Cordierite/Mullite vs. pizza stones or tiles

Description of the materials

Cordierite refractory shelves are generally combined with mullite to achieve low expansion rates.  These are most often manufactured as solid slabs, although there is an extruded version with hollow channels along the length, given the trade name corelite.

Cordierite is magnesium, iron and aluminium in a cyclosilicate form (or rings of tetrahedra).  It is named after its discoverer, Louis Cordier, who identified it in 1813.

cordierite/mullite shelves

Mullite is combined with cordierite in small amounts to increase strength and reduce the amount of expansion. It does this through the formation of needle shapes that interlock and resist thermal shock. It also provides mechanical strength.

Mullite was first described in 1924 and named for an occurrence on the Isle of Mull, Scotland, although it occurs elsewhere, usually in conjunction with volcanic deposits.   

Pizza Stones and Tiles

Pizza stones are a variant of baking stones where the food is placed on (sometimes heated) stones.  Baking stones are a variation on hot stone cooking, one of the oldest cooking techniques. The stones are normally unglazed tiles of varying thicknesses.  What is said of pizza stones also applies to tiles.

Characteristics

Pizza stones  

Ceramic tiles and pizza stones are essentially the same things.  Some tiles may be thinner, especially if they are not large. In both cases, the ceramic is a poor heat conductor and the thermal mass means care needs to be taken in rapid heating and cooling of tiles and of baking stones. These are dry pressed which give a coarser surface texture than cast shelves.  All these ceramics are generally fired at about 1100C, so they can withstand kiln forming temperatures.  They are adequate as small shelves, but will deform over larger areas over time.

Cordierite-Mullite kiln shelves and furniture.

This formulation of materials has an extremely low coefficient of thermal expansion that explains the outstanding thermal shock resistance of these kiln furniture materials. They are also strong although heavy. Cordierite/mullite shelves are sintered, to allow the mullite needles to form, and fired at 1400C+, higher than tiles.

This material can be cast, dry pressed or extruded.  Cast shelves are the cheapest of the methods and provides a smooth surface.  These are used for kilnforming glass, and low temperature ceramic firing. 

Dry pressed shelves have a higher temperature resistance than cast. For this reason, these are often marketed as ceramic shelves, even though the cast shelves are fine for smaller areas.  These are more expensive than the cast shelves.

Corelite, a brand name for extruded shelves with hollow channels, is often used where larger shelves are required, as the weight is less than the solid cordierite. Extruded shelves are ground smooth after forming.

pizza stones

Preparation

Pizza Stones and Tiles

Due to the thermal mass of pizza stones and the material's property as a poor heat conductor, care must be taken when firing.  Firing quickly can break the stone or tile.  The stone or tile should be fired slowly to just under the boiling point and soaked for a couple of hours to eliminate any dampness in the material.  This probably should be done each time kiln wash is applied.  Because it is porous, a baking stone or tile will absorb any liquid applied, including detergent. They should be cleaned with a dry brush and then plain water if further cleaning is necessary.

Pizza stones and tiles should be checked for having straight and level surfaces. It is not a priority for these to have flat surfaces as for glass and ceramics shelves.  If by placing a straight edge on the surface you can see slivers of light, the shelf needs to be smoothed.  You can do this by grinding two of the proposed shelves together with a bit of coarse grit between.  This best done wet to avoid the dust getting into the air.

Cordierite

Cordierite/mullite shelves do not need this level of preparation, unless they have been stored outside.  It is possible to kiln wash and air dry for a few hours before placing glass on the shelf and firing.  This difference is the low rate of expansion (CoLE 19, if you are interested).

corelite shelves

Corelite

The extruded corelite shelves are made with cordierite/mullite, but are more delicate due to the hollow channels along their length.  They should be fired slowly to just under the boiling point of water - to eliminate the moisture - then continue the slow rise to at least 260C/500F to avoid the crystobalite inversion.  It should be fired to 540C with a pause before going to the top temperature.  The shelf should be supported at 30cm intervals under the shelf to minimise breakage.  The whole surface of the shelf should be filled rather than having just one heavy piece; again this is to minimise breakage.

Revised 23.2.25

Flat Kiln Shelves

A question has been asked about using tiles in addition to standard kiln shelves to fire glass upon.  Yes, you can use the unglazed backs to fire on, assuming they are not ridged or in other ways not a regular surface.

It is important to have flat shelves, as ones with even small shallow depressions can promote bubbles at higher temperatures. Tiles for walls and floors do not need to be flat to do their intended job and so are not checked for be flatness.

A magnified view of a shelf surface that is not perfectly even

You can do a quick check for flatness, by placing a ruler on edge across the tile or shelf to see if any light comes through underneath the ruler.  The light areas are the places where the surface is lower than the rest.  A more certain way to determine flatness is to sprinkle a black powder on the shelf.  Then draw a straight edge across the shelf to reveal any black areas.  These are the places that the kiln has a depression.  

If the depressions are few and small you can make corrections in the surface of the tile by grinding.  Put two tiles back to back and grind them together. The initial grind will show you the high spots as they will have the grinding marks there. 

You can eliminate these higher areas by rubbing the tiles together with a coarse grit (ca. 80) between the tiles to speed the grinding. If you are concerned about the dust or don’t have good ventilation, you can make a slurry of the grit by adding water. When the whole surface has the same marks, both will be flat. To double check, sprinkle black powder on the shelf and repeat the test for flatness with a straight edge.  If it is not fully flat, repeat the grinding process and checking until the tile or shelf is flat.

This sounds time consuming and lots of effort, but you will be surprised at how quickly you can achieve flat smooth surfaces even on larger tiles.  This also works for larger kiln shelves.

Revised 23.2.25

Multiple Firings of Kiln Wash

Many people report that they fire multiple times on kiln wash that has not been renewed.  Most add coats over existing kiln wash.  They only remove all the kiln wash when it begins to crack, stick to the glass or gets divots.

We all know that kiln wash fired a second time to full fuse is likely to stick to the glass.  We also know that kiln wash fired to slumping temperatures lasts almost indefinitely.  The kaolin in the kiln wash that allows easy spreading, undergoes a gradual change from platelets to crystals with increasing temperature.  This begins at around 600C/1115F and is complete by 900C/1655F.  The crystalline version of kaolin sticks kiln wash to glass, but as the transition from platelet to crystal is so slow at the lower end of the range, kiln wash on slumping moulds does not exhibit the sticking behaviour even over very many firings.  But, as the temperature rises, the risk of there being enough crystals to stick the kiln wash to the glass also increases.  By full fuse temperatures the proportion of crystalline kaolin is high and becomes complete on the next firing.

. 

credit: Immerman Glass

It is possible to fire several times to tack fusing temperatures without experiencing the sticking behaviour of kiln wash.  However, the more times and the higher temperature used, the greater risk of kiln wash sticking.

Some people continue firing without adding additional layers of kiln wash until cracks, divots, or sticking occurs.  This leads to creating a fix after the failure of the kiln wash. This requires both finding a means of cleaning the kiln wash residue from the glass, and fixing the firing surface.

Others paint a layer of kiln wash on top of the existing separator before high temperature firings. This continues each firing with a fresh layer of kiln wash.  However, the same cracks, divots, and sticking occurs at some point, requiring a complete re-coating of the shelf, and getting the kiln wash off the glass.

credit: Sue McLeod Ceramics

Re-coating of a shelf takes a couple of minutes and can be done with simple tools.  A broad scraper will remove most of the kiln wash.  This can be followed by rubbing with an open weave sanding sheet as used for plaster board or other dry walling.  If you are worried about the dust – which has less risk than fibre papers – you can dampen the surface before beginning the cleaning process.

If the kiln wash has been on the shelf for many firings, it is more difficult to remove, requiring more effort than a single firing.  High temperature firings as for melts also make the kiln wash more difficult to remove. But the same process is used in these cases.

       

Kiln wash in firings at slump and low temperature tack fuses can be reused as many times as it remains smooth and undamaged since the temperature is not high enough to cause the chemical changes.

The ultimate benefits of renewing kiln wash are that not only less effort is required to clean and re-coat, than to fix pieces, and also the cost of kiln wash is significantly less than fibre papers.

Removing kiln wash

Applying kiln wash

Revisde 18.1.25

Rigidisers - Application and Use

credit: Scarva

 

Material

Rigidisers are colloidal solutions of silica or quartz with a carrier of some form.  It is also available as a powder to mix with water according to the instructions.

Health and Safety 

Silica and quartz (sometimes referred to as flint) in dry powdered form are a serious health risk.  Wear good respiratory protection and long sleeves and gloves against its skin irritant.  Work outside with the powdered form to keep the dust out of the studio. Clean clothing immediately after working with the powdered form of rigidiser.  Wearing gloves is a good idea whenever working with rigidisers, as the wet form is also a strong skin irritant.

Application

Mix up the powdered form as 1 part powder to 4 parts water, by volume.  Do this masked and gloved, and outdoors if possible.  If not, have a HEPA vacuum running next to your work area.  Mix thoroughly and allow to slake for 24 hours.  Then mix very well by hand or with a blender.  Strain the mix to remove any clumps - they can be made into a paste and added to the main solution.

Liberally paint the solution onto the refractory fibre.  Stir prior to use and frequently throughout the application to keep the silica/quartz in suspension.  Depending on permanence, coat one or both sides of the paper/blanket/board.  It is not necessary to soak the fibre completely.  The object is to provide a hard surface.  It does not need to be hard throughout.

Flat Board

It is best to apply rigidiser on both sides of refractory board.  If rigidising both sides, allow one side to air dry before turning over to coat the other side.  By coating both sides, the warping from heating on one side is reduced. 

Slumping forms 

Cover the shape you are taking the mould from with an impervious separator such as Vaseline or thin plastic film.  Prepare the fibre blanket by coating both sides of the fibre with the rigidiser.  It does not need to be completely soaked.  Press the fibre firmly into/onto the shape and especially into any depressions and around any protrusions to be certain of a faithful replication.

Curing  

Allow the refractory fibre to air dry.  Or if needed quickly, you can kiln dry at 90˚C – 110˚C / 194˚F – 258˚F for several hours.  But only if the master mould can withstand the heat.  If not, demould only after the fibre is dry and can hold its shape without the master.  Be sure to remove the master mould from the fibre before proceeding to heat cure.

When air dried, cure in the kiln by firing to 790˚C/1454˚F for 20 minutes.  Before firing, place the dry form on a refractory fibre separator to avoid the silica/quartz sticking to the shelf. A rapid rate straight to the top temperature is acceptable.  After the soak, turn the kiln off, as the rigidised refractory material is not subject to thermal shock.

In Use

Coat the hardened fibre in kiln wash, or cover with shelf paper or refractory fibre paper, to avoid glass sticking to the hardened board.  The bare surface of the rigidised form is now coated in glass fibres and they will stick to the glass unless a separator is applied.

When used as a shelf, it is best to turn the board over after a few dozen firings. This helps counteract the warping tendency that rigidised boards have.

Thermocouple Placement

Photo credit: Kiln Frog

Sometimes it is difficult to replace a kiln shelf back into the kiln with work on it.  This is normally because the thermocouple sticks out from the wall of the kiln.  Questions have been asked if the thermocouple can be below the shelf to make it easier to place.  Others have asked about reducing the distance into the kiln that the thermocouple projects into the kiln.

These are both bad ideas.

 

Thermocouple Under the Shelf

It is not a good idea to have thermocouple under the shelf because it will then measuring the air temperature under the shelf.  The air temperature under the shelf can be as much as 100ºC/180ºF lower than above the shelf where the work is being fired.  This will cause an overfire on the way up.  Setting the top temperature for 790ºC/1454ºF may give an actual air temperature of up to 890ºC/1634ºF!

 

On the way down at annealing temperature, the air temperature below the shelf is hotter than the air temperature above. It might be annealing at 582º/1080ºF.  This will result in improper annealing at too high a temperature.  The cool will start too early.  The time at the appropriate annealing temperature will be too little.  And the cool finish at too high a temperature.

 

It would be a disaster of a firing.  Don't do it!

 

Reduce the Distance into the Kiln

Another suggestion is to reduce the distance the thermocouple is into the kiln.  This produces inaccurate readings too.  If the the thermocouple is moveable, you can pull it out while inserting the shelf, but it must be re-inserted to the original length before firing, to avoid overfiring.

 

If the thermocouple is not fully inserted, it records a lower temperature than when fully inserted.  I know this from bitter experience.  This results in the pieces being overfired.  But also in inadequate annealing, just as when the thermocouple is under the shelf.

 

 

The placement of the thermocouple is critical to the reading of the air temperatures in the kiln.  The thermocouple should not be moved unless absolutely necessary.  If it is moved, it must be checked to be in the same location as originally placed, because if it is not replaced exactly the temperature readings will be different than previously.

Care of Ceramic Kiln Shelves

Mullite kiln shelves
credit: IPS Ceramics

The most popular and easily available ceramic shelves are made from Mullite, Cordierite, and CoreLite. Other hard specialist kiln shelves are available. They are made of other materials. Shelves are also made from other materials such as refractory fibre board, vermiculite, and fire-resistant ceiling tiles. This concentrates on the care of ceramic shelves.

Composition and Characteristics

This table gives some information about the characteristics of the materials involved in these shelves.

 

Name	Thermal Shock Resistance	Brittle	Strength	Composition
CoreLite	Low	Yes	Moderate	Ceramic with a high silica content
Cordierite	High	Yes	Strong, but heavy	Magnesium, iron, aluminium oxide, silica
Mullite	High	Yes	Strong. but heavy	Silica, Aluminium oxide

 

CoreLite is a trade name for an extruded ceramic shelf. It is strong, but brittle. It is subject to thermal shock below 540ºC/1000ºF. This suggests the ceramic has a high silica content as the quartz inversion is at 573°C/1063°F, where the ceramic has a sudden expansion on heating and an equal contraction on cooling. The cooling rate at this temperature is normally slow enough to avoid breakage.

credit: Clay Planet

cordierite - composed of magnesium, iron, aluminium oxide, and silica. hard, brittle, and with low expansion characteristics.

credit: refractorykilnfurniture.com

Mullite –composed largely of silica and aluminium oxide. It is strong, brittle, and has good thermal shock resistance.

Care

There is enough information from considering the composition of these shelves to indicate they are all brittle and have differing vulnerabilities. These have implications for storage, use and cleaning.

Storage

If storing vertically, take care to avoid setting down on hard surfaces. If they are in a rack, have a separate slot for each shelf. This avoids friction between shelves and possible surface scratches. The most useful material for these racks is wood, or harder materials covered with wood. These racks can be horizontal or vertical.

If it is not possible to have a separate rack for each shelf, do not lean them on each other. Shelves leaning against others or against hard surfaces can become scratched. Provide a cushion against scratches such as cardboard, or thin plywood.

When moving the shelves, avoid setting them down on their corners, or bumping the shelf anywhere against hard structures.

Use

Reduce firing speeds to less than 220ºC/430ºF per hour up to 540ºC/1005ºF, especially for CoreLite shelves. Cordierite and Mullite shelves are not as sensitive, but still can be broken by fast firing rates in this temperature region.

Cover a large portion of the shelf at each firing to avoid uneven heating of the shelf. It is best to evenly distribute moulds and other things that shade the heat from the shelf around the shelf to help avoid thermal shock breaks.

If you cannot or do not want to cover the whole shelf, elevate the mould(s). This helps to keep the whole shelf at the same temperature when only small parts of shelf are covered. It does not seem to matter so much when flat glass is in contact with the shelf. But continue to observe the moderate ramp rates below 540ºC/1005ºF.

It is even more important to elevate damp or heavy moulds from the shelf. These kinds of moulds shade the heat from the shelf immediately below them while the rest of the shelf heats rapidly. This difference in expansion over parts of the shelf becomes too great for the shelf to resist.

Another thing to avoid is cutting fibre or shelf paper on top of the shelf. It often creates long shallow scratches in the shelf. These can be the source of bubbles, but more often, flaws on the back of the fired pieces.

Cleaning

Care is needed to avoid mechanical damage during cleaning. Scraping can create scratches in the shelf. These are difficult to remove or fill smoothly. So, scraping needs to be done carefully.

Any sanding also needs to be done carefully. If you use power tools, it is very easy to create shallow depressions that will be the source of bubbles in future firings. It is slightly more time consuming to manually sand the kiln wash with a sanding screen with or without a holder. But it preserves the flatness of the surface.

If it is decided to wash the shelf primer off the shelf, consider how difficult it is to wash a very persistent baked on substance. It requires thorough scrubbing to remove all the hardened material. Power washers are not advised since the high water pressure can abrade the surface of the shelf.  But if you do decide on washing, you need to air dry for several days afterwards. Then kiln dry slowly to just below boiling point of water. Soak at that point for several hours, or until a mirror held above the open port does not fog up.

There is more information on removing kiln wash here and here.

Summary

Ceramic kiln shelves are hard, but subject to scratches, impact breaks, excess dampness, failure due to uneven temperatures, and to rapid rises in temperature below 540ºC/1005ºF.

Wet shelves

 "Was the shelf completely dry? I’ve had pieces practically crumble from a wet mold or shelf."

There is a lot of speculation about wet shelves causing problems. And not just this one. The reported problems centre around large bubbles and glass sticking to the shelf. Generally, the dampness is the result of applying kiln wash. Although the mould or shelf can be damp for other reasons too.

Kiln Wash

I assure you that kiln wash is dry long before the glass sticks together. It is dry before the glass forms a seal to the kiln shelf or mould. The moisture has sufficient time and space to move from under the glass during moderate first ramp rates.

There is a precaution about wet shelves and moulds, though. You need to be careful in placing glass on top of wet kiln wash. It is possible to scrape kiln wash off areas of the shelf when placing the glass. So, the glass must be placed directly onto the supporting surface without any subsequent movement.

Wet Moulds and Shelves

However, if it is the mould or shelf which is wet, rather than just the dampness from kiln wash, different considerations apply.

If a mould is wet, it will need days of air - and then careful kiln - drying before using. It is best to avoid getting shelves and moulds wet. Washing or soaking of these items is not recommended.

The difficulties relate to the nature of wet porous structures. Not only is there free water in the structure of the mould/shelf, but there is also chemical water. Free water is what makes things feel or look wet. Chemically bound water is molecules of water lightly bonded to molecules of the structure. An item can appear to be dry and still contain this chemically bound water.

Both need careful removal. Air drying for up to a week is good for removing the free water. If you do not want to wait that long, you can kiln dry. But this needs to be done carefully. A slow ramp to just under the boiling temperature of water is required to allow the water to evaporate without creating steam. This rate should be less than 100˚C/180˚F per hour. The length of the soak needs to be related to the size of the piece and how wet it is. But one hour is a minimum.

Then another slow ramp needs to follow to remove the chemically bound water. This temperature is around 250˚C/480˚F. Hold that temperature until no fogging of a mirror or glass held above the open port occurs. This will ensure the mould is completely dry and free of the chemically bound water too.

Conclusion

The best advice is to avoid wetting shelves or moulds. It takes a lot of care and time to get them completely dry. The dampness created by applying kiln wash is easy and quick to remove. It can be done during a firing with a moderately slow rise in temperature to 250˚C/480˚F or beyond.

 

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.

 

 

Drying Kiln Washed Moulds

A question about kiln wash. Do you have to let each coat dry while applying before applying the next coat?

 There seems to be a popular notion that newly kiln washed moulds must be cured before use.  I'm not sure where the information comes from, and no reasoning is given.  It is suggested that that quickly heating newly kiln washed moulds to 550°F (290°C) is important.

 If you want to make sure the mould is dry, this may not be the best way to do it.  All ceramics have a cristobalite inversion at around 225°C/437°F.  This a very rapid increase in volume of 2.5% that often leads to cracks and breaks in ceramics when the rate of advance is quick.  The mould will react better and last longer if the rate of advance is slow until that inversion temperature is passed.  But also note there is a quartz inversion at around 570°C/1060°F that is significant.

 

 This is another reason to advance the temperature slowly when slumping or draping with a ceramic mould.  A further reason to heat slowly is to avoid steam formation within the ceramic body.  If the steam is created over a short time, the force can be great enough to break the ceramic.  To ensure the water evaporates, a soak at 95°C/203°F for a significant amount of time is a better, safer option.

 But in addition to all these precautions, it simply is not necessary to cure kiln wash on slumping and draping moulds made of ceramics.  The glass does not begin to move until after 540°C/1000°F. Therefore, the kiln wash will be dry long before the glass gets near slumping temperatures.  Any vapor caused by evaporating water will escape through the vent holes in the mould or under the glass at the rim, as it will not form a seal until higher temperatures.

 

Newly kiln washed mould beside others already fired

 If you want to be sure your kiln wash is dry before you put the mould in the kiln, you can leave it in a warm ventilated space, or even on top of your kiln while it is being fired.  Using either drying method will dry the kiln wash sufficiently before the glass is placed on the slumping mould.

 The other part of the question was about drying the kiln wash between applying coats. It is not necessary to dry between coats of kiln wash.  In fact, a better result is obtained by applying all the coats at one time. It is not like painting wood. The result of applying all coats is a smoother surface.  There is no dragging of the dry powder along with the wet kiln wash as it is being applied over the existing coats.

 Kiln drying ceramic slumping and draping moulds is not necessary. It only adds another, unnecessary step in kilnforming preparations.  There are exceptionally good reasons to avoid rapid firing of damp moulds. 

 Some extra care could be taken with texture moulds and those intended for casting.