Elevation of Moulds in the Kiln

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

Hot and Cool Spots

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

Effect of Elevation of Mould

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

ΔT Shelf vs. Elevated Moulds (Celsius)

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

 

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

ΔT Difference Between Air and Elevated Mould (Celsius)

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

 

Large, Heavy, Wet Moulds

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

 

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

Coe and Annealing

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

 

I have several problems with this statement.

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

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

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

Annealing 

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

Annealing Range

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

Annealing Point

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

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

Annealing Temperature

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

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

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

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

Firing with different anneal points

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

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

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

 

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

 

Changing Coldworking Grit Sizes

 

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

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

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

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

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

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

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

Kiln Wash Sticking to Glass

Causes and avoidance

 

Photo credit: Immerman Glass

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

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

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

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

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

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

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

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

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

Spider Web Cracks

 

Credit ASTM

 The nature of the cracks - and spider web describes it perfectly - shows an adhesion problem. It is not an annealing problem as that shows a single sinuous line with a hook at each end. It is not a compatibility problem, as that shows as cracks or breaks along the edges of the combined glasses. It is not a thermal break, as those show as breaks where the glass has separated to some amount.

 

Glaze crazed in a ceramic vessel

 

The cracks are exactly like crazed glazes on ceramic objects. And for the same reason. The glass is trying to contract more than the underlying ceramic. It is stuck to the pores of the ceramic and creates a crack where there is a slightly weaker part of the glass. These cracks in ceramic glaze propagate across the surface as it wears, or in the kilnforming case as it cools.

 

Glass puddled in ceramic

 Most usually it results from a lack of separator in that area of the shelf, or uncoated kiln furniture. It indicates either the glass has adhered to the shelf or mould, or (rarely with fusing glass) that the glass has suffered severe devitrification.

 

 

 Occasionally there will be the appearance of shards of glass. This will be where the glass has stuck to some particle on the shelf. Sometimes it can be a speck of something resistant to the temperatures we use in kilnforming that “grabs” the glass and breaks it into shards from that point as the glass cools.

 It is not the schedule that causes the breaks. It is in the shelf preparation.

 The shelf should be cleaned of all the kiln wash and lightly sanded down to smooth. It should then be coated with four thin layers of kiln wash painted in a different direction for each layer. No drying is necessary or even advisable. All kiln furniture must be completely coated with kiln wash.

 If you are re-using a shelf, it must be swept clean before any glass is laid on it.

 Crazing results from the glass sticking to the surface it is resting on.

 

Some additional information:

https://glasstips.blogspot.com/2019/05/kiln-cleanliness.html

https://glasstips.blogspot.com/2020/07/crazing.html

 

Effect of Air Space Around Shelves

The Bullseye research on annealing thick slabs indicates that it is important to have a 50mm space between the shelf and the kiln walls. This is to assist even distribution of the air temperature above and below the shelf.

I decided to learn what the temperature differences are between ventilated and unventilated floors of kilns. The recording of the temperatures was conducted using pyrometers on the floor of the kiln and in the air above the kiln shelf. The pyrometer above the shelf was at the height of the kiln’s pyrometer. The recording was done during normal firings of glass. The graph below shows temperature differences during a typical firing.

The blue line indicates the air temperature, the orange line the floor temperature and the grey line the difference in the two over the whole firing. Each horizontal line is 100C

The next graphs show in more detail the differences between having no significant space and another firing with space between shelf and kiln walls.

Horizontal axis legend:

1.  = 300°C
2.  = Softening point
3.  = Top of Bubble Squeeze
4.  = Top temperature
5.  = Start of anneal soak
6.  = start of first cool
7.  = start of second cool
8.  = start of final cool
9.  = 300°C
10.  = 200°C
11.  = 100°C
12.  = 40°C

The general results are that there is a greater difference during the rise in temperature and a reducing difference in floor and air temperature during the anneal cool. However, there are significant differentials at various points during the firings.

Space between the shelf and kiln walls:

• Smaller temperature difference is experienced on the heat up.
• Floor stays hotter than the above shelf air temperature during the anneal soak.
• This difference gradually equalises during the anneal cool

Without space between the shelf and kiln walls:

• Significantly greater difference on heat up is experienced – over 100°C cooler than ventilated floor area.
• Floor temperature is less than air until the final cool.
• During the anneal soak the floor temperature difference becomes larger than at start of anneal. This seems to be the consequence of heat continuing to dissipate through the kiln body, while the air temperature above the shelf is maintained at a constant temperature.
• The difference between the air and floor temperature gradually reduces during the anneal cool as the whole kiln and its contents near the natural cooling rate of the kiln.

 

This appears to indicate that space between the shelf and kiln walls helps to equalise the temperature during the critical anneal soak and first stage of the anneal cool. This will be particularly important when annealing thick slabs.

These tests were done in a kiln of 50cm square. It is likely that the differences would be greater in a large kiln, making it more important to have the air gap between shelf and kiln wall. Smaller kilns and thinner glass seem to be less affected by these differences.

Note that the air temperature and shelf temperature differences in these firings maintain the same character whether the floor has good circulation or not. The shelf temperature lags behind the air temperature throughout the heat up.

The fact is that floor and air temperatures are nearer each other with air space around the shelf. The difference reduces during the bubble squeeze and the top temperature soak. The difference in temperature on cool down is small. During the anneal soak and cool, the shelf tends to be a few degrees hotter than the air temperature.

There was no difference in the amount of stress in the glass in these tests on a small kiln whether there was a gap or not between the shelf and the kiln walls.

Implications for kilns with multiple shelves

Those using multiple shelves in a single firing load should take note of the implications from this. It is important to have significant ventilation between layers to get consistent results from firings.

The ideal would be to have larger than 50mm/2” gap around the upper shelf. Possibly 100mm/4” would be a good starting point. This would allow sufficient heat circulation to compensate a little for the lack of radiant heat from the elements.

If you have a really deep kiln and are using three shelves, the ideal would be to start with a 50mm/2” gap around the bottom shelf. Then a 100mm/4” gap around the middle shelf and finally a 150mm/6” gap around the top shelf. This will assist the heat to circulate to the bottom layer.

 

There are greater differences in temperature between the floor and above shelf air temperature when there is no ventilation space around the shelf. This is especially the case during the anneal soak.

Causes of Large bubbles

 Let’s think about moisture and large bubbles from under the glass. It is not the water, but the gasses created by the decomposition of materials that can cause the bubbles. There are other causes of large bubbles too. The most common causes are discussed here.

The usual explanations are:

• ·        Uneven shelf
• ·        Heat resistant particles under the glass
• ·        Uneven heating
• ·        Glues
• ·        Organic material
• ·        Moisture
• ·        Amount of gas

 

image credit: Warm Glass

Uneven shelf

Shallow depressions in shelves can cause large bubbles. Occasionally, the shelf can be damaged in various ways causing scratches or dings in the shelf. Air can be trapped in these depressions. And it does not take much volume of trapped to be a problem. The heat of kilnforming causes the air to expand. As the glass becomes less viscous with increased temperature, the pressure from the expanding air forces the glass upwards. The amount of air and the amount of heat work combine to create bubbles from simple uprisings to large thin walled or even burst bubbles.

There are some things that can be done to detect and avoid bubbles from forming. It is possible to screed powdered kiln wash over kiln washed shelf. This gives pathways for the air to escape. It does leave a more marked bottom surface than kiln wash.

Using 1mm or 2mm fibre paper allows air from under glass. You can maintain a relatively smooth surface with Papyros or Thinfire over the fibre. Even Thinfire or Papyros on its own will allow air from under the glass.

Checking for depressions can be done by spreading kiln wash powder over shelf and drawing a straight edge over the shelf. Depressions will be shown by the presence of the powder. It can also be done with powdered glass frit.

Particles under glass

Any particle resistant to kilnforming temperatures holds the glass up while it is forming so creating an air space. It is important to ensure the shelf is clean as well as flat. Small pieces of grit or dirt that are resistant to high temperatures will hold the glass up from the shelf enough to create a bubble – small or large depending on the temperature. Vacuuming the shelf before adding anything to the surface before each firing is important to bubble free results.

Uneven heating

This is sometimes cited as a cause of bubbles. If so, the heat would need to be very localised. This is possible if the glass is very near elements. In general, the temperature is equalised at a distance equal to the width of the elements.

Glues

A wide variety of glues are used in kilnforming. Those available to enthusiasts all burn away leaving gasses between layers. These gasses - if trapped - can thin the glass below as well as above the glue’s position. This will give the impression that the bubble has come from between the shelf and the glass. Most often the bubble forms between the glass layers, pushing a bubble only into or through the top layer. The solution is to avoid using glue or minimise it and place it only at the edges.

Organic material

Organic materials can be a problem. When you are using a large or thick fibre paper sheet under a piece of glass, occasionally the gasses from burning out of the binder can be great enough to create a bubble. Although normally, this only leaves a grey to black mark on the underside of the glass. Vermiculite boards need to be fired before use, as they contain significant amounts of binder.

Inclusion of organic materials such as leaves, twigs, or bones, leads to bubbles. Very long soaks below the softening point of the glass are required to allow the organic material to burn out of the objects.  The time required increases from an hour for leaves to 24 for bones.

Moisture

Moisture is very often cited as the source of bubbles. It is possible that the steam from water may be trapped in shelf depressions, or the areas held up from the shelf. And anytime there are no precautions to allow the air from under the glass, or between sheets bubble formation can be promoted. If adequate precautions are taken (flat shelf, clean shelf, bubble squeeze) the moisture will evaporate before the glass is hot enough to form a seal around the edges and trap any steam. It is another good reason for moderate ramp rates at the beginning of a firing.

Amount of gasses

Of course, if there is a lot of moisture there can be problems. Simply applying kiln wash in four coats does not leave enough water in the shelf to be a problem.

If you have washed the kiln wash off a mullite shelf, there will be a lot of water in it even after it feels dry. Then it does need to be kiln dried before use. To avoid breaking the shelf you need to fire slowly to 99°C/210°F and soak there for a couple of hours with the vents open or lid propped up a little to allow the moisture out of the kiln.

 

 

Lustres

Lustres are metallic colourants in colloidal suspension. They provide intense reflective colour. They are most effective when used sparingly as accents. They are supplied as a dark brown viscous liquid in small bottles. They are widely available from ceramics suppliers.

A bar with gold lustre. Credit: Bath Potters

 The application of these is important, and not only because they are expensive.

 They must be applied to clean dry surfaces with a smooth brush. The gold, platinum, copper, and bronze lustres do not need dilution. The brush should not have a lot of lustre, nor too little. Too much causes burning, flaking, dullness or clouding during and after the firing. The application must be uniform in thickness. “Application of lustres is possibly the most important factor in achieving the best results. The more richly coloured lustres require a fairly thin coat while other lustres (particularly the metallic lustres) require an even thinner coat.” Bath Potters.

silver lustre brushed on.  Credit: Pottery Crafts

 The kiln should be vented until the carrier has burned off. The absence of the smell will indicate when this has been achieved. The firing of lustres can be between 586°C/1088°F and 733°C/1353°F. Metallic lustres usually fire between 586°C/1088°F and 617°C/144°F. These lower ones are in the slumping range of temperatures and can be applied to the flat blank before slumping or to the completed flat blank depending on the requirements of the lustre.

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.