In my view the protection you need at slumping - or combing - temperatures are:
Face and hair protection – a face shield with a coating to reduce the infrared glow is ideal, but expensive.
Good heat protecting gloves – do not cut corners here. You need something more than leather or welders’ gloves.
Heat reflecting sleeves – the aluminised ones work very well.
A reversed denim jacket to protect your chest - you don't want hot buttons!
A helper to hold the lid/door open.
The tools
Wet timbers - at least 40mm in one direction, square is probably the best. They need to be at least 600mm long - a metre is better.
Wet wood handles on any metal tools.
In the case of manipulating glass at slumping temperatures, you need two soaked pieces of timber. If you try to manipulate with one stick you will just push the piece around the kiln. Two sticks allow you to move the glass on opposite sides of the mould toward or away from each other without shoving things around. Glass is amazingly stiff at slumping temperatures. You will wonder how it moves at all when you have to use so much pressure to affect the piece!
Note also that the timbers must be soaked. If they are not they will certainly leave marks on the glass.
When the timbers began to smoke, it is time to close down the kiln - whatever manipulation stage you are at - and let the kiln heat up again while your sticks get wet again.
You need a second person to help with this kind of manipulation in the kiln. They are the door holders, fetch and carriers and anything that needs to be done while you are pushing the glass around the mould. Perhaps after a few trials, you would be able to do it without help, but it is always easier and safer to have help.
Showing posts with label kiln forming. Show all posts
Showing posts with label kiln forming. Show all posts
Saturday 22 June 2019
Strain Points and Annealing Ranges
I received the following question a while back and thought my response might be useful, although very informal.
“Can you dumb down the concepts of 'annealing point' and 'strain point'? I understand anneal point to be a fixed point (depending on the glass) but the strain point is a range...is this correct? I understand the concept of a hold at the anneal point but I'd like to understand how to bring it down through the strain point.”
I really dislike the idea of dumbing down concepts in kiln forming glass. Glass chemistry is incredibly complicated. Glass physics is still little understood. Glass is a very complicated subject. The marketing of glass for kiln forming has led us all to think it is a simple matter of recipes. Well it's not.
Having got that rant out of my system.... Let’s go ahead.
The annealing point is roughly defined as the temperature at which the glass (if it is the same temperature throughout) will relax most quickly. In the practical kiln forming that we do, it is not possible to ensure that the glass is that temperature throughout. So it is better to think of an annealing soak at the annealing point to allow the glass to become a more even temperature throughout its thickness. As thicker glass means the heat has further to travel from the centre to the surfaces, a longer soak is needed for thicker glass.
The annealing occurs during the slow cool past the lower strain point. The annealing occurs best with a slow, but steady drop in temperature. So annealing is occurring over a range, not at a point. We all rely on a combination of the manufacturers' recommendations, various writings we read, and experience to determine that rate, although Bullseye have published a chart which is most helpful, whichever glass you use.
Strain points.
There is an upper and lower strain point, although this is disputed by some. There are mathematical definitions for these as well as observational definitions. I do not understand the mathematics of either. In lay terms, the lower strain point is that temperature below which no further annealing can take place. It is safe to assume this is 50C below the annealing point (I think it actually is 43C, but I'm not certain of this number).
So it is safest to control the cooling to at least 5C below the lower strain point. Bullseye find that cooling from the annealing soak to 370C is best - this is much more conservative than is theoretically required – 146C below the annealing soak point. This does take care of any problem of thermal shocking of the glass during the cooling.
The upper strain point might be more properly described as a softening point. This also has scientific definitions. The way I think of it is as being the temperature above which no annealing can occur. Another is to think of it as a point beyond which the molecules of the glass are in relatively free motion - which increases with temperature. This again can be considered (on the rise) as 50C above annealing. However on the way down it is safer to consider it to be not more than 30C above annealing. This is because the glass temperature lags behind the air temperature (which is what our controllers measure).
So there is no point in soaking more than 30C above annealing in an attempt to equalise the temperature throughout the glass. However, if you really need to equalise temperature at some point above the annealing point, it might be better to slow the cooling from the working/top temperature and do the final equalisation of temperature at the annealing point.
To answer directly, the strain point by definition of language cannot be a range. There are two points which form the possible annealing range, although the lower one is the critical one. The upper one I described earlier as the softening point. The softening point forms the upper part and the strain point forms the lower part of a range in which the annealing can occur. So the concepts are the opposite of what you propose. They are points which are the boundaries of the annealing range.
To complicate things further, not all glass from one manufacturer has the same annealing point. The published annealing point is a compromise that their experiments and experience have shown to be most suitable. Bullseye glass for example has three annealing points, 532C for opals, 505C for cathedrals and 472C for gold bearing glasses. NOTE: these figures may not be exact; they come from memory rather than documents. Since this list of annealing points was published, Bullseye have conducted further experimentation that shows the best annealing soak occurs at 482C which is below transparent and opalescent, but above gold bearing annealing points.
Schott recommends a range for annealing, not a point, to accommodate these variables. Bullseye, Uroboros, and Spectrum have published annealing points that are practical for people kiln forming in smaller kilns that are less well controlled than the factory lehrs.
If you look at the Bullseye site - education section, you will find a lot of useful information. Especially informative are their tech notes. Spectrum - to a lesser extent than Bullseye - gives helpful information. The information from both sites should be absorbed and the principles applied to other glasses.
Finally, kiln forming is deceptively simple. I have spent 29 years discovering how much more there is to learn. This is one of the reasons that glass is such an exciting medium - people keep discovering new things.
Reviewing the above, I realise that I have not answered your question "... how to bring {the temperature} down through the [lower] strain point". My answer is that you should look at the manufacturer's site for each glass that you use. Look at their rates for annealing for different thicknesses of glass (some also take into account the size). Consider them. Then look at some of the other sites for their published annealing rates for various thicknesses. Comparison of their rates will reveal differences. Think about what they are, how they relate, and whether they reveal that they are using the same principles with slight variations.
Also, if you can, get a copy of Graham Stone's book "Firing schedules for glass, the kiln companion". It provides a handy guide to annealing rates. But DO NOT use it as a book of recipes. Read all the commentary about the schedules, as they (combined with the introductory parts) give principles and tips about how to think about the cooling of the glass. Bob Leatherbarrow has recently published an excellent book on kilnforming schedules, available from his website.
By the way, experience is so often lost, or misremembered, that keeping a log is essential. My first log consisted of loose leaf binder, so I could file all the same kind of firings for various glasses together (this was in the days when there was not much fusing compatible glass, and I couldn't afford Bullseye at UK prices. I was discovering lots about glass firing and using some schedules that I now wonder how I had any success. I did learn a lot from my failures and recorded them too. Now I use a log, usually an out-of-date A4 size diary, sometimes a manuscript book that is big enough to record observations and illustrations. Bullseye have a good record form on their site.
I congratulate you on your desire to understand the processes. Too many only want to put the glass in and turn the kiln on. That is the desire a number of kiln manufacturers pander to when they put pre-programmed schedules on the controllers. So, don't take any of this as criticism of you or your comments. It is meant in a constructive manner - even though I am told frequently that the manner is blunt, even rude.
Best wishes on continued successful kiln forming.
Revised 22/06/19
“Can you dumb down the concepts of 'annealing point' and 'strain point'? I understand anneal point to be a fixed point (depending on the glass) but the strain point is a range...is this correct? I understand the concept of a hold at the anneal point but I'd like to understand how to bring it down through the strain point.”
I really dislike the idea of dumbing down concepts in kiln forming glass. Glass chemistry is incredibly complicated. Glass physics is still little understood. Glass is a very complicated subject. The marketing of glass for kiln forming has led us all to think it is a simple matter of recipes. Well it's not.
Having got that rant out of my system.... Let’s go ahead.
The annealing point is roughly defined as the temperature at which the glass (if it is the same temperature throughout) will relax most quickly. In the practical kiln forming that we do, it is not possible to ensure that the glass is that temperature throughout. So it is better to think of an annealing soak at the annealing point to allow the glass to become a more even temperature throughout its thickness. As thicker glass means the heat has further to travel from the centre to the surfaces, a longer soak is needed for thicker glass.
The annealing occurs during the slow cool past the lower strain point. The annealing occurs best with a slow, but steady drop in temperature. So annealing is occurring over a range, not at a point. We all rely on a combination of the manufacturers' recommendations, various writings we read, and experience to determine that rate, although Bullseye have published a chart which is most helpful, whichever glass you use.
Strain points.
There is an upper and lower strain point, although this is disputed by some. There are mathematical definitions for these as well as observational definitions. I do not understand the mathematics of either. In lay terms, the lower strain point is that temperature below which no further annealing can take place. It is safe to assume this is 50C below the annealing point (I think it actually is 43C, but I'm not certain of this number).
So it is safest to control the cooling to at least 5C below the lower strain point. Bullseye find that cooling from the annealing soak to 370C is best - this is much more conservative than is theoretically required – 146C below the annealing soak point. This does take care of any problem of thermal shocking of the glass during the cooling.
The upper strain point might be more properly described as a softening point. This also has scientific definitions. The way I think of it is as being the temperature above which no annealing can occur. Another is to think of it as a point beyond which the molecules of the glass are in relatively free motion - which increases with temperature. This again can be considered (on the rise) as 50C above annealing. However on the way down it is safer to consider it to be not more than 30C above annealing. This is because the glass temperature lags behind the air temperature (which is what our controllers measure).
So there is no point in soaking more than 30C above annealing in an attempt to equalise the temperature throughout the glass. However, if you really need to equalise temperature at some point above the annealing point, it might be better to slow the cooling from the working/top temperature and do the final equalisation of temperature at the annealing point.
To answer directly, the strain point by definition of language cannot be a range. There are two points which form the possible annealing range, although the lower one is the critical one. The upper one I described earlier as the softening point. The softening point forms the upper part and the strain point forms the lower part of a range in which the annealing can occur. So the concepts are the opposite of what you propose. They are points which are the boundaries of the annealing range.
To complicate things further, not all glass from one manufacturer has the same annealing point. The published annealing point is a compromise that their experiments and experience have shown to be most suitable. Bullseye glass for example has three annealing points, 532C for opals, 505C for cathedrals and 472C for gold bearing glasses. NOTE: these figures may not be exact; they come from memory rather than documents. Since this list of annealing points was published, Bullseye have conducted further experimentation that shows the best annealing soak occurs at 482C which is below transparent and opalescent, but above gold bearing annealing points.
Schott recommends a range for annealing, not a point, to accommodate these variables. Bullseye, Uroboros, and Spectrum have published annealing points that are practical for people kiln forming in smaller kilns that are less well controlled than the factory lehrs.
If you look at the Bullseye site - education section, you will find a lot of useful information. Especially informative are their tech notes. Spectrum - to a lesser extent than Bullseye - gives helpful information. The information from both sites should be absorbed and the principles applied to other glasses.
Finally, kiln forming is deceptively simple. I have spent 29 years discovering how much more there is to learn. This is one of the reasons that glass is such an exciting medium - people keep discovering new things.
Reviewing the above, I realise that I have not answered your question "... how to bring {the temperature} down through the [lower] strain point". My answer is that you should look at the manufacturer's site for each glass that you use. Look at their rates for annealing for different thicknesses of glass (some also take into account the size). Consider them. Then look at some of the other sites for their published annealing rates for various thicknesses. Comparison of their rates will reveal differences. Think about what they are, how they relate, and whether they reveal that they are using the same principles with slight variations.
Also, if you can, get a copy of Graham Stone's book "Firing schedules for glass, the kiln companion". It provides a handy guide to annealing rates. But DO NOT use it as a book of recipes. Read all the commentary about the schedules, as they (combined with the introductory parts) give principles and tips about how to think about the cooling of the glass. Bob Leatherbarrow has recently published an excellent book on kilnforming schedules, available from his website.
By the way, experience is so often lost, or misremembered, that keeping a log is essential. My first log consisted of loose leaf binder, so I could file all the same kind of firings for various glasses together (this was in the days when there was not much fusing compatible glass, and I couldn't afford Bullseye at UK prices. I was discovering lots about glass firing and using some schedules that I now wonder how I had any success. I did learn a lot from my failures and recorded them too. Now I use a log, usually an out-of-date A4 size diary, sometimes a manuscript book that is big enough to record observations and illustrations. Bullseye have a good record form on their site.
I congratulate you on your desire to understand the processes. Too many only want to put the glass in and turn the kiln on. That is the desire a number of kiln manufacturers pander to when they put pre-programmed schedules on the controllers. So, don't take any of this as criticism of you or your comments. It is meant in a constructive manner - even though I am told frequently that the manner is blunt, even rude.
Best wishes on continued successful kiln forming.
Revised 22/06/19
More detailed information is available in the e-book: Low Temperature Kilnforming.
Wednesday 3 April 2019
Slumping Breaks
“Why does my full
fused disc break when I slump it?”
There
are several possibilities. The two main ones are annealing and ramp speeds.
Inadequate
annealing in the fusing stage can lead to a very fragile piece when being
re-heated. If there is significant
residual stress in the fused piece, it is much more sensitive to heat changes
during subsequent firings whether full, tack, or slumping/draping. It is
important to thoroughly anneal any piece at every firing. If you are firing a different layup or
contrasting colours and styles, you should check for stress using polarising
filters.
The
slump – or drape – firing needs to be much slower in temperature rise than the
fuse firing. You now have a thicker
piece which takes longer to absorb the heat evenly.
If
your piece is tack fused, it needs an even more slow rate of advance. Sometimes this needs to be as though the
piece were two to four times the actual thickness of the piece. The more angular and pointed the tack fused
elements, the greater the reduction in firing speed. This post gives guidance on how the piece is designed and its thickness affects the rates and soaks in tack fusing.
Further information is available in the ebook Low Temperature Kiln Forming.
Wednesday 27 March 2019
Observation
The importance of Observation (and recording)
Observing what you, or the kiln, is doing whether you are
using a cartoon, or a schedule obtained from elsewhere – including the kiln
manufacturer – means that you will learn much more quickly as you
progress. You will be able to alter
things as you go. This applies to all
stages of the piece from design to removing the piece from the kiln.
The Design
Once you have made your design – whether as a drawing or
a mock-up – look at it. Really look at
it. Look at it from a distance, climb a
ladder if you can’t pin it on a wall and look down on it. Look at it from the sides so you have an
oblique angle view. Turn it upside down
to confuse your expectations and so see what is really there. Look at it, using a mirror to see if it still
looks good. Make the alterations you need as you go along to get the look you
want and then repeat the process until you are happy.
Assembly
Observe how you have put the piece together. Do the pieces fit?
Is everything in the right place? Are the colours right?
Does it match your vision – symmetrical or asymmetrical? Do these things as you progress, so you
become aware of the process and its rhythm.
Firing
Once you have determined your schedule, you cannot just
leave the piece. If you are new to fusing, you need to observe the stages of
firings to begin to understand what is happening to the glass at various
temperatures and rates of advance. You would not put a cake into the oven and
leave it without checking on it from time to time. Why would you fail to
observe a much more expensive process?
Even when you are experienced - observation of new layups, new processes and
anything you haven’t done several times before - you need to know how things
are progressing during the firing.
Observing a firing is relatively simple. You need to check on two things:
· Check for a too rapid rate of advance. Peek into the kiln at around 540C to see if the piece is still whole. If not, you can abort the firing and progress to fixing or move on to another project.· Check to see when the desired shape has been achieved. Peeking to see if the slump is complete or needs more time is important to getting the shape right. Peek to determine if the tack fuse has been achieved. When it has, advance to the next segment to avoid over firing. If it hasn’t, add time to the schedule to get it right.
Recording
It is not enough to simply observe. You need to record what you intended and the
results you achieved. That includes what
you did to get things right as well as wrong.
What did you do to correct elements?
These are all things that you will need to refer to in the future.
The
key to rapid learning is observation and recording what you see.
Saturday 22 December 2018
Batt Wash Sticking to the Glass
The main reasons that kiln wash sticks to glass are:
1. Firing at too high a temperature. The higher the temperature, the more likely the kiln wash will stick to the glass.
2. Firing with opalescent glass against the shelf. Kiln wash sticks to opalescent glasses more easily than to transparent glass.
3. Re-using kiln washed shelves that have been to fusing temperatures already.
4. Using kiln wash with high amounts of china clay makes for more sticking. Thus some brands stick more frequently than others.
Firing at too high a temperature is probably the worse culprit. The second is using opalescent directly on the kiln shelf.
Strategies to avoid this sticking are:
1. Fire at the lowest temperature you can to get the result you want. This often requires slow rates of advance and extended soaks at the working temperature
2. Use Bullseye kiln wash. It is among the best.
3. Have a transparent glass as the bottom layer.
4. Use iridised glass, with the iridised side down to the shelf, as the iridisation acts as a separator. Do not do this with Thinfire, as it can lead to large cavities in the glass. Fire onto kiln wash.
There are ways to get the kiln wash off but it's easier to avoid it. Using an iridised sheet on the bottom is probably the most effective prevention.
1. Firing at too high a temperature. The higher the temperature, the more likely the kiln wash will stick to the glass.
2. Firing with opalescent glass against the shelf. Kiln wash sticks to opalescent glasses more easily than to transparent glass.
3. Re-using kiln washed shelves that have been to fusing temperatures already.
4. Using kiln wash with high amounts of china clay makes for more sticking. Thus some brands stick more frequently than others.
Firing at too high a temperature is probably the worse culprit. The second is using opalescent directly on the kiln shelf.
Strategies to avoid this sticking are:
1. Fire at the lowest temperature you can to get the result you want. This often requires slow rates of advance and extended soaks at the working temperature
2. Use Bullseye kiln wash. It is among the best.
3. Have a transparent glass as the bottom layer.
4. Use iridised glass, with the iridised side down to the shelf, as the iridisation acts as a separator. Do not do this with Thinfire, as it can lead to large cavities in the glass. Fire onto kiln wash.
There are ways to get the kiln wash off but it's easier to avoid it. Using an iridised sheet on the bottom is probably the most effective prevention.
Uprisings at the Bottom of a Slumped Bowl
“I just finished slumping a dish and I got a big lump in the center of the bottom. This is not an air bubble, just a lump. What should I do to avoid this again?”
Several suggestions are possible.
Ensure there are holes at the bottom of the mould that allow air to get out into the kiln. Prop the mould up on stilts if the hole does not go directly out of the mould. Alternatively, drill a hole in the side to allow the air to escape from under the mould.
Firing for too long or at too high a temperature will cause the glass to continue sliding down. Having nowhere else to go, the weight causes the bottom to begin rising. This is a consistent experience across several kilns and with multiple users.
So keep the temperature down to the minimum required. To find that out, watch the slumping in stages (do not stare!). Look at the piece for a second or two every five minutes after you reach your desired temp.
If it already has slumped adequately, you are firing too high. Reduce your temperature in subsequent firings and watch to find what the required temp and time is. There is absolutely no substitute in slumping but to watch and learn what your mould and glass require.
If you are slumping at such a temperature to seal the glass to the mould, you are firing too hot anyway. Or put more positively, use a low temperature slump, that is, a slump at the lowest temperature to achieve the desired result over an extended period of your choice.
A low temperature slump will allow the glass to conform to the shape of the mould without softening so much that it takes up all the markings of the mould. Therefore, there are spaces for the air to escape from under the glass all the way to the top as well as through the air holes at the bottom. It also gives the most mark-free slump possible for your shape.
Several suggestions are possible.
Ensure there are holes at the bottom of the mould that allow air to get out into the kiln. Prop the mould up on stilts if the hole does not go directly out of the mould. Alternatively, drill a hole in the side to allow the air to escape from under the mould.
Firing for too long or at too high a temperature will cause the glass to continue sliding down. Having nowhere else to go, the weight causes the bottom to begin rising. This is a consistent experience across several kilns and with multiple users.
So keep the temperature down to the minimum required. To find that out, watch the slumping in stages (do not stare!). Look at the piece for a second or two every five minutes after you reach your desired temp.
If it already has slumped adequately, you are firing too high. Reduce your temperature in subsequent firings and watch to find what the required temp and time is. There is absolutely no substitute in slumping but to watch and learn what your mould and glass require.
If you are slumping at such a temperature to seal the glass to the mould, you are firing too hot anyway. Or put more positively, use a low temperature slump, that is, a slump at the lowest temperature to achieve the desired result over an extended period of your choice.
A low temperature slump will allow the glass to conform to the shape of the mould without softening so much that it takes up all the markings of the mould. Therefore, there are spaces for the air to escape from under the glass all the way to the top as well as through the air holes at the bottom. It also gives the most mark-free slump possible for your shape.
Friday 13 July 2018
Fire Polishing
Polishing of glass can be done in the flame, in the kiln, by acids or by grinding with successively fine abrasives depending on the nature of the piece and the equipment available. Fire polishing in the kiln is widely popular as it utilises existing equipment, avoiding purchasing additional cold working equipment. This post indicates some elements about fire polishing in the kiln.
Fire polishing is the technique most often available to kiln formers. This is the process of heating the glass to less than a full fuse to achieve a smoother texture on the glass. It is often used after sandblasting or hand sanding a piece in order to give a smooth shiny surface to the glass without extensive cold working with successively finer grits to get a polish. It also can be used to give a variety of textures from a sealed but almost unchanged sandblasted surface, through a satin-like finish to a very subtle difference between full polish and slightly textured surfaces in the same piece.
Fire polishing range
Fire polishing already slumped items Similarly, re-firing already slumped items to a fire polish rarely succeeds. Distortion of the piece is more likely than achieving a fire polish on an already slumped item.
Again, in these more difficult circumstances, you must observe at intervals to ensure you do not over fire and distort your piece.
Schedules
The reason that no indicative schedules are given is that different glasses, and different lay ups require different firing conditions. These are dealt with elsewhere in the blog.
Alternatives Alternatives to fire polishing include acid polishing, which can present a health hazard, and is normally an industrial process. The other common method of polishing is to cold work the piece. This often requires specialized equipment, but can be done by hand if you have the time.
Fire polishing is the technique most often available to kiln formers. This is the process of heating the glass to less than a full fuse to achieve a smoother texture on the glass. It is often used after sandblasting or hand sanding a piece in order to give a smooth shiny surface to the glass without extensive cold working with successively finer grits to get a polish. It also can be used to give a variety of textures from a sealed but almost unchanged sandblasted surface, through a satin-like finish to a very subtle difference between full polish and slightly textured surfaces in the same piece.
Fire polishing range
The temperature range that this occurs between slumping and tack fusing. The normal range is 650C to 750C depending on the glass, the soak time and the speed of advance. The purpose of this kind of firing is to get the surface of the glass hot enough to form the desired surface without soaking long at higher temperatures, as this is also the devitrification range (700C - 760C). Normally there would be a minimal or no soak at the top of the temperature range.
When to fire polish As this temperature range is above the slumping temperature, fire polishing should be done after fusing and before slumping. As this will be the last operation before forming, you also should do any work to shape the edges and deal with any other imperfections, before fire polishing. After doing any grinding or other work on the edges or surface of the piece, thoroughly wash and polish the piece dry.
Methods
You can take the fused piece that has been treated to remove the devitrification up at the same rate as for slumping the piece to the tack fuse temperature. The higher you go, the less soak time is required. Of course, the higher you go, the longer you are in the devitrification zone.
Some people advocate a quick fire polish. This is achieved by firing at a relatively slow rate until a low slump temperature is achieved. Then fire very quickly to the tack fuse temperature with no soak and return to annealing temperature as quickly as possible.
The quick fire polish does achieve a minimum of time in the devitrification zone, but it eliminates all subtely in the surface. A long soak of up to 90 minutes at a moderate slumping temperature will give a satin appearance to an abraded or sandblasted surface. A shorter soak will seal sandblasted work without eliminating the texture of the sandblasted image.
In all the cases of fire polishing you need to peek at intervals to determine when the desired surface has been achieved. This requires careful placement in relation to the place from which you will be able to peek at the surface. For a fully polished piece, you will see the reflections of the elements. For more subtle textures, you need to think about what you want to see, peek, close the lid or observation port and think about what you saw. If it is not yet what you want, peek at another interval in the same way, until you observe the surface you want.
Combining fire polish and slumping It is sometimes possible to fire polish and slump at the same time, but this is a risky technique often leading to changes in shape or an uprising of the glass at the bottom of the mould. It is possible to fire polish glass as low as 630 with a long soak – 60 minutes or more. If you are determined to fire polish and slump at the same time, it's essential that you watch the piece very carefully to prevent over-firing.
When to fire polish As this temperature range is above the slumping temperature, fire polishing should be done after fusing and before slumping. As this will be the last operation before forming, you also should do any work to shape the edges and deal with any other imperfections, before fire polishing. After doing any grinding or other work on the edges or surface of the piece, thoroughly wash and polish the piece dry.
Methods
You can take the fused piece that has been treated to remove the devitrification up at the same rate as for slumping the piece to the tack fuse temperature. The higher you go, the less soak time is required. Of course, the higher you go, the longer you are in the devitrification zone.
Some people advocate a quick fire polish. This is achieved by firing at a relatively slow rate until a low slump temperature is achieved. Then fire very quickly to the tack fuse temperature with no soak and return to annealing temperature as quickly as possible.
The quick fire polish does achieve a minimum of time in the devitrification zone, but it eliminates all subtely in the surface. A long soak of up to 90 minutes at a moderate slumping temperature will give a satin appearance to an abraded or sandblasted surface. A shorter soak will seal sandblasted work without eliminating the texture of the sandblasted image.
In all the cases of fire polishing you need to peek at intervals to determine when the desired surface has been achieved. This requires careful placement in relation to the place from which you will be able to peek at the surface. For a fully polished piece, you will see the reflections of the elements. For more subtle textures, you need to think about what you want to see, peek, close the lid or observation port and think about what you saw. If it is not yet what you want, peek at another interval in the same way, until you observe the surface you want.
Combining fire polish and slumping It is sometimes possible to fire polish and slump at the same time, but this is a risky technique often leading to changes in shape or an uprising of the glass at the bottom of the mould. It is possible to fire polish glass as low as 630 with a long soak – 60 minutes or more. If you are determined to fire polish and slump at the same time, it's essential that you watch the piece very carefully to prevent over-firing.
Fire polishing already slumped items Similarly, re-firing already slumped items to a fire polish rarely succeeds. Distortion of the piece is more likely than achieving a fire polish on an already slumped item.
Again, in these more difficult circumstances, you must observe at intervals to ensure you do not over fire and distort your piece.
Schedules
The reason that no indicative schedules are given is that different glasses, and different lay ups require different firing conditions. These are dealt with elsewhere in the blog.
Alternatives Alternatives to fire polishing include acid polishing, which can present a health hazard, and is normally an industrial process. The other common method of polishing is to cold work the piece. This often requires specialized equipment, but can be done by hand if you have the time.
Wednesday 11 July 2018
Smooth Kiln Wash on Shelves
There are a number of ways of applying separators to the kiln shelf.
These go by a variety of names - kiln wash, shelf primer, batt wash, etc. - all are separators to keep the glass from sticking to the shelf. They are all combinations of alumina hydrate and china clay (or kaolin or EPK) in various amounts. The china clay provides a high temperature binder for the alumina hydrate which does not stick to glass.
These are some examples of glass separators. The Primo Primer has very little china clay, and is easy to remove. It is particularly good for small casting moulds.
The object in applying the separator is to achieve a smooth surface a possible. Remember there will always be some texture because of the particle size of the wash. For the smoothest surface, use the finest powder you can find. You can, if you want to spend the time and effort, put the powder into a rock tumbler with ceramic balls to get an even finer powder. Avoid shelf primer that is intended for ceramics, as it is coarser than that sold as a separator for glass.
It also is important to prepare the mixture some hours before application to ensure all the particles of the powder are wetted. Immediate use often leads to a gritty surface.
There are several methods for applying the kiln wash to the shelf. The two I use are spraying and brushing. Which I use depends on circumstances - spraying requires more set up time.
Spraying the separator onto the shelves can give an even coating without brush marks, runs or ridges. In this example a mould is being sprayed. To ensure an even covering on a shelf, it should be horizontal and leveled so the kiln wash is evenly distributed. Numerous light passes with the sprayer is best, as in air brushing.
Applying the kiln wash with a very soft brush such as a hake brush in a variety of directions will ensure full coverage.
The brush should lightly touch the shelf and provide a number of thin layers. Applying in four directions - horizontal, vertical, and the two diagonals will ensure full even coverage. There may be some residual brush marks.
To reduce the application marks further, you can brush or spray hot water over the still damp kiln wash. This helps to remove brush marks or the stippling that often comes from spraying and brushing. It is important that the shelf is perfectly level for this operation.
Another way to reduce the texture after the shelf primer dries is to lightly polish the kiln wash with a ball of old nylons or rub a flat piece of paper with the palm of your hand over the shelf. Be sure to remove the dust that may be left behind from this polishing.
Boron Nitride
Another separator that has become popular in spite of its expense is boron nitride, often referred to by the trade name Zyp. This is a high temperature lubricant for industrial kiln operations that has been adapted for the generally lower glass forming temperatures. This is not suitable for kiln shelves, as it completely seals the porous surface of the shelf. It is difficult to go back to the cheaper kiln wash separator as the water of the kiln wash solution will not be absorbed into the shelf, leaving a patchy coverage of the kiln wash. Although both separators should be renewed after each firing (above low temperature tack fusing) the boron nitride is much more expensive and cannot provide a smoother surface than the shelf already has. My recommendation is that boron nitride use should be confined to moulds or other surfaces where the glass may slide or move in the forming process.
These go by a variety of names - kiln wash, shelf primer, batt wash, etc. - all are separators to keep the glass from sticking to the shelf. They are all combinations of alumina hydrate and china clay (or kaolin or EPK) in various amounts. The china clay provides a high temperature binder for the alumina hydrate which does not stick to glass.
These are some examples of glass separators. The Primo Primer has very little china clay, and is easy to remove. It is particularly good for small casting moulds.
The object in applying the separator is to achieve a smooth surface a possible. Remember there will always be some texture because of the particle size of the wash. For the smoothest surface, use the finest powder you can find. You can, if you want to spend the time and effort, put the powder into a rock tumbler with ceramic balls to get an even finer powder. Avoid shelf primer that is intended for ceramics, as it is coarser than that sold as a separator for glass.
It also is important to prepare the mixture some hours before application to ensure all the particles of the powder are wetted. Immediate use often leads to a gritty surface.
There are several methods for applying the kiln wash to the shelf. The two I use are spraying and brushing. Which I use depends on circumstances - spraying requires more set up time.
Spraying the separator onto the shelves can give an even coating without brush marks, runs or ridges. In this example a mould is being sprayed. To ensure an even covering on a shelf, it should be horizontal and leveled so the kiln wash is evenly distributed. Numerous light passes with the sprayer is best, as in air brushing.
Applying the kiln wash with a very soft brush such as a hake brush in a variety of directions will ensure full coverage.
To reduce the application marks further, you can brush or spray hot water over the still damp kiln wash. This helps to remove brush marks or the stippling that often comes from spraying and brushing. It is important that the shelf is perfectly level for this operation.
Another way to reduce the texture after the shelf primer dries is to lightly polish the kiln wash with a ball of old nylons or rub a flat piece of paper with the palm of your hand over the shelf. Be sure to remove the dust that may be left behind from this polishing.
Boron Nitride
Another separator that has become popular in spite of its expense is boron nitride, often referred to by the trade name Zyp. This is a high temperature lubricant for industrial kiln operations that has been adapted for the generally lower glass forming temperatures. This is not suitable for kiln shelves, as it completely seals the porous surface of the shelf. It is difficult to go back to the cheaper kiln wash separator as the water of the kiln wash solution will not be absorbed into the shelf, leaving a patchy coverage of the kiln wash. Although both separators should be renewed after each firing (above low temperature tack fusing) the boron nitride is much more expensive and cannot provide a smoother surface than the shelf already has. My recommendation is that boron nitride use should be confined to moulds or other surfaces where the glass may slide or move in the forming process.
Sunday 3 June 2018
Home Made Devitrification sprays
You can buy a number of devitrification sprays. Some of them are lead bearing and will not be suitable for food and drink containers. Many times people apply them before firing the first time to prevent devitrification. More often these sprays are applied after a piece has become devitrified. However it is applied, these sprays are not cheap.
It is possible to make your own devitrification solution. It is made from borax which you can buy from your local chemicals supplier, or sometimes as a washing powder – but make sure it has no additives!
To make a solution, boil a few cups of water. Take the water off the boil and put in 4 – 5 tablespoons of borax. Stir and allow to stand until cool. Pour off the clear liquid and you have a saturated solution of borax. The sediment in the bottom can be added to more hot water to make more of the borax solution. You will have to break up the remaining crystals of borax to enable suspension in the hot water.
You can spray this solution onto the glass, just as the commercial sprays. Or you can brush it on as you do kiln wash on a shelf. It requires an even application to ensure there are no streaks left on the finished glass.
This works because borax is one of the fluxes used in glass making to reduce the melting temperature of glass batch and so serves to soften the surface of the glass enough to overcome mild devitrification.
 |
| borax in powder form |
It is possible to make your own devitrification solution. It is made from borax which you can buy from your local chemicals supplier, or sometimes as a washing powder – but make sure it has no additives!
 |
| An example of a borax washing powder |
To make a solution, boil a few cups of water. Take the water off the boil and put in 4 – 5 tablespoons of borax. Stir and allow to stand until cool. Pour off the clear liquid and you have a saturated solution of borax. The sediment in the bottom can be added to more hot water to make more of the borax solution. You will have to break up the remaining crystals of borax to enable suspension in the hot water.
Add a couple of drops of washing up liquid to the solution. This is enough to break the solution's surface tension. It helps to give an even distribution of the solution across the clean glass by reducing the surface tension and therefore, beading of the liquid that otherwise occurs.
You can spray this solution onto the glass, just as the commercial sprays. Or you can brush it on as you do kiln wash on a shelf. It requires an even application to ensure there are no streaks left on the finished glass.
This works because borax is one of the fluxes used in glass making to reduce the melting temperature of glass batch and so serves to soften the surface of the glass enough to overcome mild devitrification.
https://glasstips.blogspot.com/2016/02/borax-characteristics.html
https://glasstips.blogspot.com/2009/06/borax-solutions.html
Revised June 2018
Revised June 2018
Polarising Filters
Using polarized light filters to show stress works on the principle that stressed glass rotates the polarisation direction of the light as it comes through the glass. As polarized light filters placed at right angles do not allow any light through, only unstressed glass will continue to appear dark.
If there is stress the light is rotated slightly and becomes visible through the filters.
You can buy stress testing kits that incorporate a light source. You can also make your own. You need polarizing lighting gels. These come in sheets and are available from theatrical lighting sources. You will need to frame these in stiff card to keep them flat.
You use them over a light source. Place one filter down above the light source. Place the piece to be tested on top. Then orient the top filter so that the minimum amount of light shows through the filters. Any stress will show up as a light source. The amount of light rotation depends on the stress direction, magnitude and light path length. The greater the intensity of the glow, the greater the stress the glass is exhibiting. The amount light visible through the filters is wavelength dependent, as the filter transmits light with a particular polarisation direction. If there is large stress, different colours will be visible.
 |
| This example shows extreme stress by the rainbow effect of light rotated in multiple directions |
Also note that the filters are normally on plastic sheets and easily scratched, so the glass should always be lifted and placed, rather than slid, to a new position.
A description of the compatibility test can be seen here.
revised June 2018
Tuesday 15 May 2018
Tin Bloom
Using float glass sometimes produces partial clouding as
though devitrification were present. Although float glass is prone to
devitrification, not all the cloudy film on the surface is due to
devitrification.
Float glass, which these days, is almost all clear smooth
glass, gets its name from the process of floating the glass on molten tin. The
tin in compression gives an apparent devitrification effect which is called tin
bloom.
it is different from devitrification, to which float glass is
particularly subject. Devitrification sprays and solutions will not have an
effect on this surface defect called tin bloom.
When the tin layer is stretched, it does not create a tin
bloom on the surface. Therefore, it is
important to have a means to detect which is the tin surface. Always fire the glass with the tin in the same
relative location to each other. I.e.,
on several layers of glass have all the tin side down or all up, but not mixed.
This example of a test by Glass Art by Margot shows the tin bloom on the outer portions of the platter where the tin side was up, causing the tin too be compressed and show. The flatter portion of the piece did not show this tin bloom as there was not the same extent of compression. You can visit the description of the experiment here.
When forming the glass (slumping, draping, kiln carving) make sure the tin sides will be stretched rather than compressed. Of course, you can take advantage of the tin bloom by controlling the compression of the tin layers.
When forming the glass (slumping, draping, kiln carving) make sure the tin sides will be stretched rather than compressed. Of course, you can take advantage of the tin bloom by controlling the compression of the tin layers.
Wednesday 11 April 2018
Copper inclusions
Inclusions of metals can be achieved with care. Copper is a very good metal, as it is soft, even though its expansion characteristics are very different from glass. This note provides some things you might consider when planning to include copper in your fused pieces.
The copper sheet should be stiff, but not thick. If the metal can be incised with a scribe and maintain that through gentle burnishing, it is suitably thick. The usual problem is that the copper is too thick rather than too thin. Copper leaf can be very faint if a single layer is used. Placing several layers of leaf improves the colour, but often provides wrinkles. In summary, the requirement is to get a thickness of copper that will retain its structure, but not be so thick and stiff as to hold the glass up during the fusing process.
Do not use the copper foil as used for stained glass applications. The adhesive backing produces a black colour from the adhesive and many bubbles - sometimes a single large one.
Copper can provide several colours.
Copper sheet normally turns burgundy colour when oxidised. This means that there is enough air reaching the copper to oxidise it to deep copper red. This most normally happens, because a lot of air can contact the metal during the extensive bubble squeeze usually given to inclusions.
To keep the copper colour, clean the metal well metal well with steel wool or a pot scrubber. If you use steel wool, wash and polish dry the metal before fusing. Reduction of air contact with the metal helps to retain the copper colour. There are two methods I have used. Addition of a glass flux like borax or other devitrification spray will help prevent the air getting to the surface. Another method of avoiding oxidisation, is to cover the copper with clear powdered frit, as well as the surrounding glass.
In certain circumstances you can get the blue green verdigris typical of copper in the environment. This is an extent of oxidisation that is between the clean copper coloured metal and the burgundy colour of extensive oxidisation. The key seems to be be a combination of restricted air supply, shorter bubble squeezes and lower temperatures. Experimentation is required to achieve this consistently.
The spaces under and over the copper give the opportunity for bubbles to form.
This means that the copper needs to be as flat as possible for one thing. Burnishing the copper can have a good effect on reducing the undulations in the copper. Thinner copper is easier to make flat than thicker. If you can stamp a shape from the copper with a stamper designed for card making, it is a good indication that it will burnish flat. Thicker copper sheet holds the glass up long enough in the temperature rise during the bubble squeeze to retain air around the metal. This remains the case even after burnishing to be as flat as possible.
The second element that can help to reduce bubbles around the copper is to sprinkle clear powder over the copper sheet once in place on the glass. The spread of the powder over the glass assists in giving places for the air between layers to escape.
These two things combined with a long slow squeeze can reduce the amount of bubbles you get. It cannot totally eliminate them.
Of course, a longer bubble squeeze allows air to be in contact with the copper and promotes the change to a blue green or burgundy colour.
The copper sheet should be stiff, but not thick. If the metal can be incised with a scribe and maintain that through gentle burnishing, it is suitably thick. The usual problem is that the copper is too thick rather than too thin. Copper leaf can be very faint if a single layer is used. Placing several layers of leaf improves the colour, but often provides wrinkles. In summary, the requirement is to get a thickness of copper that will retain its structure, but not be so thick and stiff as to hold the glass up during the fusing process.
Do not use the copper foil as used for stained glass applications. The adhesive backing produces a black colour from the adhesive and many bubbles - sometimes a single large one.
Copper can provide several colours.
Copper sheet normally turns burgundy colour when oxidised. This means that there is enough air reaching the copper to oxidise it to deep copper red. This most normally happens, because a lot of air can contact the metal during the extensive bubble squeeze usually given to inclusions.
To keep the copper colour, clean the metal well metal well with steel wool or a pot scrubber. If you use steel wool, wash and polish dry the metal before fusing. Reduction of air contact with the metal helps to retain the copper colour. There are two methods I have used. Addition of a glass flux like borax or other devitrification spray will help prevent the air getting to the surface. Another method of avoiding oxidisation, is to cover the copper with clear powdered frit, as well as the surrounding glass.
In certain circumstances you can get the blue green verdigris typical of copper in the environment. This is an extent of oxidisation that is between the clean copper coloured metal and the burgundy colour of extensive oxidisation. The key seems to be be a combination of restricted air supply, shorter bubble squeezes and lower temperatures. Experimentation is required to achieve this consistently.
The spaces under and over the copper give the opportunity for bubbles to form.
This means that the copper needs to be as flat as possible for one thing. Burnishing the copper can have a good effect on reducing the undulations in the copper. Thinner copper is easier to make flat than thicker. If you can stamp a shape from the copper with a stamper designed for card making, it is a good indication that it will burnish flat. Thicker copper sheet holds the glass up long enough in the temperature rise during the bubble squeeze to retain air around the metal. This remains the case even after burnishing to be as flat as possible.
The second element that can help to reduce bubbles around the copper is to sprinkle clear powder over the copper sheet once in place on the glass. The spread of the powder over the glass assists in giving places for the air between layers to escape.
These two things combined with a long slow squeeze can reduce the amount of bubbles you get. It cannot totally eliminate them.
Of course, a longer bubble squeeze allows air to be in contact with the copper and promotes the change to a blue green or burgundy colour.
Sunday 4 March 2018
CoE Varies with Temperature
Information from Bullseye shows that the Coeficient of Linear Expansion changes rapidly around the annealing range.
The following is from results of a laboratory test of Bullseye clear (1101F)
Temperature range.......................COE
20C-300C (68F - 572F).................90.6
300C-400C (572F - 752F).............102.9
400C-450C (752F - 842F).............107.5
570C-580C (1058F-1076F)............502.0
Bullseye glass is probably typical of soda lime glasses designed for fusing.
The change of CoE by temperature is further illustrated by Kugler (a blowing glass) who state their CoE by temperature range. Remember CoE is an average expansion over a stated range of temperatures)
CoE 93 for the range 0C-300C
CoE 96 for the range 20C - 300C
CoE 100 for the range 20C - 400C
The extension of the range by 100C has a distinct effect on the average expansion over the (larger) range.
This shows why it is not helpful to refer to CoE without also mentioning the range of temperature.
In addition, here is an illustration of the effect.
(If the owner of this illustration comes across this, please let me know, as I have lost the source)
The following is from results of a laboratory test of Bullseye clear (1101F)
Temperature range.......................COE
20C-300C (68F - 572F).................90.6
300C-400C (572F - 752F).............102.9
400C-450C (752F - 842F).............107.5
570C-580C (1058F-1076F)............502.0
Bullseye glass is probably typical of soda lime glasses designed for fusing.
The change of CoE by temperature is further illustrated by Kugler (a blowing glass) who state their CoE by temperature range. Remember CoE is an average expansion over a stated range of temperatures)
CoE 93 for the range 0C-300C
CoE 96 for the range 20C - 300C
CoE 100 for the range 20C - 400C
The extension of the range by 100C has a distinct effect on the average expansion over the (larger) range.
This shows why it is not helpful to refer to CoE without also mentioning the range of temperature.
In addition, here is an illustration of the effect.
(If the owner of this illustration comes across this, please let me know, as I have lost the source)
Wednesday 30 August 2017
Comparisons of "CoE" and Temperatures
This table shows the lack of correlation between CoE and temperature characteristics of the glasses. See the previous post for the discussion.
| Nominal | Temperatures (celsius) | ||||
| Manufacturer | CoE | anneal | slump | full fuse | |
| Pilkington UK Float | 83 | 540 | 720 | 835 | |
| USA Float | 83 | 548 | 720 | ||
| Australian Float | 84 | 505-525 | |||
| Wissmach 90 | 90 | 482 | 638 | 771 | |
| Bullseye | 90 | 482 | 630-677 | 804 | |
| Uroboros FX90 | 90 | 525 | 649-677 | 771-788 | |
| Kokomo | 93 | 507-477 | 565 | ||
| Artista | 94 | 535 | 565 | ||
| Spectrum | 96 | 510 | 663 | 796 | |
| Uroboros | 96 | 510 | 664 | 767-774 | |
| Wissmach 96 | 96 | 510 | 638 | 771 | |
| Sorted by annealing point, averaged
as necessary CoE Anneal Slump Full fuse |
|||||
| Kokomo | 93 | 482 | 565 | ||
| Wissmach 90 | 90 | 482 | 638 | 77 | 1 |
| Spectrum | 96 | 510 | 663 | 796 | |
| Uroboros | 96 | 510 | 664 | 771 | (ave) |
| Wissmach 96 | 96 | 482 | 638 | 77 | 1 |
| Australian Float | 84 | 515 | |||
| Bullseye | 90 | 482 | 654 | 804 | (ave) |
| Uroboros FX90 | 90 | 525 | 663 | 780 | (ave) |
| Artista | 94 | 535 | 565 | ||
| Pilkington UK Float | 83 | 540 | 720 | 835 | |
| USA Float | 83 | 548 | 515 | ||
| Sorted by Slump point, averaged as
necessary CoE Anneal Slump Full fuse |
|||||
| USA Float | 83 | 548 | 515 | ||
| Artista | 94 | 535 | 565 | ||
| Kokomo | 93 | 492 | 565 | (ave) | |
| Bullseye | 90 | 482 | 654 | 804 | (ave) |
| Spectrum | 96 | 510 | 663 | 796 | |
| Uroboros FX90 | 90 | 525 | 663 | 780 | (ave) |
| Uroboros | 96 | 510 | 664 | 771 | (ave) |
| Wissmach 90 | 90 | 482 | 638 | 77 | 1 |
| Wissmach 96 | 96 | 482 | 638 | 771 | |
| Pilkington UK Float | 83 | 540 | 720 | 835 | |
| Australian Float | 84 | 515 | (ave) | ||
| Sorted by full fuse, averaged as necessary | |||||
| Uroboros | 96 | 510 | 664 | 771 | (ave) |
| Wissmach 90 | 90 | 482 | 638 | 771 | |
| Wissmach 96 | 96 | 482 | 638 | 771 | |
| Uroboros FX90 | 90 | 525 | 663 | 780 | (ave) |
| Spectrum | 96 | 510 | 663 | 796 | |
| Bullseye | 90 | 482 | 654 | 804 | (ave) |
| Pilkington UK Float | 83 | 540 | 720 | 835 | |
| Artista | 94 | 535 | 565 | ||
| USA Float | 83 | 548 | 515 | ||
| Australian Float | 84 | 515 | (ave) | ||
| Kokomo | 93 | 492 | 565 | (ave) | |
Saturday 15 July 2017
Needling
Needling is a description of the fine points emerging from the edges of glass.
This occurs in two conditions mainly.
The one that is most commonly seen is in the fusing of single layers of glass. The surface tension of the glass pulls the glass in from its original size, trying to achieve the 6-7mm that is a thickness equilibrium at full fusing temperatures. If the surface the glass is resting on has any rough areas, and most surfaces do, some of the glass will stick and the rest retract. This leaves short, thin and extremely sharp “needles” extending from the edges.
Two common surfaces allow these sharp edges. Fibre paper of 0.5mm and greater is rough enough to allow the hot glass to stick to tiny depressions in the paper. Kiln wash is often not smooth enough to prevent this kind of sticking either. You can smooth powdered kiln wash or aluminia hydrate over these surfaces to reduce the grabbing of the surface by the hot glass. However, the powder is often drawn back with the contracting glass. Thinfire or Papyros paper is fine enough to avoid the needling most of the time without any addition of powders.
The other main condition is in casting, mainly box casting or damming. In this case, the stack of glass sheets or cullet is higher before firing than its final thickness. This means the glass flows out to the dams and sinks down to its final thickness during the firing process. As the glass touches the fibre paper or other separator it behaves just as the single layer of glass does. Some sticks to the surface while the rest is dragged away by the surface tension and reducing thickness of the stack of glass.
Prevention of Needling
Lining dams
Separators for dams
This occurs in two conditions mainly.
The one that is most commonly seen is in the fusing of single layers of glass. The surface tension of the glass pulls the glass in from its original size, trying to achieve the 6-7mm that is a thickness equilibrium at full fusing temperatures. If the surface the glass is resting on has any rough areas, and most surfaces do, some of the glass will stick and the rest retract. This leaves short, thin and extremely sharp “needles” extending from the edges.
Two common surfaces allow these sharp edges. Fibre paper of 0.5mm and greater is rough enough to allow the hot glass to stick to tiny depressions in the paper. Kiln wash is often not smooth enough to prevent this kind of sticking either. You can smooth powdered kiln wash or aluminia hydrate over these surfaces to reduce the grabbing of the surface by the hot glass. However, the powder is often drawn back with the contracting glass. Thinfire or Papyros paper is fine enough to avoid the needling most of the time without any addition of powders.
The other main condition is in casting, mainly box casting or damming. In this case, the stack of glass sheets or cullet is higher before firing than its final thickness. This means the glass flows out to the dams and sinks down to its final thickness during the firing process. As the glass touches the fibre paper or other separator it behaves just as the single layer of glass does. Some sticks to the surface while the rest is dragged away by the surface tension and reducing thickness of the stack of glass.
Prevention of Needling
Lining dams
Separators for dams
Wednesday 17 May 2017
Compatibility Tests
These procedures are based on the observation that glasses compatible with the base glass are compatible with each other. This means that you can test opaque colours’ compatibilities with each other by testing each of them on clear strips.
Annealing test
These tests must be combined with an annealing test. This consists of putting two pieces from the same sheet of glass together - so you know they are compatible - and firing them along with your compatibility test.
Viewing the results of your annealing through the polarised filters shows whether there is stress left in your annealing. If there is, the compatibility tests are inconlusive as there is no difference in appearance of stress whether from incompatibility or from inadequate annealing. Once you have the annealing right, you can then interpret the compatibility tests done at the same time.
Strip test
Cut a strip of base glass 75mm/3" wide and as long as convenient for you or your kiln.
Cut clear glass squares of 25mm/1" to separate the colours.
Cut 25mm/1" squares of the colours to be tested.
Start with a clear square at one end of the clear strip and alternate colours and clear along the strip finishing with a clear square.
Annealing test
These tests must be combined with an annealing test. This consists of putting two pieces from the same sheet of glass together - so you know they are compatible - and firing them along with your compatibility test.
Viewing the results of your annealing through the polarised filters shows whether there is stress left in your annealing. If there is, the compatibility tests are inconlusive as there is no difference in appearance of stress whether from incompatibility or from inadequate annealing. Once you have the annealing right, you can then interpret the compatibility tests done at the same time.
Strip test
Cut a strip of base glass 75mm/3" wide and as long as convenient for you or your kiln.
Cut clear glass squares of 25mm/1" to separate the colours.
Cut 25mm/1" squares of the colours to be tested.
Start with a clear square at one end of the clear strip and alternate colours and clear along the strip finishing with a clear square.
Place two strips 25mm/1" wide either side of the clear and coloured squares.
Add a stack of two layers of clear to the kiln before firing as a test for adequate annealing. If the annealing is inadequate, then the whole test is invalid.
Test the result with polarising filters. Start with the clear annealing test square. If no stress is apparent, go to the test strip. But if stress is apparent in the annealing test, look to your annealing schedule as something needs to change. Usually the requirement is a combination of a longer soak at the annealing temperature and a slower annealing cool.
To test for compatibility, look carefully for little bits of light in the clear glass surrounding the colour. These are indications of stress – the more light or the bigger the halo, the greater the stress. Really extreme stress appears to be similar to a rainbow, although without the full spectrum.
You can use this test to determine if you annealing is satisfactory for larger pieces. In this case you should use at least 100mm squares. Stack them to the height of your planned project and dam them with fibre board or other refractory materials to prevent spread. Fire to full fuse and anneal. When cool check for stresses.
The tile method looks at compressive factors too.
Cut a 100mm/4" square clear tile
Cut two strips of glass 25mm/1" wide and 100mm/4" long for each test
Cut two rectangles of 25mm by 50mm (1" by 2") of the same glass for the two remaining sides
Cut a square of 50mm/2" for the centre. The glass in the middle is normally the test glass. To be very certain of what has happened you can do the reverse lay up at the same time. You put coloured glass around the outside, but in this case the inside needs to be clear or transparent. At least one element needs to be transparent enough to view the stress patterns, if any. So you could have a clear middle and black exterior, and vice versa.
This test is a more time consuming process and you may wish to use it only for larger projects.
Also look at the use of polarising filters
Add a stack of two layers of clear to the kiln before firing as a test for adequate annealing. If the annealing is inadequate, then the whole test is invalid.
 |
| In this test there is very mild stress showing from the dark brown and dark green. This is well within the limits for compatibility. |
To test for compatibility, look carefully for little bits of light in the clear glass surrounding the colour. These are indications of stress – the more light or the bigger the halo, the greater the stress. Really extreme stress appears to be similar to a rainbow, although without the full spectrum.
You can use this test to determine if you annealing is satisfactory for larger pieces. In this case you should use at least 100mm squares. Stack them to the height of your planned project and dam them with fibre board or other refractory materials to prevent spread. Fire to full fuse and anneal. When cool check for stresses.
The tile method looks at compressive factors too.
Cut a 100mm/4" square clear tile
Cut two strips of glass 25mm/1" wide and 100mm/4" long for each test
Cut two rectangles of 25mm by 50mm (1" by 2") of the same glass for the two remaining sides
Cut a square of 50mm/2" for the centre. The glass in the middle is normally the test glass. To be very certain of what has happened you can do the reverse lay up at the same time. You put coloured glass around the outside, but in this case the inside needs to be clear or transparent. At least one element needs to be transparent enough to view the stress patterns, if any. So you could have a clear middle and black exterior, and vice versa.
This test is a more time consuming process and you may wish to use it only for larger projects.
Also look at the use of polarising filters
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