Writing Slumping Schedules

 

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.

Avoiding Slumping Breaks

Most slumping breaks are due to scheduling.  The piece to be slumped has survived the fuse, and with good practice will have been tested for stress. It has passed all the compatibility and annealing complications, so it is sound. 

There are things you should think about when determining the schedule for slumping. General considerations are thickness, and degree of fuse. There are many other factors to be considered – such as depth, mould detail, span, colour contrasts, etc. These will affect the scheduling in detail rather than the general approach.

Ramp Rates

In general, the scheduling for the first ramp rate is done by taking note of profile (degree of fuse), and so, its effective thickness.

Each profile of fused glass has its own considerations.  Full fused pieces can be fired at the rate recommended by the many schedules for slumping fused items. Tack fused and other glass configurations need further precautions.

The ramp rate for slumping should be no faster than a rate to ensure the glass is evenly heated throughout the rise to the slumping temperature. I recommend that this rate of advance should be a steady single rate all the way to the slumping temperature.  There is no need for soaking at any point during this temperature rise. 

But as much of the breaking of glass occurs below 300°C (573°F), a precaution can be added. An additional slower first ramp can be inserted with a 20-minute soak at 260°C/500°F before proceeding. This also helps protect ceramic moulds which have a cristobalite inversion at that temperature. 

The rates for moulds that are large relative to kiln size, that are heavy, or may be damp, should be considerably slower than for other glass. 

Force of Breaks

If the glass has broken during the forming process, take note of the distance between the pieces.  The amount of space between the broken pieces shows the relative force that caused the break.  Greater space is related to more stress; lesser space or only partial cracks indicate lower levels of stress.  The separation distance indicates the degree of change required in scheduling. A small parting of the glass requires only a little reduction in the rate.  Large spaces indicate that much slower rates are required, and possibly a complete rethink of the schedule.

This approach can be used for breaks on the heat up or the cool down.  Whether the glass is rounded or sharp, the force of the break will still be an indicator of the degree of change required.  On a rounded edge break, it is the heating rate that needs to be slowed.  Sharp-edged breaks indicate that the anneal soak needs to be lengthened and the anneal cool slowed.  The rounded versus sharp edges are more difficult to establish at these low temperatures and need to be combined with how well the formed pieces match.  Of course, there will be some experimentation required to determine the exact amount of change needed. 

“It hasn’t happened before” Scenario.

Often people experience breaks even though the set up was similar and the schedule was the same for successful pieces in the past.  There are two responses to this – “what did you change for the setup and firing of this piece from others?”, and “You have been skating on the edge of disaster for a while”.  Glass behaviour is predictable. Since the break occurred when the setup was similar, and the schedule was the same, something else has changed.

Consider what was different.  Review the differences in set up of the piece – colours, arrangement, thickness, volume of material used – everything that might be different at each stage of the layup.  Note these differences and review them one by one. 

• ·   Could have any one element been sufficient to make the firing conditions different? 
• ·   Could a combination of these differences have been significant? 
• ·   Are there any differences in the firing schedule? 
• ·   Have you made any little tweaks in the schedule? 
• ·   What is different? Different times of the day, different power supply, plugs in or out, venting, peeking, different shelves (or none) – any small thing that could have introduced a variable in the firing conditions. 

For each of these differences consider what needs to be altered, if anything, for a successful firing.  Combine these small tweaks into a full schedule and run it as an experiment.