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.

Annealing Range for Unknown Glass

It is possible to anneal unknown glass with some degree of certainty by using what is known as the slump point test.  This will not be as accurate as a factory determined test, so you do have to extend the range over which you do the annealing.  

The annealing of glass with unknown characteristics is possible in two ways - shotgun and calculated.  The examples here are for 6mm thick glass.  The soak and cooling times need to be extended for thicker glass.  

Both the shotgun and calculated approaches exemplified here assume glass of 6mm thickness.  For thicker glass the soak time needs to be extended and the anneal cool rate slowed more than indicated above.  Using the Bullseye chart for annealing thick slabs will give you an indication of the relationship of thickness to speed.

1)  One is the traditional shotgun approach – pick an arbitrary, but slightly high temperature, and soak for a minimal amount of time there. Then go very slowly through the next 55°C.  This may be as slow as 25°C per hour, followed by a doubling of that rate for the next 55°C. Then double again to 300°C or less.

2)  By using the slump point test and the calculations, you will be sure of the annealing point/temperature equalisation point within 10°C.  The approach here would be to soak for half an hour at the calculated temperature, followed by a slow drop of 50°C per hour to 55°C below annealing soak and then at 100°C/hr to 110°C below your chosen temperature equalisation point. The final cooling could be at 200°C to room temperature.

2a) An additional tweak to the slump point test calculations is to use the Bullseye concept behind their recommendations for thick slabs.  Using their concept, you reduce the calculated annealing point by 30°C from the calculated annealing point to do the temperature equalisation soak at the lower end of the annealing range.  Having calculated the annealing point, you reduce that temperature by 30°C and soak for  a longer time of 60 minutes and at a slower rate as noted in the chart.

In using the chart for unknown glass you substitute the calculated temperatures, but continue to use the rates and times indicated.  An example:

• You have calculated that the annealing point is approximately 535°C.
• Subtract 30°C from that to get a equalisation temperature of 505°C.
• Assume the piece is uniformly 12mm thick or 6mm tack fused (when you want to use rates for  twice the actual thickness to account for the difficulties in tack fusing). 
• For a 12mm thick piece your soak time at 505°C will be two hours.
• The cooling rate for the first 55°C is given as 55°C per hour according to the chart. Therefore the first cooling segment will be 55°C from 505°C to 450°C.  The second will be 99°C per hour from 450°C to 395°C.  The third rate will be 330°C per hour from 395°C to room temperature.

You can see that the times and rates are taken as given by the chart (as determined by the thickness of your piece), but the temperature set points are determined by the calculations for the glass you have tested.

When determining what temperature you should use to anneal a glass about which you are uncertain of its characteristics, you can use one of two basic approaches.  Pick an arbitrary temperature and soak for some time there and then proceed slowly in 55°C segments to about 370°C.  A second more certain method is to use the slump point test to determine the annealing point and then apply the Bullseye chart for thick slabs for the soak times and cooling rates.

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

How Close to the Edge

“How close to the edge of my shelf can I place a large piece?”

It depends in one sense how thick the piece is.  A 6mm piece that maintains the same footprint after firing as before, does expand beyond that footprint by about half a centimetre during the firing, so it would be safe to have a full centimetre space to the edge.  Thicker pieces will need more space – 9mm will need about two centimetres to accommodate the expansion at the top temperature. 

But

The real answer to this question is: When you know the heat characteristics across your shelf, you will know how close you can go to the edge for a relatively large piece. 

This Bullseye Tech Note number 1 tells you how to test the variations of temperature across your kiln. - http://www.bullseyeglass.com/methods-ideas/technotes-1-knowing-your-kiln.html

The objective in cooling glass is to have less than a 5C difference in temperature over the whole of the glass piece – top to bottom, and side to side.

If you have greater differences in temperature than that at the edges of your kiln shelf, you need to avoid placing large pieces in the danger area. Small pieces will not suffer by being close to the edges as their temperature differentials will be small.

I have found that the temperature differential in one of my kilns is great enough at the edges that I cannot have the edge of a relatively large piece of glass nearer than 50mm (2") from the edge.

Controlled cooling

It is sometimes stated that you can simply turn the kiln off below 370C and let the kiln’s natural rate of cooling take over the cool down.

This works for most flat 6mm pieces in most kilns, but as you work thicker or with greater contrasts in thickness, lots of tack fused elements or in a small rapidly cooling kiln, you do need to control the cooling toward room temperature.

The first thing you need to know is the natural cooling rate of your kiln.  

The rate of cool is not just about the annealing soak. The soak at annealing temperature is to equalise the temperature throughout the blass to have a differential of not more than 5C. 

The rate of cool is about avoiding thermal shock, too. The glass needs to maintain the temperature variation to less than 5 degrees Celsius difference throughout the glass as it cools.  This requires a slow controlled cool.  

You may program a cool of 100C to 370C thinking that the kiln will maintain that rate or less.  If the natural cooling rate of your kiln at 370C is 200C/hour, you risk thermal shock due to the rapid increase in the cooling rate.

You really do need to know the natural cooling rate of the kiln from the point you turn the programmer off to room temperature to be safe from thermal shock.

The alternative to turning off at 370C is to program the schedule all the way to room temperature.  The kiln will use no energy unless the kiln cools too quickly on its own.  At which point the program will kick in to slow the cooling of the kiln.

Finding Your Kiln’s Natural Cooling Rate

You need to observe how your kiln behaves while cooling without any power to be sure when you can safely &turn it off and let it cool without power.

Assuming you have programmed your kiln for a shut off at 370C, you need to observe every quarter hour or so to record both time and temperature.  From those observations you can calculate the cooling rate at the various temperatures.

Say at 6:00 your kiln was at 370C;

At 6:15 it was at 310C;

At 6:30 it was at 265C;

At 6:45 it was at 230C;

At 7:00 it was at 200C;

At 7:30 (you missed the quarter hour) it was at 160C;

At 8:00 it was at 140C;

At 9:00 it was at 125C;

At 10:30 it was at 110C.

To calculate the rate, you divide the temperature difference by the proportion of an hour between observations, as demonstrated in the following table.

Kiln Name/Description
Size
Shelf composition
Amount of glass
Observations
	Time	Temperature	minutes	Proportion	temperature	Rate of
1st	06:00:00	370	difference	of an hr	difference	cooling
2nd	06:15:00	310	15	0.25	60	240
3rd	06:30:00	265	15	0.25	45	180
4th	06:45:00	230	15	0.25	35	140
5th	07:00:00	200	15	0.25	30	120
6th	07:30:00	160	30	0.50	40	80
7th	08:00:00	140	30	0.50	20	40
8th	09:00:00	125	60	1.00	15	15
9th	10:30:00	110	90	1.50	15	10

Although this is an example, it shows how the cooling rate slows down as the kiln cools. 

If you were cooling a flat piece 12mm thick, you might get away with turning the kiln off at 370C, as a flat piece can cool as quickly as 300C/hr.

If you were cooling a piece 19mm thick, the natural cooling rate of the above kiln is too fast. 19 mm thick pieces need a cooling rate of 150C/hr, so according to the figures above you need to programme this kiln down to 230C to get the appropriate final cooling rate.

If it is a tack fused piece with a 6mm base and areas of two layers of tack fusing, you should fire as though it is 24mm thick.  In this case, the final cooling rate needs to be 90C/hr.  For the kiln in the example above, that rate is not achieved until below 160C, so that is the minimum temperature for switch off.

This method can be used for any temperature range.  For example, you may want to know the rate of cooling from the top temperature to the annealing temperature.  This method will work there too. You may want to record the temperatures more frequently than every quarter of an hour though.

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

You really need to know your kiln’s natural cooling rate before you can be confident of switching the kiln off at 370C.  This blog shows a method of determining the natural rate of cooling. 

Strain Points and Annealing Ranges

I received the following question a while back and thought my response might be useful, although very informal.

“Can you dumb down the concepts of 'annealing point' and 'strain point'? I understand anneal point to be a fixed point (depending on the glass) but the strain point is a range...is this correct? I understand the concept of a hold at the anneal point but I'd like to understand how to bring it down through the strain point.”


I really dislike the idea of dumbing down concepts in kiln forming glass. Glass chemistry is incredibly complicated. Glass physics is still little understood. Glass is a very complicated subject. The marketing of glass for kiln forming has led us all to think it is a simple matter of recipes. Well it's not.

Having got that rant out of my system.... Let’s go ahead.

The annealing point is roughly defined as the temperature at which the glass (if it is the same temperature throughout) will relax most quickly. In the practical kiln forming that we do, it is not possible to ensure that the glass is that temperature throughout. So it is better to think of an annealing soak at the annealing point to allow the glass to become a more even temperature throughout its thickness. As thicker glass means the heat has further to travel from the centre to the surfaces, a longer soak is needed for thicker glass.

The annealing occurs during the slow cool past the lower strain point. The annealing occurs best with a slow, but steady drop in temperature. So annealing is occurring over a range, not at a point. We all rely on a combination of the manufacturers' recommendations, various writings we read, and experience to determine that rate, although Bullseye have published a chart which is most helpful, whichever glass you use.

Strain points.

There is an upper and lower strain point, although this is disputed by some. There are mathematical definitions for these as well as observational definitions. I do not understand the mathematics of either. In lay terms, the lower strain point is that temperature below which no further annealing can take place. It is safe to assume this is 50C below the annealing point (I think it actually is 43C, but I'm not certain of this number).

So it is safest to control the cooling to at least 5C below the lower strain point. Bullseye find that cooling from the annealing soak to 370C is best - this is much more conservative than is theoretically required – 146C below the annealing soak point. This does take care of any problem of thermal shocking of the glass during the cooling.

The upper strain point might be more properly described as a softening point. This also has scientific definitions. The way I think of it is as being the temperature above which no annealing can occur. Another is to think of it as a point beyond which the molecules of the glass are in relatively free motion - which increases with temperature. This again can be considered (on the rise) as 50C above annealing. However on the way down it is safer to consider it to be not more than 30C above annealing. This is because the glass temperature lags behind the air temperature (which is what our controllers measure).

So there is no point in soaking more than 30C above annealing in an attempt to equalise the temperature throughout the glass. However, if you really need to equalise temperature at some point above the annealing point, it might be better to slow the cooling from the working/top temperature and do the final equalisation of temperature at the annealing point.

To answer directly, the strain point by definition of language cannot be a range. There are two points which form the possible annealing range, although the lower one is the critical one. The upper one I described earlier as the softening point. The softening point forms the upper part and the strain point forms the lower part of a range in which the annealing can occur. So the concepts are the opposite of what you propose. They are points which are the boundaries of the annealing range.

To complicate things further, not all glass from one manufacturer has the same annealing point. The published annealing point is a compromise that their experiments and experience have shown to be most suitable. Bullseye glass for example has three annealing points, 532C for opals, 505C for cathedrals and 472C for gold bearing glasses. NOTE: these figures may not be exact; they come from memory rather than documents. Since this list of annealing points was published, Bullseye have conducted further experimentation that shows the best annealing soak occurs at 482C which is below transparent and opalescent, but above gold bearing annealing points.

Schott recommends a range for annealing, not a point, to accommodate these variables. Bullseye, Uroboros, and Spectrum have published annealing points that are practical for people kiln forming in smaller kilns that are less well controlled than the factory lehrs.
If you look at the Bullseye site - education section, you will find a lot of useful information. Especially informative are their tech notes. Spectrum - to a lesser extent than Bullseye - gives helpful information. The information from both sites should be absorbed and the principles applied to other glasses.

Finally, kiln forming is deceptively simple. I have spent 29 years discovering how much more there is to learn. This is one of the reasons that glass is such an exciting medium - people keep discovering new things.

Reviewing the above, I realise that I have not answered your question "... how to bring {the temperature} down through the [lower] strain point". My answer is that you should look at the manufacturer's site for each glass that you use. Look at their rates for annealing for different thicknesses of glass (some also take into account the size). Consider them. Then look at some of the other sites for their published annealing rates for various thicknesses. Comparison of their rates will reveal differences. Think about what they are, how they relate, and whether they reveal that they are using the same principles with slight variations.

Also, if you can, get a copy of Graham Stone's book "Firing schedules for glass, the kiln companion". It provides a handy guide to annealing rates. But DO NOT use it as a book of recipes. Read all the commentary about the schedules, as they (combined with the introductory parts) give principles and tips about how to think about the cooling of the glass.  Bob Leatherbarrow has recently published an excellent book on kilnforming schedules, available from his website.

By the way, experience is so often lost, or misremembered, that keeping a log is essential. My first log consisted of loose leaf binder, so I could file all the same kind of firings for various glasses together (this was in the days when there was not much fusing compatible glass, and I couldn't afford Bullseye at UK prices. I was discovering lots about glass firing and using some schedules that I now wonder how I had any success. I did learn a lot from my failures and recorded them too. Now I use a log, usually an out-of-date A4 size diary, sometimes a manuscript book that is big enough to record observations and illustrations. Bullseye have a good record form on their site.

I congratulate you on your desire to understand the processes. Too many only want to put the glass in and turn the kiln on. That is the desire a number of kiln manufacturers pander to when they put pre-programmed schedules on the controllers. So, don't take any of this as criticism of you or your comments. It is meant in a constructive manner - even though I am told frequently that the manner is blunt, even rude.

Best wishes on continued successful kiln forming.

Revised 22/06/19

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

Firing Bullseye and Oceanside Together

Is it possible to fire Oceanside (formerly Spectrum) and Bullseye at the same time?

Yes, it is possible to fire pieces made of Oceanside and pieces made of Bullseye in the same firing – as long as the glass is not mixed in one piece.

There will be differences in profile as the temperatures for Spectrum are a little less than for Bullseye at all stages.  A rounded tack for Spectrum will be a much sharper edged tack for the Bullseye, etc.  If you can accommodate those differences you can continue to fire.

It is a bit easier on slumping operations as you can use the lower slumping temperature for Spectrum and extend the soak for the Bullseye glass.  Or, choose a mould for the Bullseye that requires less time than the Spectrum, so they complete the slump at the same time.

The annealing points are different, of course.  But not by much – Spectrum is 510°C and Bullseye 516°C (for any but thick pieces).  These are not far away from each other.

There are two main approaches to annealing different glass in the same firing.

One is to use a shotgun approach.  This means that you choose your upper anneal soak – in this case 516°C – and hold the temperature for the required amount of time.  Then proceed more slowly than usual – say 50°C /hour rather than 80C/hour – until about 55°C below the lower anneal point.  Then proceed to the rest of the cooling.

The other approach is to anneal soak at both annealing points before proceeding to the anneal cool.  This approach is probably best with thicker than 6mm pieces than the shotgun method.  It is also required if you use the Bullseye lower annealing point of 482C.  You would anneal at 510°C and again at 482°C and soak at each point for the required time for thickness.  This doubles the annealing time, thus reducing the advantage of one over two firings.

There is a third approach for pieces less than 9mm that will eliminate the double anneal soak.  Choose a single annealing temperature.  The two annealing points for Bullseye and Spectrum are so close (510°C and 516°C) that you could chose a mid-point between them (say 513°C) and soak there before proceeding to the anneal cool.  

It might be even better to choose a temperature midway between 510°C and 482°C (say 499°C) and soak both glasses for a longer period to ensure the temperature is equalised before proceeding to a slow rate of anneal cool.  This will be especially applicable for tack fused pieces, which require more care than full fused pieces.  Remember that you should be soaking at the temperature equalisation hold for at least twice the thickness of the thickest part of the piece.  Then reduce the temperature at the rate recommended for the thickness indicated.  Look at the Bullseye chart for annealing thick slabs for the rates. 

The reason that you can anneal at different temperatures is that annealing occurs over a range of temperature.   The annealing point is the temperature at which annealing can most quickly occur.  There are several of physical changes that are affected by temperature and rates of cooling. 

If you cool too quickly after the anneal soak, you will induce stress and probable breakage.  The cooling after the anneal soak is an essential part of the whole annealing process.  Annealing at a lower temperature requires more certainty that the glass is all equal in temperature.  This means a longer anneal (or temperature equalisation) soak is required.  It is also a good bet to slow the anneal cool to be less than you would use for a single glass.

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