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.
