Relieving Stress at Corners

The most frequent locations of high stress in a piece is at corners or points.  The stress seems to be concentrated there and thus they become the most vulnerable parts of the piece.

Although the above image is of a plastic drawing triangle, it illustrates the point. The stresses are concentrated at the points and right angles whether inside or at the edge. The rainbow effect of some of the stress points show that those are the location of extreme stress.  If you see any of that in your glass, you need to check for compatibility and certainly anneal it again more slowly if it is compatible.  Remember though: slow annealing of incompatible glass will not enable incompatible glasses to fit together and become compatible.

Of course, the main thing that we do is to ensure the anneal is adequate to reduce the stress at these points.  It is important in a piece that has points, right angles and other abrupt changes in angle that you are more conservative in your annealing soak and cool. 

Further, if you are tack fusing, the stresses will be greater than on a full fuse. This is because the pieces of glass are not fully incorporated and tend to expand and contract independently of each other and of the main piece.  Also, the lower glass is shaded from the heat by the upper pieces on heat up. On cool down, the lower glass looses heat more slowly.  These two main effects, although there are others, require that the annealing is done much more slowly - two to four times more slowly than a piece of the same thickness.

One simple means of reducing stress before the start of the fusing process is to nip the corners off.  And slightly round the internal angles.  This requires only a very small piece to be taken from the corner or point to reduce the stress in the final piece. This is particularly important in tack fusing projects.

This nipping of the corners also removes the frequentl sharp points that some right and more acute angles develop during the cool down.  Glass, even of 6mm and more expands with the heat of the fusing.  As it cools toward the annealing temperature, it contracts.  The glass at the corners has to contract further than the edges, and so leaves a sharp point where it was unable to fully round. Removing only a small piece of glass from the corner removes enough mass to counteract this effect of contraction.

Hot Short Firings - Kiln Forming Myths 28

The hottest temperature for the least time always gives you best results.

It is difficult to imagine where or how this instruction arose.  Just as “low and slow” is not always the answer, so this also has its application, but not as a general practice.

In general, I try to get my fusing work done in 10 minutes at the working temperature.  Any less time there and I feel I am trying to go too fast. 

Advancing very fast normally requires a higher temperature than a slow advance, to get the same result.  Also with a higher temperature you do not need to have as long a soak as at a lower temperature.

It is more difficult to get repeatable results with fast firings.  A more controlled rate of advance will allow the controller to cope with any variations (e.g., power, or mass of material being fired) present. 

But you need to know why you are doing the AFAP for as short a time as possible.  It can be useful for small and jewellery scale items.  It certainly is not applicable to larger or thicker items. 

For slumping, it may be that the reverse of the headline suggestion could be the appropriate response.  Slow advances allow the glass to gently conform to the mould without excessive stretching.  This is also helped by using a low temperature and a long soak. 

These observations show that the injunction may be appropriate for some work, but most kiln work is better done with a slower, lower, longer approach.  This means slower rates of advance, lower target temperatures, longer soaks.

Spacing of Pieces on the Shelf

It is natural that we should want to put as much onto the shelf as we can to maximise the number of pieces from each firing.  But, when you are placing the pieces remember that glass expands as it heats up. When the glass is at its maximum expansion, it will be much less viscous than at lower temperatures and so will stick very easily to any neighbouring piece it touches.

Although the final size of a two-layer piece is the same at the end as the beginning, they do expand to a larger size during the fusing process.  My experience shows me that a 6mm piece can expand as much as 5mm, depending both on temperature and size.  This means that I treat 10mm as the absolute minimum space between pieces. But, because of the size of my fingers, my normal minimum placing is 20 mm apart as that is a comfortable space between my fingers and the other glass.

Thicker pieces expand to become larger after fusing than they were at the start. These pieces spread more during the firing than the 6mm piece.  A 9mm piece may expand by about 3mm at the finish – depending on size and temperature.  But during the firing, it may expand as much as 9mm. This means that 20mm is an absolute minimum between pieces that are 9mm thick at the edges, even though they may be only 6mm over most of the area.

The tip is to avoid over-filling your kiln shelf.  By trying to get too much production in one firing you may find a number of pieces stuck together at the end, thus eliminating any savings on glass or space. 

Didymium Lamp Working Glasses - Kiln Forming Myths 27

Lamp working glasses can be used to look into the kiln at high temperatures.

Definitely not! 

Didymium glasses are used by lamp workers to protect eyes against sodium flare – the yellow glow coming off the glass in the torch flame.

In kiln forming, the radiation that our eyes need protection against is infrared.  Welders’ goggles do this, but didymium glasses do not.  Welders goggles and helmets are much cheaper too.

Edge Working Options for Glass

There are a number of standard options for the worked shape of edges.  The simplest is to have a seamed edge, where just enough sanding is done to take the sharpness from the edge.

The next is to have an arris where more glass is removed, usually as a chamfer, but sometimes in a rounded, bullnose effect.  These are commonly used for glass that is to be toughened.

Flat chamfered and often polished edges are quite common also.

Bevelled glass is very common on mirrors as this reduces the reflection of the inside of the frame holding the glass.

As you can see from the attached illustration, there are a number of standard edge treatments, although some of them are uncommon.

The seamed, arrised and flat polished edges are easiest to create by hand grinding.  The other more fancy edges require machines.

Slump Point Test

Revised 7.7.21

At a time when we are all going to be trying a variety of glass of unknown compositions to reduce costs of kiln working, the knowledge of how to determine the slump point temperature (normally called the softening point in the glass manufacturing circles) and the approximate annealing temperature becomes more important.  This is called the slump point test.

This test can be used to determine both the slumping point and the annealing soak temperature. This used to be required when the manufacturers did not publish the information. It continues to be useful for untested glasses.

The method requires the suspension at a defined height of a strip of glass, the inclusion of an annealing test, and the interruption of the schedule to enter the calculated annealing soak temperature.

A strip of 3 mm transparent glass is required. This does not mean that it has to be clear, but remember that dark glass absorbs heat differently from clear or lightly tinted glass. The strip should be 305 mm x 25 mm.  If you are testing bottles, you may find it more difficult to get such a long strip.  My suggestion is that you cut a bottle on a tile saw to give you a 25 mm strip through the length of the bottle.  Do not worry about the curves, extra thickness, etc.  Put the strip in the kiln and take it to about 740C to flatten it. Reduce the temperature to about 520C to soak there for 20 minutes.  Then turn the kiln off.  

Suspend the strip 25 mm above the shelf, leaving a span of 275 mm. This can be done with kiln brick cut to size, kiln furniture, or a stack of fibre paper.   Make sure you coat any kiln furniture with kiln wash to keep the glass from sticking.

The 305mm strip suspended 25mm above the shelf with kiln furniture.

Place some kiln furniture on top of the glass where it is suspended to keep the strip from sliding off the support at each end. Place a piece of wire under the centre of this span to make observation of the point that the glass touches down to the shelf easier.

The strip held down by placing kiln furniture on top of the glass, anchoring it in place while the glass slumps.

Also add a two layer stack of the transparent glass near the suspended strip of glass to act as a check on whether the annealing soak temperature is correct. This stack should be of two pieces about 100 mm square. If you are testing bottles, a flattened side will provide about the same thickness.  This process provides a check on the annealing temperature you choose to use.  If the calculated temperature is correct there should be little if any stress showing in the fired piece.

The completed test set up with an annealing test and wire set at the midpoint of the suspended glass to help with determining when the glass touches down.

The schedule will need to be a bit of guess work.  The reasons for the suggested temperatures are given after this sample initial schedule which will need to be modified during the firing.
Ramp 1: 200C per hour to 500C, no soak
Ramp 2: 50C per hour to 720C, no soak
Ramp 3: 300C per hour to 815C or 835C, 10 minute soak
Ramp 4: 9999 to 520C, 30 minute soak
Ramp 5: 80C per hour to 370C, no soak
Ramp 6: off.

Fire at a moderate rate initially – 200C/hr to 500C - and then at 50C/hr until the strip touches down. This is to be able to accurately record the touch down temperature.  If you fire quickly, the glass temperature will be much less than the air temperature that the pyrometer measures.  Firing slowly allows the glass to be nearly the same temperature as the air.  

Observe the progress of the firing frequently from 500C onward, unless it is float glass you are testing. Then you can start observing from about 580C. Record the temperature in Celsius when the middle of the glass strip touches the shelf. The wire at the centre of the span will help you determine when the glass touches down.  This touch down temperature is the slump point of your glass.  You now know the temperature to use for gentle slumps with a half hour soak.  More angular slumps will require a higher temperature or much more time.

Once you have recorded the slump point temperature, you can skip to the next ramp (the fast ramp 3).  This is to proceed to a full fuse for soda lime glasses. Going beyond tack fusing temperatures is advisable, as tack fuses are much more difficult to anneal and so may give an inaccurate assessment of the annealing. Most glasses, except float, bottles and borosillicate will be fully fused by 815C. If it is float, bottles or borosilicate that you are testing, try 835C. If it is a lead bearing glass, lower temperatures than the soda lime glass should be used. In all these cases observation at the top temperature will tell you if you have reached the full fuse temperature. If not add more time or more heat to get the degree of fuse desired.

While the kiln is heating toward the top temperature you can do the arithmetic to determine the annealing point.  To do this, subtract 40C from the recorded touch down temperature to obtain an approximate upper annealing point.  The annealing point will be 33C below the upper point.  This is approximate as the touch down temperature is by the nature of the observation also approximate.  

The next operation is to set this as the annealing soak temperature in the controller. This will be the point at which it usually possible to interrupt the schedule and change the temperature for the annealing soak that you guessed at previously. Sometimes though, you need to turn the controller off and reset the new program.  Most times the numbers from the last firing are retained, so that all you need to do is to change the annealing soak temperature.

The annealing soak should be for 60 minutes to ensure an adequate anneal. This may be excessive for 3 mm glass, but as the anneal test is for 6 mm, the longer soak is advisable. The annealing cool should be 83C/hr down to 370C. This is a moderate rate which will help to ensure the annealing is done properly. The kiln can be turned off at that temperature, as the cooling of the kiln will be slow enough to avoid any thermal shock to the annealing test piece.

When cooled, check the stack for stress. This is done by using two polarised light filters. See here for the method. 

Squares of glass showing different levels of stress from virtually none to severe
 (no light emanating for no stress to strong light from the corners indicating a high degree of stress.)

If the anneal test piece is stressed there is a problem. There could be a number of reasons for the inadequate annealing. It could be that the glass has devitrified so much that it is not possible to fuse this glass at all. If you also test the suspended strip for stresses and there is very little or none, it is evidence that you can kiln form single layers of this glass. You now know the slumping temperature and a suitable annealing temperature and soak for it, even though fusing this glass is not going to be successful.

Other reasons for stress due to inadequate annealing could be that the observations or calculations were incorrect.  

• Of course, before doing any other work, you should check your arithmetic to ensure the calculations have been done correctly. I'm sure you did, but it is necessary to check.  If they are not accurate, all the following work to discover the difficulties will be fruitless.

• The observation of the touch down of the suspended strip can vary by quite a bit - maybe up to 15C. To check this, you can put other annealing test pieces in the kiln.  This will require multiple firings using temperatures in a range from 10C above to 10C below your calculated annealing soak temperature to find an appropriate annealing soak temperature.

• If stress is still showing in the test pieces after all these tests, you can conduct a slump point test on a strip of glass for which there are known properties. This will show you the look of the glass that has just reached touch down point as you know it will happen at 73C above the published annealing point.  You can then apply this experience to a new observation of the test glass. 

Fast Ramps - Kiln Forming Myths 26

Firing AFAP harms your kiln.

This may be a hangover from the time when ceramic kilns were being used commonly.  There certainly is a tradition of this kind in ceramics practice.  However, nowadays we are firing in kilns with light weight bricks or fibre, or a combination of the two, making this less relevant.

The light weight bricks are much less subject to temperature shock than the dense ones.  Fibre is completely unaffected by rapid changes in temperature.

Firing as fast as possible is much more likely to damage the glass you have in the kiln than the kiln itself.  It is also likely to have over runs in temperature.  The controllers compare the actual increase in temperature with that requested by the schedule.  It takes time for the controller to “learn” the rate of advance being achieved within the kiln.  On fast rises in temperature, it does not have the capacity to stop the input of energy early enough to prevent the kiln temperature rising beyond that which is programmed.  This can lead to unexpected and unexplained results (unless you think about the effects of an AFAP rate on the controller's computer).

Dog boning in Slumping

Often even in shallow rectangular moulds the sides pull in during the slump.  To know what things to try to correct this effect, you need to understand why this effect is occurring.  These two pieces show the effect in different ways.

ebay 0916_slump_01

 This slump shows that even with thick glass the sides curve inwards even on shallow slumps.

theglassundergroundnj.org

This slump shows the interesting effect that the further up the piece you look, the greater the curvature. This relates to the greater amount of movement required by the glass to conform to the mould at the outer edges.

Why

During the slump of a rectangle or square the whole shape of the glass sheet is changing.  It is slightly stretching to form into the “hollow” of the mould, but it cannot stretch evenly all over, especially at the corners.  If you think of the analogy of Draping a piece of cloth into a rectangular depression, you will find it wrinkles up at the corners if you smooth it at the sides. This indicates the material is attempting to overlap there as it does not have a dart to take up the excess cloth.

This similar to what is happening to the glass sheet.  It is relatively thicker at the corners than along the sides.  Therefore, it does not slide down the mould at the corners as on the sides. It is simply thicker and is compressed by the movement of the glass at the sides.

Prevention

The question is how to use that knowledge to avoid or minimise the dog boning during the slump.  There are probably lots of methods, but three have occurred to me and others.

Add more material along the sides.  This involves fusing a piece with shallow arcs rather than straight sides.  This gives more material to counteract the dog boning effect when slumping a rectangle.  The difficulty is getting the proportions of the arc correct in relation to the length of the sides. You also need to ensure the arcs on the sides are not so much larger than the mould that they slump over the edge.  This means the whole piece will need to be cut smaller than the mould.

Remove material at the corners.  This takes the opposite approach.  To avoid the increased amount of glass at the corners, you remove some of it.  That is, you round the corners of the pieces to be fused. How much you will need to round the corners is a matter of experience, but is a shorter learning curve than cutting the edges in an arc.

Reduce the temp and increase soak time.  This approach requires less skill in cutting a shape.  It relies on giving the glass time to relax into mould with a minimum of stretch.  You need to find the lowest practical temperature at which to slump.  This will be the temperature at which you can first see the deformation of the glass in the mould.  Hold the temperature there for as long as it takes – possibly one or two hours. It is likely that you will still need some rounding of the corners of the glass, but only your experience will determine that, and if so how much.

Cold work the edges until straight.  This can be done by hand or by machine.

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

You Can Re-fire 3 Times Only - Kiln Forming Myths 25

Bullseye claims that you should only fire a piece 3 times

       

No. They only say the glasses are tested three times and that you are on you own after that.

There is not a general answer that can be given for the number of times you can fire a piece.  In general, Bullseye glass (and probably others, although they do not state what their limits of confidence are) can be fired three times with confidence.  Beyond that you need to do your own testing.

Bullseye states: 

At Bullseye, glasses known to be fairly stable are tested by firing to a top temperature of 1500°F (815°C) and soaking for 15 minutes before annealing. Once cooled, these tests are viewed for stress through polarized light and graded accordingly. We fire glasses known to be less stable three times to make sure they'll perform well under multiple firing conditions, such as those used to fuse and slump a plate.

If you have plans for multiple re-firings, tests are needed. The tests should replicate the temperatures, colours and thickness of the proposed project.  You probably do not need to reproduce the size of the project in these tests though.

Results from each firing should be tested for stress and these tests should include a test for annealing each time. 

You may wish to note that I have fired up to 7 times on several two layer with powder pieces.  Many people fire more times successfully.  It is my belief, but I have no proof, that multiple firings of a piece to slightly lower than full fuse will be more successful than each of them being to the full fuse.  My practice is to go to a rounded tack each of the firings subsequent to the first full fuse, but the final firing will be to a full fuse if I wish a gloss finish.  If I do not, my final firing will be about 10C - 15C below full fuse.

Dog Boning Causes

I fired a one-layer piece of glass and it shrank. What did I do wrong?

Cause

This result relates to the thickness that glass, under kiln forming circumstances achieves.  The combination of gravity and viscosity lead to this effect.  As the glass becomes less viscous (more runny), the surface tension is greater than gravity and so it becomes thicker at the edges.  This additional glass is supplied from the edges and to some extent from the interior. The glass in the middle becomes thinner, allowing in certain circumstances bubbles or holes to appear.

This illustration from Fusedglass.org shows the effects of gravity, which is related to mass, and viscosity.  The lack of mass means the surface tension allows the glass to draw up to be come thicker, forming the classic dog boning appearance.

Prevention

Knowing why this occurs allows you to take come precautions, when firing single layer pieces, to help prevent the shrinkage, often known as dog boning.

Fire larger

You can cut the glass larger than the final piece will be.  After firing, you cut it down to the size you want.  You may have to do a bit of cold working to get a rounded edge to the glass before any further processing.

Fire lower  

You can fire at a lower temperature for a longer time.  You will need to observe to determine when the glass begins to shrink. Either stop the temperature rise and soak there for a time, or reduce the temperature a little and soak for as long as needed to get the surface texture wanted.

Fire oval or circular pieces.  

With these shapes the shrinking is not so obvious, as it occurs all the way around.  With rectangular pieces, as the glass shrinks, the corners become thick more quickly and so do not shrink as much, giving that dog bone appearance.  Rounded pieces become thicker all the way around more evenly and the shrinkage is not so obvious.  However, you still get thinning in the interior which can lead to holes or bubbles, so observation is still necessary to prevent excessive thinning and bubble formation.

Fire thicker

The real prevention is to fire two layer pieces as that is the thickness at which viscosity, surface tension and gravity are in balance.  So the glass does not change size at kiln forming temperatures.

Cold work

Alternatively, you can cold work the edges back to straight parallel edges.  This can be done by hand grinding or by machine.

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

Pre-programed Schedules - Kiln Forming Myths 24

Don’t use the pre-programed schedules that come with your kiln. 

As a universal approach, this does not stand up.  They do have the disadvantage of trying to cover all possibilities at once. This means they will fail if used uncritically. But everyone needs a place to start. 

An analogy might be the oven temperatures and times in recipes for cooking.  You have to start somewhere.  After a little experience you modify the schedule to fit the equipment you have and the material you are cooking.  This is similar to what happens with people starting in kiln forming.  Prior to the time when manufacturers began putting programs into the controllers, we all copied schedules from text books, guides and other workers.  We put them into the controller and tried them out.

I use pre-programmed schedules all the time – but they are built from my own from observations. They have been based on what others have done, writings and research, but modified by my equipment, the style of work I am doing and many other considerations as indicated in another post.

The instruction should be more about understanding what your schedule does than just dumping the pre-programed schedules.  You should know what your pre-programed schedule does. It is not enough to say “I used full fuse #1.”  You need to know what that schedule does.  You have look at the steps and temperatures and times that the schedule instructs the kiln to do.  Only in this way can you know what is working.  If it is not possible to see what the program is doing by reviewing the steps on the controller, then you need to delete it and copy a program from the glass manufacturer.  This is a reliable indicator of what will work in a wide variety of situations and can later be modified to meet your needs.

 The following are schedules for fusing and slumping.  You need to look at these and decide how you want to modify them - if at all - for your purposes.

An example of a fusing schedule

For this program, you have to decide, on the Goldilocks principle: 

• Is the rate of advance is too fast, too slow or just right.  
• Do I need a soak at 200C? 
• Is the next rate of advance right? 
• Do I need a bubble squeeze? 
• Is the top temperature right and the soak long enough?  
• Is the anneal soak long enough? 
• Is the anneal rate too slow, too fast or just right?  
• Do I need to control the rate of fall below the initial anneal cool, or just let the kiln cool naturally?

An example of a slumping schedule

Again apply the Goldilocks principle:

• You need to think about the speed of the rate of advance. Too fast, too slow or just right?
• Is the top temperature right? Too high, too low?
• Is the soak too long, too short, just right?  
• Is the annealing soak right, too short, too long?
• Is the annealing cool too fast, too slow?

When you have thought about these things, you are well on the way to writing your own programs.

Scheduling Relates to the Piece

My piece cracked, but I've always used this schedule and it has worked.

One schedule is not for all pieces. A number of factors affect the scheduling of a firing.  Some of them are:

Thickness

• The thicker the stack of glass, the slower the advance and anneal should be. 
•  The more layers of glass there are, the slower the rate of advance should be. 
•  The more uneven the thickness, the slower the temperature changes should be.

Angularity

• Glass with right angles or even more acute angles needs slower schedules than round or oval shapes.  

Degree of fuse

•  Laminated and tack fused pieces need much slower annealing and re-firing schedules.  

Contrasting colours

• Pieces with strongly contrasting colours of glass need slowing in heating and annealing.

Size

• To some extent the increased size will need some slowing of the schedule. Size becomes more important as you near the edge of the shelf or nearer to the sides of the kiln. Jewellery scale items can have an accelerated schedule.  

Mould base

• The size and shape of the mould will affect the speed and temperature of the scheduling.         
• The type and style of mould affect the schedule.  Drapes and especially over steel moulds require slower schedules. 

Position in the kiln

• The closer the glass is to the elements whether top or side, the slower the schedule must be.
• The less central on the shelf, the more care must be taken in scheduling.  

Type of kiln

• A kiln constructed for ceramics needs different scheduling considerations than one for fusing.  
• A kiln with side elements needs more careful firing than one with only top elements.

Re-Firing Schedules

Pieces need to be fired after their initial firing for various reasons – additions, corrections, fire polishing, etc.  You need to think about how this next firing differs from the previous one when thinking about the schedule to use.

The most common need for re-firing is after the full fuse or tack fuse to do the slumping.  On the first firing you had two independent pieces, so they could be fired faster than the fused piece.  It is now at least six millimetres thick – at least in parts. As glass is a poor conductor of heat, it needs a slower initial rate of advance than the assembly of thinner pieces did.

If you have fused a blank and now want to add tack fused elements to it, you need to consider how the pieces on the top will shade the heat from the glass below.  Unless the upper pieces almost completely cover the base, you will need to go much slower than the two-layer piece.  The blank is not only thicker, it also is shaded from the heat by the upper pieces.  If they are of both dark and light tones over the same base, the differential shading will be even greater, requiring slower rates of advance.

If you are adding layers of powder, you are not adding much to the thickness or unevenness of the glass.  So no additional reduction, other than that used for previous powder layers, in firing rates is required.

You need to think about the changes you have made to an already fired piece.  If you have made significant changes in thickness or are going to a tack fuse, you need to slow the rates of advance.  Some advice is given on rates of advance for tack fused items here.  If you have added only a layer of powder or thin coating of frit evenly spread, you will not need reductions in rates of advance.

Of course, the annealing soak will need to be longer for thicker or more complicated pieces, and the annealing cool will need to be slower. This blog post gives information on annealing considerations.

Bubble Squeeze

What is a bubble squeeze?

The term bubble squeeze refers to the process of allowing the glass to relax gradually allowing the air to escape to the edge of the piece.

The exact temperature is dependent on the softening point of the glass, its weight, and the complexity of the layup.  Normally the bubble squeeze is performed with a soak of about 30 minutes at the slumping temperature. 

Of course, glass being glass, the slumping point of any glass is a range temperatures.  This can be taken advantage of for complex layups or potentially difficult projects.  Pick the temperature about 50°C below the standard slumping point.  For example, Bullseye recommend 677°C as the slumping point.  Programme a slow rise - say 50°C per hour - from 625°C to 677°C where you also soak for at least 30 minutes.  This slow rise allows an even more gradual and progressive relaxation of the upper glass toward the lower.

For more information look at this post. 

Mica - Kiln Forming Myths 23

Mica will not stick to glass unless it's capped with clear.

Almost by definition, any material that needs to be encased, does not stick to glass.

However, mica does stick to glass.  But it is only the surface that is in contact with the glass that sticks.  Mica shears into very fine sheets and particles (almost microscopic), meaning that there many layers of mica even with a thin layer.  So only a minor portion of the mica you sprinkle, sift or paint onto the glass can stick. 

It is possible to add a flux such as borax to the mica solution to soften the surface of the glass, allowing more mica to sink into and stick to the glass.

Of course you can encase much more mica than will stick to the surface.  However, you have to be very careful about avoiding bubbles.  There is so much air (relative to the volume of the mica) that bubbles in encased mica is a constant problem.  Very good bubble squeezes and supporting the edges on shards of glass to keep the glass open while beginning to slump are required.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Scum on Ground Edges

Almost without exception, ground edges show scum after fusing.  This scummy appearance is devitrification. This is caused by the powdered glass from grinding remaining in the pits caused by the action of refining the shape of the glass with a grinder. 

The suggestion that the glass should be placed in water immediately is of course a good precaution, although addition of vinegar is less efficacious than grinder lubricant added to the soak water.  This lubricant helps to keep the glass in suspension rather than settling into the scratches and pits of the grinding marks. The vinegar, which is often recommended, will etch the glass if left to soak and  smells up the place.  A better solution to soak the glass in is a 6% solution of tri-sodium citrate.

The glass needs to be made smoother than the standard grinding bit will achieve.  Normally, a 600 grit grinding bit will be sufficient to allow a good fire polish without any devitrification. Sometimes 400 grit will be enough. You will need to step down in grit from the standard (about 100) to fine (about 200) to at least super fine (about 400) grit.  If you can find a 600 grit bit, that can be your final smoothing before cleaning and placing on your piece for fusing.  Of course, this grinding can be done by hand with wet and dry sandpaper without any great labour.

There is, of course, a more simple solution - don't grind. I rarely grind any pieces for kiln forming.  Often, this is because I am working thicker than 6mm and know the gaps will fill during the forming.  If I need to make adjustments for 6mm pieces, and I often do, I groze the edges of the glass.  This gives a much cleaner break of the glass than grinding.  Of course, the edges are not as precise as when ground, but the glass remains absent of all the scratches that harbour the devitrification.  Often the fit does not need to be precise anyway. 

When the fit does need to be precise, the parts that do not fit perfectly can be filled with the appropriate colour of powder. This should be kept as near the gap as possible and piled up only a little over the gap to compensate for the lack of mass that powder has in comparison with sheet glass.  This powder technique, of course, does not work well on tack fused pieces.  There, the grinding and smoothing needs to be pursued.

Use of Untested Glass - Kiln Forming Myths 22

You must use art glass rather than recycled glass.

This seems to refer to the use of untested glass in kiln forming.  If you are going to use untested glass for kiln forming, it does not much matter which you use.  Because, in every case you will need to test for forming and annealing temperatures to be able to make use of the glass with unknown properties. 

Of course, people use glass that is not tested fusing compatible in many circumstances.  Float glass is frequently used in many kiln forming applications.  And bottle glass is of very little different in composition.  So-called art glass can be used in a variety of ways also.  There are many other variations of glass including handmade, casting, lamp working, and borosilicate, among others.  Each has their own set of characteristics, which overlap with each other.  The forming and annealing temperatures must be determined to enable you to use them. Some of this information is often available from the manufacturer’s web site or other sources.  Many times you have to do the testing for yourself.  One guide to help determine the critical temperatures is here. 

One characteristic that all untested glasses share is a tendency to devitrify by the second or third firing, so attempting to get the most work done in the fewest firings is a good idea.  This tendency to devitrify is frequently shown when manipulating bottle glass.

Peeking Without a Vent

What can I do if my kiln does not have a plug?

To understand thoroughly what is happening to your glass while firing, observation is key.  This means that an observation port is an ideal feature of any kiln.

However, many kilns are made without ventilation or observation ports.  This means that several possibilities need to be considered.

The easiest is simply to open the door or lid a small amount to make a brief observation.  This means that you have to set up the piece to be fired in such a place it can be seen from a small opening of the door/lid.  This brief opening of a small space will not normally cause any problem to the glass or kiln.  At the higher temperatures, you need to take personal safety precautions against the heat and light from the kiln.

It is possible to be more radical and drill an observation port through the metal casing and brick or fibre lining of the kiln.  This is then filled with a piece of fire brick or roll of fibre blanket.  This is sufficient to insulate the heat from the external part of the kiln.  This port should be about 50mm diameter to give a decent field of view.

A further refinement is to place a quartz viewing window in the hole you have drilled.  This viewing piece will become very hot, but not visibly red.  So, you must provide some insulating cover over the window.

But best of all, is to purchase a kiln with a viewing port in the first place.

Powders Burn Away - Kiln Forming Myths 21

Glass powders will burn off at high temperatures.

No.  The powder is glass.  Glass does not evaporate or otherwise combust at kiln forming temperatures.

The appearance of glass powders fading at fusing temperatures is related to the different appearance before and after firing.  Before firing, the powder looks both denser and paler than the final colour.  The initial experience with glass powder always is to put less on than needed. 

You need to remember that a thin film of powder is a tiny fraction of the thickness of the glass it is made from, so the colour will be much fainter.  A considerable amount of powder is required to give the colour shown by the colour charts – as much as 2mm for paler and transparent colours.  Opalescent colours show a little better with thin applications, but still require significant amounts.

This shows the application of powder on a piece where the powder provides almost all the colour for the piece.

The best procedure is to make test tiles with varying amounts of the powder to determine the thickness required for your desired result.  This gives a visual reference and experience in laying down the powder in appropriate thicknesses.

The appearance of the glass powder burning off, is merely the application of too little powder.

Organic and Mineral Inclusions

Encasing organic materials adds a new level of complexity to inclusions.  In addition to bubble formation, you need to consider how to eliminate the combustion gases from the vegetable matter.  On the other hand you don’t need to worry about expansion differences.

Some examples of metal and glass inclusions

The requirement is to burn out all of the vegetable matter to avoid creating big bubbles from the burn off of the material.  There are two elements to this burnout.  One is the amount of moisture contained in the object and the second is the volume of dry material that has to burn out.

Drying

Unless you have dried the material before including it, you will need to leave enough soak time before the glass begins to move to ensure all the water is removed.  It is also advisable to place small shards of glass at the corners of the piece, to allow easy ventilation both for the moisture to evaporate and the vegetation to burn easily.  You can estimate the time required and then put a witness piece of glass or better mirror above the vent or peep hole to see if there is any fogging on the glass from escaping moisture.  You need to continue soaking until there is no fogging.

Burnout

The second element is to give the vegetable matter enough time to burn out.  The burn out should occur at about 400°C.  This is high enough to ignite carbon based materials, but not so high that an extended soak will allow the glass to sufficiently deform to seal the un-burnt material inside.  If you have a really good sense of smell you can tell when the carbon has burned away by the absence of the smell.  For the rest of us, we need to open the peep hole and use a strong light to tell how much is left to burn away.  The burning is much more like a smouldering with very little light coming from it.

An incompletely burned out leaf in a large trivet with felt feet at the corners. 150mm square

The length of time you need to soak below the softening point of the glass is directly related to both the water content and the amount of combustible material you have included.  The burning will not begin until everything is dry.  If the material is not dry, the time for this needs to be added to the burnout time.  The length of soak for burnout is much more difficult to determine and needs periodic observation beyond the time when the smoke stops coming out of the kiln ports.

Bubble squeeze
Once the drying and burnout are completed, you need to advance to the bubble squeeze.  This will need to be longer or slower than usual to ensure all the combustion gasses are out for organic materials.  Minerals will normally be thicker than the organic materials and so need long bubble squeezes. These can be at or just below slumping temperature, or a slow rate of rise, taking an hour or more, from about 50C below the slumping point.

It is possible to include other minerals such as bone, or ash, or other inert particles that will not stick to the glass. Materials that contain silica are not suitable, as they stick to the glass and cause breakages.  So most stone, which contain silica, however thinly sliced will not be suitable as an inclusion.