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.

Crash Cooling - Kiln Forming Myths 20

Crash cooling will harm your kiln or break your glass.

Crash or flash cooling was often a requirement in the early days of fusing to avoid devitrification. The kilns used were ceramic ones that did not lose heat very quickly.  The glass also was more subject to devitrification than the glass being made now.  Since those early days, kiln design has advanced so the kilns lose heat more quickly, although still well insulated; and the glass is more resistant to devitrification.  Thus, crash cooling is no longer advised.

If you have a brick lined kiln, crash cooling is hard on the bricks.  The cold air causes rapid shrinking of the brick.  The more rapidly the brick heats and cools, the more fractures will develop in the brick.  This effect will take place over many firings before there is any noticeable damage to the structure of the brick.  However, if you have brick tops or lids, there is the increasing likely development of crumbs of brick falling onto your work.  Brick lids and tops should be vacuumed frequently to remove the crumbs as they form.

Crash or flash cooling from top temperature toward annealing temperature is unlikely to break any glass other than thick glass pieces.  However, when using glass formulated for kiln forming, you do not need to crash cool. The crash cooling may be more useful when using glass that is not formulated for kiln forming.  The purpose in this case would be the same as that for the early fusing – avoiding devitrification by moving as quickly as possible through the devitrification range.

Sometimes flash/crash cooling is required to fix a free drop in place.  If allowed to cool on its own, the glass will continue to move for a while.  If the extent of the drop is critical, crash cooling is required.  This should be to a point below the slumping but above the annealing temperature.  The flash cooling will cool the outer portions of the glass, stopping any further movement. Meanwhile the inner portions are still hot.  This sets up significant stresses.  By stopping the cooling just below the slumping temperature, you allow the internal and external temperatures of the glass to approach one another before going into the anneal soak where the temperature equalises throughout if the differentials are not too great from the flash cooling.

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”.