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.

Large Tiles for Kiln Shelves

Pizza stone in use

People frequently wonder if other materials than mullite can be used for kiln shelves.  Mullite is used for its strength and very small expansion, even at high temperatures, as used in ceramics firing.  There are other materials that can be used in kilnforming of glass such as refractory fibre board, and ceramic pizza stones,  the best of which are made from mullite.  This post is about using ceramic floor tiles.

An unglazed floor tile, 11 x 11 inches

Structural Soundness

A major element in obtaining and using a floor tile is how sound it is.  Tapping the tile to determine whether the sound is a low toned ring or a dull thud is important.  There may be invisible cracks within the tile.  A dull thud is an indication that the whole tile has one or more cracks in it, or that it has not been fired high enough to completely vitrify the clay. A low frequency tone indicates there are no cracks and that it has been fired sufficiently high.

Flatness

The first thing you need to do is make sure the ceramic tile is flat and without undulations before using it. To test this, get a straight edge and move it along the tile to look for any slivers of light coming through underneath the straight edge. Any light or variation in the amount indicates depressions that can produce bubbles during the firings. Do this test at least twice at right angles to each other.  Take note of the depressed areas (or even possibly high areas) to know where these uneven areas are to work them out of the tile. 

You can do the above test in the showroom.  Another more accurate means of checking is more difficult to do in a sales area.  Place a line of dark powder, say black glass powder, and with a straight edge held vertical to the shelf, drag the powder across the shelf.  Where there are dark patches is an indication of depressions.  The area and depth can be seen from the spread of the visible powder and to some extent the density of the colour.

Making Shelf Flat

If you buy two of these large tiles, you can rub them together face to face in circular motions. The abrasion marks will show the high spots, with the low spots clear of those marks.  This will indicate the amount of work needed to get the whole surface even.  The smaller the unmarked areas, the less grinding will be required. You can add an abrasive with some water to form a slurry and continue to grind until everything is even. The use of water with the abrasives is important to eliminate dust which might be harmful, and to ease the grinding process.

The above is a manual process.  If you have a large enough flat lap, you can mechanise the flattening process.  Using decreasing grit sizes, you can grind the shelves level with a high degree of smoothness. You do not have to use a grit of less than 200, as the tile structure is even more coarse than that.

If you can't find unglazed floor tiles, you need to look at the back of the tiles.  Many floor tiles have a grid pattern on the back to ensure sufficient adhesive is used.  This makes getting the back, unglazed side flat more difficult or time consuming, because they will need to have the grid ground down to the lower surface.  In this case, it may be that you need to sandblast the glazed side before making sure it is flat.  The sandblasting can make a flat tile uneven by unequal times spend on various parts of the tile, so you have to check after sandblasting for the flatness.

Materials for making dams

Rectangular or straight sided shapes

Broken shelves

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

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

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

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

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

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

Credit: Bullseye Glass Company

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

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

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

Curved and circular pieces

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

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

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

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

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

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

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

Removing Shelves for Slumping

There are those who advocate removing the kiln shelf(s) before slumping.  The advantages claimed include:

Better heat distribution around mould.  The shelf acts as a heat sink. During the firing the shelf absorbs heat and during the cooling the heat is released, so slowing the cool down. 

Additional height. For kilns with little head room, greater height is provided by this practice.

Observations

My observations on this practice lead me to some questions about the necessity, desirability and in some cases the practicality of it.

Elevation of mould above the shelf

This is a widely recommended practice.  I haven’t found the need, but many people do.  One of the points of this is to allow increased air circulation around the mould and under the bottom.  Another is to let air out from under the bottom of the mould to avoid creating air pockets between the mould and the glass.

If the elevation of the mould allows air circulation, what is the necessity to remove the shelf?  There is air circulation around the bottom of the shelf and of the mould. If the mould is placed on the floor of the kiln, the mould will still need to be raised from the bed of the kiln to allow air circulation under the mould. Of course, if the kiln does not have enough space for the height of the mould, it will be necessary to remove the shelf, but not for circulation purposes.

There is also the fact that the floor of the kiln is most often made of refractory bricks even if the walls and top are of refractory fibre.  This also is a heat sink.  I don’t see the advantage of removing the shelf to avoid a heat sink when the base of the kiln works in holding heat in the same way as the shelf.

Difficulty of removing shelves from some kilns

It is difficult to remove shelves from many kilns.  This can be avoidance of damage to the thermocouple; difficulty of getting fingers around the shelf; weight; size; or even depth of the kiln.  It is impractical to remove the shelves from kilns of this nature.  It is still possible to get a good slump in these kilns.

Uneven cooling of the glass

Research shows long soaks lead to a cooler bottom of the glass than top during the anneal – sometimes greater than the +/- 5°C for adequate annealing.  This is a consequence of the fact that the hot air above the glass is not balanced by the same amount of heat below the glass.  So, there may be good arguments for retaining that heat sink of a shelf under the mould to more evenly balance the cooling of the upper and lower surfaces of the glass during the anneal soak and cool.

Height

I don’t have any argument that when extra height is needed, as removing the shelf will provide some.

Some consideration needs to be given on whether to remove the kiln shelf when slumping.  Research implies that increased cooling of the bottom of the glass may go outside the parameters for the even cooling of the glass.

Multiple Firings of Kiln Wash

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

We all know that kiln wash fired a second time to full fuse is likely to stick to the glass.  We also know that kiln wash fired to slumping temperatures lasts almost indefinitely.  Somewhere between the two temperatures the kiln wash undergoes a chemical change that makes it more likely to stick to the glass on the next full fuse firing. 

credit: Immerman Glass

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

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

credit: Sue McLeod Ceramics

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

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

       

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

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

Removing kiln wash

Applying kiln wash

Large Bubbles

As you move up from smaller pieces to pieces that occupy most of the shelf, you sometimes begin to get large rounded bubbles at tack fuse and burst ones at full fuse.

Image from B Stiverson

You have to go back to basics to discover the cause.

Schedule

It is not likely to be the schedule. It has worked for smaller items. But it is important to review the schedule.  Is it like others you have seen? Is it similar to what the glass manufacturer recommends?  Both these will reassure you that the schedule is OK, if not perfect, or to revise it.

Cleanliness

Going back to the basics relates to the cleanliness of your kiln, among other things.  Even a small speck of material under the glass can result in a bubble. Although the grit lifts the glass off the shelf only a fraction, as it heats up the glass slumps around that and creates an air pocket.  That grows as the glass heats up and creates a large diameter bubble. If there is no grit in evidence, you need to check another element of your kilnforming practice.

Shelf

The large bubble might often occur in the same relative place in the kiln, although different places on the glass pieces, depending where they are placed.  This is an indication that you may have a hollow in the shelf. It may not have been obvious with smaller pieces.  You need to check the shelf with a straight edge. If any light is seen between shelf and edge, you have a depression in the shelf.  It may only be a sliver of light, but that indicates a depression which is enough to create a large bubble. That must be fixed.

Image from Suze

There are temporary and permanent fixes for avoiding bubbles due to depressions in the shelf. 

The temporary fix is to use 1mm fibre paper on the shelf, to allow air out from under the glass.  This can be topped with Thinfire or Papyros. Alternatively, a thin layer of powdered kiln wash can be smoothed over the fibre paper to give the smoothest back possible in the circumstances. You can use a plasterer’s float, or simply a piece of float glass.

The permanent fix is to sand the shelf smooth and level.  A method for doing this is here.

Single Layer Bases

If you are firing with single layer bases, there may be nothing wrong with the shelf.  It is typical in tack fusing to use single layers with glass placed decoratively around the surface of the base.  This leaves gaps where the base glass is exposed.  Even though the whole piece may survive the differential heat up of the exposed base glass and the covered parts, there is the possibility of creating an air pocket under the exposed base.  This comes from the weight of the stacked glass pressing any air out to the side.  If the design is unable to provide a route out for the air, the possibility of creating an air bubble increases.

It is possible to create conditions to reduce the possibility of these large bubbles developing. 

One solution is to use a layer of fibre paper as for a shelf with slight depressions.  This allows air out from under the glass, even with a single layer layup.

The other solution is to change the rate and temperature of the firing.  By using the low and slow principle, you can reduce the risk of bubbles.  Use a much slower rate of advance to a lower temperature with a longer soak you can achieve the look you want without bubbles.  This utilises the concept of heat work.  It does require observation to determine when the effect you desire is achieved and then advance to the next segment.

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

Kiln Maintenance

Switch off the kiln before doing any maintenance.

Before or after each use

Vacuum the inside of the kiln. Use a low suction setting, especially on fibre walls and ceilings. Stronger suction is possible when cleaning a brick floor.

Example of vacuuming around elements

Example of vacuuming lid without elements

An alternative to vacuuming the elements is to use the air compressor hose at low power to gently blow out any dust settled in the element grooves.  Do not do this for fibre insulated kilns, only brick.

Check on the kiln furniture – including shelves, boards, supports. Are they kiln washed and without scrapes, scratches, gaps? Has the kiln wash been fired to full fuse temperature? In both cases, clean the used kiln wash off the shelf and renew.

Check that the shelves and other kiln furniture are without cracks.

Clean kiln furniture of dust and debris.

Check the level of any item newly placed in the kiln - e.g., mould, or shelf replacement - with a spirit level.

Two examples of two-way spirit levels

 Check on the conditions and placement of the thermocouple.

Check on the elements.  Some may be sagging or hanging out of their channels.  Use tweezers to bring the coils closer together.  This shortens the length of the element and it then can be pushed back into the channel.  It may not have to be done after each firing, but checking will catch things before sagging becomes a major problem.

When the shelf paper is exhausted lift out the thicker papers and vacuum the shelf.  The Thinfire and Papyrus papers can be vacuumed directly or gently swept up and placed in a container for disposal.  Do not introduce any moisture to help reduce the dust.  This is not good for the kiln or you, as it could induce shorting out of the elements.

Monthly

Electrical parts: check the elements and their connections (normally at back or side).

First unplug or switch off the power to the kiln.

Check the screws on the connectors for the element tails are tight. Loose connections cause the wire to vibrate at the connection during the power phase. They heat up enough to melt the wire at the connection. For a single element kiln, it will simply lose power.  In multiple element kilns the remaining elements work much harder to achieve the temperature and provide uneven heating.

If the connectors are badly corroded , they need to be replaced.  This can be done without replacing the elements. Unscrew the connectors and put new ones on.  If the connector is fused to the element wire, you need to cut the wire as close to the connector as possible to maintain a length of wire for the new connector to be fixed.

Check the condition of leads and plugs supplying power to the kiln.  Make sure they are sound, not frayed and not kinked. Replace any frayed parts.  Take out any kinks in the power supply cable.

Any support pins or wires should be firmly seated in the brick work or supported by sound hangers.

Check the level of the kiln floor and internal shelves on a regular basis and every time the kiln and its internal furniture is moved.

Making a schedule of maintenance checks and noting on it the dates checked is a good idea for those who need reminders.

Equalising Effects on Both Sides of the Glass in the Same Firing

The desire is to have the same degree of fusing on both sides of the glass.  An example is where a person wants to have their colourline paints equally matured on both sides of the glass in one firing.  This is difficult and requires a different strategy than normal fusing.

Background

A bit of background first. Glass is a very good insulator. This means that heat travels slowly through the glass. Its practical effect is that we have wavy lines on the top and very crisp lines on the bottom.  This results from the temperature differential between the two surfaces.  This can be many degrees different during the plastic phase of the glass.  It is dependent on how fast the temperature rise is.  The faster the rise in temperature, the greater the difference as the glass transmits the heat from top to bottom so slowly.  The problem is how to keep the temperature differential as small as possible.

Heat Work

The concept of heat work relates to the way heat is put into the glass.  It can be done quickly to a high temperature, or slowly to a low temperature and still get the same effect.  This shows glass reacts to the combination of temperature and time. Putting heat into the glass slowly allows lower temperatures to be used to achieve the desired effect, than fast rises in temperature.

The insulating properties of glass means that the heat work needs to be applied slowly to achieve similar temperatures on both sides of the glass.  The thicker the glass the longer it will take to temperature equalisation.

The mass of materials also needs to be considered.  The glass will normally be on a ceramic shelf of 15mm to 19mm.  This mass also needs to heat up to the temperature of the top of the glass.  Until it does, it will draw heat from the glass.  This also points to the need for slow heat input.

The question that prompted this note was how to get glass strainers paints to have the same degree of maturation on both sides at the same time.  The maturation temperature of Reusche tracing paints is around 650°C.  If you use a normal rate of advance – say, 200°C – the bottom of the glass will be considerably cooler than the top.  This is both because of the insulating properties of the glass and the mass of the shelf.

Methods to achieve the effect.

Some methods are worthy of consideration separately or in combination.

Use refractory fibre board as shelf.  This dramatically reduces the mass of the shelf to be heated up.  This kind of shelf requires more care to avoid damage than a ceramic shelf.  It would be possible to place smaller fibre shelves on top of the standard ceramic shelf rather than having one large fibre board shelf.  This will not be so efficient an insulating mass as fibre board on its own.  Also, it will not be sufficient on its own to obtain equal temperatures on both sides of the glass.

Use 3-6mm refractory fibre paper between shelf and glass.  This again reduces the heat sink effect of the ceramic shelf, but not as much as a fibre shelf on its own.  Again, the fibre paper on its own is not enough. The scheduling is important.

Use very slow rates of advance.  A slow rate of advance in temperature is important to achieving equal temperatures throughout the glass.  Even using 3mm glass, the rate of advance might need to be as slow as 50°C per hour.  The corollary of this is that you will not need to use as high a temperature to achieve the effect.  Heat work means that it is not an absolute temperature that will achieve the effect.  The slower you put the heat into the glass the lower temperature required.  The understanding of this relationship will require experimentation to establish the relationship to the rate of advance and the top temperature required.  For example, a satin polish of a sandblasted surface can occur at 650°C, if held there for 90 minutes.

In this case, a 50°C rate of advance will probably not require more than 600°C – and probably less - to achieve the shiny surface normally achieved at 660°C with a 200°C rate of advance.  At 50°C per hour, it will take 12 hours to reach 600°C, although a little more than 3.25 hours at an advance of 200°C to reach 660°C.  The input of heat acts upon the glass throughout the process, making lower working temperatures possible.  The reduction in temperature required is not directly related to the reduction in the rate of advance.  You will have to observe during the experimental phase of this kind of process.

If it was desired to fire enamels that mature at 520°C to 550°C you could put the sheets in vertical racks to allow the heat to get to both sides equally as Jeff Zimmer does.  But this will only work for very low temperatures and for quick firings, otherwise the glass will begin to bend.

There are limits to this strategy of getting upper and lower surfaces to the same temperature, both in terms of physics and practicality.  There are temperatures below which no amount of slow heat input will have a practical effect, for example,  due to the brittle nature of the glass.  Even where it is possible, it can take too long to be practical.  For example, I can bend float glass at 590°C in 20 minutes into a 1/3 cylinder.  I could also bend it at 550°C (just 10°C above the annealing point), but it would take more than 10 hours – not practical.

How Close to the Edge

“How close to the edge of my shelf can I place a large piece?”

It depends in one sense how thick the piece is.  A 6mm piece that maintains the same footprint after firing as before, does expand beyond that footprint by about half a centimetre during the firing, so it would be safe to have a full centimetre space to the edge.  Thicker pieces will need more space – 9mm will need about two centimetres to accommodate the expansion at the top temperature. 

But

The real answer to this question is: When you know the heat characteristics across your shelf, you will know how close you can go to the edge for a relatively large piece. 

This Bullseye Tech Note number 1 tells you how to test the variations of temperature across your kiln. - http://www.bullseyeglass.com/methods-ideas/technotes-1-knowing-your-kiln.html

The objective in cooling glass is to have less than a 5C difference in temperature over the whole of the glass piece – top to bottom, and side to side.

If you have greater differences in temperature than that at the edges of your kiln shelf, you need to avoid placing large pieces in the danger area. Small pieces will not suffer by being close to the edges as their temperature differentials will be small.

I have found that the temperature differential in one of my kilns is great enough at the edges that I cannot have the edge of a relatively large piece of glass nearer than 50mm (2") from the edge.

Sand Beds for Kilns

Sand beds can be used for shaping directly into the sand or as a bed for a large kiln to avoid having to place a number of shelves together and patch cracks. It also saves on the purchase of a large shelf, at the expense of some labour before each firing.

Mix (by weight)The mix is largely sand with powders to coat the sand particles to reduce the amount of sticking to glass. The mix ranges from 75% sand 25% alumina hydrate, up to 67% sand, 33% alumina. The proportions can be adjusted by experience.

Some kiln formers include plaster or china clay (kaolin) in various proportions so the whole mix can range from 70% sand, 25% alumina, 5% plaster/kaolin up to 60% sand, 20% each alumina and plaster/kaolin. Some have found that the plaster started to scum up the glass after a number of firings, and this can happen with china clay too.

UseThe use of sand as the bed of the kiln requires screeding of the sand before each firing. It is possible to smooth the sand with a plaster’s float. This presses the sand down and allows a fine film of powder to coat the surface. A more certain way of avoiding the sand sticking to or texturing the glass is to lay fibre paper over the sand.

You should be aware that a sand bed requires a little different cooling than glass on a suspended shelf does. Effectively you are cooling the glass on the bed of the kiln. Thus, you should use annealing and cooling schedules for one or two thicknesses more than is actually being fired to be really safe.

Sand formingIf you are using the sand for mild shaping, sprinkle alumina hydrate over formed sand to reduce sand particle take up and textures onto the glass.

Renew sand regularly if you are using powders on the sand. An annual renewal using half the old sand mix and the rest new will be sufficient in most cases. The effect of too much powder is to promote large bubbles as air cannot move through the sand with the same ease as when there is less powder. An easy way to tell if you have too much powder is that the mix flows ahead as you screed it.

Revised 22/6/19

Applying kiln wash

Applying kiln wash to shelves and moulds have the same requirements.

The kiln wash must be applied evenly

You can use a soft bristled brush like a hake and trail on the kiln wash in four directions – top to bottom, side to side, and diagonally left to right and right to left.

The pigmented kiln wash turns white, indicating that the shelf has been fired

You can also spray the kiln wash onto the shelf or mould. This can provide an even coating, but you must be careful to avoid puddling the kiln wash. Pause for a few seconds after each coat to allow the water to be absorbed before applying the next coat. Apply until the shelf or mould surface is not obvious through the kiln wash. There is no need to dry between coats.  The best coverage is achieved by applying all four coats in different directions at the same time.

You can use a sponge to apply to flat surfaces. A light touch is required. You need to apply in various directions as with the brush.    

Metal moulds are a special case. The water carrying the kiln wash will simply bead up or run off the metal. First clean the metal to get rid of oils and dirt – sandblasting will do well. Then you need to heat the metal to about 125C – 150C (ile., above boiling) and brush or spray the kiln wash onto the mould, one coat at a time. Return the mould to the heat source and then apply another coat of kiln wash. Do this until you have an even layer of kiln wash. Be careful not to put so much liquid on the mould that it begins to run. If this happens, you really need to start again.  Also if the metal is too hot, the water will boil off, leaving bare patched.

Apply the kiln wash thinly

You need enough kiln wash to separate the glass from the carrying surface. Any additional kiln wash will not make for a better separation, but may begin to flake off the surface and adhere to the glass more or less tightly.

The usual recommendation for brushing is one part kiln wash to five parts water. I recommend ten parts water to one part kiln wash if you are spraying the kiln wash. If you have a really absorbent surface, such as a vermiculite mould, you can reduce the water to two and a half parts water to one kiln wash. All these measurements are by volume.

The kiln wash finish must be smooth

There are several ways to smooth the surface.

You can rub your hand over the shelf or mould to remove high spots/streaks. You need to remove the dust before using though.

You can smooth the surface using a rolled up nylon stocking. This relatively open weave allows the powder to be captured in the material. It works well on irregular surfaces like a mould. Again you must clear off any remaining dust.

Another way is applicable to flat surfaces. After applying the kiln wash, but before it has dried, make sure the surface is level. Then brush or spray on a layer of hot water. This both puddles and evaporates quickly, leaving a smooth surface on thinly applied kiln wash. If the kiln wash is thick, the drying process will leave cracks as in a dried-out river bed.

Multiple firings of kiln wash