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.

Mould Elevation

 

The expansion characteristics of glass and ceramics components

Many people advocate the elevation of moulds.  Mainly for air flow to equalise temperatures above and below the mould.  But also, to prolong the life of the mould.  My observation on these reasons for elevating the mould are that they are not harmful, but not necessary, except for investment moulds.

My experiments have showed insignificant differences in temperatures above and below whether elevated or not.  Since the air temperature under the mould is much the same whether elevated or not, it indicates that elevation of the mould has no significant effect.  But, of course, elevation of the mould does no harm either. 

More important than elevation of the mould, is consideration of the nature of the ceramic mould.  Ceramics have two expansion/contraction temperatures called inversions.  The first is at 226˚C/439˚F, and the second around 570˚C/1058˚F.  The ceramic expands rapidly at these temperatures.  There is a 2.5% increase in volume at 226˚C and a slightly more gradual 1% increase around 570˚C.

This a main reason to use slow ramp rates up to at least 570˚C/1058˚F.  Slower rates ease the ceramic expansion speed and reduces the risk of breaking.  So, slower rates will lengthen the life of ceramic moulds. The cool down for annealing and cooling is slow enough that it presents no risk for the ceramic.

There are occasions when the mould must be elevated, though.  These are when the mould is large, heavy, or damp.  This is to protect the shelf rather than the mould or glass.

 

Deep Slumps with Bubbles

 

Photo Credit: Rachel Meadows-Ibrahim

The main causes of the large thin bubble is most probably  too high a temperature combined with a long soak.

Elevation of the Mould

The poster indicated there are eight holes total – four on the sides and four under the glass. This means any air has an exit out from under the glass and from the inside of the mould. So, in this case it does not need to be elevated for exit of air.  In my practice l have never, except in tests, elevated my slumping moulds. I have not had failures. My experiments involved in writing the eBook Low Temperature Kilnforming  showed no significant temperature differences between elevating, or not, below the mould.

Effect of Fast Rates

Slow rates to low temperatures with long soaks avoid sealing the glass to the mould. This means air can move out from under the glass during the slump. 

• Fast rates, and elevated temperatures can restrict air movement from under the slumping glass.  
• Fast rates and high slump temperatures can each cause uprisings because the glass slides down the mould during the soak, and that weight pushes the bottom upwards.

Temperature and Uprisings

This uprising is different from the bubble at the bottom on this piece. It is possible to see the glass bubble is thinner than the surrounding glass. As there were holes for air to escape, it seems the temperature and or speed was great enough to allow the glass to form to the mould at the bottom.  This covered the air holes and allowed the remaining air to push upwards on the glass.  A lower top temperature may have avoided this bubble formation.  Certainly, a combination of a slower rate and a lower temperature would have avoided the formation of the bubble.

Observation

Further, observation during the firing would have caught this bubble formation early enough to skip to the annealing and result in a piece with only a slight uprising, and before it became a bubble. Peeking should start at the beginning of the slumping soak and be repeated at 5 to 10 minute intervals.

Uneven Slumps on Deep Rectangular Moulds

 "Can anyone please tell me why this mould always comes out wonky and devitrifies and pulls in on the edges. I used Primo Primer; my kiln is level, and this is the slump schedule I use for 3mm base with 6mm in places [temperatures in Celsius]: 100/593/30 mins; 66/663/25 mins; 204/482/60 mins; 66/371/10 mins; END.*   12cm square."

 GlassAlisa

The suggestion has been made that having a 6mm base would lessen the irregular slump in the mould.  I am not convinced that making the base thicker will sort the problems.

 

1)  This is a very deep mould in relation to the span.  The mould sides are steep.

 

2) The glass slides down and picks up marks from sliding down the walls of the mould.  The marks are not devitrification.

 

3) Deep slumps are prone to going off centre. One fix is to watch and be prepared to reach in with wet sticks to readjust the glass placement on the mould.  

 

4) Deep moulds (deep is relative to the span of the mould) require two or more stages of slumping. Start with shallow a slump, and progress through steeper ones.

5) The sides dog bone on many rectangular moulds.  One way to reduce this is to round the corners with a 10mm radius.

 

6) Reducing the forming temperature, and extending the soak time dramatically, will go some way to alleviating the previous problems. I suggest trying a 620C slump temperature and soak for 2 - 3 hours.  Peek at intervals to see when the slump is complete, then advance to anneal and cool.

In my view, it is a mould from a maker that does not fully understand glass behaviour.

And in passing, the ramp speed from top temperature to annealing should be as fast as possible, to avoid any risk of devitrification on the way down.  

* Schedule in Fahrenheit for the Americans.  

• 198 to 1100, 30' 
• 119 to 1225, 25'
• 367 to 900, 60'  [ASAP is the recommended rate.  As it is a tack fused piece, I would anneal as though 12mm/4 layers.  This would use a 2 hour soak, cool at 100 to 800, 180 to 700, off ]
• 120 to 700, 10'
• End

Slumping Breaks on “go-to” Schedules

 An "It has always worked for me before" schedule implies a single approach to slumping regardless of differing conditions.  Layup alterations, thickness variations, colour contrasts, mould variations all affect the scheduling.  The schedule for each piece needs to be altered when there are changes from the schedule for the “standard” piece, or mould.

Photo credit: Emma Lee

In the example shown, we are not told the schedule, but it shows that the rate was a little too fast. If it had been faster the glass would have separated further apart. The heat was enough to appear to recombine at the edges where it was not slumping so much. 

Review your "go to" schedules whenever something changes. It may still be a good base from which to work. But you need to assess the layup, thickness, and any other variations to help adjust the schedule to fire each piece.

Some of the variations from the “standard” to be considered are:

Single layer slumping 

Weight

Mould sizes

Relative Slumping Depth

Mould shape and detail

Fold Moulds

 

These moulds are available in stainless steel forms in various sizes

You can create your own mould for self-supporting display items. Fibre board and vermiculite board are suitable.

 

I chose 25mm/1” fibre board because I had a suitable piece lying around. It is possible to use thinner fibre board, but the thicker board is more likely to resist deformation over a long use period. The 15mm/0.675” board is suitable for light use. These do not need to be rigidised unless you desire to for a more robust structure. They do not need to be kiln washed unless you feel a better surface will be achieved.

Angled Surface

The 25mm/1” vermiculite board is more durable. It does need to be kiln washed to avoid glass sticking to it. Otherwise’ it is treated just the same as the fibre board.

The width and length of the board are determined by the width and length of the piece you are currently making or envisage making. You can make it longer than current needs and use a stop of a piece of fibre board or other kiln furniture to ensure the glass does not slip down the slope. This allows you to adjust the mould to different lengths for a variety of projects.

Both materials need to have an angle cut from one end. This is the end that will be elevated. It allows the glass to bend directly from the end of the angled board. This angle does not need to be more than 30 degrees from vertical, as most self-supporting items have angles of about 15 degrees or less.

Support

Then a support piece needs to be made. If it is not of fibre board, it needs to be kiln washed to prevent the glass from sticking. This support needs to be as wide as the angled board. The height of support will determine the angle of the finished piece.

It needs to be aligned vertically and directly under the top of the angled board. A try square can help with this alignment. This support also stops the draping glass from curving under the top. It would be interesting for a rocking horse kind of item, but not for a stable decoration.

The support under the elevated end can be made to various heights to obtain various angles on the piece. Also, different heights of support will be required to maintain the same angle on different lengths of the standing piece. This makes the home-made mould much more versatile than the steel ones.

The Stop

The stop is a piece of kiln furniture placed on the slope at the end of the glass to ensure the glass does not slip down during the firing. It is not fixed to the sloped board so that it can be repositioned. If you are using fibre board for the slope and the stop, you can pin the stop to the sloped board. Or you can use heavier kiln furniture, propped as appropriate to form the stop.

Firing notes

Glass lengths

The length of base in relation to upright needs to be determined before firing. You can, of course, cut the excess base length off after firing. I make the base to be the same length as the top leans back. This ensures the piece will not become top heavy.

A spirit level can be used to determine how long the support needs to be. You already know how long the sloped piece of glass is. Place the stop at that distance from the top end of the sloped board. Use a spirit level to indicate the length the base will need to be. When levelled, make a mark on the support. Then measure the distance from the mark to the top of the slope. That length plus the length of the sloped glass will equal the total length of the flat glass.

Scheduling

Use a moderate ramp rate for the thickness of the glass. The top temperature should be about 650˚C/1200˚F. Set the soak time for an hour. Peek frequently from the start of the hold to be sure the glass has draped vertically. When it has advance to the next segment and proceed to anneal.

Dog Boning During Slumping

Does the size of the rim affect the amount of dog boning when slumping rectangular items?

This question was prompted by previous testing on the amount of distortion by adding additional elements. I found that single layer pieces stacked 15mm/0.6” or more from the edge do not affect its shape.

This led me to think: “how wide a rim would be required to avoid dog boning of rectangular pieces while slumping?” The premise was that there must be some relation to the width of the rim and the amount of dog boning.

Method

The method I chose was to make two vermiculite moulds. One with an almost square aperture and the other with a rectangular one. These were not large pieces. 

• One was 27cm by 22cm/ 10.6” by 8.66” with an opening of 10cm by 10.5cm/4” by 4.12”. 
• The other was 25cm by 22cm/9.84” by 25cm/8.66” with an opening of 19.5cm by 13cm/7.68” by 5.1”. 
• Both had a drop of 25mm/1”.

The sizes of the rim were proportional to the opening of the mould. The remainder of the mould was merely a support to the rim.

The firing schedule for all pieces was kept the same.

• Ramp 1   220˚C/396˚F to 677˚C/1252˚F     hold for 1.75 hrs
• Ramp 2   Full to 482˚C/900˚F                     hold for 1.0 hours
• Ramp 3   83˚C/150˚F to 427˚C/800˚F         Hold for 0 hours
• Ramp 4   150˚C/270˚F to 371˚C/700˚F        Hold for 0 hours
• Ramp 5   300˚C/540˚F to 50˚C/122˚F         Off

Results for single layer slumping

Various widths of single layer rim were tested from 1cm/0.4” to 3cm/1.18” at 2.5cm/1” deep. The 2cm/0.79” rim was also tested at 3cm/1.18” and 3.8cm/1.5” deep.

Square openings

The results showed there is no further reduction in dog boning with rims greater than 2cm/0.79” for square apertures of this size. The dog boning of a 1cm/0.4” rim was 1.5mm/0.6”. The amount of deflection from straight was 0.5mm/0.02” for both 2cm/.079” and 3cm/1.18” rims.

There was no effect of increasing the depth of the slump to 3.8cm/1.5” on a 2cm/0.79” rim.

Rectangular openings

The results were different for slumps into rectangular apertures. The glass on the long side of the opening had greater dog boning at all rim widths from 1.25cm/0.5” to 3cm/1.18” than the shorter side.

• ·   A 1.25cm/0.5” rim deformed 3mm/1.18” on the long side and 2.5mm/0.98” on the short one.
• ·   With a 2.5cm/1.0” rim the deformation on the long side was 2.5mm/0.98”. The short side of the opening was 1.5mm/0.6”.
• ·   A rim of 3cm/1.5” deformed 1mm/0.02” on the long side. The short side of the opening deformed 0.5mm/0.02”.

Results for Two Layer Slumping

The big surprise for me was the greater amount of dog boning on the slumping of two layers. I expected less.

The two-layer slumping was done on the same moulds with the same schedule. The results of greater rim widths showed gradual reductions in the amount of dog boning. But there was significant sensitivity to the difference in the square opening.

Square Opening

The square opening is only slightly rectangular by 5mm/0.02” but the 6mm/0.25” glass reacted to that small difference. The amount of dog boning with a 2cm/0.79” rim was 4.5mm/0.18” on the long side. But 2mm/0.18” on the side only 5mm/0.02” shorter. 

This amount of dog boning reduced gradually until with a 5cm/2” rim the deflection was 3mm/0.12” on the long side. The deflection was too small to measure on the short side.

Rectangular openings

The rectangular opening was 1.5 times longer than wide. This had significant effects on the extent of dog boning. Although increasing the rim width did reduce the deformation, the long side continued to exhibit greater deformation than the short one.

• ·   With a 3cm/1.5” rim, the long side deformed by 4.5mm/0.12”. The short side by 3.5mm/0.14”.
• ·   A rim of 3.5cm/ reduced the deformation to 4mm/0.16 on the long side. But 2mm/0.08” on the short side.
• ·   At 4cm/1.57” the rim deformed 2mm/0.12” on the long side and 1mm/ on the short one.
• ·   Strangely, a 4.5cm/1.77” rim had a little larger deformation than the 4cm/1.57” rim. It was 3mm/0.12” on the long and 2mm/0.08” on the short side. It may be that the greater length of the rim contributed to increased dog boning.

 

A general reflection on the two-layer tests. 

It is possible that there was too long a hold at 677c for 6mm. I did not do a check on the time it took to reach full slump. The long soak was required to get the single layer to conform to the mould. At the time, my requirement was to keep the firing of single and double layer slumping the same for comparison. Perhaps keeping that hold constant was the wrong decision. Further testing will be required.

 

Summary

I learned some things from these (incomplete) tests that I did not expect. This is good for my learning. The things I found out are:

• ·        In general, the wider the rim is, the less dog boning occurs.
• ·        The extent of dog boning is more sensitive to the dimensions of the opening than to the size of the rim for both single and double layers.
• ·        The depth of the slump of a single layer has less influence than the size of the rim. Once the rim is of sufficient size to minimise the dog boning, the increase of the depth by 20% or 50% did not affect the dog boning.
• ·        Thicker glass with the same schedule deforms more than single layers. This does need more investigation, though.

 

More Informaton:

The basic cause of dog boning is related to volume control.

The causes of dog boning other than volume control.

More about the effects in slumping.

Much more information is available in the eBook Low Temperature Kilnforming.

Stuck Kiln Wash

 

Moulds

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

Preparation

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

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

Removal Methods

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

Metal

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

Ceramic

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

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

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

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

More information on removing kiln wash from moulds.

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

Shelves

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

Dry Methods

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

 

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

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

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

Wet methods

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

 

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

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

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

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

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

and here.

Boron Nitride

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

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

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

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

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

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

More information on removal of boron nitride is given here. 

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

Steel Moulds

Credit: Slump and Fuse

Do steel moulds need to be kiln washed for slumping?

Some prominent people in the kilnforming milieu like to promote the extremes of the craft. The argument seems to be that the glass does not get hot enough to stick to the metal at slumping temperatures. It could be argued in the same way that it is possible to slump glass on bronze or unglazed ceramic moulds.

This proposal may come from glass blowing where glass is pressed into metal moulds. The difference is that the glass is in contact with the metal for a short time. And in industrial processes the steel is water cooled.

Also, the higher the slumping temperature, the “softer” and “stickier” the glass becomes. The metal is also heating up and expanding, unlike in glass blowing. If the glass adheres to the metal at all, the greater contraction of the metal during cooling will ensure the glass is stressed and likely break. Therefore, it is usable only in low temperature slumping – below about 630˚C/1167˚F – or for short times. The break patterns that occur when slumping on bare steel show glass most often sticks to the steel and becomes crazed or even shatters on cooling.

Bare steel as a slumping mould is always a risky practice. Just because it can be done - or is done - in glass blowing and other industry settings, does not mean it should or can be done in studio settings. The practice comes with high risks of failure.

To be safe, a separator needs to be used between any supporting structure and the glass. Why risk glass into which you have put significant amounts of work for a few pennies worth of kiln wash, fibre paper, or boron nitride?

Flows

 

Credit: Marcy Berman

I have not had much success [with] the Patty Gray mould despite using the recommended firing schedule. I always have holes or bubbles and the edges are not smooth.

The schedule for Oceanside was:

• 111°C/200°F per hour to 537°C1000°F for 15 minutes
• 167°C/300°F per hour to 662°C/1225°F for 30 minutes
• 195°C/350°F per hour to 798°C/1470°F for 20 minutes
• 9999 to 510°C/950°F for 120 minutes
• 55°C/100°F per hour to 371°C/700°F off

 Your picture shows a bottom view of the piece - made of cullet pieces - as fired. Two large bubbles show to have been created from the bottom rising through the glass to the top.

 Although a long bubble squeeze will not prevent this, it will help to reduce the number of bubbles, and especially large ones. Because of the number of pieces and the thickness of the glass put into the mould, a longer bubble squeeze would benefit this piece.

 The bubble squeeze can be as you have done this – at a single temperature – with a soak. In this case, I would have used 60 to 90 minutes as the soak.

 The other bubble squeeze method is to start the squeeze about 55C/100F below the top of the bubble squeeze. Most people use a soak of about 30 minutes there. They then proceed at a rate of between 30C/55F and 55C/100F to the top of the bubble squeeze and soak there for another 30 minutes. The rates and soak times will vary according to the thickness or complexity of the piece.

 I dispense with the soak at the beginning of the bubble squeeze on the grounds that at 610/1130F so little movement will be created that it is a waste of time. I would prefer to have a slower ramp rate to the top temperature and a longer soak there. I know the glass will be moving at those temperatures. Many people find the soak at the beginning of the bubble squeeze successful.

 The schedule to the top of the fuse is faster than the rest of the schedule. When I want a piece to flow, and especially, to fill gaps, I slow the rate. In this case a rate of between 100C/180F and 167C/300F would be slow enough to allow the glass to flow to fill gaps.

 I want to ensure the glass has enough time when it is flowing most freely at the top temperature to level out. This requires scheduling a longer soak at the top and observing how well the glass is levelling out. If more time is required you can add it on the “run,” and advance to the next segment when the surface is as wanted. Read up in your kiln manual how to do both these things.

 Yes, the rate is one which will enable devitrification to form on flat glass. The soak at top temperature is even more likely to promote it. However, as the glass is flowing, less devitrification has an opportunity to form. The crystallisation – which is what devitrification is - of the glass takes time to form. The movement of the glass surface is sufficient to reduce the formation of those crystals. It is of course likely there will be some devitrification, but not as much as the slow rates and long soaks would lead you to think. 

 But for these flows there always is the possibility of devitrification. You have to plan a method of removing it. Unless the surface is very flat, grinding the top is not a fast way to remove it. Sandblasting is a quick way to remove devitrification. Another way is to sift a thin layer of clear glass powder over the surface. This is an increasingly popular way to deal with devitrification for those without access to sandblasting facilities. When fired again, the powder melts and forms a new shining surface. The piece will need to be fired fire again whether sandblasted or covered in glass powder.

The summary for flows:

• Slow down to top temperature.
• Give sufficient time there to get the flow needed.
• Observe the progress as you near the top temperature.
• Extend the soak or advance to the next segment when the surface is smooth.
• Anneal soak for the calculated thickness.
• Use a three-stage cool – as outlined in the Bullseye chart for annealing thick slabs - to ensure no temporary contraction stresses are created.
• Accept there will be devitrification.

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.

 

Elevation of Moulds in the Kiln

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

Hot and Cool Spots

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

Effect of Elevation of Mould

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

ΔT Shelf vs. Elevated Moulds (Celsius)

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

 

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

ΔT Difference Between Air and Elevated Mould (Celsius)

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

 

Large, Heavy, Wet Moulds

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

 

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