Drapes over cylinders

Draping glass over cylinders or similar shapes presents some ordinary problems in a problematic combination.

• ·        In general, the glass is a long rectangle
• ·        The glass is supported on a long thin part of the mould
• ·        The glass is usually high in the kiln
• ·        The mould is heated unevenly
• ·        The material of the mould influences the way the glass is heated
• ·        The characteristics of the glass interacting with the mould material

Narrow glass

Especially in smaller kilns, a long rectangle will receive uneven heat.  The short edges of the glass are nearer the sides of the kiln than the long edges are.  This means that the ends nearest the sides are in relatively cooler parts of the kiln in a top fired kiln.  It is the opposite in a side fired one.

Long thin support

A drape on a cylindrical mould means the glass is supported on only a long thin part of its substance.  This further increases the temperature differential in the glass.   The unsupported glass receives both radiant heat and heat transmitted through the air, allowing the unsupported glass to heat faster than where the glass is in contact with the mould.

Elevated glass

Glass high in the kiln – the effect of placing glass on top of a cylindrical mould – heats more unevenly than on the shelf. 

Uneven mould heating

The mould directly under the glass will be shaded from radiant heat, but will continue to be heated by convection of along the lower sides.

Mould material

The two common mould materials are steel and ceramic.  These gain heat at different rates.  The steel generally heats more quickly. The ceramic is usually thicker, so with a greater mass, and the heat transfers more slowly through the ceramic than an equivalent mass of steel.

Glass characteristics

Glass is a good insulator of heat.  This means that heat transfers to the mould supporting the glass more slowly than through the air.

The question becomes how to overcome or at least alleviate these limitations.

Relatively narrow glass sheets that extend near side elements will heat those narrow edges more quickly than on the long sides.  Top fired kilns often have the opposite problem, as the short sides may be in the cooler part of the kiln. The usual solution is to reduce the rate of advance, or to baffle the hot parts.  Either of these should work well in this circumstance.

The long thin support of the glass creates the problem of a heating differential.  The glass may be in contact with half a centimetre of the mould all along its length. The glass and mould heat at different rates.  The normal solution to this is to slow the rate of advance.  The slower rate of advance can be combined with periodic soaks 100⁰C intervals.

Elevated glass

Glass high in the kiln needs special care, as the heat is more uneven there than most parts of the kiln on the heat up.  A general rule of thumb is that the radiant surface temperature given by the elements evens out at a distance from the elements.  This distance is determined by the distance between the elements.  The radiant temperature evens at a distance that is one half the distance between the elements.  If your elements are 100mm apart, the radiant temperature will only be even 50mm below the element.  Any glass closer than this will require slow schedules to overcome this uneven heating.

Uneven mould heating

As described earlier the mould will be heated by convection current of the hot air, rather than directly the radiant heat from the elements.  To reduce this difference, the rate of advance needs to be slow.

Mould materials

Although there are other materials, steel and ceramic are the most common materials from which moulds are made. Steel gains heat much more quickly than ceramic.  In the forms used for glass draping, ceramic has much more mass to heat than steel.  Steel also transmits the heat more quickly.  This means that a steel mould can give a hot line under the glass, and ceramic a cool line.  Reduction in the rate of advance will assist in overcoming this differential heating.

Scheduling

Experience has shown that a very slow rate of advance to a soak of 20 minutes at 100⁰C will allow the temperature to equalise between the glass and mould.  However, too fast a rise after that will cause thermal shock possibilities.  So, increase the rate of heating by 50% to another 20 minute soak at 300⁰C.  Follow this by a rate twice the initial rate to 500⁰C for another 20 minutes as a precaution.  Then proceed to fire at a normal rate.

These precautions are not necessary on the annealing cool as the glass will be in contact with the mould.

Glass characteristics

Glass is a good insulator, so the heat passing to the mould will be less than through the air.  With steel, this will give a hot line and with ceramics a cool line.  Slowing the rate of advance will help reduce this differential.  Experience has shown that placing a sheet of 1mm fibre paper over the mould will also help to reduce the effect of the temperature differences.  You can place a sheet of Thinfire or Papyrus over the fibre paper to retain as smooth a surface as possible.

Summary

The best defence to the thermal shock of glass on a cylindrical mould is to reduce the rate of advance with periodic soaks to equalise the temperature.  The addition of fibre paper to the cylinder is an added protection against uneven heating from a hot or cold spot on the mould.

But why does the glass break at right angles to the length of the mould?

I have talked of the long thin contact line between the mould and the glass. “Why does the glass not break along the length of the glass?” I hear you ask.

In thermal shock, the break will occur on the line of least resistance.  In these cases that is on the short sides.

Rounded Bottom on Drapes

Sometimes drapes, such as the handkerchief drape over a cocktail shaker, finish with a rounded base.

The base is rounded because not enough time or heat was allowed to get it flat. The glass will benefit from a moderate, but steady advance in temperature all the way to the top temperature.  This rate will be around 100°C to 150°C per hour.  There is no need to speed the rate of advance at any time during the process of the drape.  Too rapid an increase in temperature may even give uneven drapes if there are differences in thickness or colour.  There is no need for a soak at the strain point on the way to the top temperature. Any thermal stress from the rate of advance - that some suggest may occur - will already have taken place by this temperature.

This slower rate of advance will mean that the glass will not dome so much on the drape.  It will have time both to conform to the top (which will become the bottom of the piece) of the mould support during the drape stage. 

You need to visualise what the glass is doing during the forming process. As the glass begins to drape, the glass on the support rises because it is not yet soft enough to stay flat on the supporting mould. It is only later at higher temperatures, that the glass on top of the support can conform to it.

If you watch the process – a really good practice - you will be able to tell when you have a good drape. And with this reduced rate of advance, you should have a flat bottom. And all of this may happen at a lower temperature than you expected.

Scheduling for a New Slump Mould

Often you will see statements that imply a single temperature and time is suitable for all slumping or draping.  This is not so.  In fact, slumping is a delicate balance of layup, time, gravity, shape and temperature. This applies to draping operations too.

Factors in glass forming

The balance of colour arrangement has an effect on how the glass forms.  In an extreme example of white on one side and black on the other, the forming will begin on the black side first, leading to an uneven slump. Read on - there are ways to make this effect less severe.

The length of time you are willing to wait for the piece to slump is a factor in the temperature required.  Patience is rewarded.  Longer soaks mean that lower temperatures can be used. Lower temperatures lead to less marking on the back.

The mass (often thought of as the thickness) of the glass affects how quickly the glass will form. The greater the mass, the sooner the glass begins to form. This means with heavy glass lower forming temperatures can be used, because of gravity effects.

To get glass conform to a mould with complicated shapes takes longer soaks or higher temperatures than simple shapes.  This is because the glass requires to be more plastic to get into multiple shapes, small details or sharp angles.

General Principles

Since all these factors interact, any one schedule will not do for all occasions.  The general principles for a good slump are:

• Use a steady rate of temperature increase (rate of advance).
• Use the lowest practical temperature to get the forming done.

The reasons for using a single steady rate of advance in kiln forming are:

• It is much simpler to program a single rate of advance all the way to slump temperature.  
• Glass reacts best to steady inputs of heat, allowing the whole substance to be at the same temperature as it heats up.
• It helps avoid uneven slumps.  Glass that is the same temperature across and throughout itself is more likely to begin to form all at the same time.
• It helps ensure that the whole of the thickness of the glass is at the same temperature, thus avoiding splits on the bottom.
• The slow steady input of heat means the glass can be formed at a lower temperature because of the heat work put into the glass on the rise in temperature.

The reasons for using the lowest practical temperature to slump and drape are:

• It allows the glass to begin moving before it gets sticky, and so dragging on the mould producing stretch marks and sometimes needles.
• A low temperature slump reduces the risk of uneven slumps.  At low temperatures the glass is less likely to react to colour variations that absorb heat more quickly than others.  Where there is uneven weight, the forming is more likely to be even as it cannot react so quickly to the differences in weight.
• The glass will be less marked on the mould side at lower than at higher temperatures.  The glass, being less plastic, will take up less of the mould texture.

Calibration of Schedules

As each mould is different, there are as many schedules applicable as there are moulds.  Bullseye has recognised this by publishing suggested schedules for their moulds.  But there are lots more moulds than the Bullseye ones.  And even for the Bullseye moulds there are a variety of variables in the glass put on top.

The point is to find a way to determine the appropriate schedule for the mould and the glass it supports.  This involves the main variables - rate of advance, top temperature and soak time - although there are others such as lay up, degree of fusing, weight and its distribution, colour variation, etc.

The rate of advance will depend on:

• The thickness of the piece.  Thicker glass needs slower rates of advance.
• The degree of fuse.  A tack fused piece will require a slower rate than a full fused piece.

The top temperature depends largely on the complexity of the mould shapes, although it is very closely related to the soak time.  One of the principles of slumping given above is to use the lowest practical temperature. The reason for this is to get a good result with the minimum of mould marks.

The main means of determining forming temperature and time is observation. I determine my slump temperature (normally) by what temperature I have to use for the particular mould to get the glass fully slumped in half an hour.  For more complicated moulds such as a candle bridge I would use 1.5 hours as the soak time.

There are two main methods of doing this observation.  One is to set the “one size fits all” schedule and modify it. The other is to create a new schedule by working up from the lowest temperature to the practical temperature.

Modification of existing schedule

To prepare for the modified schedule, you need to do several things.  

Get your kiln log out ready to record the information about the firing.  Record the mould shape and separator (and add a picture of the set up if you can) and include the lay up of the blank to be formed.  Also record anything you think may be relevant to the forming process for this firing.

Set your single rate of advance all the way to the top (forming) temperature and record it in the kiln log.  Begin observing the progress of the slump from 60°C below the top temperature you have set.  This involves quick peeks at approximately five to ten minute intervals.  You may not see much movement at first, but at later peeks you will see the glass progressively forming.  When the glass appears to have just touched down at the bottom, you can use that as the top temperature.  Advance the schedule to the soak portion (read the controller manual if you do not already know how to do that).  Note the temperature and time in your log book when you do this.

Continue to observe the progress of the slump but at about ten minute intervals to check on the progress of the slump.  When the slump appears complete, advance to the next segment of the schedule and note the time.  Subtract the start of the slump soak from the present time and you know how long the soak needs to be for that layup in the mould. Record that in your log book. 

When cool, inspect the slumped piece to determine if it is fully formed. Record the results in your kiln log.  If it is not fully formed, you can decide if it is practical to add additional soak time or if you need to increase the top temperature.  Only you can determine what is a practical soak time.  If you are soaking while you are away or asleep, it does not really matter how long a soak you need at the chosen temperature.  However, there are times when you need to have a piece out of the kiln to be able to put in the next.  Somewhere between these two is the practical soak time.

You may find that the glass does not need as much time as you gave it.  Record this result too.  In this case, you can reduce the top temperature in future firings until you find the best combination of temperature and time. You will have experience from watching the forming (whether slump or drape) to give an indication of the lower temperature to choose.  A general guide would be to reduce the temperature by 10C, and extend the time by at least 50% more than  what you used in the higher temperature firing.  

Record each firing with the lay up, rates, temperatures and soak times, plus the results.  When you have determined the ideal combination of factors, record the determined temperature and soak time together with the layup in your log book and on something in your mould box.  I have also used vitreous paint on the underside of the mould to indicate my standard temperature and soak time so that I don't loose the information.

Development of a new schedule

This is not as difficult as might be imagined.  It does involve a lot of peeking into the kiln, though. You start with an appropriate rate of advance for the thickness and style of fusing.  Remember that thicker glass and tack fused glass require slower advances than thinner and flat fused glass.  Set this rate all the way to your predicted top temperature.  No rapid rises with short soaks are required or desirable. Set a predicted soak time. If you are not certain, use 30 minutes as a general average. Then set the anneal soak and cool rates.

As you observe, you will see when the glass on the mould begins to form. It will generally start at about 600°C.  Peek at about 10 minute intervals from that temperature onwards toward the target.  When you see the glass begining to change shape, Change the top temperature to be about 20C higher than the initial forming temperature, and then observe after 15 mins at the new temperature. If it hasn't moved much, add 5°C more to the temperature and observe. Repeat as necessary. When the glass has a significant curve, stop the rise and soak at that temperature with the 30 minute soak.  Continue to observe at 10 – 15 minute intervals to determine when the slump is complete.  Then proceed to the anneal cool. Record rates, temperatures and times in your log book.

When you remove the piece from the kiln, check it over.  If it is not fully slumped, you can add time or temperature.  Adding time is likely to give a better surface to the glass on the mould side.  Sometimes, but not often, adding temperature will be the choice. 

It is possible that the piece will show evidence of too high a temperature or too long in the mould.  This will be clear from extensive mould marking, sometimes needles at the edges, stretch marks, or uprisings at or near the bottom of the mould.  In these cases, the temperature needs to be reduced.  Reducing the time is not advisable, as quick slumps can often be distorted or unbalanced.

Glass Types

Remember that these tests for the best forming schedule for you and your mould are only relevant to the kind of glass you are using at that moment. There will be only minor variations between Bullseye, Uroboros and Wissmach. There will be major variations between these and float glass. Separate schedules will need to be worked out for it, remembering that there are a variety of manufactures of float and they do not all behave the same as each other.  Float and other glass that is not formulated for fusing will not provide such consistent results as fusing glass, but successful schedules can be determined in the same way as for the fusing compatible glasses.

Records

Once you have calibrated the temperature and time for the mould and the layup, you will know how to schedule for that mould. Record it in your log book and also along with your mould, either in the box or on the mould.

It will be for you to decide whether you use longer times and therefore lower temperatures.  When making the decision remember the principles of slumping – steady rate of increase to the working temperature, and use of the lowest practical temperature.

These actions will give you the standard forming temperature for the mould.  It is a base from which to make variations when you use a different thickness, lay up, or degree of fusing.  

You should continue to record each of your firings with full details, because sometimes things change. This will give you a basis to diagnose what has become different. It will help avoid the cry of "this has always worked for me before."  It means you have the possibility of working back to see what, if anything has changed. If nothing has changed in your level of fusing, thickness, lay up, schedule and all those other things you record, then you can begin looking at your kiln to see what might be different.

Slumping and Draping materials - Kiln Forming Myths 12

Always slump into ceramic, drape over steel

This myth is based on the fact that steel expands and contracts more than glass and ceramic expands and contracts less.

So, the myth goes, slumping into steel means the glass will be trapped or crushed by the contracting steel.  But draping over means the steel will contract more than the glass making the removal of the glass easy.

The reverse is the expectation for ceramic.  Slumping into the ceramic allows the greater contraction of the glass to be removed from the mould without sticking.  But draping over means the glass traps itself against the ceramic as a result of its greater contraction.

These things are true.  But….

The most important thing in considering a mould is the draft.  This not about cold air, but the angles of the mould. A mould with vertical sides will not release the casting or kiln formed object even if the expansion characteristics of the two materials are identical. To release, the mould must have a slight angle from the vertical away from the glass.  This applies whether a slump or a drape.  This is called a positive draft, as illustrated.

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 And here

If the draft is sufficient, it does not matter whether you are slumping or draping into steel.  In using a stainless steel mixing bowl for draping, you can only use the lower portion where the angle is shallow.  If you rest the glass on the rim, the draft will be too steep to allow the glass to slide upwards as the steel contracts on cooling.

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Even when draping over steel, you need to have a draft to aid the easy removal of the glass, as in this example:

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When draping over ceramic, you need to be careful that you have sufficient draft over the whole of the mould. In the case of this ceramic draping mould you need to make sure the glass is not fully formed as the steep portion at the top will be where the glass grabs the mould.

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And if you were to use this casting mould as a slumping mould, the steep straight sides would make it difficult to get the glass out of the mould. 

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Although the facts behind the statement “slump into ceramic, drape over steel” are established, you need to understand that the draft of the mould is as important as the way in which you use the material.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Thinfire as a Separator on Moulds

"I was told that it was possible to put a piece of thinfire paper under a circular piece of glass that is to be draped over a mould....  Has anyone done this or heard of doing it?"

Yes this can be done. Some caveats are in order though.

It is important to put a separator on the mould -normally kiln wash - before using it. Once coated, it will not need to be re-coated unless the surface is damaged, scratched, etc., for a very long time. If the kiln wash seems to be rough you can smooth it in various ways as noted in this tip.  The advantage of kiln wash is that it does not cost much and lasts a long time. The thinfire or similar is a one-time-use product.

The binder in the thnifire burns away during the heat up and allows the now unbound separator to drift down to the mould before the glass begins to conform to the mould. In that way it is very similar to a fine dusting of kiln wash powder over the mould surface before firing.

I don't see the point of using Thinfire or similar during slumps (although I can see that there are occasions where this method would be useful). For drapes, placing a bit of thinfire over the mould - especially if it is metal – is an additional precaution.

My view is that you have to kiln wash the mould anyway. KIln wash is cheap and long lasting while thinfire needs to be replaced after each firing.

Complex Drapes

A question has been asked that relates to draping over a face mask mould, but this response relates to any drape with compound or multiple shapes.

The rate of advance should be a steady one all the way to the forming temperature. This should be 150C/hr or less. Thicker glass requires a slower rate of advance to allow the glass to heat all the way through. The point is to get the glass all the same temperature by the time it reaches the forming temperature.

Draping over an undulating mould takes more heat or time or both than simple drapes, because there is a much greater variety of form for the glass to conform to.

You have a choice about the top temperature and soak times. You can choose a low temperature with a long soak time, which most often leads to a minimum of marks on the underside of the piece, but requires long vigils at the kiln to determine when the drape is finished. The other strategy is to go for a higher temperature and shorter soak, which leads to more marks, but less time observing the firing. The higher temperature may be as much as 720C.

Note that there are a group of considerations about the size of the drape and the thickness of the glass being draped. 

Weight 

Span 

Complexity of shape 

There are some things you can do to assist the progress of the drape. One, already mentioned, is to increase the forming temperature.

Use a longer time, or as much time as required. Watch the draping progress because it is never certain how long the piece will take to conform to the mould sufficiently.

Use props. Place the props where the glass first starts to form just barely supporting the edge of the glass. As the glass begins to bend, it will slip off the supports - assuming they are well covered in separators, even pieces of kiln paper. This means the folds will start somewhere else than at the nose on a face mould or other high point on any other form.

Manipulate the glass. Reach in with wet wood sticks and push the glass about. One stick will be needed to keep the glass in place while the other pushes the glass about. The sticks do need to be both wet and strong. If you use dry sticks they will mark the glass as well as go on fire. Of course, you need protective gear to avoid burns to your skin and hair if you do this.

Separators for Steel

Steel moulds, whether stainless or other steel, require separators as glass will stick to hot metal.

The preparation of steel is a little different from ceramic or fibre moulds. First the steel has to have the oil coating removed. This can be done by sanding, sandblasting or by heating to about 550C.

The separators can be just normal kiln wash, boron nitride, or fibre. When using kiln wash or boron nitride, the steel needs to be heated – about 200C should be enough. Too hot and the water will boil leaving gaps in the coating. If the metal is too cool, the kiln wash will drip. A little experimentation will be needed to find the right temperature for your purposes. Remove the mould from the kiln and brush or spray on the separator. Return to the kiln to heat up again and apply the separator again. Continue this until an even complete covering of the metal has been achieved. It does not have to be thick.

Fibre paper works best on cylinder or wave moulds with curves in one plane only.  Bowl shapes lead to wrinkling of the fibre papers and marking of the glass with wrinkles.  This applies to the Bullseye Thinfire too.

Just as on shelves and ceramic moulds and as long as you fire below 700C, you can use the coated mould over and over with no problems. Once you fire hotter than 700C, it is best to remove the old kiln wash and put a fresh coating on before firing again.

Single Layer Slumping

Contrary to expectations, single layers are more difficult to slump than multiple layers, as you do not have so much mass for the gravity to act upon. Especially if the pieces are small, the slump will be slower than for large pieces.

It is possible to take the glass up in temperature very quickly without breaking it, but that will not give you much control over the slump. You may not get the curve you want, and you may over fire it so that it distorts.

If you feel the need to go quickly, slow down from 630C to a rate of advance of 40 or 50C/hr. For the first firings and new moulds, observe how the glass settles into the mould. This will give you the top temperature.  In future you can back off 10C or more and soak for about 30 minutes to get less marked pieces.

You can drape or slump a single layer over a mold or drape two overlaid layers on or over a mold – assuming the layers are 2 mm to 3 mm thick.  Draping double layers together is a popular way to make two colour double layered handkerchief vases with art glass. But when you fire one piece of glass on top of another, the air between the two pieces acts as an insulator, slowing the heat transfer between the two pieces of glass. So you should advance at about half the rate you use for a single layer. Anneal soak and cool for 6 mm glass.

Note that there are different behaviours between single and double layered pieces.

Draping and slumping processes work by taking advantage of the action of gravity on the just pliable glass. This means that thicker, heavier glass slumps and drapes more quickly than thin light glass does. You will need longer soak times for thin glass than for thick glass. You can go higher, but the effects become less controllable.

The size of the opening of the mould has a strong effect when slumping single 3 mm layer glass. You need much more time – often as much as three times as long as for a 6 mm piece over a small diameter.

The effect of the shape of the mould has little effect on simple shapes. But when angular or complex curves are part of the mould the single layer will have much more difficulty in conforming to the mould than the thicker pieces.

Annealing the 3mm piece can be a bit quicker than for 6 mm, but a reduction of more than a third in the soak time and more than doubling the cooling rate may cause problems.

Fusing with Rocks

Rocks contain a lot of water and so require a long gentle drying process of days or weeks to avoid steam building up within the rock and breaking the rock and so also breaking the glass in any attempt to fuse the rock into the glass.

Rocks and glass have radically different expansion rates and viscosities from glass. Also rocks are not consistent in their characteristics as they are made up of different proportions of materials even within one site. So fusing rocks to glass is likely to be unsuccessful.

However, you can use the rock as a master for a mould. Make a mould of the rock, then pour investment material into the mould and use that to drape the glass around. Then glue the rock into the shaped glass.

Ramp Speeds for Slumps and Drapes

I find that a steady slow heat up gives best results for both slumping and draping. Many do a two stage heat up with a slower initial rise to a temparature above the annealing point and then faster afterwards.

A slow steady increase in temperature allows all the glass to be nearly the same temperature both across the piece and from top to bottom.

Speed in draping, especially where there are different colours and thickness, promotes "wings" as any unevenness in the heat across the piece will translate into variable drape speeds. Allow all the glass to get to the same heat, by going slowly.

I proceed at the same speed from start to the forming temperature - no speed up at all. Yes, the glass will accept it a rapid heating to the target temperature, but it does not promote even slumps or drapes.

Candle Bridge Moulds

A candle bridge mould is one of the most difficult moulds to use successfully. The problems relate to the kinds of work you are trying to do, the size of the glass to put onto the mould, the shape of the piece and the stability of the resulting piece.

With a candle bridge you are trying to do a combination of slumping and draping at the same time. You are slumping into the middle and draping over the curved sides.

Additionally the candle mould requires the glass to fall into a small opening and this requires long soak times. Long soak times mean the glass that is draping stretches while the central portion is trying to fall into the opening. Of course, if you don't want the depression to be flat, you don't have to soak so long and the stretching effects on the draping part of the of the glass won't be so great.

It would seem logical to measure the mould around the drapery curve (or arc of the mould) and to the shape of the ends, but experience has shown me that this leads to glass that is too long along the sides and bent at the ends. So I cut my blanks for candle moulds as a rectangle without curved ends and then round the corners of the rectangle just a little by nipping them with my grozing pliers before fusing.

If you measure along the top and along the length of the mould you have a piece of glass that will be increasing in length at the draping part of the mould, meaning that it will fall off the curve and onto the draft (or side) of the mould. The draft is an angle from the vertical. Good moulds are made with a draft so that if glass were to fall over the edge it still will be possible to get the glass off the mould.

The draft on a mould means the diameter of a circular one is greater at the base than it is at the rim. And it is common to measure only the diameter at the rim.  In the same way the dimensions at the outside base of a rectangular mould are larger than the rim of the mould.

Back to the rectangular candle mould. The draft on this means that measuring the base of the mould is slightly wider than the curved part of the mould, but less than if measured around its curved portion.

Experience has shown that in the case of the candle moulds measuring the width of the mould is sufficient. There is enough height in the moulds I have used that it does not make any functional difference if the glass does not reach the bottom of the curve on the mould. It is better than hanging off the edge.

The length of the glass should be no longer than the shortest part of the mould's length. Cutting a curve into the glass to allow a small overhang produces a depressed lip because of the length of the soak required for the slump into the small aperture of the candle depressions.

My soak for candle moulds is 90 minutes at my process temperature. This gives me a flat depressed area for the candle to sit, but it also means that the draping glass has been stretching. And it also means that the glass will drape unevenly as the various colours absorb heat differently allowing some parts of the glass to stretch more than others.

The placing of the glass on the mould is absolutely critical. It must be exactly parallel to the sides of the mould. Any slight movement from that will induce a twist in the resulting piece allowing it to rock. Arranging it exactly right and placing some kiln washed furniture at each side to keep it in place until it begins to slump is an important aid.  
The glass will begin to bend before it sticks to the kiln furniture.

I have never been able to get a stable candle mould whether from 3mm or 6mm thick glass. I always have to grind the base a little to make a stable piece. I take it as part of the process, but careful placing reduces the work.

Effect of mould shape on firing schedules

Each time you get a new mould, you should think about the firing schedule that will be needed. The existing schedule you use may need to be changed, so you need to observe the first few firings to be sure you have the correct heating pattern for the mould and the glass.

• Simple curves such as ball mould, square slumper are easiest to slump into, as they have only easy curves to take up. They need only low temperature slumps, and possibly not very long soaks. Although it is best to achieve the slump with approximately a 30 min soak, so that you are using the lowest practical temperature and so minimising mould marks on the glass.

Simple ball mould and slump mould with flat bottom

• Compound curves are those such as an ogee curve that starts in one direction and then moves into another. These require more heat or time than the simple curves. The glass begins to fall into the centre of the mould first, which will be the steepest/deepest part of the mould. The glass will first of all take up a simple curve, and only later conform to the other part of the curve. It is best to start with a low temperature slump and add time (only later increasing temperature) until you find a temperature and time that is practical for the mould.

Moulds with ogee curves and one with an angle at the foot

• The same procedure is needed for moulds with sharp curves or angles. Bowl moulds that have a sharp angle at the foot need much more time than the simple curve. The glass falls to the bottom of the mould first and then has to relax into the sharp angle at the edge of the foot. This takes considerable time. If you add lots of temperature to achieve this relaxation, you run the risk of getting an uprising of the glass near the middle of the bowl. So considerable care is needed to find the right combination of time and temperature for this kind of bowl.

• Draping moulds – those you want the glass to form over rather than into – have other requirements. The mould on which the glass rests forms a heat sink. This means the mould drains heat from the glass in that area while the rest of the glass heats up more quickly. This can lead to breakage. Draping requires more observation to get the forming right than slumping does. Each difference in span of the glass requires a different amount of time to complete the drape even though it is on the same mould. Drape moulds with steep sides require quite different considerations.