Heat Work

“Heat work” is a term applied to help understand how the glass reacts to various ways of applying of heat to the glass. In its simple form, it is the amount of heat the glass has absorbed during the kiln forming heat-up process.

There is an relationship between how heat is applied and the temperature required to achieve the wanted result.  Heat can be put into the glass quickly, but to achieve the desired result, it will need a higher temperature. If you put the heat into the glass more slowly, the reverse applies.

For example, you may be able to achieve your desired result at 816C/1500F with a 400C/hr (720F/hr) rise and 10min soak. But you can also achieve the same result by using 790C/1454F with a 250C/hr (450F/hr) rise and 10min soak. The same amount of heat has gone into the glass, as evidenced by the same result, but with different schedules. This can be important with thick glass, or with slumps where you want the minimum of mould marks. Of course, you can achieve the same results with the a rise and a longer soak at the lower temperature, e.g. a 400C/hr (720F/hr) to 790C with a 30 min soak, but you will have more marking and difficulty with sticking separators.

In short, this means that heat work is a combination of time and temperature.  The same effect can be achieved with: 
- fast rates of advance and high temperatures.
- slow rates of advance and low temperatures.

- long soaks at low temperatures.

You obtain greater control over the processes when firing at slower rates with lower temperatures.  There is less marking of the back of the piece.  There is less sticking of the separators to the back and so less cleanup.  There is less needling with the lower temperature.  More information on heat work is here.

The adage “slow and low” comes from this concept of heat work. The best results come from lower temperature processing, rather than fast processing of the kiln forming.

More information is available in the eBook Low Temperature Kilnforming available from Bullseye and Etsy.

Revised 1.1.25

Deep Slumps with Bubbles

 

Photo Credit: Rachel Meadows-Ibrahim

The main causes of the large thin bubble is most probably  too high a temperature combined with a long soak.

Elevation of the Mould

The poster indicated there are eight holes total – four on the sides and four under the glass. This means any air has an exit out from under the glass and from the inside of the mould. So, in this case it does not need to be elevated for exit of air.  In my practice l have never, except in tests, elevated my slumping moulds. I have not had failures. My experiments involved in writing the eBook Low Temperature Kilnforming  showed no significant temperature differences between elevating, or not, below the mould.

Effect of Fast Rates

Slow rates to low temperatures with long soaks avoid sealing the glass to the mould. This means air can move out from under the glass during the slump. 

• Fast rates, and elevated temperatures can restrict air movement from under the slumping glass.  
• Fast rates and high slump temperatures can each cause uprisings because the glass slides down the mould during the soak, and that weight pushes the bottom upwards.

Temperature and Uprisings

This uprising is different from the bubble at the bottom on this piece. It is possible to see the glass bubble is thinner than the surrounding glass. As there were holes for air to escape, it seems the temperature and or speed was great enough to allow the glass to form to the mould at the bottom.  This covered the air holes and allowed the remaining air to push upwards on the glass.  A lower top temperature may have avoided this bubble formation.  Certainly, a combination of a slower rate and a lower temperature would have avoided the formation of the bubble.

Observation

Further, observation during the firing would have caught this bubble formation early enough to skip to the annealing and result in a piece with only a slight uprising, and before it became a bubble. Peeking should start at the beginning of the slumping soak and be repeated at 5 to 10 minute intervals.

Visible Devitrification

"Why does devitrification appear on slumped pieces?"

A brief explanation 

Scientific research in developing a glass matrix to support bone grafts gives some information.  This kind of glass matrix requires to be strong.  Development showed that devitrification weakens the matrix.  The crystals in a matrix are not as strong as the amorphous glassy state.  So, devitrification needs to be avoided.

The research to avoid devitrification showed that it begins at about 600˚C/1110˚F.  It only begins to become visible above 700˚C/1300˚F.  The process developed was to introduce a “foaming” agent.  The process fired slowly to 600˚C/1110 ˚F and then quickly to 830˚C/ 1530˚F.  It left a strong open matrix around which bone can grow. Although the research used float glass, it is also a soda lime glass, just as fusing glasses are.  The formation of devitrification begins at the same temperature for fusing glasses as for float.

The result of this medical research shows that devitrification begins on glass before it is visible. Devitrification is cumulative. A little becomes greater with another firing.  This is so even with good cleaning between firings. The new devitrification builds on the previous.  It does this from 600˚C/1110 ˚F.

A subsequent firing can continue this devitrification to the point where it is visible. This can happen, although the temperature at which we can see it after one firing has not been reached.  This continued devitrification at low temperatures can become great enough to be visible at the end of one or multiple slumps.

Credit: Bullseye Glass Co.

What can we do?

Clean all the glass before every firing very well.

·         Avoid mineralised water.

·         Final clean with isopropyl alcohol.

·         Polish dry at each stage with white absorbent paper.

 

Soak longer at lower temperatures.

·         Use longer soaks to achieve the slump.

·         Keep the temperature low.

·         Observe the progress of the firing with quick peeks.

 

Use slower ramp rates.

·         Slower rates enable the heat to permeate the glass.

·         Enables a lower slump temperature.

 

If there is any hint of devitrification after the first firing,

·      use a devitrification spray, or

·      provide a new surface.

• o   remove the surface by abrasion on sandblasting,
• o   cap with clear, or
• o   cover whole surface with a thin layer of clear powder.

·      Fire to contour fuse to give a new smooth surface.

·      Clean very well and proceed to slump.

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.

 

Diagnosing Slump fractures

Once you have an initial idea of the source of the problem, think about it.  Test it against the evidence.  Is there enough evidence to make a call?  Make sure you have considered alternative explanations.  It is just too easy to make a snap decision about causes in low temperature processes.  The source of breaks in slumping are most often complex and stem from interrelated factors.

I give you an example of the difficulties of diagnosing a slumping break.

On a Facebook group a person showed the break of a single layer on a cyclone mould.  Others commented the same had happened to them.

Picture credit: Esther Mulvihill Pickens

Possible causes suggested on Facebook included:

• Thermal shock on the way up
• Thermal shock on the way down
• Too large on the mould and broke due to differential contraction
• Too many holds on the way up
• Too hot
• Too thin
• Follow the CPI programme
• Glass extending over the sides

Some of these suggestions were of general applicability, some in relation to the state of the broken glass.

The suggestions did not include:

• Cause of the rounded dots at the bottom of the mould.
• A cause for the state of the flat piece off the mould (it appears sharp edged.  Does it show some forming already?).
• The cause for the location of the fully formed remaining glass.
• The effect of the location of the mould and glass in the kiln.
• The consequences of a short soak at top temperature. 
• Is the kiln running hotter than most (1290ºF/698ºC for 10 minutes at top temperature was used)?

Of course, it is difficult to diagnose a problem from just one picture. It is difficult even with many pictures. And so, without handling the object, only suggestions can be made.

But….

You must spend enough time examining the piece with whatever other information is available to make specific suggestions.  The first thought may not consider all the factors.  Consider what kinds of causes there are for breaks during or after slumping.

More close inspection reveals the rounded edges of the break.  That supports the idea that the temperature was too high. It also supports the diagnosis that the break occurred on the heat up.

The edges of the piece that has fallen off the mould, and now rests on the shelf, seem to be square or sharp. This shows the extent of the difference of temperature between shelf and top of the mould – less than 100mm/4 inches.  Also, how small the differences in temperature are between slump and tack.  The extent of difference in fusing does depend on how high in the kiln the mould is placed.  That is demonstrated here by the different elevation of the two pieces. 

The conformation of the glass to the mould is complete.  This supports the diagnosis of the break occurring early in the firing, and certainly before the slump was complete.  These pieces will not fit together.  So, even if the edges were sharp the fact they will not fit together shows they conformed independently to the mould surface.  Therefore, the break was before forming temperature was reached.

The glass hangs over the mould edges on only three sides and at an angle.  This indicates the cause of the overhang was the break.  Not the reverse. An overhang at the beginning of the slump is likely to be even.

The piece on the floor of the kiln combined with the movement of the glass toward the back gives an indication that the origin of the break is at the front.  This relates to uneven temperatures and to the placement of the mould.

No one mentioned the placement of the mould and glass at the back of the kiln.  This will have an effect on scheduling.  The mould and glass are very large in relation to the kiln.  There is little space between the glass on the mould and the walls of the kiln.  Also, the mould is placed asymmetrically in the kiln – very close on three sides.  This will cause uneven heating in any kiln.  To have a successful firing of glass on this mould in this kiln will require radically different schedules to that for a centrally placed mould that is moderate for the size of the kiln.

The large size (relative to the kiln) and the asymmetrical placing are the causes of the break, in my opinion.  I admit that it took me several looks to realise the placement was a key cause of the break.

So, the generalised comments about thermal shock are correct, but not as to the cause of that shock.  The kiln will be hotter in the central part and cooler at the corners.  This is true of all rectangular kilns.  The important thing is to learn how to cope with these temperature differences.

Slow firings to low temperatures with long soaks are the three important elements.  These make up the heat work of the kiln. Applying this to a schedule means:

• slow ramp up rates – as little as one half the recommended rates for centrally placed moulds that are moderately sized in relation to the kiln.
• Low temperatures present lesser risks to the control of the outcome of the firing.  Determining the lower temperature possible requires peeking into the kiln to monitor the progress of the firing.
• Long soaks combined with low temperatures get the kilnforming done with minimal marking of the underside.  Low temperature soaks - in excess of 30 minutes - are required to minimise the marking.  Observation of the slump will be necessary to determine when it is complete.

My suggestions for the causes of other elements are:

·        Cause of the rounded dots at the bottom of the mould.

The temperature was too high. 698ºC/1290ºF is much hotter than needed for a slump. It was hot enough to round edges and small shards of glass.  Which shows excessive heat was received by the glass.

·        A cause for the state of the flat piece off the mould (it appears sharp edged. Does it show some forming already?)

The soak of 10 minutes was too short for the temperature in the kiln to equalise from top to bottom.  The glass on the shelf may not have reached 650ºC/1200ºF with such a short soak.

·        The cause for the location of the fully formed remaining glass.

The glass broke and was forced apart by the size of the expansion differences within the glass.  The movement of a piece at the front of the mould combined with the rearward and side movement of the glass indicate the origin of the break was at the front.  The distance apart shows the amount of force, and so the degree of reduction in the ramp rate required to fire this successfully.

·        The effect of the location of the mould and glass in the back of the kiln has already been discussed.

·         The consequences of a short soak at top temperature.

A high temperature is often considered necessary to pick up all the detail in moulds, whether slump or texture moulds.  The same effect can be achieved at lower temperatures with longer soaks.  The results of this strategy are fewer mould marks on the bottom of the work.

·        Is the kiln running hotter than most (Used 1290F/698C for 10 minutes at top temperature)?

This is one that cannot be answered other than by experiments carried out by the owner of the kiln.  Look at the Bullseye Tech Note #1 Knowing your Kiln for methods of testing temperatures. 

In short:

Diagnosis of slumping breaks is more complex than it appears at first.

More information is available in the eBook Low Temperature Kilnforming, an Evidence Based Approach to Scheduling.

This is available from Bullseye or Etsy

Slumping Blank Size

When you're making a piece that you intend to slump does it need to be bigger than what you're making, and by how much?

Generally,

The general advice is to make the blank the same size/diameter as the rim of the mould.  This works best for shallow moulds with a generous draft, or shallow slope to the bottom.  

There are numerous exceptions of course.

Deep moulds

Deep is a relative term.  A small round mould of 100mm/4” with a 30mm/1.25” drop can be considered deep. But this drop would be considered shallow for a 300mm/12” diameter mould.  A drop of 100mm/4” into a 300mm/12” mould would be deep.

There are two approaches to this.  We know the blank will end up with a considerably smaller diameter than the deep mould. This is because the surface of the glass does not change its dimension much.  As a result, the diameter of the slumped piece is less than the flat blank’s diameter.  Placing a flexible measuring tape on the outside of the slumped piece will show the lengths of the flat blank and curved piece are similar.

Larger Blank

As a result, we are tempted to make the blank larger than the mould.  By how much? as the questioner asks.  The safest is avoid exceeding 6-8mm/.025 – 0.375”.  With a slow slumping rate, the edges will rise as the interior bends downwards and allow the excess glass to take up the same diameter of the mould before deforming enough to catch on the edge of the mould.  With a centimetre or 0.375 inch overhang, you begin to take greater risks with the rim beginning to slump outside the mould and hanging up. 

Smaller Blank

But size matters.  The small excess works best on larger moulds (250mm/8”) or more.  Employing this oversizing on smaller moulds has problems.  The span of the smaller moulds requires higher temperatures and/or longer soaks.  The result of this greater heat work is that the rim of the glass can begin to slump outside of the mould. In extreme cases, this will cause the glass to break.  More often, the edge does come into the mould, but with heavy stretch marking and sometimes needle points where the edge drags along the mould.

It is often best to make the blank smaller than the mould.  Small enough that it fits just below the rim of the mould.  This allows forming of the glass without the frequent drag marks and needle pointing.  Placing the glass level is most important when the glass is not supported by the rim.  If the final size of the slumped piece is important, you will need to slump in a larger mould. I do not know of a method of calculating that.  It is a matter of experimentation.

Steep Moulds

Ceramic shapes from charity shop finds that are adapted to be moulds are often steep sided as well as deep. They often have no rim on which to rest the glass before the slump shape takes over.  Both these elements result in the glass being much smaller than the mould when complete.

Collar

You can counteract the effect of deep, steep moulds by placing a collar of fibre board around the mould.  

Make a cut out from the fibre board by placing the mould upside down and tracing the outline. Cut the board slightly inside the line scribed.  Then fashion a bevel to meet the angle of the outside of the mould.  Support the board at the appropriate height for the mould.  Fill any gap between board and mould with kiln wash powder or a paste of the powder, depending on the nature of the gap.  This supports the glass during the slumping while allowing it to slump into the mould.  It increases the possibility of getting a steep side on the glass.  It also allows you to put a rim on the shape if you want.

Staged Slumping

Sometimes the collar or rim is not sufficient to allow the glass to move as desired in a single slumping stage.  Then multiple slumping stages are required.  The common approach has been to purchase a three-stage slumping set.  This can limit your approach.

·        The general approach is to measure the inside surface of the final steep mould. 

·        This gives you the starting diameter for the blank. 

·        Then measure the diameter of the mould at the outside of the rim. 

·        This gives you the diameter of a slumped piece to fit into the final mould.   

Compare the largest diameter blank to existing moulds you have. Will the glass have slumped enough to reduce the diameter to fit into the final mould?  In some cases, where the answer to that question is yes, only two-stage slumps are required. 

Most times a three-stage slump is needed. You need to find an intermediate mould that will deliver a slump with the required diameter.  This will be a moderately deep mould, usually with steeper sides than the first, but less steep than the final mould.

Angular Moulds

Angular moulds are those with sudden short drops to the foot (flat part of the mould). These are often ogee curves. 

These require more time or heat to form completely to the bottom of the mould.  The glass is curving in two directions during the slump.  The glass should be only slightly larger than the mould at most.  To counteract the tendency to slide down the mould, low temperatures and long soaks are needed.

 

Rectangular moulds

The main problem of rectangular moulds is the dog boning of the straight edges of the glass during the slumping. There are some suggestions.

Cut the blank with slightly outward curves.  This will help to compensate for the tendency to dog bone.  It will require some experimentation.  Slumping a standard square or rectangular mould will give an idea of how much the glass deforms during the slumping.  That amount of curve can be added to the edge when cutting out the blank.

Round the corners of the blank.  This relies on the principle that there is more glass at the corners to slump than at the sides.  A 10mm/0.375” radius should be enough.  

There is less glass to compress along the sides than at the corners.

Another element is to provide the rectangular shapes with rims.  This forces any dog boning to the rim rather than the sides of the slump.  It can be combined with either of the previous possibilities.

These do not always work and are sometimes difficult to reconcile with the design. The additional possibility to counteract the dog boning of the shape is to use slower rates, lower temperatures, and longer soaks. This is not always successful.  Rectangular shapes remain the most difficult to get the glass to conform faithfully to the mould.

 

There are ways to get the slumping blanks to conform to the moulds, and they all involve modifications to the glass, mould, or schedules.

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.

Evaluating Top Temperature Effects

Fire for Your Kiln and Objectives

Credit: FusedGlass.org

 Often temperatures to achieve given effects are shared on the internet to be helpful to others.  Those who receive these need to evaluate the schedules used to achieve the profile at the stated temperature.

 The same effect can be achieved at different temperatures by using different rates and times in getting to the given temperature.  This is summarised as the effect of heatwork.  The longer taken to achieve the top temperature, the lower the temperature can be used.  The amount of time the holds/soaks occupy in the schedule will also affect the profile achieved.  This means that a simple top temperature can only be a point from which to begin exploration.

 It is important to evaluate each segment. What is the apparent purpose? How does it fit with the principles of applying heat and the characteristics of glass?

 When asking for help with a firing temperature, ask for the full schedule.  This will help you evaluate the suitability of the temperature given.

 The variations in schedules and kilns mean you should fire by results not by numbers. Use the rates, temperatures and soaks that will achieve what you want in your kiln.

Making Test Tiles

Creating samples or tests provides both references on firing profiles and knowledge of the characteristics of the kiln. 

General samples

 Sample tiles are normally a series of tiles with the same lay up but fired at different temperatures.  These are likely to be intervals of temperature from a sharp tack to a full fuse.  A suitable interval might be 10°C as it is easy to interpolate between these for a slightly different profile than the tiles show. This is the basic arrangement.

 You can make this more informative by including tiles in the same basic lay up but with hot and cool colours, opalescent and transparent, black and white, strikers, etc.  The addition of these will give a richer bank of information.

 Of course, these tiles must be labelled with glass types and code numbers and the temperature used.  This is not all the information required though.

 Many sample tile sets do not include the firing rate.  The heat work required to attain a specific profile is dependent on time, temperature and hold.  These are the time to get to the working temperature, the temperature, and the soak time.  If you do not record the ramp rate used, you will have incomplete information.  It is not that you have to record the entire schedule.  But the rates and any soaks on the way to the top temperature need to be recorded. This means you can take account of any slower rate of heating, any additional holds on the way up, and the length of the soak at top temperature.  Then when contemplating something more complicated than the conditions under which the tiles were made you have better information.

 It is a good idea to maintain a photographic record of the sample tiles to avoid storage problems.  These can be made from the individual tiles and photographed from several angles. 

 Another way of keeping records - without making tiles for each temperature - is to photograph the tiles through peep holes as the set temperatures are achieved.  This means the tiles need to be placed in the kiln so they can be seen from the peep hole. You will only have a physical sample for the top temperature. The other profiles will have a photographic record.  The firing conditions for these need to be recorded just as for the series of physical tiles.

 This photographic record may not be suitable for your way of working and so require making the sets of multiple tiles.  Both these methods provide a generalised record of heat work to achieve given profiles.  Note that you will need to prepare sample tiles for each kiln, as each has different characteristics. 

Specific samples

 However, there will be cases where the general conditions exemplified by the reference tiles are to be exceeded.  In this case you will need to make a sample specific to the piece you are planning.  This can be a general representation of the piece, or a scaled mock-up. 

 The general test tile may be small scale or relatively large.  It will contain only the components in terms of height and shape that will be in the planned, but more complicated piece.

 A more rigorous method is to make a full scale – or nearly so – mock-up of the piece. This is usually done in clear. Fire it to the proposed schedule to determine the exact effect.

  

Sample making gives confidence in preparing work for the kiln and scheduling to get the desired profile.