Domed drapes

 

Apologies.  I failed to keep the name of the maker.

A draped bowl didn't end up with a flat bottom. What schedule would flatten it out again and what to fix to get a flat bottom?

Drape schedule:

• 167°C/300°F – 527°C/980°F, 30’
• 111°C/200°F – 710°C/1310°F, 10’
• AFAP – 472/880°F, 1:00

Scheduling Factors

Ten minutes soak time at this top temperature is not enough time to allow the glass over the top to relax and become flat. To understand the dome formation on the top of the bowl drape mould, visualise the way the glass moves during the drape. I will attempt to describe it, but peeking at intervals from 55°C/100°F below the top temperature will show better than I can explain, of course. At the beginning of the drape, the weight of the glass at the edge bends the glass into a domed shape because it is not yet at a low enough viscosity to conform to the flat base. It is not until it has absorbed enough heat that it can begin to stretch and sink towards the mould surface at what will be the base of the glass piece.

At the rate used, the glass needs more time to absorb the heat to allow the glass to relax to the central surface. The extra time may lead to distorted edges, of course. So, a slower rate up to the top temperature may be the best solution. It is a curious fact that clear glass does not react as coloured glass does. It seems the infrared radiation goes through the clear in a way that it does not with coloured glass. This lower viscosity of the clear than coloured glass may be an additional factor in in allowing the dome to form.

The possibility of trapped air holding the glass up is not likely. At 710°C/1310°F the glass will not have made a seal with the upper edge of the mould. 

Drilling a hole in the supporting mould will not solve the problem, but it won’t create another either. Just as in draping a votive, a hole is not necessary, but of course the draping is done at a higher temperature. The amount of heat work the glass receives is important to the glass being able to relax to the surface of the mould.

The drape schedule

• 167°C/300°F – 527°C/980°F, 30’
• 111°C/200°F – 710°C/1310°F, 10’
• AFAP – 472°C/880°F, 1:00

This schedule is suspect for two main reasons. The first segment might be best maintained to 621°C/1150°F. The second segment ramp rate might be reduced to 33°C/50°F to 740°C/1364°F for up to 30 minutes to allow the glass to relax to the base part of the mould while the rest of the glass continues to drape. Observation, by peeking at intervals, from 720°C/1328°F until the glass has fully formed flat at the base and is fully draped, will indicate when the segment should be skipped to the anneal. 

Anneal

And, although the annealing temperature is not the cause of the dome, the anneal temperature used is not one normal for either Oceanside or Bullseye and could be responsible for a later break due to inadequate annealing.  

If not a typo, 472°C is below the accepted annealing temperature for any of the fusing glasses, and is so near the strain point that it will be difficult to anneal at all. If the glass is Oceanside, the anneal temperature is 510°C/960°F; if Bullseye the anneal temperature is 482°C/900°F. The soak time indicates the glass is being annealed as for two layers. The associated cool rate for 6mm is:

• 83°C/150°F to 427°C/800°F, 0 minutes, 

• 150°C/270°F to 371°C/700°F, or to room temperature.

However, this is a tack fused piece of a single layer plus 3 to 6 mm of decorative elements. So, it should be fired as though twice the 9mm thickest part, or 19mm. This profile of glass needs three hours anneal soak, and cooling rates of:

• 25°C/45°F to 427°C/800°F

• 45°C/81°F to 

• 371°C/700°F

• 180°C/270°F to room temperature

Note that this tack fuse profile needs cooling rates all the way to room temperature. If your kiln cools at a quicker rate from 371°C/700°F than 180°C/270°F, the schedule should include that final segment to room temperature.

Anneal Cooling

In the original schedule, there were no cooling rates given, as though the kiln will cool slowly enough (the natural cooling rate)to achieve a good anneal. This is rarely the case for any thing being fired for more than 3mm. The purpose of controlled cooling is to initially maintain the small difference in temperature across the whole piece to avoid it increasing. As the glass temperature falls below the strain point, it can withstand greater temperature differentials, but the temperature fall still needs to be controlled enough to avoid large contraction differentials. For thicker glass, the rate to room temperature also needs to be controlled.    

This piece is being fired as for 19mm and does need controlled cooling to avoid inadequate annealing and possible fractures from too quick cooling.

Layup

One thing that may have helped the formation of a flat bottom would be to have a tack fused a disc of clear, the same size as the base of the former in the first fuse. This would give additional weight to the glass and help to flatten the base sooner. The bending of the outer edges of the draping glass, would not pull the glass on the base into such a large dome, and possibly not at all. Think about weight when draping, as the heaviest will tend to fall first.

Glass Size for a Drape

 How big can a square be for a drape former?

The diagonal of the square for a former can not be more than twice the height of the former plus the base diameter. Of course it is better to be slightly smaller to allow for the glass stretching during the drape. But twice the height plus the base diameter is the maximum.

An example. If the former base is 2 and the height is 7, the diagonal can be no longer than 16 (7+7+2). The question then is what size square will that diagonal produce. The geometry formula of the sum of the squares of the two right angle sides equal the square of the hypotenuse (diagonal). That means the square pieces of glass cannot be more than 11.3. This is arrived at by:

• Calculate the square of the diagonal (16*16) = 256

• Divide that in two; half of the square is 128 (the square of one side)

• The square root of 128 is 11.31

• So the sides of the square are 11.3 each.

This is the largest square you can put on on that vase drop mould, because that size square has a 16 diagonal.

Or if you don't like maths:

• Mark a line at a 45 degree angle from the corner of a piece of paper.

• Mark off the length of the diagonal needed for the former on it.

• Draw a right angle from the edge of the paper to the point that is the length of the diagonal. That is a measure of the sides of the square.

This will require some drawing tools.

A no-tools method:

• Fold a piece of paper at the corner until the edges meet.

• Crease the fold to give a 45 degree angle

• Mark the diagonal length along that 45 degree crease

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• Fold the paper from the top toward the bottom where the 45 degree crease began.

• When the mark for the diagonal is reached, carefully crease the paper to the edge.

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• That gives you the length of the size of square needed.

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Circle sizes are a easier, because twice the height plus the base equals the diameter of the circle. So in the example above, 7+7+2 = 16 which is the largest diameter that can be used for that former.

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.