Is there a best separator?

 Is there a best separator?

Kilnforming separators

Separators for kiln forming come in various forms. Chemicals, liquids, sprays, refractory fibre paper, kiln wash, and others I suppose. Which is best?  Each separator has its uses. No one is useful in all circumstances.  Some will be best for one circumstance and others for another.

Boron nitride

Boron nitride (BN) is a high temperature lubricant. It can be sprayed or brushed onto the mould. It adheres to smooth non-absorbent surfaces.  BN is among the most expensive of separators for glass. It seems most useful on detailed, texture and casting moulds. BN is often recommended for steel moulds as it adheres to it better than kiln wash.  Although kiln wash will work as a separator on steel, boron nitride is easier to apply.  Various conditions apply to its use. 

Kiln wash

Kiln wash works well on slightly absorbent surfaces – ceramic moulds, and shelves, for example. It is the least expensive form of separator.  It is shipped as a powder to which five parts water is added to one of the powder.  This makes a liquid that can be applied to any appropriate surface.  It can be sprayed or brushed. The mix can be with less water on very absorbent surfaces, showing some of its flexibility. 

Almost all kiln washes contain kaolin which helps keep the alumina hydrate in suspension.  But most importantly, allows the solution to be applied evenly.  However, the same kaolin also slowly changes to a crystalline substance by 900ºC/ 1650º that sticks to glass. It needs to be re-applied after every full fuse.

Refractory fibre paper

Shelf paper works well on flat surfaces and simple moulds.  It is a moderately priced separator.  Two of the popular trade names are Papyros and Thinfire.  They both contain alumina hydrate but with different binders.  They provide a smooth surface for the shelf and cylindrical shapes. They are not so good at separating glass from irregular surfaces and incised details.  The shelf paper disintegrates after firing. Although it can sometimes be used several times if undisturbed.  The resulting powder is an irritant and should be disposed of carefully.

There are thicker refractory fibre papers.  These normally range from 0.5mm to 6mm.  Thicker versions are called blanket.  These have the same characteristics as shelf papers, although coarser.  They also do not use binders to keep them together.  These are most useful in forming moulds and insulating glass from rapid temperature changes.

 

The general statement is that there is not one separator that is best in all circumstances.  Each has its strengths.  Knowledge of the objective of the firing and its conditions will help in choosing the right one.

Stuck Kiln Wash

 

Moulds

Kiln wash on ceramic moulds lasts a very long time. But sometimes you want to use a different separator. First you need to prepare yourself and the area for the process.

Preparation

It is best to wear a mask while removing kiln wash or other separators to reduce the amount of dust you inhale. Wearing an apron or other outer wear will keep the dust off your clothing.

Spread a cloth, newspaper or other covering around the area. This is to be able to easily gather the removed kiln wash and place it in the waste.  Have a vacuum sweeper at hand to remove powder rather than blowing it around the workspace.  Of course, if you can do this outside, there is much smaller risk of contamination.

Removal Methods

The method of removing kiln wash depends in part on what the mould material is.

Metal

You can sandblast, manually sand, or wash off the kiln wash from metal moulds.

Ceramic

Sandblasting is not a safe method for ceramics, as it is so easy to damage the surface of the mould. Removing the kiln wash while dry is a good first approach. It saves having to wait long times for air drying and long kiln drying of the damp mould. You can lightly sand off the kiln wash from smooth surfaced moulds, and for detailed areas use rounded point wood and plastic tools. This can be backed up with a stiff nylon brush to clear out the narrow or detailed areas.

When these dry methods are insufficient, there are wet approaches. I recommend dampening the kiln wash rather than immersing the mould in water. The same tools can be used as for the dry removal.

Soaking or washing the mould does not remove the kiln wash as easily as you might think.  It is especially to be avoided where the mould has an internal hollow, as it may take days to dry sufficiently to apply other separators.  To put it in the kiln risks breaking the mould by the steam build up during the initial rise in temperature.

If you must soak the mould, I recommend that you use a 5% solution of citric acid because it has a chelating action on the kiln wash.

More information on removing kiln wash from moulds.

Remember that once the mould or shelf has been coated with boron nitride, it is almost impossible to go back to kiln wash again.  The boron nitride irreversibly fills the porous element of the ceramic, making it difficult for the kiln wash to adhere to the mould.

Shelves

The easiest surfaces to remove kiln wash from are flat or ones nearly so.

Dry Methods

Abrasive methods work well with a variety of tools. They can range from large paint scrapers to smaller ones with a Stanley blade inserted. 

 

Coarse open mesh plaster board (dry wall) sanding sheets are very useful. There are frames that you can fix them to, but sanding without the frame works well too.

Using power tools to sand the shelf is not advisable.  It is too easy to remove lots of material, including the surface of the shelf – even the hard, ceramic ones.  This leads to minor depressions in the shelf and consequent bubble difficulties when firing.

Do not be tempted to sandblast either, as that can easily create the small depressions in the surface of the shelf that subsequently lead to bubbles. 

Wet methods

Wet methods can be used if you are concerned about the dustiness of the process.  You can dampen the kiln wash on the shelf and sand or scrape as with the dry methods.  You will create a paste or slurry which can be bagged and put in the waste. You can also use the green scrubby washing up pads.  Unless you frequently rinse the pads, the kiln wash builds up and clogs the pads. making them ineffective.

 

Some people use vinegar or chemicals such as lime away with the water. The material that makes the kiln wash stick to the shelf is China clay and the separator is alumina hydrate. Both of these elements are almost impervious to the chemicals available to kiln workers. Instead, use citric acid. It has a chelating action which will incorporate the particles of the kiln wash. This will require some scrubbing, but avoids the smells of vinegar and the risks of other chemicals.

Do not be tempted to use pressure washers. Yes, they will remove the kiln wash. But it will also leave divots in the shelf which will cause later problems with bubble creation.

A big drawback to using wet methods, is that the shelf becomes wetted throughout and needs careful drying before use. 

Both the wet and dry methods can be used on smooth, gentle curved moulds. These include wave moulds, shallow moulds without flat bottoms, cylinder moulds, and such like.

More information on Kiln Wash Removal from shelves is available here,

and here.

Boron Nitride

A note on the reversibility of boron nitride. This is sold under a variety of trade names such as Zyp, More, MR97, etc., and sometimes under its chemical name.

Some people are applying boron nitride to ceramic moulds for the "smoother" surface.  Boron nitride is an excellent separator for metal moulds and casting moulds whether metal or ceramic. But it has limitations, including the price and the requirement for a new coating at each firing.  Some are beginning to wonder if they can go back to kiln wash after having used the boron nitride.

The general experience has been that you can't apply kiln wash on top of the boron nitride. It just beads up and flows off, because the boron nitride creates a non-wetting surface that survives relatively high temperatures. The kiln wash which is in water suspension has no opportunity to adhere to the mould.

The most accepted way to get rid of the boron nitride is by sandblasting. Sandblasting risks pitting the mould. Manual sanding seems to enable the ceramic mould to accept kiln wash. Perhaps enough of the surface is removed to reveal the porous nature of the ceramic mould. You do need to be cautious about taking the surface of the mould away when using abrasive removal methods. The ceramic is relatively soft in relation to the abrasive materials.

The difficulty of removing boron nitride from ceramic moulds means that you must think carefully about which moulds you coat with it.  If the mould has delicate or fine detail, removing the boron nitride risks the removal of the detail.  This indicates that this kind of mould, once coated, should not be taken back to the bare mould.

If you are using boron nitride to get a smoother surface to the object, consider using a lower slumping or draping temperature. This will minimise mould marks very effectively. And without the expense of boron nitride.

More information on removal of boron nitride is given here. 

More information about mould treatment is available in the ebook: Low Temperature Kiln Forming and at Bullseye ebooks

Kiln wash beading up

Sometimes kiln wash does not seem to want to stick to the mould.  There are several possible reasons. The main two seem to be a hard spot in the slip cast moulds that we use.  Another is the previous use of boron nitride or other sealant of porous surfaces.

The remedies are different for these two causes.  For hard spots you can add a bit extra kiln wash to the area.  Normally enough separator adheres to the spot to avoid sticking.  This is so even though you can see the spot more clearly than the rest of the mould.

Sealed surfaces present a little more difficulty.  It is possible to carefully sand blast off the boron nitride from the surface using low pressure and very little abrasive.  This works well for textured surfaces, if you are careful.  You can also manually sand the sealant off which works better for regularly shaped smooth surfaces.   The object of both these processes is to remove the sealed surface to reveal the porous material again.  You must remember that you are removing some of the surface of the mould in these abrasive processes.  Once removed kiln wash can be applied as before.

If neither abrasive method works, it does not mean the mould is ruined.  You can continue to use boron nitride.  Or, if you want to avoid the costs of boron nitride, you can sprinkle fine dry kiln wash over the mould.  You should give the mould a final application of boron nitride before using the dry kiln wash.

Glass Separators

Glass separators tend to be in three forms – powdered, liquid or fibre. These are applied to shelves, moulds and other surfaces that might come into contact with the hot glass.

What do they do?

Glass separators keep the glass from sticking to the shelves, kiln furniture and other supports during the higher temperature parts of the firing.  Glass as used for kilnforming reaches its softening point somewhere around 580°C. The glass will begin to stick to all surfaces as it gets warmer.  The separators are stable at high temperatures and do not stick to the glass or the materials used to separate the glass from its supports.

What are they?

       Liquid and powder separators are most often called kiln wash - or batt wash in the ceramics field.  Normally they are supplied in powder form that is mixed with water for painting onto shelves and moulds. 

They normally have a high content of alumina hydrate, some kaolin (also known as china clay) and sometimes a little silica, plus often a colouring agent that burns away on the first firing to indicate fired and unfired shelves.

       A high temperature lubricant, boron nitride, has come into use for kilnforming and has slightly different characteristics than the alumina hydrate-based kiln washes.

Sheet and blanket forms of glass separators are also widely used.  They have the general name of refractory mineral wool. They are often made from alkaline earth silicate (AES) wool, Alumino silicate wool (ASW) and Polycrystalline wool (PCW).  These have different temperature ranges and levels of health risk. The thin sheets are mainly used for covering shelves and other kiln furniture.  The blanket, which starts at about 12mm, is used mainly for insulation purposes.

Thin papers, similar in thickness to cartridge paper have been developed to give a finer texture than mineral wool separators.  These currently have the trade names Papyros and Thinfire, each with their own slightly different characteristics.

Safety

As with all refractory materials, safety precautions are needed.  In the kilnforming world the risks are not those of the industrial environment because the quantities are less, and the time of exposure is much less.  Still, breathing protection should be used. Eye protection is advisable, as the particles are hard and can scratch the eye surface.  Long sleeves and gloves are advisable when handling refractory fibres.

 

Kiln Wash

This blog concentrates on liquid and powdered separators. It draws on information from the ceramics and kilnforming communities.

Basic Kiln Wash Materials

A lot of the kilnforming knowledge of glass separators comes from the ceramics field. A brief look at the development of kiln wash by ceramicists is instructive to kilnforming. 

In order to make a good kiln wash you need to select materials that have very high melting points and that, when combined, do not create a eutectic that causes melting. Knowing a bit about the properties of materials and the principles of kiln wash allows you to choose the ingredients that make the best wash for your specific situation and avoid costly problems. 

(John Britt www.johnbrittpottery.com ceramicartsnetwork.org › firing-techniques)

The basic materials started as:

EPK Kaolin (which includes alumina)      50%

Silica                                                50%

EPK Kaolin is a high quality, water washed kaolin which is white, has unusually good forming characteristics and high green strength. In mixtures, EPK offers excellent suspension capabilities.  The source of alumina in kiln wash was often kaolin, but now is most often alumina hydrate or alumina oxide.

Silicon dioxide has a melting point of 1710°C and aluminium oxide has a melting point of 2050°C.  A mixture of these two materials will not melt, and will protect the kiln shelves at high temperatures.

This is a good kiln wash for low and mid-range electric firings [for ceramics]. The only problem is that it contains silica, which is a glass-former. So, if a lot of glaze drips onto the shelf, it can melt the silica in the kiln wash and form a glaze on the shelf. Also, when you scrape your shelves to clean them, you create a lot of silica dust, which is a known carcinogen. So, using silica in your kiln wash is not … the best choice.

Another drawback of this recipe is that, if it is used in salt or soda firings, it will most certainly create a glaze on the shelf. This is because silica, as noted above, is a glass-former. When sodium oxide, which is a strong flux, is introduced atmospherically, it can easily melt the silica in the kiln wash into a glass. This is why silica should not be used in a kiln wash recipe for wood, salt or soda kilns. 

(John Britt www.johnbrittpottery.com ceramicartsnetwork.org › firing-techniques)

For glaze firings a kiln wash with more separator and less glass former is better:

Alumina hydrate            50%

EPK kaolin                    50%

Kaolin has a melting point of 1770°C and alumina oxide has a melting point of 2050°C, so it will not melt, even in a … firing [of 1250°C to 1350°C]. These ingredients are called refractory because they are resistant to high temperatures. … This recipe can be used at all temperatures and in all kiln atmospheres. 

(John Britt www.johnbrittpottery.com ceramicartsnetwork.org › firing-techniques)

Kiln washes with kaolin, especially if applied thickly, can flake off the shelf after repeated firing.  The cause of this is the shrinking of the drying kaolin - which is a clay – similar to dried out lake beds. Adding at least half the kaolin as calcined EPK kaolin reduces this shrinkage. Calcining involves drying the kaolin at about 1000°C for some time.  This reduces the physical property of shrinkage, but retains the chemical and refractory properties of a glass separator intact.

This gives a kiln wash consisting of:

Alumina hydrate            50%

Calcined EPK kaolin        25%

EPK kaolin                    25%

You can add more calcined kaolin – up to 35% – if you want. You need to keep enough un-calcined kaolin in the recipe to suspend the other materials so that the suspended materials can be applied smoothly.  One difficulty of increasing the kaolin content of the kiln wash is that it tends to stick to the glass - especially opalescent - on a second firing.

It is, of course, possible to do away with the kaolin entirely.  You can mix alumina hydrate with water into a full milk consistency and apply that to the shelf or other kiln furniture.  It is difficult to maintain the alumina hydrate in suspension, though. After the firing you can brush the dried separator from the shelf into a container for re-use.  You do need to ensure that the powder to be reused is free of contaminants.  It is also important to find very fine grades of the alumina hydrate to minimise the texture on the base of the glass.  Most ceramic grades are coarser than wanted for kiln forming.  You can put the powder in a rock tumbler to make what you find finer than as purchased.

There are many variations on these basic kiln wash recipes. To illustrate the wide variety, some potters just dust alumina hydrate on their shelves to protect them, while some wood firing potters use 100% silica and wall paper paste to make a very thick (1/2-inch) coating that protects their shelves from excessive ash deposits. Still others, who have the new advanced nitride-bonded silicon carbide shelves, don’t even use kiln wash at all because the glaze drips shiver off when the shelves cool. Other potters, who are very neat and don’t share their space with others, may not even use kiln wash so that they can flip the shelves after every firing to prevent warping.

Kiln wash is such a ubiquitous material in the ceramics studio that we take it for granted. … There are many recipes to choose from and many solutions to common problems if we just take the time to learn about the materials we use. 

(John Britt www.johnbrittpottery.com ceramicartsnetwork.org › firing-techniques)

Variants on the traditional glass separators

There are variations in the use of alumina hydrate and kaolin, but there are also other glass separators available, although they tend to be expensive.

An example is zirconium. It is a glass separator with refractory properties, as in its zirconium oxide form it melts 2700°C.  In its zirconium silicate form it has a melting point of 2550°C.  These are available under a number of trade names. This can be added to the kiln wash mix in the knowledge that it will be stable throughout the firing.

But you must be careful in the amount you use, as zirconium silicate is used as an opacifier in glass and glazes.  Also, zirconium oxide is one of the hardest substances in the world.

Boron Nitride

Another very popular glass separator is boron nitride.  It has two forms. 

One is cubic boron nitride, a cubic structure similar to diamonds.

     

  

In the cubic form of boron nitride, alternately linked boron and nitrogen atoms form a tetrahedral bond network, exactly like carbon atoms do in diamond.  Cubic boron nitride is extremely hard and will even scratch diamond. It is the second hardest material known, second only to diamond.  Cubic boron nitride has very high thermal conductivity, excellent wear resistance and good chemical inertness, all very useful properties for a material subjected to extreme conditions. Because of its hardness, chemical inertness, high melting temperature (2973°C) cubic boron nitride is used as an abrasive and wear-resistant coating. Cubic boron nitride (CBN) is used for cutting tools and abrasive components for shaping/polishing with low carbon ferrous metals.  (http://www.docbrown.info/page03/nanochem06.htm)

Hexagonal Boron Nitride

The second form, useful in kilnforming is the hexagonal form of boron nitride.  It forms white plates of hexagons one layer thick like graphite.  These plates have weak bonds and so slide easily against one another.

https://www.substech.com/dokuwiki/doku.php?id=boron_nitride_as_solid_lubricant

It is a good insulator and chemically very inert.  It is stable to about 2700°C.

Hexagonal boron nitride (HBN) is used as a lubricant, since the weakly held layers can slide over each other.  Because of its 'soft' and 'slippery' crystalline nature, and its high temperature stability, HBN is used in lubricants in very hot mechanical working environments.  

(http://www.docbrown.info/page03/nanochem06.htm)

The slippery nature and high temperature stability characteristics make this material an excellent coating for moulds and other situations where the glass moves against its supports.

The coating of the moulds needs frequent re-coating because the layers slide from the mould. Boron nitride works very well on solid impermeable surfaces as it adheres easily to smooth surfaces. It can be used on porous surfaces, but does seal those surfaces, meaning that these surfaces cannot be returned to that porous state without significant abrasion.

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The next blog  has notes on refractory mineral wools as separators and health and safety in use.

Texture Moulds and Glass Sizes

I had an overhang [on a texture mould] and I heard a pop and opened kiln and saw it cracked off the mold. … [The piece] is 2 layers Bullseye irid placed face down and Tekta [on top]; the mold was sprayed 3-4 times with zyp and Thinfire; and I put mold on kiln posts. [I] fired to 1440[F].

Diagnosis 

The overhang of the glass caused the break. As the glass heats it expands. The ceramic does not expand as much as the glass.  This means even more glass will hang over the edge than at the start.  As the glass reaches slumping temperatures, it begins to drape over the edge. At the same time the glass on the interior is beginning to slump into the textures.  When the glass has fully taken up the texture, the overhanging glass will be touching the outer sides of the mould. This means at the end of the heating and soaking part of the firing, you have the ceramic mould partially and tightly encased in glass.  The glass has formed around the ceramic.

Credit: theavenuestainedglass.com

The physics of the two materials are that glass expands more than ceramic. On cooling, the glass grips the sides of the ceramic mould tightly. This is because it shrinks more than the ceramic.  In this case, the ceramic was stronger than the glass and the strain caused the glass to break.  Upon occasion the opposite can happen.

Two other notes.

The temperature of 781°C is higher than needed.  You will need to do a bit of experimentation to find the right combination of temperature and time for each mould.  You could consider 630°C as an initial temperature with a 90-minute soak.  Bob Leatherbarrow (p.161) describes a method of scheduling a sequence of slightly higher temperatures with soaks.  If the texture is not forming (as determined by observations), you can advance to the next segment with a higher temperature and see how that goes.  When the appropriate amount of texture has been achieved, advance to the cooling to anneal segment.

Iridised surfaces provide a very good separator.  With the iridisation down against the mould, it may be unnecessary to use Thinfire, especially when you already have used boron nitride as the separator.

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

Removing kiln wash from moulds

“How do I remove kiln wash from a mould that I have decided would work better with ZYP?”

Once coated with kiln wash, slumping or draping moulds do not need to be re-coated until the surface is damaged.  Then it is best to remove all the kiln wash to prepare a new smooth surface for the kiln wash.  You may, of course, as the enquirer above states, want to use a different kind of separator.  The cleaning of the kiln wash from the mould will be the same process whatever you want to do with the mould next.

There are many ways to get the old kiln wash off.  Some of them depend on the material from which the mould is made.

Metal

If the mould is made of stainless steel or other metal, the easiest method is to sandblast with lots of air and a minimum of grit.  You can also use sandpapers or open weave sanding screens. The methods used on ceramic moulds, as described below, can also be used on metal.

Ceramic

Sandblasting is not safe to use on ceramic moulds, as the sandblast medium can erode the surface very quickly and often unevenly.

Preparation for manual removal of kiln wash.

It is best to wear a mask during this process to reduce the amount of dust you inhale. Spread a cloth, newspaper or other covering to be able to easily gather the removed kiln wash and place it in the waste.  Have a vacuum sweeper at hand to remove powder rather than blowing it around the work space.  Of course, if you can do this outside, there is much smaller risk of contamination.

Dry

I suggest that removing the kiln wash while the mould is dry should be the first stage. 

Flat surfaces can be cleaned with a straight edged wooden stick, or wooden clay modelling tool.  Firmly push it along at a slight angle from the vertical to remove most of the kiln wash. 

On curved surfaces you will need a rounded tool such as a plastic burnisher or all nova tool for the coarse work.  This can be followed up by using a stiff sponge to clean up any stray kiln wash still adhered. If the kiln wash is persistently sticking to the mould, you can cut a small piece from an open weave sanding screen and use it to gently remove the most difficult remaining kiln wash.  Do not use more than light pressure, as with heavy pressure, the screen can begin to remove the surface of the ceramic mould.

Texture moulds and those with a lot of detail or right-angle corners need a bit more attention.  You can use a variety of non-metal tools to get into areas of detail.  Some of these are a rounded chopstick, a wooden skewer, a plastic knitting needle, and other similar items with rounded points.  These can be backed up with a small stiff nylon brush.  It is while working on these detailed areas that the vacuum sweeper will be most useful to clear out the debris and enable you to see how well the kiln wash is being removed.

Wet

Some people do not like the idea of the dust created from the removal of the kiln wash being in the air at all.  And sometimes, the dry removal is not complete.

My recommendation is to dampen the kiln wash that is on the surface of the mould.  This will cause some difficulties in removal, because a slurry is created along with the flaking of the baked-on kiln wash.  The same tools can be used to clean the mould as when dry.  The vacuum sweeper will not be of use though.  Once the kiln wash appears to be cleaned away, the mould needs to dry to enable removal of the remaining kiln wash.  Once dry, you can use dry sponges, or the small nylon brush to clean the remaining film of kiln wash from the mould.  This cleaning may reveal areas where the kiln wash is still adhering. These can be dealt with wet or dry, although I prefer dry.

Soaking or washing the mould does not remove the kiln wash as easily as you might think.  It is especially to be avoided where the mould has an internal hollow, as it may take days to dry sufficiently to apply other separators.  To put it in the kiln risks breaking the mould by the steam build up during the initial rise in temperature.

If you must soak the mould, I recommend that you use a 5% solution of citric acid because it has a chelating action on some of the components of kiln wash.

Remember that once the mould or shelf has been coated with boron nitride, it is almost impossible to go back to kiln wash again.  The boron nitride fills the porous parts of the ceramic making it difficult for the kiln wash to adhere to the mould.

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

Needle Points

Often fused glass has prickles or needle points around the edges and especially at corners after firing.

This illustration is from Glass Fusing Made Easy

The nature of glass and its interaction with the separators is the cause.  As you heat glass it expands. Once the cooling starts, the glass contracts. Often a particle of the glass sticks to the separator while the rest continues to contract. This dragging of the glass along the separator results in the creation of little sharp points developing as the glass retreats to its final dimensions.

The best solution I have found to reducing the points at corners is to blunt any points or corners before assembly. Only a tiny amount of glass needs to be removed from the corners to reduce the possibility of these points being developed.

Small needle points can also develop along the sides of the glass too.  These are more difficult to avoid.  The most successful method for me is to use a loose separator.  This can be Thinfire, Papyros or a fine dusting of alumina hydrate or powdered kiln wash.  Although less widely available, talc can be used. Talc is known to be carcinogenetic with high exposure, so breathing protection is needed. All these powders provide enough lubrication to allow the runny glass to slide without sticking. 

Of course, you can use boron nitride, which is very slippery, but the cost of it makes it expensive in comparison to the other methods, including using fine diamond pads to remove the needles.

An additional consideration is the temperature you use.  The higher the temperature, the more the expansion.  Expansion rates are almost exponential above the brittle phase of the glass.  Reducing the temperature by 20C and doubling time or more means the glass does not expand so much and the additional time allows the desired profile to be achieved.  

Of course, paying attention to volume control - using 6mm or more thickness - will help to reduce the needle points.  A 3mm sheet both expands and becomes thicker at the edges by drawing more glass from the interior and the edge while attempting to reach 6mm.  This means there is an increase in the needling effect.  Although a 6mm piece retreats on cooling, it does not have the additional thickening effect of a 3mm piece.  Even a 9mm piece retreats on cooling, although the final piece has a larger area than at the start. 

- - - -

There are various preventive measures that can be taken to avoid needle points on fused glass.  These range from altering the edges of the glass, using fibre papers that turn to powder, using refractory powders, or boron nitride. Post firing solutions relate to cold working.

Materials for making dams

Rectangular or straight sided shapes

Broken shelves

Accidents happen to mullite shelves causing breaks or cracks.  Rather than throwing them out, you can cut them into rectangles or 50mm strips with a tile saw.  The resulting shapes need to be kiln washed to keep glass from sticking.  They can be used flat or stood on their edges with supports on the outside.

Thick ceramic tiles can be used in much the same way.  You do need to remove the glaze from the tile to make sure they don’t stick to the glass.  Or you could use the unglazed side toward the glass. Again, the tiles need to be kiln washed.

Stainless steel can be used as a dam.  It will need treatment with a separator such as boron nitride or kiln wash.  In addition, it needs to be lined with refractory fibre paper to cushion the force of the greater contraction of steel than glass.

These materials cannot easily be adjusted in length to fit the size of the glass piece being dammed.  Instead, arrange them in a swastika like formation. 

This photo also shows how shorter lengths can be incorporated to make the whole dam.

Vermiculite board is a refractory material that can be used to form dams by cutting with a wood working saw.  The saw you use to cut the vermiculite will be dulled and only be useful for cutting vermiculite in the future.  Do not use any expensive cutting equipment!  

Credit: Bullseye Glass Company

Refractory fibre board is available in many thicknesses.  It can be cut with craft knives even though it dulls the blades quickly.  The thicker boards can be used without rigidising.  This avoids the need to kiln wash and allows adjustments in length.  If you do rigidise fibre board, you must coat it with a separator such as kiln wash or boron nitride.

Weighted fibre paper can be used.  It is sometimes the quickest and easiest to use, as there normally is a stash of scraps around the studio.  It is easily cut with a craft knife.  You can build up the thickness of the dam by layering pieces on top of one another.  Sometimes people put metal wire or pins in the layers to ensure there is no movement between the layers. I’ve found that if weighted, the fibres interlock enough that the layers do not shift.  But you need to line the layered fibre paper dams with vertical strips of fibre paper, so the glass does not take up the layered dam profile on its edge.

Note that you need to use breathing protection when cutting all these materials.

Curved and circular pieces

Many times, the shape to be dammed is not formed of straight lines.  Different materials need to be used in these cases.

Formed stainless steel is a good durable and reusable material.  You need to line the shape with fibre paper if it completely contains the shape, because it contracts more than the glass and can crush the piece.  It is expensive to have made and so needs to have multiple uses to justify the cost.  A cheaper alternative is to make your own shape using stainless steel strapping as used for shipping crates and pallets. 

Fibre paper is an excellent material for damming irregular shapes.  It can be cut into complicated shapes, and it can be layered to attain the required height. You can weight it if you are taking things to a high temperature and fear that the glass will flow under the fibre paper.

You can also use the thicker fibre papers upright by backing up with multiple pieces of kiln furniture to maintain the shape you desire.

Vermiculite board is a good material for making shapes, although not as complicated ones as possible in fibre paper.  Vermiculite can be shaped with wood working materials, but cheap ones should be used as they are quickly dulled. You can rough out a shape with a jigsaw and refine it with various wood working tools, including coarse sandpaper.  Because it is a relatively rigid material, a lot of inventiveness can be used in forming the edges by altering angles from the vertical, incising designs into the edge, etc.  Be certain that you have adequately kiln washed or put other separator on the board, as it will stick to the glass if left bare.

Fibre board is a less rigid material than vermiculite, but is easier to work with simple craft tools.  It is simple to use for a unique one-off shape. It only needs smoothing and does not have to have a separator applied because it does not stick to the glass.  If you create a shape that you will want re-use, you can rigidise the board after shaping, but it will require separators then.

Note that when working with refractory materials, you need to wear respiratory protection and clean surfaces with a HEPA vacuum or by dampening dusty surfaces and wiping them clean.  Dispose of cleaning materials safely.

Crazing

Crazing appears as the multiple cracks similar to what is seen on ceramic glazes.  These occur when there is a great deal of incompatibility between the glaze and the clay body.  This can also be seen in glass.

Crazing as seen on a ceramic object

I have see crazing of glass in two circumstances.  It happens with severe devitrification, to a maximum extent of crumbling under light pressure.  This usually happens with glass not formulated for fusing, and especially on opalescent glass.

The more common occurrence is where the glass has stuck to the supporting structure.  This is frequently the case where the separator has not been sufficient to keep the glass from sticking to the shelf.  This will happen on kiln washed shelves when the coating of the separator has not been even, leaving areas with bare or very thin areas.

The standard of mixing kiln wash in the ratio of 1:5 parts by volume of powder to water is important.  The application should be with a wide soft brush such as a hake brush.  The kiln wash should be painted on in four coats, one in each direction of up, down, and the two diagonals.  A well coated shelf should have an even appearance of the coating.  Only an even film of separator is required to keep the glass from sticking to the shelf, mould or other kiln furniture.