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.

To Repair or Not to Repair

 Breaks during slumping sometimes occur. What can be done?

Cause of Break

The first element in assessing the piece is to determine why it broke. 

Should it be Repaired?

The second element is whether it should be repaired or re-used. Is it worth the effort of repairing? This will be about the importance and the time and effort you have already put into the piece.

Can it be repaired?

This is a third element of assessment. If the break resulted from incompatibility, any attempt at refusing will also break for the same reason. If inadequate annealing caused the break, it may be possible.

It is sometimes suggested that those pieces which fit together exactly, should be fused together flat and re-slumped. This ignores the fact that the glass will have stretched or deformed from the flat piece it once was.

• ·   This re-fusing may be successful for shallow and simple slumps. But the piece will not be corrected by fusing the broken pieces from deep or complex slumps as a result of the stretching and thinning or thickening in the slumping process.
• ·   The glass pieces will have an imperfect join when flattened because of deformations from the changes during the slumping.
• ·   If the base is a single layer, the separate pieces will pull apart during the re-fusing process due to the lack of volume.
• ·   The fusing process will make a tack fuse much flatter than originally intended. A contour fuse - at minimum - will be required to join the pieces.

For all these reasons, any flattening, fusing and then attempting a slump again is unlikely to be successful.

Fusing in the mould

In recognition of these problems about flattening, re-fusing, and slumping again some people suggest mending by firing in the mould. This would get over the difficulty of changes of shape. However, the required contour or full fuse will leave marking on the back and may lead to thickening at the bottom. It is also hard on your ceramic moulds if you fire quickly.

Changing the Shape

If it is desired to flatten an unbroken slumped piece for use in a mould of a different shape without much change in tack profile dimensions, there are two things to do. The maximum temperature to be used to get the glass flat and retain the degree of tack is the sharp tack - or lamination - range. It will require a significantly long soak at top temperature - hours.

This long soak time is a consequence of the effects of weight and span. The effective weight is less at the unsupported edges than at an unsupported centre. The slumped piece has most of its weight on the shelf now. This makes the flattening have to use a higher temperature or a longer soak. The effective span and weight at the edge is almost zero. This requires long soaks and frequent observation to know when the flattening is complete. Both these effects make the flattening of a piece without altering the profile a lengthy process.

 

More information is available in the ebook: Further information is available in the ebook: Low Temperature Kiln Forming.

Repairing a broken slumped piece of glass requires knowing why it broke, can it be repaired, is it worth repairing. Difficulties related to the changed shape, temperature to fuse, and changes in tack profile.

Elevation of Moulds in the Kiln

The placing of the mould may have a significant effect on the outcome of a slump. The ideal placing is in the centre of the kiln to ensure it has the most even temperature. This avoids any uneven temperature that may exist within the kiln.

Hot and Cool Spots

Sometimes this is not a practical use of kiln time or space, but if the heat distribution in the kiln is uneven, the placing may be critical. If the cool areas are known, avoid them in the placing of the larger moulds. Simpler moulds, or those which do not require as much heat can go in the cooler areas of the kiln. A good and simple method to test for the heat distribution within your kiln is given in Bullseye’s Tech Note no.1.

Effect of Elevation of Mould

Elevation of the mould by a centimetre or two is often recommended to help evenly distribute the heat under the bottom of the mould as well as the top. This is viewed as a way of avoiding breakage or uneven slumps. There are differences between moulds on the shelf and those elevated. Recordings show differences up to 49°C/88°F. The differences on the cool down ramps are minimal and do not interfere with annealing. These differences appear to have no effect on breakages in the mould.

ΔT Shelf vs. Elevated Moulds (Celsius)

	Max. ΔT	Average ΔT
Rate / hour	on Rise	Start of slump	End of 30 min slump	On cool
150	49	41	30	8
120	39	31	24	5

 

These differences should be put in context. The air temperature is approximately three times any difference between the two arrangements of moulds. Much more important in breakage is the ramp rate, as it creates significant differences in expansion between the top and the bottom of the suspended glass. This much larger difference has the potential for greater effects than whether the moderately sized mould is elevated or not. This table demonstrates the air and mould temperature differences.

ΔT Difference Between Air and Elevated Mould (Celsius)

Ramp Rate	Air Minus Mould Temperature (ave)
240	138
150	112
120	97

 

Large, Heavy, Wet Moulds

The elevation of large, heavy, or wet moulds is very important. It is needed to protect the supporting shelf from breaking. The amount of shading of heat from the shelf that these kinds of moulds can do is large. Wet moulds, especially, can cause large temperature differences in the shelf. Always elevate moulds that are large relative to the kiln, contain thick glass, are heavy, or are damp to avoid difficulties with the shelf.

 

More information is available in the ebook: Further information is available in the ebook: Low Temperature Kiln Forming.

Coe and Annealing

If you have changed CoE (i.e., the manufacturer), then the annealing temperature is different. If you don't correct that, it's never going to work quite right.

 

I have several problems with this statement.

CoE does not determine the manufacturer. There are several manufacturers who claim to manufacture fusing glass to the same CoE.

No manufacturer makes to one CoE. All manufacturers have to vary the CoE of a particular glass to balance its viscosity. The CoE is a dependent variable. It depends on what the viscosity of the colour is. Spectrum at one point stated their System96 glass had a 10-point variation in CoE number. Oceanside will be no different. Bullseye have stated a 5-point difference. Other manufacturers have not stated their variations.

No manufacturer can guarantee compatibility with another’s. This is because the ingredients to make a fusing range of glass varies from one manufacturer to another. These variations can make the glass incompatible. To determine if you can combine two glasses from different manufacturers you need to do the compatibility testing yourself. The CoE number does not determine the temperature characteristics of the glass either. 

Annealing 

Having got my disagreements with the statement out of the way, I can go on to looking at differing annealing temperatures. There is a difference between annealing point and annealing temperature.

Annealing Range

Annealing occurs over a relatively small range between the softening point at the higher end to the strain point at the lower end of the range. The softening point is the temperature, above which the glass is so plastic that it cannot be annealed. The strain point is the temperature at which the glass becomes so solid than no annealing can occur below it.

Annealing Point

The annealing point is mathematically determined as the point at which the glass most quickly relieves the stresses within it. That temperature is determined by the viscosity of the glass. It is known as the glass transition point, and is expressed as Tg. In practice there are advantages in annealing at or below the published annealing point.

A soak above the annealing point is of no effect. Any equalisation of temperature that occurs on that soak is negated by the drop to the annealing point. It is better to spend the cumulative soak/hold time at the (lower) annealing temperature.

Annealing Temperature

The average annealing point for Bullseye is 516°C/962°F. Different formulations of their fusing compatible glass have different Tg temperatures. Research showed the best results for their thick glass is 482°C/900°F. Other research in academic institutions has shown that annealing at the lower part of the range provides a denser and stronger finished glass piece. This applies to thick as well as thin glass.

Bullseye has chosen to use a temperature 34°C/61°F below the average annealing point, based on their research. This is still about 7°C/13°F above the strain point. This approach can be applied to any fusing glass.

The strain point is approximately 43°C/78°F below the mathematically determined annealing point. If you know the annealing point you can choose to anneal – i.e., equalise the temperature of your glass – up to 30°C/54°F below that. 

This has a practical demonstration. Wissmach for some years designated 510°C/950°F as the annealing point for W96. A few years ago, they changed their recommended annealing temperature to be 482°C/900°F. The annealing results are good at both temperatures. The difference is that the annealing soak is for a in longer time at the lower than at the higher temperature. But it still provides a shorter annealing cool.

Firing with different anneal points

This apparent diversion - into annealing ranges - shows that it is possible to anneal glass with slightly different glass transition points at the same temperature. You may compromise a little for one glass or the other. You will also use longer times at the annealing temperature.

The annealing soak of Oceanside and Wissmach96 could both be at 482°C/900°F. Or, if it felt safer, it can be an average of the two. The average of the difference would make the annealing soak at 496°C/926°F. You would use a longer soak at this temperature than at the higher one. The safest would be to hold for an hour instead of 30 minutes for 6mm/0.25” of glass.

However, if the annealing point differs greatly, it is much more difficult. For example, float glass with an annealing point of 540°C/1005°F would be difficult to fit in the same firing with most fusing glass because of the wide range of official annealing points.

 

It is possible to anneal different glass at the same time if the annealing points are not widely different. Compromises need to be made.

 

Changing Coldworking Grit Sizes

 

One of the difficulties of coldworking is when to change grits. Checking the completeness of one grit grinding visually is difficult. It is difficult to see the effect while wet. Even when dry it can be difficult to see that all the previous marks have been ground out. This is the background to the recommendation that each successive grind should be right angles to the previous. It is easier to see marks that are in a different direction than marks that are just wider or deeper than others.

 However, there have been aids developed by cold workers in the past that are relevant today. Use a witness at each stage of grinding. The best at present are paint markers. These are the kinds used by metal workers to identify the pieces, their dimensions, etc.

 The paint needs to be dry, or it simply washes off the glass surface as soon as it is placed on the grinding surface. So, there is a process to ensure this witness works as it should.

 On a dry surface, run the marker at random across the surface to be ground. Let it dry before putting on the grinding surface. You can test by putting your finger on the paint. If some comes off on your finger, it is not yet dry. When dry, grind the surface. When all the colour has been ground away, it is time to change grits.

 Dry the glass surface again and paint it. While the paint is drying, change grits, or discs and do any other cleaning up between grits that is required. Test for dryness. Grind at right angles to the first grind. Make sure absolutely all pinpoints of colour are removed. If they are gone, dry, and paint. While letting it dry, change grits, clean up, and do the other miscellaneous tasks. Then test paint for dryness and grind.

 Repeat this process with each grit until finished. If on occasion you find you have gone to a finer grit earlier than you should have, go back to the coarser grit with you painted glass and repeat the progress back from the immediately previous grit through to the finer grits.

This process of using a witness works whether hand or machine cold working.

Kiln Wash Sticking to Glass

Causes and avoidance

 

Photo credit: Immerman Glass

 In general, kiln wash for glass is made up of aluminium hydrate with kaolin (China clay) as a carrier. I do not know the exact chemical changes of kiln wash at fusing temperatures. But I do suspect it has to do with the kaolin. The aluminium hydrate is stable to much higher temperatures (melting point of 2,072°C/3,762°F). So, I don't believe that part of kiln wash is changing.

 Some reading has led me to learn that by 600°C/1113°F the kaolin begins to go through a non-reversable chemical change. Prior to that, water can rehydrate the kaolin. In the hydrated state kaolin forms hexagonal plates that can slip over one another. Once 600°C/1113°F has been exceeded crystallisation cannot be reversed. It does not become fully crystalline until 935°C - 950°C/1717°F - 1744°F. The crystallisation stops the lubricating effect. I suspect that on the second firing these crystals (which contain silicon dioxide) interact with the glass and stick, although not fully combining with the glass. Why this does not happen in the first firing, I do not know.

 The fact that the crystallisation cannot be reversed must be the key as to why kiln wash with kaolin cannot be re-used once fusing temperatures have been used in a previous firing. It also indicates that repeated tack fusing on kiln wash will ultimately fail as the crystallisation will gradually increase with each firing.

 However, at slumping temperatures, it appears the crystal formation is so slow as to have no effect on multiple firings.

 There are of course ways to avoid kaolin. There is a kiln wash, called Primo Primer that does not have kaolin in it. And you could make your own kiln wash from aluminium hydrate. This is known as slaked alumina in ceramics. It can be used on its own, although the incorporation of  binders makes the application easier. The grades used in ceramics are usually coarser than kilnformers want. But it can be made finer by putting it in a rock tumbler with some stainless steel ball bearings. You can run the result through a fine screen to remove the ball bearings. Mix with water to brush on, or sprinkle dry over the shelf. The aluminium hydrate can be re-used, if they are kept free of contaminants. Aluminium on its own does not provide as smooth results as when the kiln wash contains kaolin.

 Chalk, also known as whiting, is calcium carbonate. This is often used as a separator in vitreous paint firings and some forming operations. It has low solubility in water, so cannot be painted onto shelves or moulds. It needs to be used as a loose or compacted powder. It goes through chemical changes too, making renewal after firing advisable. Above 800°C/1473°F calcium carbonate changes to calcium oxide, or quicklime. This corrosive form is another reason it is disposed of after any higher temperature firings.

 Kiln wash and calcium carbonate can be fired many times at low temperatures, because their chemical composition remains relatively stable. Once higher temperatures are used, chemical changes occur. This seems to enable them to stick to the glass or form undesirable compositions. This phenomenon requires removal and re-coating of shelves and moulds after full fuse firings.

Kaolin provides significant advantages in the smooth application of kiln wash.  Caution needs to be exercised in using it after it has been fired to fusing temperatures, although it can be used at low temperatures for indefinite numbers of firings.

 Methods for removal of kiln wash are in this blog post.