Diurnal Firing Practices

It is most common for people to fire overnight so they can see their piece(s) the next morning.   This is a poor practice for novices.  Not simply a lazy one. It is a practice that leads to use of others’ programs and practices, rather than building on one’s own experience and practice. Others’ programs are used because they were successful for them.  They may not be successful for you. The number of failed projects that are discovered when the kiln is opened, show that it is often not possible to transfer another’s schedule to your project.

The ability to fire while you are absent is a great advantage to kiln forming practices.  The widespread use of the controller has brought many advantages to kiln formers, not least that they can get some sleep and have a social life. They no longer need to be beside their kiln all the time it is firing.  The controller has also made it possible to set the ramp rates and soaks without calculations.  And without having to set periodic alarms to remind us to check the kiln to see if it is advancing at the correct rate.

Before controllers it was necessary to sit beside kiln to watch what was happening and adjust the ramps and soaks to conform to what was planned.  It was also necessary to observe how the glass was behaving and adjust the power input accordingly. Now we can set the controller to give what we hope will be a good result.  We find out when we open the kiln in the morning whether it is right or not.

I am not advocating returning to the days before controllers.  I enjoy my sleep and social life too much for that.

I am advocating the use of a feature almost all controllers have.  The Delay function.  On most controllers, it is the first thing that comes up on the display.  We mostly ignore that and proceed to the first ramp.  We set the controller to fire immediately, so that it will be done overnight and we can look in the morning or when we come back from work.  That way all the waiting for the piece to be finished can be eliminated.  We can go to work or to sleep knowing that the firing will be done when we can get back to the kiln.

This practice leads us to miss the real learning process that is available by observing the process of the firing.  Observing the firing can tell you when your slump is done, when it is slipping to the side, when it breaks, when more time is needed, when more heat is needed – almost everything that people ask questions about their slump – or in other instances, the fuse or melt.

People ask what temperature they should use for the kind of tack fuse they want.  Many suggestions can be made.  Trial and error will eventually tell which is the right combination of rate, temperature and time for the result you want. Observation during the firing will tell you immediately when the temperature is high enough, or the soak is long enough.  As you peek into the kiln through the observation ports you can advance to the next ramp when you have achieved the look you want. 

You do have an observation port, don’t you?  It is one of the essential features to be included in a kiln. If you don’t have one, you can open the lid or door momentarily to observe the state of the glass in the midst of the firing.  You could make an observation port by drilling through the casing and insulation.  You then place fibre blanket or a formed piece of kiln brick in the hole when not in use for peeking into the kiln.  You will not change the performance of the kiln by doing this.  Of course, if you have side elements, this retrofitting of an observation port is risky.


“I need a life.  I have to work.  I have to sleep.  I can’t be around my kiln all the time.” 

The legitimate responses to the idea that you should be around to observe the work at critical temperatures are that “I need a life.  I have to work.  I have to sleep.  I can’t be around my kiln all the time.” This is where the Delay function comes to your aid. You can use the Delay function to make sure the firing is at the critical point for observation at a time that is convenient for you.  This way, you do not disrupt your normal life.  Your social life can continue and you can get some sleep too.

An example will help understanding how you can make use of the Delay function. 

If you have time before you go to work, you can set it so that the firing comes to the critical point about an hour before you have to leave for work.  Or if it is better for you, you can set it so that time for observation is after you get home from work, or after dinner, etc.  Even if you don’t have a day job, you can use the Delay function to make sure you will be able to give the kiln the attention it needs at a convenient time for you.

How do I do this?  It is a setting of the amount of time to elapse before the kiln starts to fire on the first ramp.  Most programmes have firing times for each ramp.  You select the cumulative times up to the end of the ramp for the observation to begin.

This does not need to be difficult

This sounds complicated?  Not really.  It is a bit of arithmetic, though.  Add the times for each ramp together to get the time the kiln will take to get to the observation temperature and soak. 

E.g.:

200C/hour to 630C for 30 mins   =3.15 hours plus the 0.5 hour soak.  (divide the target temperature by the ramp rate, in this example 630/200=3.15 hours or 3 hours and 9 minutes).   Don’t include the soak time in this calculation as that is the part of the observation that is or may be variable.

Assume it is 10:00pm and you want to look at it at 7:00am.  This is 9 hours. The kiln needs 3 hours and 9 minutes to get to the temperature you want to observe the slump.  Subtract 3 hours and 9 minutes from the 9 hours you have and this gives you 5 hours and 51 minutes to set in the Delay function. 

Some controllers do not allow hours and minutes, but require only minutes.  In this case, multiply the hours by 60 and add the minutes.  In the example, there are 300 minutes in 5 hours plus 51 minutes gives 351 minutes to be put into the delay function. 

In this example, this will have the kiln at 630C at 7:00 am. Ready for you to observe the progress of the slump.  When the slump is finished, you can advance to the next segment and head off to work, allowing the cool and annealing to proceed, and knowing the slump was successful.  The same applies to other times of the day.  You could, for example load the kiln and schedule the delay to be at the critical temperature for when you come home from work.

Setting the delay function for an exact tack fuse is a little more complicated.

If you are looking to get an exact tack fuse profile, the schedule will be a little more complicated.  Say you want a rounded tack fuse that you think will be achieved at 750C in 10 minutes.  The schedule might look something like:

Ramp 1: 200C/hour to 650C for 30 minutes =3.15+0.5 hours =3.65 hours

Ramp 2: 300C/hour to 750 for 10 minutes =0.33 hours (750-650/300) + 0.167 hours soak.

Adding these two ramps together gives you 3.95 hours.  Here you include the soak at bubble squeeze temperature in the first ramp, but not in the second, because that is what you are checking on. If it is 7:00am now and you won’t be back until 6:00, that is 11hours until you will be looking in at the progress of your piece.  So you subtract 3.95 from 11 and you set the Delay as 7 hours, 57 minutes (or as 477 minutes).  Then it will be ready for observation when you come home.

Won't I loose firing time by using the delay function?

You may feel that you are going to lose a firing by using the Delay function.  You often can peek into the kiln in the morning to see how things have turned out by using the overnight firing.  But there normally is still more cooling down time required. 

If you delay the top temperature until the morning to see what is happening, you still have the rest of the day for the kiln to cool and be ready for re-loading in the afternoon or evening.  During that time, you can be preparing the next firing.  So you have not lost any kiln time, but you have gained the knowledge that the firing is OK through any adjustments you made at the critical temperatures.

If you were to fire during the day to be able to open the kiln in the evening as your normal practice, you will lose one firing at the start of this kind of practice.  As you progress with the new practice, you will find that you do not lose the number of firings you are able to complete in a week.  Again, you are preparing the next kiln load while the kiln is cooling.

Yes, it does require doing things a little differently.  But essentially it moves the kiln preparation on by 12 hours.  That’s why I call it diurnal firing.  You are just changing by 12 hours your daily practice. You make things ready for the kiln to fire overnight, and prepare the new piece(s) during the day.  Or prepare the pieces in the evening to fire during the next day.  You still are preparing the next kiln load as the kiln cools off from the previous firing.

The extra planning effort is rewarded by more rapid learning

This little extra planning is rewarded by the ability to see what is happening in the kiln, so that you can adjust during the firing, rather than having to do a firing again, or in the worst case, completely re-make a piece after a disaster. 

You also learn much faster about the desired programmes required to get specific results.  Instead of doing multiple firings to find the exact temperature needed for the desired result, you can do it in only one or two firings.  This saves you lots of time, glass and electricity. 

Observation is really necessary for free drops – aperture drops, screen melts, pot melts, etc.  These require observation to get the desired results, as their progress is so variable from one firing set up to another. Using the Delay function will enable you to have the firing at the stage where observation is important when you are best able to be there to watch.

The alteration to your working practices to make use of the Delay function will be amply rewarded by the rapid learning that observation of the firing promotes.  This essential tool to aid in designing appropriate firing programs is too often ignored in teaching and using firing schedules.

Finally, it allows you to set up programs that are pre-set for your kiln and kind of work.  You will have learned the exact rates and temperatures and soaks to put into your programs.  These become your saved schedules that are tailored to your practice.

Firing Rates

Top temperature is, to a small extent, variable between kilns, even from the same manufacturer.  But it is a small part of variations in top temperature required to get the same results in differing kilns.

An example of a firing schedule

It is, more importantly, a function of how the heat is put into the glass. Firing as fast as possible to the top temperature does not allow all the glass to be at the same temperature. This is because glass is a good insulator and the transfer of heat from the top or the sides is relatively slow.  For small things, you can fire very fast, as there is a small mass of glass to absorb the heat.  But a speed of 250°C is fast enough for anything more than 100mm square and at least two 3mm layers thick.  (Thicker glass requires slower rates of advance as surprisingly do single layer projects).  The slower rate of advance allows the glass to be all of a similar temperature from top to bottom, allowing the desired effect to be achieved at lower temperatures or shorter soak times. 

For example, a slower rate of advance will give rounded edges at shorter soak times than a rapid rate of advance will require.  Alternatively, it might require a lower temperature with the same soak time.  Keep in mind that, in general, lower temperatures with slower rates of advance, give better results.

The faster your rate of advance, the more the glass lags behind the air temperature (which is what pyrometers are measuring). Therefore, a reasonable pace will give better results than the as fast as possible rate of advance. 

In short, the variations in top temperature required and length of soak is not about the kiln firing cooler or hotter as much as it is about the firing rate.

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.

Hot Short Firings - Kiln Forming Myths 28

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

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

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

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

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

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

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

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

Spacing of Pieces on the Shelf

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

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

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

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

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.

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.

Firing AFAP - Kiln Forming Myths 19

Firing as fast as possible harms your kiln, or at least will wear out the kiln elements.

I believe this comes from the days when ceramic kilns were commonly used.  Certainly this is still the mantra among ceramicists today.

A number of people fire their kilns as fast as they can, especially small ones, all the time.  Refractory fibre kilns are not affected at all by rapid changes in temperature. 

There might possibly be some small damage to the light weight refractory brick used in glass kilns in that the frequent expansion and contraction may cause crumbs to fall from the brick.  But this will happen anyway as the brick expands and contracts the same amount every time it is fired.  There is no definitive information on whether rapid increases in temperature have any greater effect on brick than slower increases. 

Any rapid change in temperature is unlikely to affect the kiln elements.  Attempting to bend the elements while cold is very likely to break them, as a compound is formed on the surface which makes them brittle when cold.  But this is very different from rapid changes in temperature.  As an analogy, the elements in electric fires are made of the same material and are always heated as fast as possible from cold.  They have a long life, so there should be no difference in effect on kiln elements, which are generally thicker and less exposed to drafts and rapid temperature changes once hot.

It could be said that firing as fast as possible would reduce the stress on the relays in the controller, as they will be closed for the whole of the temperature rise, with no opening and closing.  Thus, the number of firings will be increased without equally increasing the number of cycles the relays have to perform.

However rapid rises in temperature affect the kiln is secondary to how it affects the glass.  Except for small pieces, extremely rapid rises in temperature increase the likelihood of the glass breaking.  This is the more important consideration when thinking about afap firings.

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”.

Absolute Firing Temperatures - Kiln Forming Myths 9

There is a given temperature for each level of fusing – slump, tack, full, etc.

You will often see statements about the temperature for achieving a particular effect.  It is as if all glass under all circumstances does the same thing at a given temperature. These temperatures can only be understood in relation to several things.

• ·         Kiln characteristics
• ·         Speed of firing – i.e., heat work
• ·         Time at forming temperature

The relevant factors about the kiln are:

·         Insulation.  The two main types of insulation in kilns are fibre blanket and insulating brick.  Fibre blanket is often the main insulating element in kilns as it does not absorb a lot of heat. It of course loses heat more quickly than refractory brick.  Most often the floors of kilns are made of brick for rigidity and resistance to damage.  (They also can be replaced individually if one is damaged.)  Refractory brick comes in two densities.  The light weight one is not rated to such a high temperature and loses heat more quickly than the higher temperature rated dense brick.  Both lose heat much more slowly than fibre blanket.  This means the top temperature can be reached more quickly in a fibre insulted kiln than in brick insulated kilns. The brick insulated kilns radiate the heat back into the kiln upon cooling, making for long safe anneal cools without much effort in controlling the cooling rate. Thus the temperatures for an effect are different for kilns with bricks all around than with fibre blanket, and no comparison is easy between kilns with different insulations.

·         Size.  The size of the kiln has an effect on the temperature cited to achieve an effect.  A small kiln can heat up very rapidly, but the glass cannot heat evenly as quickly.  A large kiln takes more time to heat up, as there is more insulation absorbing the heat input.  So working temperatures for small and large kilns are different.  The size of the piece(s) of glass also have an effect.  Small pieces can be heated much more quickly than large or thick pieces, so the top temperature for an effect will be different for the two sizes.

·         Temperature variation across the kiln shelf affects the rate of firing possible and (as noted later) will affect the top temperature.  The more even the heat the faster it is possible to go and that affects the temperature chosen.

·         Element placement.  Some kilns have only side elements, some only top elements, and some have both.  All these variations affect the temperature required to obtain an effect.  In general, top fired kilns can be fired faster than side fired kilns.  Kilns with both, require an intermediate rate, unless the side and top elements can be fired independently.

Speed of firing, i.e., heat work

·         Heat work factors make the top temperature different in different circumstances.  This is mainly about the speed at which you fire the glass.  Generally, the slower you fire, the lower temperature you need.  Allowing the glass to absorb the heat gradually usually means that you can achieve a particular effect at a lower temperature.  A fast rise in temperature requires a higher temperature.

Soak times 

·         The amount of time you soak at the working temperature will also affect the temperature chosen.  A longer soak allows a lower temperature to be used (although that can get into the risk of devitrification from spending too long at the top temperature – it is a balancing act).  A higher temperature can be used to keep the soak time reduced. 

All these variables mean that without being given the kiln characteristics and a schedule, you cannot evaluate the temperatures and rates of firing that are given out by others.  You need to know how closely their kiln fits with your kiln in its characteristics as outlined above.  When asking for a temperature or a schedule, you should indicate what kind of kiln you are using.  You need to know in any schedule what the ramp speeds are and the soak times.  They can then, of course, form the basis for your experimentation.

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”.