Scheduling with the Bullseye Annealing Chart

This post is about adapting the Bullseye chart Annealing Thick Slabs to write a schedule for any soda lime glass as used in kilnforming.

I frequently recommend that people should use the Bullseye chart for Annealing Thick Slabs in Celsius  and Fahrenheit.  This chart applies to glass from 6mm to 200mm (0.25” to 8”).

“Why should the Bullseye annealing chart be used instead of some other source?  I don’t use Bullseye.”

My answer is that the information in the chart is the most thoroughly researched set of tables for fusing compatible glass that is currently available.  This means that the soak times and rates for the thicknesses can be relied upon.

“How can it be used for glass other than Bullseye?”  

The rates and times given in the chart work for any soda lime glass, even float. It is only some of the temperatures that need to be changed.

"How do I do that?"  

My usual response is: substitute the annealing temperature for your glass into the one given in the Bullseye table.

 "It’s only half a schedule."

That is so.  The heating of glass is so dependent on layup, size, style, process, and purpose of the piece.  This makes it exceedingly difficult to suggest a generally applicable firing schedule.  People find this out after using already set schedules for a while. What works for one layup does not for another.

Devising a Schedule for the Heat Up

There is no recommendation from the chart on heat up.  You have to write your own schedule for the first ramps.  I can give some general advice on some of the things you need to be aware of while composing your schedule.

The essential element to note is that the Bullseye chart is based on evenly thick pieces of glass.  Tack fusing different thicknesses of glass across the piece, requires more caution. The practical process is to fire as for thicker pieces.  The amount of additional thickness is determined by the profile being used.  The calculation for addition depends on the final profile.  The calculation for thickness is as follows:

• Contour fusing - multiply the thickest part by 1.5. 
• Tack fusing - multiply the thickest part by 2. 
• Sharp tack or sinter - multiply the thickest part by 2.5.

The end cooling rate for the appropriate thickness is a guide for the first ramp rate of your schedule.  For example, the final rate for an evenly thick piece 19mm/0.75” is 150ºC/270ºF.  This could be used as the rate for the first ramp. 

Bob Leatherbarrow has noted that most breaks occur below 260ºC/500ºF.  If there are multiple concerns, more caution can be used for the starting ramp rate.  My testing shows that using a rate of two thirds the final rate of cooling with a 20 minute soak is cautious.  In this example of a 19mm piece it would be 100ºC/180ºF per hour.

Even though for thinner pieces the rates given are much faster, be careful.  It is not advisable to raise the temperature faster than 330ºC/600ºF per hour to care for both the glass and the kiln shelf.

Once the soak at 260ºC//500ºF is finished, the ramp to the bubble squeeze should maintain the previous rate.  It should not be speeded up.  The glass is still in the brittle phase.

After the bubble squeeze you can use a ramp rate to the top temperature of up to 330C/600F.   AFAP rates to top temperature are not advisable.  It is difficult to maintain control of the overshoots in temperature that are created by rapid rates.  

The top temperature should be such as to achieve the result in 10 minutes to avoid problems that can occur with extended soaks at top temperature.

In the example of an evenly thick 19mm/0.75” piece a heat up full fuse schedule like this could be used:

• 150ºC/270ºF to 566ºC/1052ºF for 0 minutes
• 50C/90F to 643C/1191F for 30 minutes
• 333ºC/600ºF to 804ºC/1479ºF for 10 minutes

 

If a more cautious approach to the heat up is desired, this might be the kind of schedule used:

 

• 100ºC/180ºF to 260ºC/500ºF for 20 minutes
• 100ºC/180ºF to 566ºC/1052ºF for 0 minutes
• 50C/90F to 643ºC/1191ºF for 30 minutes
• 333ºC/600ºF to 804ºC/1479ºF for 10 minutes

This approach is applicable to all fusing glasses.

 

Adapting the Bullseye Annealing Chart

After writing the first part of the schedule, you can continue to apply the annealing information from the Bullseye chart.  The first part of the anneal cooling starts with dropping the temperature as fast as possible to the annealing temperature.

The method for making the chart applicable to the annealing is a matter of substitution of the temperature.  All the other temperatures and rates apply to all fusing glasses.

Use the annealing temperature from your source as the target annealing  temperature in place of the Bullseye one.  The annealing soak times are important to equalise the temperature within the glass to an acceptable level (ΔT=5ºC).  The annealing soak time is related to the calculated thickness of the piece.  This measurement is done in the same way as devising the appropriate rate for heat up. 

Applying the Cooing Rates

Then apply the rates and temperatures as given in the chart.  The three stage cooling is important.  The gradually increasing rates keep the temperature differentials within acceptable bounds with the most rapid and safe rates.

The temperatures and rates remain the same for all soda lime glasses – the range of glass currently used in fusing, including float glass.  The soak time for the calculated thickness of your glass piece will be the same as in the Bullseye chart.  

This means that the first cooling stage will be to 427ºC/800ºF.  The second stage will be from 427ºC/800ºF to 371ºC/700˚F.  And the final stage will be from 371ºC/700˚F to room temperature.

I will repeat, because it is so important, that the thickness to be used for the anneal soak and cooling rates for your schedule relates to the profile you desire.  A fuse with even thickness across the whole piece can use the times, temperatures, and rates as given in the chart as adapted for your glass.  The thicknesses to use are for:

Contour fusing - multiply the thickest part by 1.5. 

Tack fusing - multiply the thickest part by 2. 

Sharp tack or sinter - multiply the thickest part by 2.5.

An annealing cool schedule for 19mm/0.75" Oceanside glass is like this:

• AFAP to 510˚C/ 951˚F for 3:00 hours
• 25˚C/45˚F to 427˚C/800˚F for 0 time
• 45˚C/81˚F to 371˚C/700˚F for 0 time
• 150˚C/270˚F to room temperature, off.

Many will wish to turn off the kiln as early as possible.  This is not part of best kilnforming practice.  If you still wish to do this, the turn off temperature must be related to the thickness and nature of the piece.  To turn off safely, you need to know the cooling characteristics of your kiln.  This can be determined by observing the temperature against time and then calculating the kiln’s natural cooling rate. And then applying that information to cooling the kiln.

 

The best source for devising schedules is the Bullseye chart for Annealing Thick Slabs.  It is well researched and is applicable with little work to develop appropriate schedules for all the fusing glasses currently in use.

 

 

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.