Elevation of Moulds

Putting slump and drape moulds on kiln posts is not necessary.  If you think you need air circulation under the mould, pieces of thick fibre paper are all that is needed.

The widely accepted notion that all moulds need to be elevated from the shelf is not backed up by evidence.  There is a lot of anecdotal discussion which supports the idea that it is necessary, though.  The idea is that air circulation under the mould will keep the lower part of the mould the same temperature as the upper part. That may or may not be the case. But it is the glass that needs attention.

Heating glass on moulds

Slumping

The glass is suspended either at the edges in slumps or at the centre in drops. The suspended glass will heat independently of the mould. Any difference in temperature between the glass and slumping mould will only become important at the end of the slump when it touches down onto the mould. If slow ramp rates and long soaks are used, the mould will be heated through by the time the glass touches down to the mould. But there could be an argument, based on the use of cold moulds in glass blowing, that a cool mould will impart less texture on the glass than a hot one.

Draping

In draping, the glass is supported in the centre on the mould and the differential temperature between mould and glass in the early stages of the firing may be a cause of a break. The supporting surface of the mould is being heated through the glass and, to a lesser extent, by conduction from the lower part of the mould. This requires slower heating than a slump mould does, to ensure the heat is not drained from glass by the mould which would allow the temperature differential between the supported and the unsupported parts of the glass to become great enough to break the glass.

Air Circulation

The idea that air circulating under the mould will even out the temperature between the bottom and top of the mould is not supported by two pieces of evidence.

The main effect to heat glass is infrared radiation. This is often described as the glass “seeing” the heat. Glass is only secondarily heated by the ambient temperature of the kiln. How heating the mould in the slumping process will protect from breaks or uneven slumping is not clear, and is supported only by analogies.

The idea that convection will help to heat the glass as well as the mould is countered by the general agreement that there is very little movement of air in the closed kiln. What convection of air there may be from the differential air temperature between the bottom and the top of the kiln is minor. If it were major, there would not be the large air temperature differential experienced – in deep kilns in particular, and to a lesser extent in shallow kilns – between the top and bottom of the chamber.

Temperature Differences

We all agree that there are temperature differentials from top to bottom in kilns.

The evidence is that the temperature difference is much greater between air and glass, than between mould and glass. The average temperature difference (∆T) between raised moulds and on-the-shelf moulds in tests ranged from about 50°C to 40°C (90°F to 72°F) on the rise to top temperature, depending on the ramp rate.  (a ramp rate of 150°C/ 270°F averaged a 50°C/90°F differential vs a 120°C/216°F ramp rate which gave the average 40°C/72°F differntial.)  It is also clear that by the time the slump temperature is reached, the ∆T between the two mould arrangements is about 25% less at the end of a 30 minute slump than at the beginning. So, yes, there is a difference in the temperatures under the mould versus an on-the-shelf mould.   Is it significant?

The ∆T between the raised mould and the air is approximately three times any difference between the on-the-shelf and raised moulds. This is consistent at ramp rates of 120°C, 150°C and 240°C (216°F, 270°F and 432°F). This indicates there is a greater risk to the glass with rapid ramp rates than there is from the difference in mould elevation. Slower ramp rates produce smaller differences between air temperature and mould temperatures. They also produce smaller differences between the mould bottom and top temperatures, making the argument for slow ramp rates up to the slump temperature, regardless of whether the mould is elevated or not.

Ramp Rates

The more rapid ramp rates create significant ∆T between the air and the mould. This much larger ∆T than between moulds raised or not, has the greater potential for causing breaks. The slower the ramp rate the less difference. These two elements on their own make the argument for slower ramp rates clear.

Distance from Elements

The distance between glass and heating elements is important in slumping.
The closer the glass is to the heat source, the more uneven the heat reaching the glass will be. This blog post indicates the distance required to even out the temperature. 

Where the elements are located is significant too. Side fired kilns need to use slower rates in going up in temperature, because the heat is concentrated on the edges of the glass unless baffles are in place. The uneven heating risks breaks, and this can be reduced by using rates for one to two layers thicker than actual thickness.

Moulds and Shelves

It can be argued that care in heating up ceramic moulds is required because of the quartz inversion.  If the ramp rate is fast enough to cause a mould to break, it is also too fast for the safety of the glass. The same caution in relation to the shelf needs to be employed for the glass.  Any large heat differential across the shelf will cause a break. 

Heavy, wet, damp, or large casting moulds must be elevated to keep the shelf from cracking due to a large ∆T between mould and shelf which has the real potential to break the shelf. The remedy for these kind of moulds is to elevate them on short kiln posts to maintain a relatively even temperature across the whole of the shelf.

Caution – the temperature differentials are averages and individual moulds will differ. But the principle remains that slow rises to top temperature are less risky than fast ones.

Data for these figures is available in table form and in graph form

What Formations are Made in Drapes?

The shape glass takes up during a drape seems to vary depending on whether the glass is a square or a circle at the start. The square often forms a taco shape before the “horizontal” ends soften and fall, pushing the glass into four drapes. The angle of these drapes will move more toward being straight down with additional time or heat. Circular draped glass tends to form three drapes, which also become more vertical with time and heat.

Early stages of a steep drape

One occasional problem with elevated moulds is that the draping glass is “longer” than the mould is tall. Understanding how the glass forms during a drape will help avoid this effect. The glass drape first forms a curve as it begins to drop, which continues to be in place until it touches the mould. The rest of the firing stretches the bow in the glass out.

If there is no mould at the bottom for it to touch because the mould is elevated, the curve goes beneath the bottom edge of the mould.

If the draping glass is longer than the elevated mould, the glass will drape to the shelf and can creep under the bottom edge, trapping the mould. 

 Alternatively if the mould is not elevated, the glass will flatten on the shelf. The flattened ends will spread outwards, but sometimes also creeping under the edge of the mould, trapping the mould.

The glass can be the correct length for the elevated mould, but the time and temperature combination scheduled allows the whole drape to stretch at the base, so the tips of the drape create one or other of the trapping effects.

The initial prevention of trapping the mould is to measure the length of the drape. The size of the blank should be no longer than double the length of the sides plus the width of the supporting top (the base of the mould when stood open end up). This length is the maximum diameter of a circle. 

 The diagonal of a square blank needs to be no longer than that either.  A simple visual method is to establish a square corner directly onto the glass.  Draw a 45° angle line and measure off the length on the diagonal. Draw a right angle line from the base to the point on the diagonal and you have the length of the sides ready to score.

It should be remembered that the glass will stretch to some degree during the drape. The length of that stretch is dependent on the temperature and time, so observation by frequent peeking will give the results wanted by either skipping to the next segment or extending the hold.

There is more information on scheduling for drapes in the e-book Low Temperature Kilnforming available from Bullseye  and Etsy.

Schedules for Specific Mould Shapes

A suggested temperature for a slump can only be a starting point and not all moulds will have slumping or draping temperatures given. The main factors to be considered are mould shape, span and depth, and glass thickness and profile.

Fortunately, Bullseye have a mould tips document on suggested slumping temperatures (in Celsius and Fahrenheit) for a variety of moulds.

The mould shapes are of course are those that Bullseye sell. If the shape is not one of theirs, a similar shape can be chosen and the suggested temperature and soak time can be used as a starting point.

Study of this tip sheet can give you a lot of information on various aspects of time and temperature for variations:

• Angularity – Moulds with sharper angles require longer times and sometimes higher temperatures.

• Span – The smaller the span of the mould, the longer the time required to slump into the same shape.

• Depth – It is not surprising that greater depth requires longer soaks

• Ramp rate – The rate up can affect the temperature and time of the slump.

What the Bullseye table does not give is information on the effect of the thickness of the piece. The Mould Tips document assumes a 6mm/0.25” flat fused blank. Thicker pieces will need a combination of slower ramp rates and longer soaks to fully form than given in this document. They will also need longer anneal soaks and slower cools.

It does not provide information on the effects of the profile either. Tack fused pieces need slower ramp rates and longer annealing than full fused blanks. The additional rates and times can be worked out by applying the following multipliers and firing for the resulting thickness:

• Flat fuse – the actual thickness

• Contour fuse – 1.5 times the thickest part

• Rounded tack fuse – 2 times the thickest part

• Sharp tack fuse – 2.5 times the thickest part

The key to getting the right temperature and soak length for a successful slump or drape is to observe the progress of the firing with quick peeks from the start of the top temperature. Slumping temperatures are effected by all the factors above, so that no one temperature can be given for even one mould.

My ebook Low Temperature Kilnforming gives more information on the factors in writing schedules for slumping. Available here and here.

Draping over steep moulds

 Draping over a narrow or small supporting ridge with large areas of glass is difficult.

One solution might be just to invert the whole piece and let the glass slide down into the mould. However, there rarely is enough height in a glass kiln for deep slumps, especially with a “V” shaped mould. It has to be high enough for the edges of the glass to be supported at its edges. You could also approach this by having a first mould with a shallower angle or broader support at its centre. Drape over this first, then use the steeper mould as the second draping mould. This makes the balance less critical.

The idea of supporting the glass is the key to doing this kind of slump that seems to require an impossible balancing act, if it is to be done in one go. Place kiln washed kiln furniture at the edges of the otherwise unsupported glass. Fire the kiln, but watch until the glass begins to slump. Then reach in with a wet stick and knock the kiln furniture aside to allow the glass to continue its slump and conform to the mould shape.

The lower temperature you use to do the draping and the slower your rate of increase is, the less the glass will be less marked by the mould. Frequent brief visual inspection during the drape is vital.

Also have a look at a suggestion for the kind of firing required for this here.

Using Ceramic to Drape

Characteristics

Before choosing a ceramic shape to use in draping of glass, you need to consider the characteristics of the two materials.  This is one circumstance where CoE is actually useful. 

The expansion of the two materials is different. 

• Soda lime glass typically has an expansion rate - in the 0°C to 300°C range - of 81 to 104.  
• Ceramic has an expansion rate - in the 0°C to 400°C range - of 30 to 64.  

This is important in the final cooling of the project.  As the glass expands more than the ceramic on the heat-up, so it also contracts more during the cool.  This means that the glass will shrink enough to trap the ceramic or even break if the stress on the glass is too much. 

Shape

The shape of the ceramic form will have a big effect on the usability of it as a mould.  Ceramics with right angles between the flat surface and the sides will not be suitable for draping without modifications or cushioning.  The forms suitable for draping need to have a significant draft to work well.

Ceramic forms such as rectangles, cubes, and cylinders do not have any draft in their form.  

A cube shape unsuitable for draping

Ceramic cylinders with straight sides

Although rounded at the base, the sides are too straight to be a draping mould

The glass will contract around these forms until they are stuck to the ceramic or break from the force of the contraction around the ceramic.

You can experience this trapping effect in a stack of drinking glasses.  Sometimes one glass sticks inside another even though there is a slope (i.e., a draft) on the sides of the glasses. This happens mostly when you put a cold glass inside a warm one.  On cooling the warm glass contracts to trap the cooler one. You can separate these by running hot water on the bottom glass, so that it expands and releases the inner, now cool, one. 

Effect of Shape

The ceramic contracts at about half the rate the glass contracts (on average), unlike steel which contracts faster than the glass. This means steel contracts away from the glass, while the glass contracts against the ceramic, on the cooling.

Because the glass is in its brittle or solid phase during the last 300°C to 400°C, this contraction tightens the glass against the ceramic, causing stress in the glass, even to the point of breaking.

However, if you choose ceramic forms with significant draft, you can drape over ceramic.  This is possible when the slope is great enough and the form is coated with enough separator, to allow the glass to slip upwards as it contracts more than the form. Experience with different draft forms will give you a feel for the degree of slope required. 

 

These pyramid shapes have sufficient draft to allow the glass to move up the mould during cooling.

Compensation for Lack of Draft

You can compensate for the insufficient draft of ceramic forms by increasing the thickness of the separators for the form.  The hot glass will conform to the hot ceramic, so there needs to be a means of keeping the glass from compressing the form while cooling.  This can most easily be done by wrapping the form that has little or no draft with 3mm ceramic fibre paper.  It is possible to get by with as little as 1mm fibre paper, but I like the assurance of the thicker material.

Kiln post wrapped in 3mm fibre paper with cap over the post's hole.

The fibre paper can be held to the form by thin wire wrapped around the outside of the fibre paper. The advantage of the 3mm fibre paper is that the wire will sink below the surface of the paper.  You can tie off the wire with a couple of twists.  Cut off the ends and push the twist flat to the fibre paper to keep the glass from catching onto the wire.  If you want further assurance, you can put a bit of kiln wash onto the wire.

Conclusion

The choice of ceramic shapes to drape glass over is very important.  It needs to have sufficient draft and separator to allow the glass to slip upwards as it contracts more than the ceramic during the cooling.  You often can use items with no draft if you wrap fibre paper around the sides of the form.

Draping over steep moulds

 Draping over a narrow or small supporting ridge with large areas of glass is difficult.

One solution might be just to invert the whole piece and let the glass slide down into the mould. However, there rarely is enough height in a glass kiln for deep slumps, especially with a “V” shaped mould. It has to be high enough for the edges of the glass to be supported at its edges. You could also approach this by having a first mould with a shallower angle or broader support at its centre. Drape over this first, then use the steeper mould as the second draping mould. This makes the balance less critical.

The idea of supporting the glass is the key to doing this kind of slump that seems to require an impossible balancing act, if it is to be done in one go. Place kiln washed kiln furniture at the edges of the otherwise unsupported glass. Fire the kiln, but watch until the glass begins to slump. Then reach in with a wet stick and knock the kiln furniture aside to allow the glass to continue its slump and conform to the mould shape.

The lower temperature you use to do the draping and the slower your rate of increase is, the less the glass will be less marked by the mould. Frequent brief visual inspection during the drape is vital.

Also have a look at a suggestion for the kind of firing required for this here.

Time and Temperature

credit: timeanddate.com

Credit: Shutterstock

“What are the pros and cons on turning up the max temperature slightly Vs. a longer hold time”? Lea Madsen

This is a difficult question to answer, because there are variables such as

the temperature range,

the ramp rates, and soaks,

the forces acting upon the glass at a given temperature, 

the process,

the desired outcome of the firing,

etc. 

When talking about temperature vs. time, it is heat work that we are considering.  In many processes time and temperature are interchangeable, although the temperature range is important too.  This is a brief discussion of heat work in various processes.

Slumps

Slumping temperature is generally in the range of 620˚C-680˚C/1150˚F -1255˚F *, which is below the devitrification range.  This allows the exchange of time for temperature without risk, allowing more time rather than more temperature.  Higher temperatures cause more marking from the mould since the bottom of the glass is softer than at lower ones.  Lower temperatures give higher viscosity, so the glass is stiffer, resisting marks.

Low temperature fuses

Sharp tack fusing, freeze and fuse, some pate de verre processes, and sintering occur in the 650˚C -720˚C /1150˚F - 1320˚F range, risking devitrification only at the upper end of this range.  Extending the time rather than the temperature is important to maintain detail in these processes.  Higher temperatures will smooth the surface, risking loss of detail.  

Rounded tack processes (720˚C – 760˚C /1320˚F - 1400˚F)

These are within the devitrification range making the choice between time and temperature a balance of risks.  In my experience, it takes about an hour for visible devitrification to develop.  This means that you can extend the time, if the total time between the end of the bubble squeeze and the working temperature, including the hold time, is less than an hour.  It has the advantage of a more secure attachment between the pieces of glass, without altering the surface much. 

But if extending the soak time increases the time in the devitrification zone to be more than an hour, it is advisable to increase the temperature, rather than time.  Devitrification develops in the presence of air, so reducing the time in that range reduces the risk of devitrification developing.  The glass is moving during rapid ramp rates, reducing the chance of devitrification.

Drops

This includes drapes, and other free forming processes.  Kilnformers will be observing the progress of these firings, making it easier to balance temperature and time.  There are already long holds scheduled for the processes, so it is a matter of getting the right temperature.  If, after half an hour at the scheduled top temperature, the glass has not moved much, it is time to increase the temperature by, say 10˚C/18˚F and observe after another half hour, repeating the temperature increase if necessary.   Be aware of thinning the glass at the shoulder by setting a high temperature.  Free drops may take as much as 6 – 8 hours, so patience and observation are important to get good results.

Full fuse

At full fuse try to get the work done in 10 minutes to avoid complications with devitrification.  So, increasing the temperature rather than the length of the soak seems best.

Flows

Whether frit stretching, making pattern bars, pressing, etc., low viscosity is important.  Viscosity is closely related to temperature, so increasing the temperature is the better choice.  Increasing time without increasing temperature does not change viscosity much.

Casting

Extending time at top temperature seems best for open face casting, as the temperature is already high.  A slow ramp rate to that top temperature may make adding time unnecessary, because the heat work will be increased by the slow rise.  Experience has shown that a rate of 200˚C/360˚F is enough to avoid devitrification.  With enclosed castings devitrification is not such a risk, so time can be added without concern.

 

Observation

In all these processes it is advisable to observe the progress of the firing by quick peeks to determine the effective combination of temperature and time.  Also note that heat work is cumulative, making for changes in profile with repeated firings. 

 

* The softening point of float glass is around 720°C/1328°F, so the slumping range is about 700°C/1292° to 750°C/1382°F.

Slumping a Form Flat

There are a variety of reasons that you might want to make a formed piece flat again for another kind of slump or drape.  There are a variety of things to think about when preparing to make a shaped piece flat.  I am going to assume there are no large bubbles in the piece and there are posts on Large bubbles and Bubble at bottom  including the causes.

There are five groups of things to consider when contemplating flattening an already formed shape.

Shape/form

• Shallow forms with no angles have the fewest difficulties.  Take it out of the mould, put it on the prepared shelf and fire to the slump temperature.  Observe when it is flat and proceed to the annealing.
• Forms with angles or multiple curves are a little more difficult.  If the piece has stretched in some areas to conform to the mould, you will have some distortion in the pattern and possibly some thinner areas.  It should be easy to flatten pieces on a prepared shelf with the same schedule, but a slightly higher top temperature than in the previous slump.
• Forms where the sides have pulled in will become flat, but continue to have curved sides.
• Deep forms are possibly the most difficult.  The glass may have stretched, giving thin areas.  It may be that the process of flattening the glass will cause a rippled effect as the perimeter of the piece is a smaller size than the original footprint.  These deep forms are the least likely to flatten successfully.

Orientation

• Which way up? Upside down or right side up?  Shallow forms are easiest to flatten by placing them right side up on a prepared shelf.  For deep or highly formed pieces, it may be best to put it upside down to allow the now higher parts to push the perimeter out if it is necessary.

Thickness

• Thick glass will flatten more quickly than thin glass when using slower ramp rates, so you need to keep a watch on the progress of the work to avoid excess marking of the surface of the glass.
• Very thin pieces are likely to develop wrinkles as they flatten.  Even if they do not, there will be thick and thin areas which might cause difficulty in subsequent slumping.
• Tack fused pieces are likely to tend to flatten at different places and times due to the differences in thickness and therefore weight. This makes observation of the flattening process more important.

Temperatures

• In all these processes, you should use the lowest practical temperature to flatten.  This means that you will need to peek at intervals to see when it is flat.
• Your starting point for the top temperature to use will be about the same as  the original slump, normally.  The amount of time may need to be extended significantly. The reason for this is to avoid as much marking on the finished side as possible.
• Shallow forms and thick pieces will flatten more quickly than others, so a lower temperature can be used.  You will still need to observe the progress of the flattening.
•  Angled shapes and deep forms will need more heat and time than the shallower ones. 
• Thin pieces may require more time than thick pieces.
• Tack fused pieces need more attention and slow rates of advance to compensate for the differences in thicknesses.

Separators

• Kiln washed shelves are usually adequate for flattening.
• Thinfire or Papyros are needed when flattening upside down to ease any sliding necessary.
• Powdered kiln wash or aluminium hydrate can be dusted over the kiln washed shelf when it is felt the form will need to slide on the shelf while flattening.

It may be that after all this, you feel it is not worth it to flatten.  It certainly is worth the effort, if only to learn about the characteristics of the form and its behaviour in reversing the slump or drape.

Longer Soak or Higher Temperature?

 ‘Is it better to extend the soak or add more firing time when the firing program isn’t quite enough? What are the meanings of “soak,” “hold,” “ramp,” “working temperature” and “top temperature”?’  

Let’s start with some of the terms.

“Soak” and “hold” have the same meaning in scheduling.  Schedules are made up of a series of linked segments.  Each segment contains a rate, temperature, and time.  The time is often called a “hold” in the schedules.  That time can have several effects.  It can allow enough time for a process, such as slumping, to be accomplished.

Although “soak” is entered into the schedules in the same way as a hold, it has a different concept behind it.  The hold when used as a soak allows the set temperature to permeate the whole thickness of the glass.  An example is in annealing. An annealing hold/soak is set.  This is to allow the glass to become the same temperature throughout. 

The ramp is the rate at which the controller is set to increase/decrease the temperature.  This is normally the first element in the segment.

“Top” and “Working” temperature are the same thing.  It is the temperature at which the desired effect is achieved.  They have slightly different nuances.  Top temperature is normally considered as a point where the desired profile will be achieved in a few minutes.  The working temperature is also that, but includes the idea that it will take time for the effect to be achieved.

Which should you alter first – soak time or temperature?

Most important is that you alter only one at a time.  If you alter the two elements at the same time, you do not know which was the cause of the result.

In general, you lengthen the soak if the effect is not achieved at the temperature and in the time set.  There are two reasons for this.  Glass has fewer problems at lower temperatures.  Secondly, the controllers are set up in such a way that it is easy to extend the time. Check your manual for the key sequence to extend the time.  It is more difficult to alter the temperature during a firing. 

To determine if you need more time, you peek into the kiln as the kiln approaches the top temperature.  If the profile has not been achieved by the time set at your working temperature, you enter the combination of keys to keep the kiln at the top temperature until you see the effect you want.  Then enter the combination of keys to skip to the next segment.

Whether you alter time or temperature, depends on what you are doing.  Soak plus temperature equal heat work.  With heat work you can accomplish the same effect at lower temperatures.  It may be that taking more time (usually slower ramp rates) to get to the same or lower temperature, will give the results desired.

For slumping, draping and other low temperature processes extending the hold/soak is appropriate. It reduces the amount of marking that is created by the mould or surface supporting the glass.

When tack, contour, or full fusing, you should be aiming to finish the work in about 10 minutes. Soaking/holding significantly longer increases the risk of devitrification.

For high temperature processes such as pot and screen melts and some flows, increasing the temperature is probably the right thing to do, to avoid the devitrification possibilities of long holds of open face high temperature work.

These can only be guidelines.  Your instincts and experience will help you determine which is the right thing to do in the circumstances.

 

Draping Different Thicknesses and Sizes

Scheduling for different sizes and thicknesses of drapes requires schedules specific to these factors in addition to observing the progress of the drape.

Bob Leatherbarrow's research shows 6mm drapes more slowly than 3mm. It seems the thicker glass takes longer to begin the slump.  Glass behaves in a similar way for a drape.  My experience of draping 6mm/0.25” and 3mm/0.125” in the same firing confirms that 6mm takes longer.  Or, it needs a higher temperature.  I know this goes against common sense, but tests and experience show it to be true.

If you try to drape 3mm/0.125” and 6mm/0.25” pieces at the same time, the 3mm will reach the desired shape before the 6mm. You then have the choice of an under draped 6mm piece or an over draped 3mm piece.  This indicates that draping different thicknesses in the same firing will be unsuccessful.  To a lesser extent, the size of the drape will influence the speed of the drop.  So, you are unlikely to achieve completely desirable results with significantly different sizes of drape in the same firing either.

Observation is essential in all draping operations. You cannot know how long it will take for a piece to drape or drop to your requirements.  To be sure of your result you need to observe the progress of the drape.  There is rarely a safety net of a form to drape onto as in slumping.  To observe, set your top temperature with a long soak/hold.  Start peeking at frequent intervals from the time top temperature is reached.  

When the glass has reached the desired shape, advance to the next segment.  Your controller manual will give you instructions on how to do that.

Different thickness and sizes of glass require different firing conditions.

Frit Additions to Shaped Pieces

It is possible to stick frit to slumped pieces. But soaking for a long time – several hours - at 650°C/1200°F is required to stick the frit.  The added pieces will remain relatively sharp. You need to observe frequently from 600°C/1111°F to make sure that the form of the glass is not distorting. 

Credit: Pyramid Gallery, Smyth and Zebrak

Although it is possible, adding pieces to already shaped objects is not best practice, nor will it frequently give satisfactory results.  If the slump is shallow, it is more possible to do this successfully than steep or highly shaped forms.  But the most suitable practice is to flatten the piece, then tack fuse the pieces onto it. Follow this fusing with the new slump or drape.  This flattening process will not be possible with all shapes. 

The best results will be achieved by accepting what you have and make a new piece with the planned additions from the start.

This process will not be suitable for draped glass as the glass will drape further during this low temperature soak.

 

I've a book that gives more detail. Low Temperature Kilnforming, an evidence based approach to scheduling  or at Bullseye

Square Drapes

Two pyramidical moulds. One stepped and the other smooth.

 This kind of draping mould with flat sides will never work very well as a draping mould.  The draping sides have to compress. This takes a long time and is likely to cause folds in the glass.

 The common experience is that two opposite sides drape first and conform to the mould. This displaces the compression necessity to the other two sides. This "taco" style initial drape is common in all drapes. It is usually observed in handkerchief drapes.  In the early stage of draping two sides of the glass fall, creating a taco shape. With continued heating, those long sides fall and spread the initial draped sides to become almost equal. 

 This taco formation also occurs on the pyramid style mould, giving two flat sides.  The glass on the other sides then fall. As the glass area is now larger on these sides than the mould area, a drape or fold is formed.  Imagine the drapes a square piece of cloth place on a pyramid would create. The cloth has more area than the sides of the pyramid.  The excess cloth creates folds at each corner.  The same happens with the glass.

 This draping fold can be minimised by using low temperatures and long (multiples of hours) soaks.  This allows all the sides of the glass to begin forming at more or less the same time.  I am not sure the folds can ever be completely eliminated.  With extremely long soaks, the drapes will flatten to the rest of the glass. 

 Annealing difficulties are caused by this folding.  It will create thick overlaps.  This in turn will cause the annealing difficulties. There are areas that are much thicker than others.  If you started with 6mm glass, the folds will create areas that are 18mm thick. 

 Making sure this glass - with such large differences - is all of the same temperature will require long annealing soaks.  It will also require very slow cooling segments.

 Square drape moulds are rarely successful. Folds are created at the corners, rather than fully conforming to the mould.