature without the bubble squeeze and 6.7 hours to cool.  This means that you could fire twice in one day, if organised well.  If you are planning a final tack fused layer that should be done in the last firing of the combined layers.

However, it is a much longer schedule recommended by Stone for 6 layers of 3mm glass:

• 25C/hr to 125 for 20’
• 30C/hr to 250 for 20’
• 40C/hr to 375 for 20’
• 50C/hr to 520 for 15 (a bubble squeeze could be inserted here by raising the target temperature to 650, with a 30-minute soak before continuing at the same rate to the top temperature).
• 150/hr to target temperature

This is about 18 hours to top temperature without the bubble squeeze and another 18 hours to cool.  This strategy requires 1.5 days, assuming all the layers are even.  The same amount of time is required for both strategies, but the chance of large bubbles is dramatically reduced.

He recommends for 3 layers of 6mm glass:

• 200C/hr to 250, no soak
• 340C/hr to 500, no soak
• 400C/hr to 600, no soak (a bubble squeeze could be introduced here by changing the target temperature to 650 with a 30-minute soak)
• 500C/hr to top temperature.

This is about 2.5 hours to top temperature and 18 hours to cool without the bubble squeeze.

This means that it only takes 2/3 of the time to fire 3 layers of 6mm glass than it does to fire 6 layers of 3mm glass.  Yes, you lose some time in firing the pairs of 3mm glass, but you gain in reducing the risk of creating large bubbles that will ruin your final piece.

Inclusions

If you are putting elements between the initial two-layer pieces for fusing, you need to introduce a bubble squeeze.  Putting elements between the fused pairs will also require a bubble squeeze on the final firing.

Tack fusing the final layer

Note the times indicated above are for even layers.  If you have uneven layers or are tack fusing, the times will be extended much further than the ones noted there.

For a tack fused set of top layers, you will need to add those in the last firing, or do a sharp tack firing before the last firing.  In the case of a tack fused pair for the top layers you will need to reduce the rates of advance for the last firing by about 1/3. This would mean:

• an initial rate of 135C,
• a second ramp of 230C,
• a third of 270C and
• the fourth of 335C instead of the rates for even layers. 

You will also need to reduce the top temperature.  Observation will be required to determine when the correct profile has been achieved.

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

When firing multiple layers of glass, the risk of creating large bubbles can be reduced by firing pairs of 3mm sheets, and then combining the results into one stack.

*Graham Stone. Firing Schedules for Glass, the Kiln Companion, 2000, Melbourne Australia.  ISBN 0-646-39733-8

Bubble Squeeze Temperature

“My bubble squeeze temperature is higher than my slumping temperature.“ The writer goes on to say that their bubble squeeze is at 1250°F/676°C vs. a slumping range of 1150°F/620°C - 1175°F/634°C.

I applaud the writer for doing the slumping at the lower range of slumping temperatures. This allows the glass to relax into the mould with fewer marks being picked up.  The temperatures might require significantly long soaks depending on the span, depth, shape, weight of glass, etc.   But it is a good practice to get work done at as low a temperature as practical.

 But...

There does seem to be a misunderstanding on how a bubble squeeze works. Like most things with glass, any process works over a range of temperatures.  Bullseye glass begins to soften about 540°C. This continues to about 680°C where the transformation range begins – that is, the glass is behaving more like a viscous liquid than a softening solid.  A bubble squeeze or a slump can begin anywhere in this 540°C to 680° range.  At the lower end of the range, any slump will take “forever”.  At the top end, some slumps may occur too quickly and have mould and stretch marks on the bottom. 

Credit: Fusedglass.org

However…

This note is about the relation of bubble squeeze to slumping temperatures.  If you can slump an item at 620°C, you can also perform a bubble squeeze at that temperature.  Both processes rely on the glass becoming “soft” enough to relax into the shape below it.  It may be that you will need a very long soak to press out air in a bubble squeeze at 620°C, but it can be done if you are willing to wait a long time.  

Many people begin their bubble squeeze at 620°C for fusing glass with a soak.  I am not sure that a soak is required at this point, as slowing the rate of advance over the next 50°C will have the effect of increasing the heat work the glass receives without the need of a soak at the beginning of the bubble squeeze ramp – unless you have a rapid rate of advance toward the bubble squeeze.  

They then progress slowly (maybe 50°C or less, depending on thickness) for the next 55°C to 60°C and soak at that higher temperature for half an hour, or more for difficult shapes.  This additional heat work allows the glass to gradually become more plastic and deform more slowly than at a higher temperature bubble squeeze.  This is often called a cautious bubble squeeze, since it starts at a lower temperature and moves gradually to the top of the bubble squeeze range.  It removes the single shot bubble squeeze at a higher temperature, when air might already be trapped. 

In general terms, the slump can be carried out at or below the softening point of the glass.  This softening point is also the maximum temperature for a bubble squeeze. For example, float glass has a softening point of about 720°C, so a bubble squeeze and slumping can be in the 660°C to 720°C range.  Some glasses have even higher softening points, and others have much lower softening points than Bullseye or Oceanside.

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

Pin holes in melts

Pin holes in screen and pot melts are the result of very small bubbles rising to the surface.  These bubbles are sometimes within the glass melted, but more often come from small amounts of air trapped within the flowing glass.  These are perceived to be unsightly, or make it impractical to make a functional piece from the melt.

There are ways to minimise bubble formation or to deal with the formed bubbles.

Bubble Formation

In pot and screen melts, the glass spirals as it touches down onto the shelf. This spiralling action can trap small amounts of air as each successive spiral forms beside the previous one. Efforts at prevention of tiny bubbles in the final piece need to concentrate on this fact.

Bubble Squeeze

A preliminary element in bubble prevention is to have a long bubble squeeze to allow the glass to settle in the pot or on the screen so that the rest of the process can proceed with a minimum amount of air trapped within the flowing glass.

Two-Stage Drop

In some cases. it is possible to have the glass flow from the pot onto an angled shelf where the spiralling glass has to flow from the initial touch down to the edge and then flow onto the shelf.  This allows any tiny bubbles initially trapped to escape before the final drop onto the shelf.  This provides two mixing processes and means that a lot of clear glass needs to be included to avoid a complete mix of the colours.  It requires careful selection of the original colours to avoid a brown or black result.  It also requires a big kiln with sufficient height for a two stage drop.

This two-stage drop is of course, not suitable for a screen melt where you wish the glass strands to remain.  Nor is it suitable when you wish to have many “pools” of colour mix in the final piece.

Where the two-stage drop is not practical or suitable other methods can be used.  These relate to scheduling, cold working the surface and re-firing the piece.

Schedules

Scheduling relates to using a soak at full fuse temperature before proceeding to the anneal.  The melt will occur at 850°C to 950°C.  You can cool as fast as possible to a full fuse temperature of about 810°C and soak there for an hour or more.  This allows the small bubbles to surface, break and heal.  Schedule the rapid cool to the annealing soak, once the high temperature soak is complete.  This will eliminate lots of the bubbles, but not all.

A sample friring schedule from bubble squeeze upwards and then down to a high temperature bubble reduction soak

Cold Work

Cold working the melt is about abrading the surface to open the bubbles that are just emerging to form a small dome at the surface.  Sand blasting is a common form, as usually kiln wash or fibre needs to be removed from the bottom of the melt, and some devitrification from the surface.  It would be possible to continue to grind the surface of the glass to eliminate the small depression in the glass caused by the now opened bubble, but this is likely to expose more bubbles that were at a slightly deeper level. What next?

Fire Polish

As you will need to do a fire polish firing after blasting or grinding the surface, you can use a full fuse temperature to allow the surface to become plastic enough to fill the bubble holes.  Remember to schedule the firing as though the piece were at least 12mm thick.  You may find that more bubbles are exposed in addition to the ones healed at the conclusion of this second firing.

Flip and Fire

An alternative is to fire upside down.  You will have noted that there are no bubbles on the bottom of the melt.  This is because the bubbles have risen through the heated glass.  This physical fact can be used in the second firing.  Fire with the melt surface to the shelf.  It is best to have a clean and newly kiln washed shelf, or fibre paper (not Thinfire or Papyros) under the glass. Fire the glass to a full fuse or high temperature tack fuse with a significant length of soak to allow the bubbles near the original surface to move toward the interior of the glass.  After firing, the glass will need thorough cleaning before being fire polished. This should leave you with a pin hole free piece.

Conclusion

Achieving a pin hole free pot or screen melt requires several stages of coldworking and firing.  This makes melts inexpensive in materials (it is scrap of course) but expensive in time and firings.