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.

Playing in the Sandbox

This process provides flowing, abstract images that can be used as autonomous pieces or formed into other objects, such as free drops, bowls, cut for jewellery or into pattern bars.  The appearance provided is unique to this combination of using frit and pressing.

In principle, this process is the same as creating sand pictures.  The process is in three stages: making the box, adding frit, and pressing.

The Sandbox

Determine the size of the box.  It should not be more than two-thirds the size of your kiln shelf depending on thickness.  Thicker glass pressed to 6mm will spread more than thinner.  As a guide, 12mm should have an allowance to spread to about 1.3 times the original size; 19mm should have an allowance to spread about 1.5 times the original dimensions.

Cut two sheets of the same size from clear fusing glass. One will be the front. The other will be the back.

Determine whether the image you are creating will be portrait, landscape, or square.  Orient the sheets in the appropriate way to have the top away from you.  Choose the top piece of the pair and cut two 6mm strips from the designated top.  This gives you a lip to be able to pour the frit into the box easily.

 

Box formed with bottom and sides glued to back and front.  The filling lip shows on the right.

From another piece of clear glass cut two 6mm strips for the sides.  If you cut them the same length as the side of the glass, they will stick above the back about 3mm. You can cut this off, but it really is not a worry for the construction of the box.  These strips form the spacers to allow the frit to be poured into the box.  Their thickness will determine the amount of frit needed to fill the box.

Get out the back sheet and clean and prepare it for attaching the strips. My preferred method is to glue the bottom 6mm strip on its edge with super glue.  It is advisable to wear plastic gloves when gluing the strips, to avoid sticking your fingers to the glass.  Super glue cures quickly and does not delay the construction of the box.  It burns out cleanly without any health and safety concerns.  Place a thin film of super glue on one edge of the strip.  Attach it to the bottom by placing it carefully at the edge of the sheet.  Do the same for the sides.

  

When the strips are stuck down to the back, place  a thin line of super glue on the top edge of the strips in preparation for attaching the top sheet.  Using a strip of wood placed at the bottom of the backing glass will help in placing the sheet accurately. Lower the sheet from contact with the bottom to the strips forming the sides of the box.

When the glue is cured, inspect the sides of the box for gaps. If there are gaps, use clear Sellotape to seal the gaps in the sides. It will burn off cleanly in the kiln.

Adding the Frit.

Place the box on an easel or other support so it is slightly tipped backwards.  This helps ensure the box does not fall toward you while working on it.  It also allows the frit to slide toward the bottom rather than bouncing off the other frit.

 

The early stages of filling with the box on a stand

The size of frit you choose to use will affect the final appearance.

·        Generally, powder will appear greyer and more opaque than frit. This is due to the multiplicity of tiny bubbles between the grains of powder.

·        Fine and medium frit provide more clarity than powder.

·        Coarse frit provides the most clarity, but with fizzy bubbles between pieces of frit.

When preparing to place the frit in the box, it is a good idea to take small amounts out of jars and place it into small cups to avoid contamination of the main source of the frit.

Pouring the frit into the sandbox

You can use a jeweller’s scoop or a teaspoon to move the frit from the cup to the box.  Tip the frit into the box above where you want the colour to be placed.  

Moving the frit with a skewer

If the frit does not land just where you want it, you can move it with stiff wire that is long enough to reach the bottom of the box.  Gently sweep the frit with the end of the wire toward the place you want the coloured frit to be.

Using a jewellers scoop to add the frit.

Continue adding colours to create the profile and shapes you wish.

You can make additional alterations to the way the frit is placed.  You can poke the frit from one layer into lower layers with a stiff wire by pushing the wire directly downward.  You cannot do this more than 2 or 3 centimetres deep, as the frits and powders become compacted.

A thick copper wire being used to poke down from an upper layer to the lower ones.

When filled to the top or to your desired level, use the fourth strip to close the box.  If full, glue the strip to the top.  If not full, cut strip to the length needed to drop into the opening of the box.  Place a couple of drops of super glue on the top of the already placed strip to keep it in place while moving to the kiln.

The Pressing

Prepare the shelves

You will need two shelves for each pressing. One is the base to hold the glass and the spacers.  The other is to provide the weight to press the glass thinner.

Clean off old kiln wash from the shelves. Experience shows that adding new kiln wash over old for this process promotes the sticking of the kiln wash to the glass.  Add new kiln wash that performs well at extended times at upper temperatures.  I find Bullseye shelf primer works very well.

Once partially dried, with the pink beginning to pale, you can smooth the surface brush marks.  Some use balled up material such as tights to rub over the surface.  I find very good results from rubbing lightly over the kiln washed surface with a sheet of paper between the palm of my hand and the shelf.  The advantage of doing this smoothing while slightly damp is that no dust is created that needs to be cleaned away.  The disadvantage is that too much pressure will pull bits of kiln wash from the shelf.

Do not use fibre papers as the separator.  The glass will be moving within the space between the shelves.  It will pick up and incorporate parts of the fibre paper, if used.

If you have shelves of different thicknesses, reserve the thickest shelf for the upper, pressing one.  If all your shelves are the same size, put a second on top for adequate weight, or add heavy bricks or a steel weight to the top shelf.  (Note: if you use bricks for weights, they need to be dried first.  A two-hour to three-hour soak at 95C should be sufficient.)

Placing

Place the sandbox centrally on the shelf.  If you are doing more than one, ensure there is plenty of space between the pieces and from the edge, so they don’t contact each other, or drip over the edge of the shelf.  The allowances given for the size of the sandbox are a guide.

Two sandboxes placed on separate shelves

Place spacers of the desired thickness around the four corners of the shelf to restrict the extent of thinning.  This also regulates the evenness of the glass across the whole surface.  Usually, 6mm is a desirable height for the pressing.  Other thicknesses can be chosen for different purposes.  The spacers can be steel washers, although they will spall in the cooling stages of the firing.  If you have pieces of ceramic of the desired height, they can be used.  Fibre paper stacked up to the appropriate height are surprisingly robust spacers.  They also provide a cleaner set of spacers than steel.

A corner of the shelf with the 6mm fibre spacer

Place the upper shelf gently down onto the glass piece. The glass at this stage is taking the whole of the weight of the pressing shelf.  The shelf must be placed both gently and evenly down onto the glass to avoid breakage.

Check that everything is in place. This may require additional, directional light such as from your mobile phone or a torch.  It is now ready to fire.

The Firing

This assembly of materials has a lot of mass.  It is 2 to 3 times the normal mass for a standard firing.  

Pressing shelf placed on top of the glass sandbox

This promotes variations in practice:

• ·        Even with this additional mass, you can fire quickly.  This is because the glass is in small pieces and that the mass of the shelves gains heat slowly. 
• ·        The greater mass does require longer soaks than a normal fuse firing. 
• ·        The upper temperature for a full fuse is required to get the glass to a sufficiently low viscosity to allow the glass to move.
• ·        The long soak at the top temperature does not promote devitrification as in normal fusing.  My speculation is that the glass is not exposed to the air, so the devitrification cannot form. 
• ·        A further difference in a pressing firing is that the annealing can be at the rate for the final thickness of the glass.  The mass of the shelf and weights above the glass means the glass is cooling evenly from both sides, unlike normal fusing.  The glass may be cooling more slowly than programmed, but the programmed rates limit any possibility of too rapid a cooling.

A schedule for a 12mm thick Bullseye piece with a 19mm upper shelf might look like this:

300°C/hr to    670C       for   180 minutes

300°C/hr to    816C       for   180 minutes

AFAP      to    482C       for  120 minutes

55°C/hr   to    427C       for   0 minutes

99°C/hr   to    370C       for   0 minutes

200°C/hr to    50C         for   0 minutes

Off

A piece of 19mm should be slower:

150°C/hr to    670  for   240 minutes

150°C/hr to    816  for   240 minutes

AFAP      to    482  for   120 minutes

45°C/hr   to    427  for   0 minutes

90°C/hr   to    370  for   0 minutes

180°C/hr to    50    for   0 minutes

Off  

Both these schedules assume the final thickness of the glass will be 6mm.  The schedule for glasses other than Bullseye only needs to have the top and annealing temperatures altered to the ones appropriate to the glass.

Results

The pressed glass will have the texture of the shelves on both sides.  Normally, no kiln wash will be stuck to the glass.  If there is kiln wash to be removed, you can do this by abrasive means – sandblasting, diamond pads, wet and dry sandpapers or Dremel style tools.  It is important to keep the glass damp during this process.

Untreated result of pressing

If the surface of the glass is without sticking kiln wash or other marks, you can use it with the matte surface without further kiln work.  You can also fire polish the piece to get a glassy surface, once you have thoroughly cleaned it.

Alternatives

Tape box together

After super gluing the bottom and side strips, you can bind the box together with clear Sellotape.  Pull off at least three strips of tape and set them where you can reach them easily.  Place the upper sheet on the prepared base. Move the box to the edge of the work surface so a little of the box hangs over.  The first stage is to place a strip of tape at right angles to the side to bind the top to the bottom.  Do this for each of the three sides.  When the top is securely attached to the base and sides tape along the length of each of the three sides. 

This shows on the lower left a loosened piece of sellotape on the edge of the sandbox.

This process avoids any difficulty in attaching the top.   Attempting to use only Sellotape to bind the box together is very difficult and requires at least three hands.

Spacers for the frit

Spacers do not always need to be strips on edge.  The spacers can be one or two wider strips placed on their sides to provide the needed height.  They can be coloured, forming a border; but remember the border will become curved. The strips will need to be glued to the back.  The top can be attached with super glue, or taped to the sides and back.

Pressing without a box

It is possible to use the pressing technique without a box or frit.  You can arrange clear and coloured cullet on the shelf.  The arrangement needs to be such that there are no gaps between the pieces.  This means that the glass will probably be 3 to 4 layers thick.  Be careful to avoid creating thick layers of dark colour by interfiling with clear. Place the spacers at the corners of the shelf for the thickness desired and fire.  The slower rate of firing (as for 19mm) should be used.

This sandbox process is a combination of arranging frits and pressing.

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