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 below 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.

Slumping Schedules by Profile (Celsius)

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)

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	540		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”

 

*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.

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 relating to the first ramp rate have shown firing as for two layers thicker than indicated by the profile schedule provides the best results. It is then possible to increase the rate as determined by the profile schedule.

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 for 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 difficult to devise 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 20°C/36°F – to ensure the slump is stopped when it is complete.  If it is not complete, the soak can be extended. The controller manual will give the information on how to do these operations. In general, you 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 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.
• ·        Increase the rate after that to one for a single layer thicker than calculated all the way 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)
• ·        Initial ramp rate for 31mm/1.25” (two thickness greater)

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)
• ·        Initial ramp rate for 38mm/1.5” (two thickness greater)

 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 Temperature Kilnforming. and at Bullseye eBooks