Bubbles on Single Layer Fusing

“I'm making 3mm French Vanilla sconce covers; …

·        [initially they were] fine, but now 1.5" bubbles form during the full fuse.

·        I pop the bubbles and fill the holes with frit and refire,

·        [The]… edges draw in and distort the design…

·        The shelf is flat,

·        I fire on Bullseye paper, and

·        the 13.5 hour long firing schedule [in F] is:

200 to 1150, hold 30 minutes.

50 to 1225, hold 30 minutes.

300 to 1490, hold 30 minutes.

9999 to 990, hold 60 minutes.

100 to 750, hold 1 minute.

Does anyone know what I can do to avoid the large bubbles? 

A critique of the schedule. 

 This is for a single sheet of 3mm glass, so the hold at 621˚C/1150˚F is unnecessary as is the slow rise to and hold at 663˚C/1225˚F, because it is a single sheet and does not need the traditional bubble squeeze. 

 The hold of 30 minutes at 810˚C/1490˚F is excessive. 

·        The temperature may be too high.

·        Ten minutes at top temperature is sufficient in most cases. 

·        A soak of 1 minute would be enough. 

·        The anneal soak at 990˚F is most probably a misprint for                          516˚C/960˚F. 

·        The anneal soak is longer than the half hour necessary, but not a             bubble creating problem.

 It means the schedule could have been:

111˚C/200˚F to 796˚C/1465˚F for 5 minutes

AFAP to 516˚C/960˚F for 30 minutes

83˚C/150F˚ to 370F˚/700F˚, 0 minutes

Off

 

Different firing strategies are possible.

•         Reduce the time at top temperature to no more than 10 minutes. 
•         Reduce top temperature by 55˚C/100˚F or more and extend the soak to 20 minutes, if necessary.  Peek frequently to see when the kiln work is complete.
•         Fire on fibre paper covered with Thinfire to allow air out from under the glass.

These strategies can be mixed as desired, and the reasoning for the strategies is:

• Excessive time at the top temperature allows the glass to thin as it migrates to form thicker areas/edges. This makes the glass too thin to resist the air pressure from below.
• Reducing the top temperature will increase the viscosity, so              resisting the migration of the glass, and maintain the original            thickness. 
• Also, single layers are prone to dog boning, but there are ways of reducing it.

Ways to reduce the risk of bubbles appearing in general are:

•    Reduce the time at the top temperature,
•    Reduce the top temperature,
•    Provide ways for the expanding air to migrate from under the glass.

Pressing glass

I have been looking for a different way than flows or melts to mix colours and thought glass pressing might be a promising way to achieve what I wanted.

Weight vs Temperature

I conducted some experiments attempting to thin 1.25 kg/2.75 pounds of glass to 3-4mm.  One and then two 40x40cmx15mm thick shelves were placed on top of the glass cullet with 3mm spacers at the corners. The glass was fired at 220ºC/396ºF to 825ºC/1517ºF and initially held for 30 minutes, later extended to 90 minutes.  The thickness stubbornly remained between 5 and 7mm. 

A few other attempts with different times and temperatures gave inconsistent results.  Perhaps the uneven piling of cullet had an influence on the outcomes, but I was still looking for a flow and mixing of colours different to that obtained by melts.

Other experiments were being conducted in parallel, relating to viscosity. These indicated that glass became thinner than 6-7mm at higher temperatures without pressing.  These experiments lead me to think there are four elements controllable by kilnformers in pressing: size, weight, time, temperature.

The same weight of press with the same temperature and time will make small amounts thinner than large amounts, and this is not surprising.  More time with the same temperature, weight, and amount allows some slight decrease in thickness. 

Higher temperatures with the same weight, and time will allow thinner pressings of the same amount of glass.   Viscosity decreases with temperature, so higher temperatures make glass easier to thin.

More weight is required get the same thickness when pressing a greater volume of glass.  Of course, more time and temperature can be added to increase the effect of the weight.

However, the main factor in pressing large amounts of glass is higher temperatures, which results in reducing the viscosity and the resistance to thinning. 

 

Annealing and Cooling

An important aspect of pressing is the annealing requirements.  It is sensible to anneal for a longer time than normal for thick glass, because of the heat retention of the pressing weights. 

This image shows the stress in an 8mm/0.3” (or 5/16”) after annealing as for 16mm/0.63” (5/8”).  There is widespread low level stress with 30mm thick pressing weight.

Indications are that extending the annealing to at least 3 times the target thickness is a minimum annealing soak requirement.  Alternatively, if it is possible to remove some, or all, of the weight from the glass at the beginning of the anneal soak, the annealing time can be reduced.

 

Veiling

The stress picture above shows there is visual element too.  This veiling is most apparent in clear glass, and less obvious in coloured and opalescent glass.  Small volume stacks, which are pressed thin will exhibit less of the veiling.

 

 

Four factors that kilnformers can control in pressing glass to less than 6mm are weight, size, time, and temperature.  The main one is temperature.

Rapid Ramp Rates with Soaks

I have seen many schedules with initial rates of advance interrupted by soaks.  These kinds of schedules that are written something like this:

250°C/450°F to 200°C/482°F, soak for 10 (or 20 or 30) minutes

250°C/450°F to 500°C/933°F, soak for 10 (or 20 or 30) minutes

300°C/540°F to 595°C/1100°F, soak for 10 (or 20 or 30) minutes

300°C/540°F to 677°C/1250°F, soak for 10 (or 20 or 30) minutes

330°C/600°F to working temperature (1450°, 1500° etc.)

When I have asked, I’m usually told that these are catch up pauses to allow all the glass to have an even temperature.  There are occasions when that may be a good idea, but I will come to those later.  For normal fusing, draping and slumping these soaks are not needed.

To understand why, needs a little information on the characteristics of glass.  Glass is a good insulator, and therefore a poor transmitter of heat.  Glass behaves better with a moderate steady input of heat to ensure it is distributed evenly throughout the glass.  To advance the temperature quickly during the initial heat up stages where the glass is brittle risks thermal shock. 

The soaks at intervals do not protect against a too rapid increase in temperature.  It is the rate of heat input that causes thermal shock.  Rapid heat inputs cause uneven temperatures through and across the glass.  When these temperatures are more than 5°C different across the glass, stress is not relieved.  As the temperature differential increases, so does the stress until the glass is not strong enough to contain those stresses and breaks.  At higher temperatures these stresses do not exist as the glass is less viscous.

If, as is common and illustrated in the schedule above, you advance at the same rate on both sides of the soak, the soak really does not serve any purpose – other than to make writing schedules more complicated.  If the glass survived the rate of heat input between the soaks, it will survive without the soaks.

But you may wish to be a little more careful. The same heating effect can be achieved by slowing the rate of advance.  Just consider the time used in the soak and then slow the rate by the appropriate amount.  Take the example above using 30-minute soaks:

250°C/450°F to 200°C/482°F, soak for 30 minutes

250°C/450°F to 500°C/933°F, soak for 30 minutes

This part of the schedule will take three hours.  You can achieve the same heat work by going at 167°C/300°F per hour to 500°C/933°F.  This will add the heat to the glass in a steady manner and the result will be rather like the hare and tortoise.  If you have to pause periodically because you have gone too quickly, you can reach the same end point by steady but slower input of heat without the pauses.

But, you may argue, “the periodic soaks on the way up have always worked for me.”  As you work with thicker than 6mm glass, this “quick heat, soak; quick heat, soak” cycle will not continue to work.  Each layer insulates the lower layer from the heat above.  As the number of layers increase, the greater the risk of thermal shock. Enough time needs to be given for the heat to gradually penetrate from the top to the bottom layer and across the whole area in a steady manner.

To be safest in the initial rate of advance, you should put heat into the glass in a moderate, controlled fashion.  This means a steady input of heat with no quick changes in temperature.  How do you calculate that rate?  Contrary as it may seem, start by writing out your cooling phases of the schedule.  The cooling rate to room temperature is the safe cooling rate for the final and now thicker piece.  If that final cool rate is 300°C/540°F, the appropriate heat up rate is one third of that or 100°C/180°F. 

This “one third speed” rate of advance will allow the heat to penetrate the layers in an even manner during the brittle phase of the glass.  This rate needs to be maintained until the upper end of the annealing range is passed.  This is normally around 55°C/100°F above the annealing point.

Then you can begin to write the rate of advance portion of your schedule.  It could be something like:

100°C/180°F to 540°C, no soak

225°C/405°F to bubble squeeze, soak

330°C/600°F to working temperature, soak 10 minutes

Proceed to cool segments 

I like simple schedules, so I normally stick to one rate of advance all the way to the bubble squeeze.  This could be at the softening point of the glass or start at 50°C below with a one hour rise to the softening point with a 30-minute soak there before proceeding more quickly to the working temperature.

Exceptions.

I did say I would come back to an exception about soaks on the first ramp rates  segment of the schedules.  When the glass is supported – usually in a drape – with a lot of the glass unsupported you do need to have soaks.  The kind of suspension is when draping over a cylinder or doing a handkerchief drop.  This is where a small portion of the glass is supported by a point or a long line while the rest of the glass is suspended in the air.  It also occurs when supported by steel or thick ceramic.

The soaks are not to equalise the temperature in the glass primarily.  They are to equalise the temperature between the supports and the glass.  A thick ceramic form supporting glass takes longer to heat up than the glass.  The steel of a cocktail shaker takes the heat away from the glass as it heats faster. 

The second element in this may already be obvious.  The glass in the air on a ceramic mould can heat faster than that on the mould.  The glass on a steel mould can heat faster over the steel than the suspended glass.  Both these cases mean that you need to be careful with the temperature rises.

Now, according to my arguments above, you should be able to slow the rate of advance enough to avoid breakage.  However, my experience has shown that periodic soaks in combination with gradual increases in the rates of advance are important, because it is more successful. 

An example of my rates of advance for 6mm glass supported on a steel cylinder is:

100°C/180°F to 100°C/212°F, soak 20 minutes

125°C/225°F to 200°C/392°F, soak 20 minutes

150°C/270°F to 400°C/753°F, soak 20 minutes

200°C/360°F to draping temperature

Call me inconsistent, but this has proved to be more effective than dramatically slowing the rates of advance.  This exception does not apply to slumps where the glass is supported all around by the edge of a circular or oval mould, or where it is supported at the corners of a rectangular or square one.

Another exception is where you have a lot of moisture in a mould, for example. You need to soak just under the boiling point of water to dry the mould or drive out water from other elements of your work before proceeding.  This also applies to situations where you need a burn out, of for example vegetable matter at around 500°C/933°F for several hours.

In both these cases, these are about the materials holding or contained in the glass, rather than the glass itself.

Revised 23.2.25

Time and Temperature

credit: timeanddate.com

Credit: Shutterstock

“What are the pros and cons on turning up the max temperature slightly Vs. a longer hold time”? Lea Madsen

This is a difficult question to answer, because there are variables such as

the temperature range,

the ramp rates, and soaks,

the forces acting upon the glass at a given temperature, 

the process,

the desired outcome of the firing,

etc. 

When talking about temperature vs. time, it is heat work that we are considering.  In many processes time and temperature are interchangeable, although the temperature range is important too.  This is a brief discussion of heat work in various processes.

Slumps

Slumping temperature is generally in the range of 620˚C-680˚C/1150˚F -1255˚F *, which is below the devitrification range.  This allows the exchange of time for temperature without risk, allowing more time rather than more temperature.  Higher temperatures cause more marking from the mould since the bottom of the glass is softer than at lower ones.  Lower temperatures give higher viscosity, so the glass is stiffer, resisting marks.

Low temperature fuses

Sharp tack fusing, freeze and fuse, some pate de verre processes, and sintering occur in the 650˚C -720˚C /1150˚F - 1320˚F range, risking devitrification only at the upper end of this range.  Extending the time rather than the temperature is important to maintain detail in these processes.  Higher temperatures will smooth the surface, risking loss of detail.  

Rounded tack processes (720˚C – 760˚C /1320˚F - 1400˚F)

These are within the devitrification range making the choice between time and temperature a balance of risks.  In my experience, it takes about an hour for visible devitrification to develop.  This means that you can extend the time, if the total time between the end of the bubble squeeze and the working temperature, including the hold time, is less than an hour.  It has the advantage of a more secure attachment between the pieces of glass, without altering the surface much. 

But if extending the soak time increases the time in the devitrification zone to be more than an hour, it is advisable to increase the temperature, rather than time.  Devitrification develops in the presence of air, so reducing the time in that range reduces the risk of devitrification developing.  The glass is moving during rapid ramp rates, reducing the chance of devitrification.

Drops

This includes drapes, and other free forming processes.  Kilnformers will be observing the progress of these firings, making it easier to balance temperature and time.  There are already long holds scheduled for the processes, so it is a matter of getting the right temperature.  If, after half an hour at the scheduled top temperature, the glass has not moved much, it is time to increase the temperature by, say 10˚C/18˚F and observe after another half hour, repeating the temperature increase if necessary.   Be aware of thinning the glass at the shoulder by setting a high temperature.  Free drops may take as much as 6 – 8 hours, so patience and observation are important to get good results.

Full fuse

At full fuse try to get the work done in 10 minutes to avoid complications with devitrification.  So, increasing the temperature rather than the length of the soak seems best.

Flows

Whether frit stretching, making pattern bars, pressing, etc., low viscosity is important.  Viscosity is closely related to temperature, so increasing the temperature is the better choice.  Increasing time without increasing temperature does not change viscosity much.

Casting

Extending time at top temperature seems best for open face casting, as the temperature is already high.  A slow ramp rate to that top temperature may make adding time unnecessary, because the heat work will be increased by the slow rise.  Experience has shown that a rate of 200˚C/360˚F is enough to avoid devitrification.  With enclosed castings devitrification is not such a risk, so time can be added without concern.

 

Observation

In all these processes it is advisable to observe the progress of the firing by quick peeks to determine the effective combination of temperature and time.  Also note that heat work is cumulative, making for changes in profile with repeated firings. 

 

* The softening point of float glass is around 720°C/1328°F, so the slumping range is about 700°C/1292° to 750°C/1382°F.

Sample Tiles

credit: Tia Murphy

There are advocates for making tiles as references for future work.  

• They show the profiles achieved at different temperatures.  
• They can be stored for easy visual reference when planning a firing.  
• It is a useful practice for any kiln new to the user.  

These tiles are assembled in identical ways to enable comparisons.  They should include black and white, iridised pieces- up and down, transparent and opal, and optionally stringers, confetti, millefiori, frit and enamels.  

The tiles are fired at different top temperatures with the same heat up schedule with the top temperature of each at about 10C or 20F intervals.  These show what effect different temperatures give.  Start the temperature intervals at about 720C or 1330F.

This is a good practice, even if time consuming.  It gets you familiar with your kiln and its operation.  It gives a reference for the profiles that are achieved with different temperatures at the rates used.

Ramp rate and time

But, as with many things in kilnforming, it is a little more complicated.  The effect you achieve is affected by rate and time used as well as the temperature.

The firing rate is almost as important as the temperature.  

• A slow rate to the same top temperature will give a different result than a fast rate.  
• The amount of heat work put into the glass will affect the temperature required.  
• Slow rates increase the time available for the glass to absorb the heat.  
• Glass absorbs heat slowly, so the longer the time used by slower rates, the rounder the profile will be.

Since time is a significant factor in achieving a given profile, any soaks/holds in the schedule will affect the profile at a set temperature.  A schedule without a bubble squeeze will give a different result than one with a bubble squeeze at the same temperature.

To help achieve knowledge of the rate/time effect, make some further test tiles.  Use different rates and soaks for the test tiles of the same nature as the first temperature tests. But vary only one of those factors at a time. Consider the results of these tests when writing the schedule for more complex or thicker layups. 

Mass

Also be aware that more mass takes longer to achieve the same profile.  Slower rates and longer times will help to achieve the desired profile at a lower temperature.  It is probably not practical to make a whole series of test tiles for thicker items.  But, a sample or two of different thicknesses and mass will be helpful to give a guide to the amount of adjustment required to achieve the desired outcome.

The results of sample tiles are due to more than just temperature.  They are a combination of rate, time, and temperature (and sometimes mass).  These factors need to be considered when devising or evaluating a schedule, because without considering those factors, it is not possible to accurately evaluate the relevance of a suggested top temperature.

See also: Low Temperature Kilnforming, available from Bullseye and Etsy

Heat Up Soaks

Photo credit: Bullseye Glass Co.

It is often advocated that there should be a soak at the strain point to even out the temperature throughout the glass.

My question continues to be why? 

The glass has survived whatever rate has been used up to that point during its brittle phase.  So, it already has every chance of surviving a rapid rate during the plastic phase.

Instead of a soak at the strain point, Bob Leatherbarrow indicates a soak during the brittle phase will be more successful in avoiding heat up breaks.  He has observed that heat up breaks are most likely to happen around 260ºC/500ºF.  Therefore, a soak in that region is most likely to be of use in evenly distributing the heat effectively through the glass rather than at a higher temperature.  He recommends up to a half hour soak there before proceeding at the same rate to the strain point (about 540ºC/1004ºF).  The ramp rate to this heat up soak in the brittle phase should be related to the thickness of the glass and the intended profile.

The thickness to be fired for is determined by the profile.  Rates for full and contour fusing can be as for the thickness before firing.  Rounded tack fuse needs to be fired as though twice as thick, and sharp tack or laminated fuse need to be fired as though 2.5 times.  More information on initial ramp rates to the strain point can be found in Low Temperature Kilnforming available from Bullseye and from Etsy

Visible Devitrification

"Why does devitrification appear on slumped pieces?"

A brief explanation 

Scientific research in developing a glass matrix to support bone grafts gives some information.  This kind of glass matrix requires to be strong.  Development showed that devitrification weakens the matrix.  The crystals in a matrix are not as strong as the amorphous glassy state.  So, devitrification needs to be avoided.

The research to avoid devitrification showed that it begins at about 600˚C/1110˚F.  It only begins to become visible above 700˚C/1300˚F.  The process developed was to introduce a “foaming” agent.  The process fired slowly to 600˚C/1110 ˚F and then quickly to 830˚C/ 1530˚F.  It left a strong open matrix around which bone can grow. Although the research used float glass, it is also a soda lime glass, just as fusing glasses are.  The formation of devitrification begins at the same temperature for fusing glasses as for float.

The result of this medical research shows that devitrification begins on glass before it is visible. Devitrification is cumulative. A little becomes greater with another firing.  This is so even with good cleaning between firings. The new devitrification builds on the previous.  It does this from 600˚C/1110 ˚F.

A subsequent firing can continue this devitrification to the point where it is visible. This can happen, although the temperature at which we can see it after one firing has not been reached.  This continued devitrification at low temperatures can become great enough to be visible at the end of one or multiple slumps.

Credit: Bullseye Glass Co.

What can we do?

Clean all the glass before every firing very well.

·         Avoid mineralised water.

·         Final clean with isopropyl alcohol.

·         Polish dry at each stage with white absorbent paper.

 

Soak longer at lower temperatures.

·         Use longer soaks to achieve the slump.

·         Keep the temperature low.

·         Observe the progress of the firing with quick peeks.

 

Use slower ramp rates.

·         Slower rates enable the heat to permeate the glass.

·         Enables a lower slump temperature.

 

If there is any hint of devitrification after the first firing,

·      use a devitrification spray, or

·      provide a new surface.

• o   remove the surface by abrasion on sandblasting,
• o   cap with clear, or
• o   cover whole surface with a thin layer of clear powder.

·      Fire to contour fuse to give a new smooth surface.

·      Clean very well and proceed to slump.

Slow Rates to Annealing

"I have seen recommendations for slower than ASAP rates from the top temperature, but most schedules say 9999 or ASAP.  Which is right?"

Slow drops in temperature from top to annealing temperatures risk devitrification. Accepted advice is to go ASAP to annealing temperature to avoid devitrification forming.

Breaks do not occur because of a too rapid drop from top temperature to annealing. The glass is too plastic until the strain point has been passed to be brittle enough to break. On the way down that will be below an air temperature of 500˚C/933˚F.

credit: ww.protolabs.com

Different kilns cool from top temperature at different rates. Ceramic kilns are designed to cool more slowly and may need assistance to cool quickly.  This is usually by opening vents or even the door or lid a little. Glass kilns are designed to lose temperature relatively quickly from high temperatures. They do not need a crash cooling as ceramic kilns may need in certain circumstances.  Of course, crash cooling may be necessary for some free drops and drapes.

The length of the soak at annealing is determined by the effective thickness of the piece.  Tack fusing needs to be annealed for thickness as a factor of 1.5 to 2.5, depending on profile.

The extent to which you control the cooling to room temperature after the anneal soak is dependent on the calculated thickness of the piece you are cooling. The objective is to keep the internal temperature differential to 5˚C/10˚F or less to avoid expansion/ contraction differences that are great enough to break the piece. Those rates are directly related to the required length of the anneal soak.  Those rates can be taken from the Bullseye chart for Annealing Thick Slabs. The Fahrenheit version is is available too.

An example.  If you have a 2 layer base with 3 layers (=15mm) stacked on top for a rounded tack fuse, you need to fire as for at least 30mm. This will require controlled cooling all the way to room temperature.

• ·        The rate to 427˚C /800˚F will be19˚C /34˚F
• ·        The rate to 370˚C /700˚F will be 36˚C /65˚F
• ·        The final rate 120˚C /216˚F to room temperature.

You may need to wait a day before any coldworking. An example from my experience shows the necessity.  I checked a piece for stress a few hours after removing the piece from the kiln when it felt cool to the touch. It puzzled me that stress showed, although it didn't on similar pieces.  The next morning, I went to check if I misunderstood the reading. Now, a full 15 hours after coming out of the kiln, there was no stress.  The example shows that the glass internally is hotter than we think. And certainly, hotter than the air temperature.

In the temperature regions above the strain point, the glass needs to be cooled quickly. In the annealing region and below the glass needs to be cooled slowly.

More information is available in the eBook Low temperature Kilnforming.  This is available from Bullseye or Etsy