Slumping Schedules
When slumping fired pieces, it is most often appropriate to
use a slow ramp rate to avoid too rapid expansion of the glass that might lead
to a break. Most glass breaks on the ramp up are above 300°C/573°F. It is in
this range that there is a rapid expansion of ceramic. This means a slow rate
is protective for both glass and ceramic moulds.
This slow first ramp rate is followed by the rate determined
as appropriate for profile and thickness. The table below gives rates and times
for different profiles that are 6mm/0.25” thick. Of course, the slumping
temperature will be altered for the glass according to the manufacturer’s
stated range. The nature of the mould will also have a big effect on
temperature and time. The soak times at the slump soak are those appropriate
for the mould. The annealing soaks are related to the profile of the glass.
Slumping Schedules by Profile (Celsius) 6mm thick
Flat Fuse and Contour Tack
Actual thickness |
Ramp 1 rate to
260°C |
Soak time (min) |
Ramp 2 rate |
Slumping temp. for mould * |
Soak time (min) |
Anneal as for
contour: |
6 |
240 |
20 |
240 |
|
30 |
9mm |
Rounded Tack
Actual thickness |
Ramp 1 rate to
260°C |
Soak time (min) |
Ramp 2 rate |
Slumping temp. for mould * |
Soak time (min) |
Anneal as for
round tack: |
6 |
150 |
20 |
150 |
|
30 |
9mm |
Sharp Tack
Actual thickness |
Ramp 1 rate to
260°C |
Soak time (min) |
Ramp 2 rate |
Slumping temp. for mould * |
Soak time (min) |
Anneal as for
sharp tack: |
6 |
120 |
20 |
120 |
|
30 |
9mm |
Slumping Schedules by Profile (Fahrenheit) .025" thick
Flat Fuse and Contour Tack
Actual thickness |
Ramp 1 rate to
500°F |
Soak time (min) |
Ramp 2 rate |
Slumping temp. for mould * |
Soak time (min) |
Anneal as for: |
0.250” |
432 |
20 |
432 |
|
30 |
0.375” |
Rounded Tack
Actual thickness |
Ramp 1 rate to
500°F |
Soak time (min) |
Ramp 2 rate |
Slumping temp. for mould * |
Soak time (min) |
Anneal for: |
0.250” |
270 |
20 |
270 |
|
30 |
0.375” |
Sharp Tack
Actual thickness |
Ramp 1 rate to
500°F |
Soak time (min) |
Ramp 2 rate |
Slumping temp. for mould * |
Soak time (min) |
Anneal for: |
0.250” |
216 |
20 |
216 |
|
30 |
0.375” |
Rates
It is most often best to use a slow ramp rate to at least 500°C/933°F.
This avoids the risk of inducing a too rapid differential expansion within the
glass as it heats up. Experiments about the first ramp rate have shown firing
as for two layers thicker than indicated by the profile schedule provides the
best results.
The rates for the anneal soak and cool are those that are
one layer thicker than determined by the schedule for the profile. This has
been shown by experimentation to give the best annealing result – i.e., least
stress.
Temperatures
The slumping
temperature needs to be altered for two factors:
- ·
the
glass according to the manufacturer’s stated range, and
- ·
the
nature of the mould.
Many manufacturers are giving recommended temperatures and times for slumping in their moulds. An example is the Bullseye “Quick Tip” which gives suggested temperatures and times for various sizes and natures of moulds that can form the basis of your independent scheduling of slumps. The rates are normally for flat uniformly thick pieces. This will need alteration for tack profile pieces.
Take note of
the soak time in these recommendations. If it is less than 10 minutes, it is
possible to reduce the temperature by about 10°C/18°F by using a 30-minute soak.
This will reduce marking on the back of the glass.
Soaks / Holds
Slumping schedules
tend to be more imprecise than many other operations in kilnforming because of
variations in moulds and what is placed on them. This, consequently, makes observation
of the slump more important. It is needed from a point below the target
temperature – say 22˚C/40°F – to ensure the slump is stopped when it is
complete, or extended if not. The controller manual will give the information
on how to do both of these operations. In general, schedule slower ramp rates
for thicker pieces in combination with the half hour soak. This means for each
thickness greater than 6mm, the top temperature can be reduced slightly and still
achieve a full slump.
The schedules here are applicable for pieces up to 9mm actual
thickness.
Slumping of thicker pieces needs to apply the underlying
scheduling method:
- · Apply the rate for two layers thicker for the advance to 260°C/500°F.
- · Continue the next ramp rate as for two layers thicker than calculated up to the slumping temperature.
- · For annealing, also select the rates and times for one layer thicker than indicated by the profile.
For example:
- · Rounded Tack of Bullseye, 12mm/0.5” thickness
- · Schedule for 25mm/1” (2 times multiplier)
Celsius schedule for up to 9mm actual thickness:
Segment > |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
Rate |
150 |
150 |
ASAP |
15 |
27 |
90 |
off |
Temp |
260 |
Top |
482 |
427 |
370 |
RT |
|
Time(mins) |
20 |
30 |
240 |
0 |
0 |
0 |
|
and
in Fahrenheit:
Segment > |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
Rate |
270 |
270 |
ASAP |
27 |
49 |
162 |
off |
Temp |
500 |
Top |
900 |
800 |
700 |
RT |
|
Time(mins) |
20 |
30 |
240 |
0 |
0 |
0 |
|
A further example:
- · Sharp Tack of Bullseye, 0.5” thickness
- · Schedule for 31mm/1.25” (2.5 times multiplier)
Celsius schedule for up to 9mm actual thickness:
Segment > |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
Rate |
78 |
78 |
ASAP |
11 |
20 |
65 |
off |
Temp |
260 |
Top |
482 |
427 |
370 |
RT |
|
Time(mins) |
20 |
30 |
300 |
0 |
0 |
0 |
|
and
in Fahrenheit:
Segment > |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
Rate |
140 |
140 |
ASAP |
20 |
36 |
117 |
off |
Temp |
500 |
Top |
900 |
800 |
700 |
RT |
|
Time(mins) |
20 |
30 |
300 |
0 |
0 |
0 |
|
These examples show that considerable differences in
scheduling are needed for different tack profiles. It also shows longer
annealing soaks and slower cooling rates are required for sharp than rounded
tack pieces.
More information is given in the e-book Low TemperatureKilnforming.
* Of
course, the slumping temperature will be altered for the glass according to the
manufacturer’s stated range. The nature of the mould will also have a big
effect on temperature and time.
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