Strain Points

A critical range is the temperature around the annealing point. The upper and lower limits of this range are known as the softening and strain points. The higher one is the point at which glass begins to bend.  It is also the highest temperature at which annealing can begin. The lower one is the lowest point at which annealing can be done. Soaking at any lower temperature will not anneal the glass at all. This temperature range is a little arbitrary, but it is generally considered to be 55C above and below the annealing point. The ideal point to anneal is thought to be at the annealing temperature, as annealing occurs most rapidly at this temperature.

Annealing Range

However, glass kiln pyrometers are not accurate in recording the temperature within the glass, only the air temperature within the kiln. The glass on the way down in temperature is hotter than the recorded kiln atmosphere temperature. A soak within the annealing range is required to ensure the glass temperature is equalised. If you do a soak at 515°C for example, the glass is actually hotter, and is cooling and equalising throughout to 515°C during the soak. The slow cool to below the lower strain point constitutes the annealing, the soak at the annealing point is to ensure that the glass is at the same temperature throughout, before  the annealing cool begins.

Strain Point and Below

No further annealing will take place below the strain point. If you do not anneal properly, the glass will break either in the kiln or later no matter how carefully you cool the glass after annealing.

It is still possible to give the glass a thermal shock at temperatures below the lower strain point, so care needs to be taken.  The cool below the anneal soak needs to be at a slow controlled rate that is related to the length of the required anneal soak. Too great a differential in contraction rates within the glass can cause what are most often referred to as thermal shock.  The control of the cooling rate reduces the chance of these breaks.

Softening Point

The glass is brittle below the softening point temperature, although it is less and less likely to be subject to thermal shock as it nears the softening point.  It is after the softening point on the increase in temperature that you can advance the temperature rapidly without breaking the glass.  So, if you have a glass that gives its annealing temperature as 515C, you can safely advance the temperature quickly after 570C (being 55C above the annealing point).

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.

Overlaying Fibre Paper with Thinfire

Some people use a 1mm or 2 mm fibre paper with Thinfire or Papyros laid on top to get a smoother surface. It also allows assurance that the air can migrate from under the glass through the fibre paper. 

However, if the Thinfire or Papyros is laid down too many times, it fills the spaces between the fibres of the paper, and resists the passage of air.  This results in the large bubbles your are trying to avoid.  Experience will show how many times you can layer Thinfire without blocking the passage of air through the fibre paper.

This effect can also happen on sand beds.  The separating powders can build up and fill all the spaces between the sand particles. This resists the movement of air through the sand.  It leads to large bubbles just as compacted Thinfire on fibre paper does.

Is this practice of repeated layering of Thinfire and Papyros sensible?  The tipping point between achieving a smooth surface and creating bubbles cannot be known with certainty.  This means there is always a risk of bubble formation.

It is possible to use a vacuum sweeper with variable suction control  to remove the Thinfire or Papyros residue.  Using the lowest power and holding the end of the hose above the paper can lift the Thinfire or Papyros without affecting the underlying fibre paper.  Then a new sheet of Thinfire can be placed over the fibre paper to maintain the smooth surface.

Repeated layering of thin fibre papers can lead to a compaction that no longer allows the passage of air from under the glass during fusing. 

Single Layer Circle with Decorative Rim

A question arose:

If you fuse a single 20cm diameter sheet of 3mm glass to full fuse, [with a decorative rim] what happens? … Would the lack of two layers in the centre be a problem for the 6mm rule?

This layup risks trapped air and a large central bubble.  The explanation involves the combination of volume control and weight.

Volume control

The volume control relates to the single 3mm layer in the centre.  The glass will thin in the centre and thicken at the perimeter.  This leads to the risk of thinning to the degree that bubbles are created in the centre.  The edges will also draw in as the viscosity - surface tension - of the glass pulls the glass toward a 6mm thickness.

Weight

The explanation is also about weight.  The decorative rim adds weight to the outside of the piece.  This weight will “seal” the rim of the glass to the shelf, reducing the possibility of air escaping from under the central portion of the piece.  This weight effect on the rim increases the risk of a large central bubble.

Profile

Another influence on the result of the fuse is the degree of fuse.  At full fuse the viscosity of the glass is less and so resists the force of expanding air much less than when cooler. Even at rounded tack fuse, the glass will be unable to resist the formation of bubbles. As the glass thins and viscosity decreases, any air at all will cause a bubble.

Changes for the future

Avoidance of bubbles in this piece relate to design, scheduling and technique.

Design

It is possible to design a piece of this nature to avoid the volume control issue.  The base piece could have a smaller circle or rectangle centralised on top inside the proposed perimeter.  The rim can then have the decorative elements placed.  If they are spaced widely, frit can be used to fill significant gaps.  The piece can then be placed in the kiln for a full fuse.

Scheduling

You can also fire the piece as originally described very slowly to a low temperature.  This uses the concept of heat work. By applying the heat over a long period, you can achieve the same effect as would be achieved by a faster rate of advance to a higher temperature. 

There are at least two ways to increase the heat work.  You can use a very slow rate of advance to a point slightly above the softening point of the glass.  This will be the lower end of the slumping temperature range of your glass.  The soak may be for hours.  You will need to observe when the effect you want is achieved.

You also can choose the same lower slumping temperature and reach it in your standard fashion.  This will require an even longer soak time to achieve the same result.

In both these low firing approaches, you will need to observe to determine when the piece is finished.

Technique

The “flip and fire” technique may also work on the single layer with an added rim.  To do this you build the piece upside down on the shelf.  It helps to draw an outline of design on Thinfire, or Papyros.  Place the decorative elements and cap them with the clear.  Take the whole to a rounded tack fuse.  When cool, clean well and fire to a tack fuse again.  This will give something less than a full fuse, but it will be more than a tack, as the heat work is cumulative.

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

Summary

A single layer piece with a decorative rim is most likely to produce bubbles in the centre.  There are some ways to overcome this: design, scheduling, and technique. Design is the most likely to be successful.

Bubble formation

Question:

I had 2 kiln loads where every piece had huge bubbles. The 3rd time I did a test fire and put 2 pieces on Thinfire and one on the kiln shelf. The one without thin fire got a bubble. Theories?

Response:

Your experiment showed that the Thinfire prevented the bubbling.  The question you are asking is why.

Fibre paper is a porous material allowing air to move through it and from under the glass.  If the shelf has only slight depressions, the Thinfire or Papyros will allow air out from under the glass avoiding bubbles.

Thicker fibre paper can provide a different and more level surface if it is thick enough to span any depressions in the shelf, while allowing air out too.

First, it is apparent that your shelf is not absolutely smooth and level.  This has been shown by your experiment where Thinfire prevented bubbles where previously there had been bubbles.  It showed that without the additional cushion that the depressions, although slight are enough to cause bubbles without additional fibre paper separators.

Second, although you do not give your schedule, the firing is too hot.  There is dog boning of your thin glass.  The bubbles on the thinner glass have burst and thinned greatly.

Firing hot causes the glass to become much less viscous than needed to perform a full fuse and allows the trapped air to push bubbles into and through the glass.  Lower temperatures with longer soaks/holds enable the glass to better resist the formation of large bubbles.  Also firing more slowly enables air to escape and allows the use of lower temperatures while still being able to achieve the fuse you want.

More information is available in Low Temperature Kilnforming, an evidenced based guide to scheduling.

Tack fusing multiple layers

The question:

Full fused 6mm base, with 3mm tacked pieces. It is to be tack fused and slumped now.  Does the number of fuse firings affect the rate of advance, and how long a soak will be required to slump it? 

Multiple firings

If properly annealed each time the glass is fired, the number of firings does not affect how the glass should be fired.  This assumes the same number of layers are being fired.

Tack fusing

Tack fusing this piece will need some care.  The portion to be tack fused is 3 layers thick – overlapping white pieces surmounted by the yellow balls.  The base layer is shaded from the heat by the white, which is generally slower to transmit the heat than many other colours. Bullseye suggests doubling the total height and firing for that thickness. Bob Leatherbarrow suggests 1.5 times the total height for creating the schedule.  Firing Schedules for Kilnformed Glass,  p. 124-6

In this case, because of the amount of white, I would go with the Bullseye suggestion.  My researches for "Low Temperature Kilnforming" also indicated that a tack fuse requires a schedule for two times the thickness. Other levels of tack fusing require different calculations.  The total height of 15mm will be treated as 30mm for scheduling purposes.  This is midway between the thicknesses in the published table.  The rates and times in the table are linear. You can calculate a mid-point in the schedule to get the numbers for your piece.  Half the difference between 25 and 38 is 6.5mm giving 31.5mm.  Using the half-way point will be slightly more conservative than using exact calculations.  It is so close as to make no significant difference.

You will notice that the table gives only annealing times and rates.  There is way you can use this table for the getting initial heating rates.  Look at the final cooling rate for the thickness. If the glass can survive the cooling rate given without showing stress, it will also survive that rate of increase.  The mid-point between 90 and 45 is 67.5°C.  This gives an initial rate of advance (68°C) which can be applied for this piece that has so much shading of the base layer. It should allow the heat to transfer through the white to the base layer without great temperature differences between the covered and the uncovered base layer.

As there is a lot of work in this piece, and it is for someone else, you can be cautious.  Introduce a soak at 260°C of about 30 minutes.  This will help to ensure the heat is distributed to the bottom layer.  If you want to be even more cautious, you can introduce a second 30-minute soak at 371°C before continuing to 540°C.

At 540°C you have passed out of the brittle zone of glass and can increase the rate of advance to 167°C per hour.  The amount of heat work you have put into this piece by the slow rate of advance may enable you to complete the tack fusing with a soak at 720°C.  You will need to observe when the appropriate amount of rounding has been achieved. You will then be able to advance to the annealing portion of the firing. 

For this piece, the annealing soak will be for 5 hours with a cool of 11°C per hour for the first 55°C. Then 20°C per hour for the next 55°C and a final cool of  65°C per hour. This anneal and cool will be about 21 hours, in addition to the ca. 21 hours, in addition to about 10 hours heat up, so don’t expect a quick firing.  Plan two days for the tack fuse.

Slumping

Slumping will need care too.  The piece has uneven layers and the same care is required as for tack fusing.  Experimentation has shown me that scheduling for an additional 3mm (1/8") is needed to ensure the piece is thoroughly heated throughout its thickness.  In addition, the white is stiffer than the other colours and will not bend so easily.  This kind of slow schedule means the glass will be at the same temperature throughout as the slumping starts.

Because of the slow rates of advance, you may be able to slump this piece at 620°C with a significant soak time.  You will need to observe when the piece is fully slumped.  Be prepared to advance to the annealing and cool segments of the schedule.  Some times you need to extend the hold time.  Be prepared for this too. The annealing time and cooling rates will be the same as for the tack fusing.

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

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.

Incomplete definition on texture moulds

An enquiry on incomplete definition in the glass from texture moulds:

I have this texture mould, but I’m not getting much definition. I’m using a single 3mm layer of 96. Do I need to go hotter for longer?

155C - 750C, 15 minutes

Full - 516C, 140 minutes

49C - 371C, 0 minutes

Full - 49C, off

My response:

You have sensibly increased the temperature at a moderately slow rate (for 3mm).  This ensures the glass is evenly heated through by the time it reaches the working temperature.  It is slow enough for you to be confident that most of the air has been squeezed out.

If you wanted to be more cautious about bubbles,  you could introduce a slow increase in temperature - maybe 50°C - from 600°C to 650°C.  You may want to soak there for 30 minutes, although it may not be necessary.  Once that segment is finished you can resume at 75°C to the top temperature.

I would not increase the temperature as you are already at risk of dog-boning the glass.  I would extend the soak time to 180 minutes at 750°C. You need to check frequently after the top temperature is achieved.  A quick peek is all that is required to see if the texture is fully reflected in the top surface.  You may find success by using a lower temperature, say 730°C, but it will require at least an hour more soak time.

The piece above conformed completely to the 12mm depth of the mould with a soak of three hours at 750°C. There was incomplete formation of another test piece at 740°C for three hours. So the 10°C made enough difference for complete formation over this depth.  With less extreme heights, a lower temperature or a shorter soak would be possible.

Once the texture is assured, you need to advance to the next segment.  Or, if it is not achieved by 10 minutes before the end, extend the soak.  Check your controller manual on how to skip to the next segment, or to extend the soak.

As an aside, your annealing soak and cool is very long and slow for 3mm.  You can regain the time used in the slow ramp rate and soak. Review the requirements for a single sheet of glass.  A 60-minute annealing soak and a cool rate of 83°C/hr. to 370°C is an adequately slow anneal cool for a 3mm piece.

You may find more success with a 6mm sheet.  The weight of glass helps it conform to the texture mould.  I have found that a slow ramp rate (about 150°C) to the strain point of ca. 540°C, followed by half that rate to top temperature allows a reduction in soak time to achieve the required definition. This reduction in soak time can be one half hour less than the time required to get good definition on a 3mm sheet.

The strategy outlined here for the scheduling is using the principle of slow and low and long firings.  It is much easier to control the results of the firing by using moderate ramp rates to lower temperatures combined with longer soaks and periodic peeking to check on progress.

If you do not have the time to devote to peeking when the schedule is at the top temperature, you should investigate the method of programming a delay to the start of the firing.  Your controller manual will give the method of using this function.

Texture moulds work well with the slow and low principle of kilnforming.  Long soaks may be required with periodic observation to determine when the process is complete.

Further information is available in the e-book: Low Temperature Kilnforming.

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.