Scheduling for a New Slump Mould

Often you will see statements that imply a single temperature and time is suitable for all slumping or draping.  This is not so.  In fact, slumping is a delicate balance of layup, time, gravity, shape and temperature. This applies to draping operations too.

Factors in glass forming

The balance of colour arrangement has an effect on how the glass forms.  In an extreme example of white on one side and black on the other, the forming will begin on the black side first, leading to an uneven slump. Read on - there are ways to make this effect less severe.

The length of time you are willing to wait for the piece to slump is a factor in the temperature required.  Patience is rewarded.  Longer soaks mean that lower temperatures can be used. Lower temperatures lead to less marking on the back.

The mass (often thought of as the thickness) of the glass affects how quickly the glass will form. The greater the mass, the sooner the glass begins to form. This means with heavy glass lower forming temperatures can be used, because of gravity effects.

To get glass conform to a mould with complicated shapes takes longer soaks or higher temperatures than simple shapes.  This is because the glass requires to be more plastic to get into multiple shapes, small details or sharp angles.

General Principles

Since all these factors interact, any one schedule will not do for all occasions.  The general principles for a good slump are:

• Use a steady rate of temperature increase (rate of advance).
• Use the lowest practical temperature to get the forming done.

The reasons for using a single steady rate of advance in kiln forming are:

• It is much simpler to program a single rate of advance all the way to slump temperature.  
• Glass reacts best to steady inputs of heat, allowing the whole substance to be at the same temperature as it heats up.
• It helps avoid uneven slumps.  Glass that is the same temperature across and throughout itself is more likely to begin to form all at the same time.
• It helps ensure that the whole of the thickness of the glass is at the same temperature, thus avoiding splits on the bottom.
• The slow steady input of heat means the glass can be formed at a lower temperature because of the heat work put into the glass on the rise in temperature.

The reasons for using the lowest practical temperature to slump and drape are:

• It allows the glass to begin moving before it gets sticky, and so dragging on the mould producing stretch marks and sometimes needles.
• A low temperature slump reduces the risk of uneven slumps.  At low temperatures the glass is less likely to react to colour variations that absorb heat more quickly than others.  Where there is uneven weight, the forming is more likely to be even as it cannot react so quickly to the differences in weight.
• The glass will be less marked on the mould side at lower than at higher temperatures.  The glass, being less plastic, will take up less of the mould texture.

Calibration of Schedules

As each mould is different, there are as many schedules applicable as there are moulds.  Bullseye has recognised this by publishing suggested schedules for their moulds.  But there are lots more moulds than the Bullseye ones.  And even for the Bullseye moulds there are a variety of variables in the glass put on top.

The point is to find a way to determine the appropriate schedule for the mould and the glass it supports.  This involves the main variables - rate of advance, top temperature and soak time - although there are others such as lay up, degree of fusing, weight and its distribution, colour variation, etc.

The rate of advance will depend on:

• The thickness of the piece.  Thicker glass needs slower rates of advance.
• The degree of fuse.  A tack fused piece will require a slower rate than a full fused piece.

The top temperature depends largely on the complexity of the mould shapes, although it is very closely related to the soak time.  One of the principles of slumping given above is to use the lowest practical temperature. The reason for this is to get a good result with the minimum of mould marks.

The main means of determining forming temperature and time is observation. I determine my slump temperature (normally) by what temperature I have to use for the particular mould to get the glass fully slumped in half an hour.  For more complicated moulds such as a candle bridge I would use 1.5 hours as the soak time.

There are two main methods of doing this observation.  One is to set the “one size fits all” schedule and modify it. The other is to create a new schedule by working up from the lowest temperature to the practical temperature.

Modification of existing schedule

To prepare for the modified schedule, you need to do several things.  

Get your kiln log out ready to record the information about the firing.  Record the mould shape and separator (and add a picture of the set up if you can) and include the lay up of the blank to be formed.  Also record anything you think may be relevant to the forming process for this firing.

Set your single rate of advance all the way to the top (forming) temperature and record it in the kiln log.  Begin observing the progress of the slump from 60°C below the top temperature you have set.  This involves quick peeks at approximately five to ten minute intervals.  You may not see much movement at first, but at later peeks you will see the glass progressively forming.  When the glass appears to have just touched down at the bottom, you can use that as the top temperature.  Advance the schedule to the soak portion (read the controller manual if you do not already know how to do that).  Note the temperature and time in your log book when you do this.

Continue to observe the progress of the slump but at about ten minute intervals to check on the progress of the slump.  When the slump appears complete, advance to the next segment of the schedule and note the time.  Subtract the start of the slump soak from the present time and you know how long the soak needs to be for that layup in the mould. Record that in your log book. 

When cool, inspect the slumped piece to determine if it is fully formed. Record the results in your kiln log.  If it is not fully formed, you can decide if it is practical to add additional soak time or if you need to increase the top temperature.  Only you can determine what is a practical soak time.  If you are soaking while you are away or asleep, it does not really matter how long a soak you need at the chosen temperature.  However, there are times when you need to have a piece out of the kiln to be able to put in the next.  Somewhere between these two is the practical soak time.

You may find that the glass does not need as much time as you gave it.  Record this result too.  In this case, you can reduce the top temperature in future firings until you find the best combination of temperature and time. You will have experience from watching the forming (whether slump or drape) to give an indication of the lower temperature to choose.  A general guide would be to reduce the temperature by 10C, and extend the time by at least 50% more than  what you used in the higher temperature firing.  

Record each firing with the lay up, rates, temperatures and soak times, plus the results.  When you have determined the ideal combination of factors, record the determined temperature and soak time together with the layup in your log book and on something in your mould box.  I have also used vitreous paint on the underside of the mould to indicate my standard temperature and soak time so that I don't loose the information.

Development of a new schedule

This is not as difficult as might be imagined.  It does involve a lot of peeking into the kiln, though. You start with an appropriate rate of advance for the thickness and style of fusing.  Remember that thicker glass and tack fused glass require slower advances than thinner and flat fused glass.  Set this rate all the way to your predicted top temperature.  No rapid rises with short soaks are required or desirable. Set a predicted soak time. If you are not certain, use 30 minutes as a general average. Then set the anneal soak and cool rates.

As you observe, you will see when the glass on the mould begins to form. It will generally start at about 600°C.  Peek at about 10 minute intervals from that temperature onwards toward the target.  When you see the glass begining to change shape, Change the top temperature to be about 20C higher than the initial forming temperature, and then observe after 15 mins at the new temperature. If it hasn't moved much, add 5°C more to the temperature and observe. Repeat as necessary. When the glass has a significant curve, stop the rise and soak at that temperature with the 30 minute soak.  Continue to observe at 10 – 15 minute intervals to determine when the slump is complete.  Then proceed to the anneal cool. Record rates, temperatures and times in your log book.

When you remove the piece from the kiln, check it over.  If it is not fully slumped, you can add time or temperature.  Adding time is likely to give a better surface to the glass on the mould side.  Sometimes, but not often, adding temperature will be the choice. 

It is possible that the piece will show evidence of too high a temperature or too long in the mould.  This will be clear from extensive mould marking, sometimes needles at the edges, stretch marks, or uprisings at or near the bottom of the mould.  In these cases, the temperature needs to be reduced.  Reducing the time is not advisable, as quick slumps can often be distorted or unbalanced.

Glass Types

Remember that these tests for the best forming schedule for you and your mould are only relevant to the kind of glass you are using at that moment. There will be only minor variations between Bullseye, Uroboros and Wissmach. There will be major variations between these and float glass. Separate schedules will need to be worked out for it, remembering that there are a variety of manufactures of float and they do not all behave the same as each other.  Float and other glass that is not formulated for fusing will not provide such consistent results as fusing glass, but successful schedules can be determined in the same way as for the fusing compatible glasses.

Records

Once you have calibrated the temperature and time for the mould and the layup, you will know how to schedule for that mould. Record it in your log book and also along with your mould, either in the box or on the mould.

It will be for you to decide whether you use longer times and therefore lower temperatures.  When making the decision remember the principles of slumping – steady rate of increase to the working temperature, and use of the lowest practical temperature.

These actions will give you the standard forming temperature for the mould.  It is a base from which to make variations when you use a different thickness, lay up, or degree of fusing.  

You should continue to record each of your firings with full details, because sometimes things change. This will give you a basis to diagnose what has become different. It will help avoid the cry of "this has always worked for me before."  It means you have the possibility of working back to see what, if anything has changed. If nothing has changed in your level of fusing, thickness, lay up, schedule and all those other things you record, then you can begin looking at your kiln to see what might be different.

Slump Depth - Kiln Forming Myths 30

You can slump only 5cm or so per firing. If so, why don't drop rings fail?

This is really about a comparison of deep slumps and free drops.

Multi-stage Slump

·         The stages of a deep slump give more control of the design

·         The shape of the result is determined by the shape of the mould

·         The wall thickness will be much more consistent

·         For its size the deep slump is lighter than the drop. 

·         The multiple stage deep slump is designed to have the minimum of cold work.

Single Stage Slump

·         If you attempt to do a deep slump all at once you run the risk of ruffle around the upper edge. 

·         There are likely to be a large number of stretch marks on the outside.

·         The design will be distorted to varying degrees along the surface.

·         The wall thickness will vary greatly.

·         You lose the advantages of the multi-stage slump without gaining the advantages of a drop vessel.

Drop Ring Vessels

·         Drop rings require higher temperatures. 
·         The glass thins as it stretches through the ring, so you need to start with a much thicker blank than slumping. 
·         There will be a thick base in relation to the sides.
·         The design will stretch, and if properly designed will be very pleasing.
·         The walls of a drop vessel will vary from thick at the bottom to thin at the top.
·         The collar needs to be cut off the vessel and cold worked to smooth.
·         The process of falling through the ring needs to be monitored to avoid an excessive drop causing distortion or an insufficient drop causing the need for grinding a flat base for the vessel.

Super Glue - Kiln Forming Myths 29

The use of super glue in the kiln causes cyanide gas

This is not true.  But because it is such a persistent belief, a lot of detail is given below.  In short the precautions are: 

• use the minimum amount, 
• use an organic gas face mask, 
• do not wear natural fibres or gloves, 
• let the glue cure before placing it in the kiln, 
• have the solvents at hand while using the glue.

Super Glue Safety

Super glue is frequently used as a temporary fixative in assembly of kiln forming projects. There is some concern about safety, as it is known that super glue is made from cyanoacrylate, which it is feared will break down in the kiln into cyanide gas.

Greg Rawls, a certified industrial hygienist says

"I looked at the MSDSs for several forms of super glue. The main component is Ethyl 2-cyanoacrylate, which has a TLV of 0.2 ppm which is relatively toxic. [However,] the thermal decomposition products are carbon monoxide and carbon dioxide. I did not see a reference to cyanide gas. However, as I recall cyanide gas dissociates into elemental carbon and nitrogen at about 800 F. Since you use it in such small quantities, I would not worry about it. In my opinion the worst thing that could happen is you glue your fingers to the glass."

Safety issues

To treat the safety issues seriously and determine if you feel Greg Rawls' view is justified, you need to look at the issues of toxicity, reactions, adhesion of tissue, ventilation, first aid and decomposition products in the whole context.

Toxicity

The fumes from cyanoacrylate are a vaporized form of the cyanoacrylate monomer that irritate sensitive membranes in the eyes, nose, and throat. They are immediately polymerized by the moisture in the membranes and become inert. These risks can be minimized by using cyanoacrylate in well ventilated areas. About 5% of the population can become sensitized to cyanoacrylate fumes after repeated exposure, resulting in flu-like symptoms. It may also act as a skin irritant and may cause an allergic skin reaction. On rare occasions, inhalation may trigger asthma. There is no single measurement of toxicity for all cyanoacrylate adhesives as there is a wide variety of adhesives that contain various cyanoacrylate formulations.

The United States National Toxicology Program and the United Kingdom Health and Safety Executive have concluded that the use of ethyl cyanoacrylate is safe and that additional study is unnecessary. 2-octyl cyanoacrylate degrades much more slowly due to its longer organic backbone that slows the degradation of the adhesive enough to remain below the threshold of tissue toxicity, so the use of 2-octyl cyanoacrylate for sutures is preferred.

Reaction with cotton

Applying cyanoacrylate to some materials made of cotton or wool results in a powerful, rapid exothermic reaction. The heat released may cause serious burns, ignite the cotton product, or release irritating white smoke. Users should not to wear cotton or wool clothing, especially cotton gloves, when applying or handling cyanoacrylates.

Adhesion of the Skin

Various solvents and de-bonders can be used. These include:

·         Acetone, commonly found in nail polish remover, is a widely available solvent capable of softening cured cyanoacrylate

·         Nitromethane

·         Dimethyl sulfoxide

·         Methylene chloride

Commercial de-bonders are also available.

Warnings include:

·         It is a mild irritant to the skin.

·         It is an eye irritant.

·         It bonds skin in seconds.

·         Any skin or eye contact should be copiously flushed with water and medical attention be sought immediately.

·         Do not attempt to separate eye tissues – the bond will separate naturally within a few days.

Precautions

·         Use goggles.

·         Do not wear cotton or wool clothing while using super glue

·         Ventilate the area well. Since cyanoacrylate vapours are heavier than air, place exhaust intake below work area. Activated charcoal filters using an acidic charcoal have been found effective in removing vapours from effluent air so the bench top air filters are suitable for use while using super glue.

·         Avoid use of excess adhesive. Excess adhesive outside of bond area will increase level of vapours.

·         Assemble parts as quickly as possible. Long open times will increase level of vapours.

Evaporation Effects

·         The effects of heating cyanoacrylate are not completely known. The flash point is known to be greater than 85ºC. As a precaution do not remain in the area of the kiln after that temperature has been reached.

·         The decomposition products are carbon monoxide and carbon dioxide. There is no reference in the literature to cyanide gas. It is highly unlikely that heat will cause the release of cyanide gas at any time during the heating. To be certain, you should make sure the evaporation of the glue is complete before firing the kiln.

See this tip for the use of super glue in kiln forming. http://glasstips.blogspot.co.uk/2013/12/super-glue.html

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.  I try to get my fusing work done in 10 minutes at the working temperature.   Any less time there and I feel I am going 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 are the aim of most kilnforming.

Revised 30.12.24

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.

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.