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.