Slower Ramps on Additional Firings

"Every time you fire a previously fired piece you need to slow down."

This is not accurate. If you have not changed anything significant, the annealing does not need to be extended.  The clearest example is a fire polish.  Nothing has been added. The physics and chemistry of the piece have not changed.  If only confetti or a thin frit/powder layer is added, nothing significant for scheduling has been added.  As nothing significant has changes the annealing used in the previous firing can be used again.

Of course, you do need to slow the ramp up rates on the second firing.  This is because you are firing a single thicker piece.  On the first firing, the pieces are individual and can withstand slightly faster rates. But on third and subsequent firings, if nothing significant has been changed, there is no need to slow rates further.

There is a post which describes this further.

"When adding more thickness more time is needed."

This is the occasion when the annealing soak needs to be extended.  Placing a full sheet of clear glass on the bottom, or less usually, the top, and taken to a full fuse, requires slower ramp rates.   The annealing time for a full fuse can be taken directly from the annealing tables for thick slabs.  

The fusing profile for any additional items has a strong affect on the length of the annealing soak.  If the glass is now of uneven thicknesses, and greater care in assigning ramp rates is needed.  The profile for the piece also determines the amount of additional annealing time required.  A sharp tack of a single additional layer will require annealing as for 2.5 times the total height of the piece at the start or the firing. A rounded tack will need two times and a contour fuse will require 1.5 times.  A full fuse can be carried out for the new total height of the piece without any multiplying factors.

 If the intention with multiple firings is to achieve a variety of profiles within one piece, a slightly different approach is required.  A blog post here describes the process.

The general approach to multiple firings is that unless there are changes to the thickness or profile of the glass, no changes in annealing time is required.  

 

Annealing Previously Fired Items

“Double the annealing soak time for each firing” and “Slow the rate of advance each time you fire” are common responses as a diagnosis when a piece breaks in the slumping process.  It may come from the fact that once fired, It is now a single piece that needs a slower rate of advance on the second firing.  I’m not sure where the idea of doubling the annealing process originates.

You need to think about why you would slow the rate of advance and double the anneal for each subsequent firing of the piece.  This is an investigation of the proposals.

Thickness determines ramp rates and annealing

Annealing soak lengths and cooling rates are related to thickness and complexity.  If no additions or complications are added between the previous and the current firing, there is no reason to extend the soak or decrease the rate of cooling.

You of course, need to consider what lay-up and process you are using in the additional firing.  Have you added any complexity to the piece in the previous or the current firing?  If so, you do need to consider how those changes will affect the firing requirements.

Fire polishing

The question to be asked is, “if the piece was properly annealed in the first firing and shows no significant stress, why do I need to change the firing?”

The answer is, “you only need to slow the heat up because it is a single piece now.”  You do need to know that the existing stress is minimal, of course. A note on stress testing is here.  If there is little or no stress from the previous firing, the annealing and cooling can be the same as the previous firing.  Nothing has changed. You are only softening the surface to a shine.  The anneal was adequate on the first firing, and it will be on the second.

If you are firing a pot or screen melt, you have added a complexity into the firing. This is because of the high temperatures used in the first firing.  It means you may wish to be more cautious about a re-firing to eliminate bubbles, or for a fire polish for the surface.

Frit layers

If you are adding confetti or thin layers of frit or powder you have not significantly changed the piece.  You can re-fire the piece as though you are fire polishing any other piece of the same dimensions.

Additional layers

If you are adding more full layers in subsequent firings, you need to reduce the rate of advance to top temperature.  You also need to extend the soak and reduce the cooling rate according to the new thickness of the piece.  This is because the piece is thicker, so the rate of advance needs to be slower, the time required to adequately anneal is longer, and the cooling rate needs to be slower.  All of these changes in scheduling are to accommodate the additional thickness.

Tack fusing additional pieces

If you are tack fusing pieces to the top of an already fired piece, you need to go slower than you would by just adding a full layer.  Tack fusing pieces to an existing piece adds a significant complication to the firing.  Tack fusing requires a firing for thickness between 1.5 and 2.5 times the actual total height of the piece.  The complexity added is the shading of the base glass from the heat radiating from the elements. 

For example, if your piece from the melt is 9mm/0.375", it would have been annealed with a 90 minutes soak. The first cool would be at 69C/127F per hour, and the second at 125C/225F per hour with the cool to room temperature at 415C/750F. If it shows no significant stress, you can fire polish and anneal in the same way as your initial firing.

But

If you tack fuse pieces on top, then you need to treat the piece as though it were between 15mm/0.625" (a little over 1.5 the thickness) and 25mm/1.0" (a little over 2.5 times) thick.  This would require a soak of 3 or 4 hours.  A cooling rate of between 40C/72F and 15C/27F per hour for the first cooling stage is needed. The second stage between will need a rate between 72C/130F and 27C/49F per hour. The final cooling to room temperature will be between 90C/162F and 240C/432F to room temperature.

Conclusion

If you have made no significant changes in thickness or complexity, the second firing can be the same annealing as the first firing. If you have altered the thickness or complexity of the piece, the second firing will need to be slower.

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

Achieving the Striking Colour

"Is there anything special I have to do to fire striker glass?  Can I mix striker and non-striker in the same kiln or piece?"

Strikers generally need a two-hour soak at slumping temperatures, about 660C.  This heat soak helps ensure full development of the colour. If the soak is not long enough, the colour may not achieve the target colour at all, or be paler than anticipated.

The rate of advance to the heat soak is not critical.  But it does need to be the appropriate rate for the thickness and nature of the assembly of glass being fired.

If you were to have too short a heat soak, you can fire again to help mature underdeveloped colours.  This will, of course, change the profile of the finished piece.

Strikers are compatible within their manufacturer’s own range. So, they can be combined in the same piece as any other of the glass in the fusing compatible range.  That means that they can be fired in the same kiln load as non strikers.

The two-hour soak at slumping temperature will not harm the later stages of firing, but it might lead to use of a slightly lower temperature tack fusing than without the long heat soak.  That is because of the heat work put into the glass at the lower temperature.   Only observation will tell you how much less temperature is required.  It may be possible that only a little less time at the forming temperature is required.  Again, only observation will tell you that.

Strikers require a heat soak to mature the final colour.  These striking glasses are compatible with the rest of the fusing range from a single manufacturer. Glass from different manufacturers must be tested for compatibility before combined into a project.

Soaks Below the Softening Point

There are frequent suggestions that holds in the rise of temperature for glass are required.  Various justifications are given.  A few notes before getting to the explanation of why they are uncessary.

A note is required about the softening point sometimes called the upper strain point. There is a reasonable amount of discussion about the lower strain point.  So much that it is often simply referred to as the strain point.    Below the lower strain point, the glass becomes so stiff and brittle that no further annealing can occur.  Thermal shock can happen though, so the cooling needs to be controlled.

There also is an upper point at which the behaviour of the glass is different.  Above this temperature, no annealing can occur either, because the glass has become plastic and the molecules randomly arranged.  It is only just pliable, of course, but its molecules are no longer strongly bound to one another.  This is the temperature at which much of slumping is done.

It is disputed whether such a point exists.  Still, in practical terms it is where the glass becomes so plastic that it cannot be temperature shocked.  The temperature of this “point” is approximately 45°C above the annealing point, rather than the temperature equalisation soak. 

Note that the temperature at which Bullseye recommends that the annealing soak should occur is a temperature equalisation point, which is about 33°C below the glass transition temperature - the point at which glass can be most quickly annealed.  The average glass transition point for Bullseye is 516°C.  Most other fusing glasses use the glass transition (Tg) point as the annealing temperature for the soak.  They or you could employ the Bullseye technique on thicker slabs of the glass by setting the temperature equalisation point 33°C below the annealing point, and soaking for the same kinds of time used in the Bullseye chart for annealing thick slabs.  In fact, this is what Wissmach has recently done with its W90 and W96 fusing glass ranges.  They now recommend 482C (900F) as the anneal soak temperature.

Now to the point of the post.

The soaks that are often put into schedules on the rise in temperature are justified as allowing the glass to equalise in temperature.  Glass in its brittle phase is an excellent insulator.  This means that heat does not travel quickly through the glass.  Consequently glass behaves best with steady and even rises in temperature (and correspondingly on the reduction in temperature).  Rapid rates risk breaking the glass on the temperature rise, no matter how many or how long the holds are.  

This means a slower rate of advance will accomplish the heating of the glass in the same amount of time, and in a safer manner, than rapid rises with short soaks/dwells/holds.  The slower rate of temperature increase allows the glass to absorb and distribute the heat more evenly.  This slow heating is most obviously required in tack fusing where there are different thicknesses of glass.  

This means that it is possible for thin areas of glass to heat up much more quickly than glass covered by different thicknesses of glass.  It also applies to strongly contrasting colours such as black and white, because they absorb the heat differently - black more quickly than white.

There are, of course, circumstances where soaks at intervals are required – usually because of mould characteristics, in slumping, and in pate de verre.

Sometimes people add a soak at the annealing temperature on the way up in their schedules.  This is unnecessary.  If the glass has survived up to this point without breaking, it is highly unlikely it will break with a further increase in the rate of advance unless it is very fast.  The temperature after all, is above the strain point meaning the glass is no longer in the brittle phase.

Many people add a soak at around 540°C (ca. 1000°F) into their schedule on the increase in temperature, before their rapid rate of advance to the top temperature.  The choice of this temperature relates to the lower strain point.  This also is unnecessary, except possibly for very thick pieces. By this time the glass has reached its plastic stage and if it hasn’t broken by then, it won’t with a rapid rise in temperature either.

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

Soaks at various temperatures during the advance to the upper strain points of glass are not necessary.  What is necessary is a knowledge of when the glass becomes plastic in its behaviour, and an understanding of how soaks can overcome characteristics of moulds, or how to achieve specific results and appearances of the glass.

As Fast as Possible Firings

I have long advocated that it is best to avoid as fast as possible firings because the way controllers work.  They compare the temperatures several times a minute (the number depending on the manufacturer) to determine the rate of increase.  This allows big overshoots at the top temperature with fast rises.  This was reinforced this morning by observing a different factor.
 
I took a piece out at 68°C to put another in.  During the time the kiln was open, the air temperature dropped to 21°C.  I filled the kiln and closed the lid and idly watched the temperature climb before switching the kiln on for another firing.  It took a bit more than two minutes for the thermocouple to reach 54°C with the eventual stable temperature being 58°C.  I had not been aware how long it takes the thermocouple to react to the change in temperature.  Yes, it takes a little time for the air temperature in the kiln to equalise with the mass of the kiln, but not two minutes.
 
With a two-minute delay the recorded temperature can be significantly behind the actual air temperature.  For example, a rate of 500°C per hour is equal to 8.3°C (15°F) per minute or 16.6°C (30°F) overshoot of the programmed temperature. Even at 300°C it is a 10°C (18°F) overshoot.  This effect, added to the way the controller samples the temperatures, means the actual overshoot can be significant for the resulting glass appearance.
 

This is just another small element in why moderate ramp rates can be helpful in providing consistent results for the glass.

Sintering

This is a process used in glass to stick glass together without any change in appearance of the separate pieces.  It has various names - fuse to stick and lamination are two.

General description
“Sintering or frittage is the process of compacting and forming a solid mass of material by heat or pressure without melting it…. Sintering happens naturally in mineral deposits [and] as a manufacturing process used with metals, ceramics, plastics, and other materials.

“The atoms in the materials diffuse across the boundaries of the particles, fusing the particles together and creating one solid piece. Because the sintering temperature does not have to reach the melting point of the material, sintering is often chosen as the shaping process for materials with extremely high melting points such as tungsten and molybdenum….
 
“An example of sintering can be observed when ice cubes in a glass of water adhere to each other, which is driven by the temperature difference between the water and the ice.”
https://en.wikipedia.org/wiki/Sintering
 
Applied to glass this means that you can make a solid piece out of multiple touching or overlapping pieces that do not change their shape.  This is done by using low temperatures and very long soaks. 
 
The usual process is to take the glass at a moderate rate up to the lower strain point.  The rate of advance is slowed to 50°C or less to a temperature between slumping and the bottom of the tack fuse range.  The operator must choose the temperature, largely by experimentation. 
 
The slow rate of advance allows a lot of heat work to be put into the glass.  This, combined with a long soak (hours), gives the molecules time to combine with their neighbours in other particles.
 
Sintering can be done in the range of 610°C to 700°C.  The lower limit is determined by the strain point of the glass being used and practicality.  

The upper limit is determined by the onset of devitrification. This  has been determined by the scientific studies of sintered glass as a structure for growing bone transplants.  Devitrification reduces the strength of the bonds of the particles at the molecular level.  These studies showed that the onset of devitrification is at 700°C and is visibly apparent at 750°C regardless of the glass used.  Therefore, the choice was to use 690°C as the top sintering temperature. 
 
For reasons of practicality the lowest temperature tested was 650°C.  Indications were that at least an additional two hours would need to be added to the sinter soak for each 10°C reduction below 650°C.  This would make for a 12-hour soak at 610°C.  For me this was not practical.
 
My recent testing has indicated some guidelines for the sintering process:
 
The ramp rate has significant effects on the strength of the resulting piece. 

• A moderate rate (150°C) all the way to the sintering temperature needs a two-hour soak at the top temperature. 
  
• A rapid rate (600°C) - as used in medicine – to the sintering temperature requires approximately six-hours soaking.
• A rapid rise to the strain point followed by the slow 50°C per hour rate to the sinter temperature requires a three-hour soak.

 
The temperature range of 610°C to 700°C can be used for sintering.  The effects of the temperature used have these effects:

• With the same rates and soak times, lower temperatures produce weaker glass.
• The lower the temperature, the longer the sinter soak needs to be for similar strengths.  Generally, the soak at 650°C needs to be twice that of sintering at 690°C.
• Lower temperatures produce more opaque glass.  In this picture all the glass is clear powder and fine frit in the ratio 1:2, powder:frit.

 

The annealing of sintered objects needs to be very cautious. The particles are largely independent of each other, only joined at the contact points.  The annealing soak needs to be longer and the cool slower than for simple tack fusing. 

• Testing showed that annealing as for 12mm is adequate. 
  
• There was no advantage of annealing as for 25mm as that did not increase the strength.

 
Porosity
Although the structure of the sintered glass appears granular, it is not porous except at or below 650°C.  At the lower temperatures, the glass becomes damp on the outside and weeps water.  At 670° and 690°C the outside became cool to touch but did not leak water.  This observation depends on evenly and firmly packed frits.
 

Grain structure at 650C

Grain structure at 690C

The keys to successful sintering of glass are the use of a heat work through slow ramp rates, and long soaks throughout the whole firing.

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

Firing multiple layers

Glass Stela
Credit: Stephen Richard

Fusing multiple layers is prone to the creation of multiple large bubbles.  It also needs a strategy to schedule for thick layers.

Avoid bubbles

A widely recommended strategy for stacks of glass is to fire in pairs of layers. Then combine the fused two-layer pieces in a final firing. 

It is easier to fire two layers of glass than 6, 8 or 10 layers. The heat up is easier and less time consuming for multiples of 6mm than multiples of 3mm. The bubble squeeze schedule is simpler.  It also allows inclusions between the initial two-layer sheets and then between the layers of 6mm sheets.

This multiple firing strategy reduces the risk of large bubbles in a stack of multiple pieces. It seems the weight of the 6mm layers forces the air out from between the thicker glass more effectively than thinner layers. 

It is also a simpler set of firings.  If you were to want to make up a 12mm thick piece from 3mm sheets, your heat up will be very long compared to firing two layers in three firings.

E.g. Stone* recommends a heat up for 2 layers of 3mm glass:

240C/hr to 250C, no soak

400C/hr to 500C, no soak (a bubble squeeze could be inserted here by raising the target temperature to 650, with a 30-minute soak)

500/hr to top temperature.

This is about 2.3 hours to top temperature without the bubble squeeze and 6.7 hours to cool.  This means that you could fire twice in one day, if organised well.  If you are planning a final tack fused layer that should be done in the last firing of the combined layers.

However, it is a much longer schedule recommended by Stone for 6 layers of 3mm glass:

• 25C/hr to 125 for 20’
• 30C/hr to 250 for 20’
• 40C/hr to 375 for 20’
• 50C/hr to 520 for 15 (a bubble squeeze could be inserted here by raising the target temperature to 650, with a 30-minute soak before continuing at the same rate to the top temperature).
• 150/hr to target temperature

This is about 18 hours to top temperature without the bubble squeeze and another 18 hours to cool.  This strategy requires 1.5 days, assuming all the layers are even.  The same amount of time is required for both strategies, but the chance of large bubbles is dramatically reduced.

He recommends for 3 layers of 6mm glass:

• 200C/hr to 250, no soak
• 340C/hr to 500, no soak
• 400C/hr to 600, no soak (a bubble squeeze could be introduced here by changing the target temperature to 650 with a 30-minute soak)
• 500C/hr to top temperature.

This is about 2.5 hours to top temperature and 18 hours to cool without the bubble squeeze.

This means that it only takes 2/3 of the time to fire 3 layers of 6mm glass than it does to fire 6 layers of 3mm glass.  Yes, you lose some time in firing the pairs of 3mm glass, but you gain in reducing the risk of creating large bubbles that will ruin your final piece.

Inclusions

If you are putting elements between the initial two-layer pieces for fusing, you need to introduce a bubble squeeze.  Putting elements between the fused pairs will also require a bubble squeeze on the final firing.

Tack fusing the final layer

Note the times indicated above are for even layers.  If you have uneven layers or are tack fusing, the times will be extended much further than the ones noted there.

For a tack fused set of top layers, you will need to add those in the last firing, or do a sharp tack firing before the last firing.  In the case of a tack fused pair for the top layers you will need to reduce the rates of advance for the last firing by about 1/3. This would mean:

• an initial rate of 135C,
• a second ramp of 230C,
• a third of 270C and
• the fourth of 335C instead of the rates for even layers. 

You will also need to reduce the top temperature.  Observation will be required to determine when the correct profile has been achieved.

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

When firing multiple layers of glass, the risk of creating large bubbles can be reduced by firing pairs of 3mm sheets, and then combining the results into one stack.

*Graham Stone. Firing Schedules for Glass, the Kiln Companion, 2000, Melbourne Australia.  ISBN 0-646-39733-8

Firing Uneven Layers

There are a number of people firing stacked layers of glass in a pyramidical fashion to melt the layers down, in a nod to the 1950’s.

Annealing of a full fused platter of this nature is easier than a tack fused one. The degree of contour still in evidence will be important in determining what the annealing schedule should be.

Full or Tack Fuse

In this example shown by Vicki Urbich there are at least four layers - five if the base is two layers, although in this case there is only one.  A full fuse at 800°C will not be enough to give a flat piece for a trivet or platter.  You could extend the soak at 800°C but, it is better to go to 816°C for ten minutes rather than an extended soak at lower temperatures to avoid devitrification.

Damming

The edges of this piece will be wavy, unless dammed, because of the uneven layering.  Placing dams around will give crisp edges to the piece, even though the stacked pieces will round and spread. 

Annealing stacked pieces

Anneal this set-up for at least 9mm for a full fused piece. The pieces will spread and attempt to fill the gaps between the stacks.  Even with an 816°C fuse, the pieces will not be perfectly even in thickness.  To be safer, and account for the remaining unevenness, anneal as though it were 12mm thick.  Other lay-ups will have slightly different requirements.

If it is to be tack fused, you will need to anneal considering the different thicknesses across the piece.  You will have nearly 12mm thick at the thickest and only 3mm at the thinnest.  The generally accepted recommendations are to anneal for twice the thickest part - 24mm in this case.

The anneal is more than the length of soak. It is a combination of the soak and the rate at which you cool to at least 370°C. The cooling rate is directly related to the length of the soak.  If you require twice the length of soak at the temperature equalisation soak, you will require half the speed of anneal cool.  The Bullseye Chart forAnnealing Thick Slabs will give you the relevant rates regardless of the glass you are using. The temperature points will change for other glasses, of course, but the rates remain the same.

Rate of Advance

The earlier problem this lay-up gives you is the heat up to avoid thermal shock. 

The heat up of 4 layers of glass stacked on a single or even two-layer layer base is more difficult than for even layers across the whole piece. Each upper piece shades the heat from the lower ones, making for cool and hot areas next to each other.  With four layers, each layer needs to heat through to transfer its heat to the one below.  This means the bottom of the stack will take a long time to become as hot as the top layer.  Meanwhile, the uncovered glass is getting as hot as the top of the stack.  This often leads to the bottom layers breaking from the stress of the uneven heating.  

Graham Stone suggests 100°C (180°F) per hour for four, even layers. As this is four uneven layers, the rate of advance should be at least half that. This should be used all the way up to the bottom of the bubble squeeze to allow all the glass to heat at the same rate. Glass generally reacts better to a slow, steady contant rate of advance in heat, than faster rates with multiple soaks.

Bubble Squeeze

The bubble squeeze for this single layer base piece can be as quick as 50°C per hour over the 50°C range.  It does not need to be slower, as the weight of the stacks pushes the air out between layers more quickly than large, even and lighter layers.  A double layer base requires a slower bubble squeeze because the weight of the stacks will push the air out to be between the two base layers.  This means a rate of 30°C or even 25°C through the range.

Then you can go faster to the top temperature.

Firing uneven layers requires extreme care on the initial heat up to avoid thermal shock.  A high fusing temperature is needed to get an even thickness across the piece.  Annealing is easier to calculate for even pieces, but must be much more cautious for tack fused items.

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

Long Annealing Times

I’ve noticed a lot of people are using long annealing times for full or contour fused glass of 6mm thickness.  Whether this comes from the advice for tack fused pieces to be annealed as though there were twice the total thickness of the piece, or not, is uncertain. 

Temperature Equalisation

This term employs the concept of what happens at the annealing point chosen for your glass.  The soak at the annealing point - or even at some degrees below - is the process of ensuring the glass is all at the same temperature before proceeding to the annealing cool.

A “flattish” fused piece of 6mm needs only a half hour when the temperature equalisation soak is at the annealing temperature.  When using a temperature equalisation soak about 30°C-35°C below the annealing temperature – as Bullseye, and now Wissmach, does – you may need an hour.

Tack fusing is much more difficult to anneal properly than a full or contour fuse.  The general rule of thumb has become that you must schedule the firing as though it is twice the actual maximum height. 

E.g., if you have a two-layer piece with other pieces distributed around this flat base, you have a 9mm thick piece in total height. Scheduling for this piece as though it is 18mm thick requires longer soaks and slower cools.  In this case, you should schedule for a three-hour soak at temperature equalisation, whether using the annealing point or a lower temperature. 

Other kilnformers have found that firing tack fused pieces as though they are one and a half times the actual maximum thickness provides a perfectly adequate result.  It will be up to each person to decide which approach they take.  Once the current lock down is over, I am going to do some work on the actual requirements.

Annealing Cool

People seem to cool a tack fused piece at a rate suitable for thinner pieces. It appears that the general practice is to use rates suitable for a for 12mm thick piece regardless of the calculated thickness. 

But the annealing cool is still too rapid for a tack fuse.  If you need an anneal soak for 19mm, you need to cool for that thickness too. An annealing cool for a 19mm piece is one half of the rate of that for a 12mm piece.  It is during this too rapid cool that stress can be induced, especially on a tack fused piece.

The temperature equalisation soak and anneal cool can take as much as twice - or more – as long as a flat piece of the same total height.  This needs to be allowed for in the scheduling of the firing.


The length of anneal soak and the rate of anneal cool both need to be related to the appropriate thickness.  There is the Bullseye chart for annealing thick slabs the rates of which, can be applied to any fusing glass.  You only need adjustment to the temperatures for your glass, if not Bullseye.

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

Odd Schedules

Schedules appear on the internet which do not seem to have a logical sequence in the firing schedule.  Some have multiple soaks at intervals up to 540°C.  Others have kind of dance toward the top temperature – slow, quick, slow.  Some initially cool at a given rate and then slow to about half that initial rate.

Multiple soaks

These schedules have been referred to as catch-up schedules.  They tend to look something like this:

200°C to 150°C for 20 minutes

250°C to 300°C for 20 minutes

300°C to 590°C for 20 minutes

50°C   to 677°C for 30 minutes

330°C to 804°C for 10 minutes

AFAP   to 482°C for 60 minutes

60°C   to 370°C for  0 minutes

Off

The justification for the first two soaks is given as allowing the glass to catch up to the air temperature.  It would be much safer for the glass to have a moderate steady advance in temperature rather than risking the heat shocking of the glass.  You could achieve the same work in the same amount of time by altering the rate of advance to a single one of 198°C to 590°C.  This achieves the same temperature in the same amount of time, but has less risk of heat shock, as there is a steady input of heat.  

Secondly, if the glass can survive the initial rate of heat up without breaking, there is no need to soak at an arbitrary temperature.  The first relevant point where a change in temperature makes sense is above the softening point, which for most fusing glasses is about 540°C. The equivalent softening point for float glass is about 700°C

Slow, quick, slow

This kind of schedule alters rates up and down with little justification as far as I can see.  This is an example:

139°C  to 560°C  for 30 minutes

222°C  to 621°C  for 30 minutes

139°C  to 786°C  for 15 minutes

9999 to  515°C  for 120 minutes  

60°C   to 427°C  for 10 minutes

115°C  to 350°C  for 10  minutes

The question for me is why the slow down to top temperature. There is a lot of heat work being put into the glass, so that the higher top temperature may not be required.  The slower rate from 621°C does allow a form of a bubble squeeze to occur, but is not the traditional one.  A 139°C rate from 621°C to 677°C with a soak would be faster than usual, but may be acceptable.  I would prefer 50°C per hour with a 30-minute soak at the end.  Then advancing at 300°C per hour to top temperature.  The anneal soak and cool of this schedule are acceptable, even though different than I would do it.

Erratic Slumping Schedule

The fusing schedule above was followed by this slumping schedule:

83°C to 148°C  15 minutes

167°C to 590°C  10 minutes

83°C to 720°C  10 minutes

222°C to 410°C  120 minutes

83°C to 427°C  10 minutes

This schedule seems to have a catch-up phase in that it goes at half speed for the first 148°C and then doubles the speed to 590°C (a little above the brittle phase of the glass).  It then slows the rate and continues that to a very high slump temperature.  It is, of course, necessary to have a slower rate of advance in the slumping than the fusing, as the piece is now thicker. Slowing the rate of advance as much as in this should be able to achieve the slump at around 620°C (100°C) less than the target temperature used by the schedule. 

Once the top temperature soak is finished, a very slow cool to the annealing soak is used in this schedule.  This is not ideal as it invites devitrification to form.  The kiln and its contents should be allowed to cool as quickly as possible to the temperature equalisation soak at the annealing point.

The schedule then uses an annealing soak temperature 100°C below that used for the fusing. This does not make sense. The annealing soak should be at the same temperature for both firings.  The length of the soak is not in question, but the early turn off the kiln at 427°C is questionable. The anneal cool of the fused piece extended down to 350°C.  The anneal cool on slumping should be almost the same as the fuse.  Almost all anneal cools extend to 370°C at least.

Anneal Cools

Some anneal cools have erratic rather than progressive cooling.  In this example the early part of the schedule is eliminated:

……………..

AFAP to 482°C 120 minutes

110°C to 427°C 0 minutes

55°C to 370°C 0 minutes

200°C to 100°C 0 minutes

off

Here the schedule is faster in the most critical part of the anneal cool than in the next, cooler part.  This will not provide as good an anneal as if the first two segments after the equalisation soak were reversed.  Start slowly in the anneal cool and then you can speed up (approximately twice the previous segment rate) on each of the following segments.

Rationale

This critique of the schedules above is not to batter anyone.  It is to make clear that there needs to be a conscious rationale for each of the segments in relation to the others.  If you take a schedule from a source, it is a good idea to see if there is a reason for each segment and how it relates to the next. 

·        The scheduling must take account of the nature of the glass.  Glass is a poor conductor of heat and needs steady moderate input of heat.

·        Glass is brittle until approximately 55°C above the annealing temperature when you can accelerate the rate of advance.

·        Time is required to allow air out from between the layers of glass. This usually done in the range of 620°C to 675°C and is known as the bubble squeeze.

·        You need to go relatively quickly through the devitrification range of temperatures – approximately 700°C to 760°C - both up and down.

·        Glass needs a temperature equalisation soak at the annealing point (or nearby) related to its thickness.

·        The rate of cooling needs to be progressive.  The first 55°C below the annealing soak is the most important.

·        Cooling rates must be related to thickness.

·        The second cooling rate can be up to double the initial one.

·        The final cooling rate can be double the previous one.

·        The rate of firing will affect the required top temperature.