Crazing

Crazing appears as the multiple cracks similar to what is seen on ceramic glazes.  These occur when there is a great deal of incompatibility between the glaze and the clay body.  This can also be seen in glass.

Crazing as seen on a ceramic object

I have see crazing of glass in two circumstances.  It happens with severe devitrification, to a maximum extent of crumbling under light pressure.  This usually happens with glass not formulated for fusing, and especially on opalescent glass.

The more common occurrence is where the glass has stuck to the supporting structure.  This is frequently the case where the separator has not been sufficient to keep the glass from sticking to the shelf.  This will happen on kiln washed shelves when the coating of the separator has not been even, leaving areas with bare or very thin areas.

The standard of mixing kiln wash in the ratio of 1:5 parts by volume of powder to water is important.  The application should be with a wide soft brush such as a hake brush.  The kiln wash should be painted on in four coats, one in each direction of up, down, and the two diagonals.  A well coated shelf should have an even appearance of the coating.  Only an even film of separator is required to keep the glass from sticking to the shelf, mould or other kiln furniture.

Drying Kiln Washed Moulds

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

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

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

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

newly kiln washed mould beside others already fired

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

Kiln drying ceramic slumping and draping moulds is not necessary. It only adds another, unnecessary, step in kiln forming.  There are exceptionally good reasons to avoid rapid firing of damp moulds. The exceptions can occur with texture moulds and those intended for casting that do not have vents.

Pot Melt Contamination

Pot melting occurs at temperatures above that for which kiln washes are designed. This means the kiln wash most often sticks to the back of the melt.

If you put only fiber paper – Thinfire, Papyros, or standard 1mm or 2mm fibre paper – at the bottom, the dripping glass will tear and move it about.  It also tends to incorporate fibers from the refractory papers into the melt.  It is best to avoid fibre papers of any kind on the base.  Using fibre paper around the edges of dams, if you use them, is better than simple kiln washing of the dams.

From wikihow

If you have a sandblaster, it is easy to take the kiln wash off leaving a matt surface. You can live with this for many purposes, but if you want a more polished surface you can take the melt up to fire polishing temperature to shine up the surface. You will need to flip this over and fire again, if the original top surface is what you want to present.  Or if you like the new shiny surface, use it as is.

If you are going to cut the pot melt up for other uses, there is no need to fire polish as the surface does not matter, only the cleanliness, and removal of contaminants.


There is another thing you can do to avoid kiln wash contamination.

The best solution appears to be to put a disk or rectangle of glass on top of fibre paper. It can be clear or any colour you wish, but needs to fill the area enclosed by the dams. This seems to keep the fiber paper from tearing and being incorporated into the glass, even though the base will have the fibre paper marks.

It also works very well when you are confining the melt to get a thicker disk. Make sure you have kiln washed the sides of the container or dam very well, in addition to 3mm fibre paper arranged so that it is 3mm narrower than the expected final thickness, or any excess glass may stick to the dams. The means of arranging the fibre paper around the dams is given here. You may need to grind the marks off the edge of the disk, but this is much easier than grinding it off the bottom.

Bubble Mystery

A question was asked about a collapsed bubble. There were two pieces in the kiln and one (strips) was fine and the other (flat plate) had the collapsed bubble.  Both on the same dried shelf.  The question also asked if the collapsed bubble piece could be flattened by fusing again.

Collapsed Bubble

The bubble collapsed because it had not burst by the time the cool toward annealing had begun.  As the air pressure under the bubble dropped, and the weight of the thinned glass bubble sank down as there was not enough air pressure to hold it up.

The glass is now thinner at the centre of the bubble than the main part of the piece, and thicker at the edge of the bubble. I don't think it is possible to successfully flatten it to become an even thickness across the whole piece. To get the same thickness across the whole piece would require high temperatures and long soaks there. 

Another possibility is to use a pressing solution. 

My suggestion is to add elements or repurpose it. I don’t think any repairs would present a good-looking piece.

Diagnosis

The on-line diagnosis of the possibilities for the cause of the bubble was extensive and sometimes inventive.  It was finally determined the bubble was from under the glass, that is, between the glass and the shelf. A slight depression in the shelf is the usual explanation.  The user tested the shelf for smoothness and found no depressions.

It was clear the bubble came from under the glass.  All the suggestions about how bubbles can form under glass were given, but none seemed to apply.

How can you get a bubble on a dry shelf that is perfectly flat and that has not been subjected to too rapid or too high a temperature?

Solution

The answer is that a little spot of grit or tiny ball of fibre paper can keep the glass raised up enough for air to be trapped.

Prevention

It is not enough to test the shelf is flat.  You need to use clean kiln wash with a clean brush to avoid any grit being brought to the shelf. It is also a good reason to vacuum the shelf before each use in case any dust or grit has fallen onto the shelf. Covering the shelf or putting it into a cupboard will also reduce the possibility of small bits of grit falling onto the shelf.

Of course, if you smooth the kiln wash with a nylon or similar fine cloth, you will remove any specks of grit.  A vacuum of the shelf after smoothing is still a good idea.

Conclusion

It is as important to keep tools and materials clean as it is to clean the glass you are going to kilnform.

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

Kiln Washing Kiln Surfaces

“Having just got my first kiln, I was wondering how often I have to add kiln wash to the bottom of the kiln.”

It has become common practice to kiln wash the bottom of a new kiln.

This may be fine for brick lined kilns.  Kilns with fibre blanket or fibre board do not need to be kiln washed at all.  The fibre is a separator already and does not need additional material which will turn to powder and need to be carefully cleaned to avoid damaging the refractory fibre lining.  This has led me to reconsider the value of kiln washing the bottom of the kiln.

I have followed the practice kiln washing of the bottom of the kiln in the past.  However, I have found that small glass pieces falling to the floor, do not stick to the bare brick or to fibre.  They can be vacuumed or picked from the surface of the kiln without creating any damage.  This means that at fusing temperatures, the brick and fibre does not stick to the glass and kiln wash is not needed.

The main idea seems to be to help protect the kiln surfaces from the molten glass if a relay becomes stuck, raising the kiln temperature to very high levels. Kiln washing the bottom of the kiln does not protect the brick or fibre from a large amount of glass running off the edge of the shelf onto the bottom. Of course, pieces of glass resting on the floor of the kiln may become stuck when higher temperature work is being conducted, such as combing, the various melts, and casting.  The solution is not to kiln wash the floor, but to clean the floor of the kiln before entering the high temperature processes.

There is not really a need to kiln wash the bottom of the kiln at all.  The kiln wash will not protect the kiln brick or fibre in the event of a high temperature accident.  The kiln wash turns to powder which needs to be cleaned from the kiln to avoid contamination, as with other dusts, of the glass being fired.  The main objective is to keep the kiln clean and free of dust rather than adding another source of dust.  A dusty atmosphere in the kiln can promote devitrification, so anything which avoids introducing dust will be beneficial in reducing the incidence of devitrification.

I suppose if you really want to protect the bottom of the kiln from molten glass, you can add a high temperature separator such as a refractory fibre board, or a thin layer of sand.  The sand will resist the molten glass and can be scooped out of the bottom before adding new. 

After some consideration, I no longer think kiln washing the bottom of kilns is worth the potential for dust accumulation, as it doesn’t really protect the kiln floor during high temperature accidents. Low temperature spills of frit, glass powder and shards will lift off the kiln surfaces easily without damage to surfaces.

Kiln Elements - Aging

As elements age, they generally increase in their resistance. This increase in resistance decreases the amount of amperage and, so, the amount of heat given off by the elements. This explains  why older kilns sometimes go so slowly and may not reach their maximum temperature.

There are several factors which affect the longevity of elements and so have implications for firing practices.

• ·        Contaminants such as silica which is contained in kiln wash and some glazes attack the aluminium oxide coating of the wire.
• ·        Allowing the wires to become tightly wound increases overheating of sections of the element.
• ·        Powders, paints and kiln wash accidentally touching the elements cause rapid corrosion of the elements if not cleaned off before firing.
• ·        Firing close to the elements allows fumes to contact the elements.
• ·        Subjecting elements to reducing atmospheres will age the elements quickly.  This would be done by introducing organics or oils into the kiln without venting.  Among the things that will attack the aluminium oxide coating of the elements are carbon, wax, halogens (such as chlorine or fluorine), molten metals (such as zinc, aluminium), lead glazes, alkaline metals, borax compounds.

All these elements attack the element coating.  And each time you fire the slight difference in expansion between the core of the wire and the coating creates cracks in the coating.  The exposed core forms new coating to fill the gaps.  This over time reduces the thickness of the element wire.  As the wire thins, the resistances increases, causing more fissures in the coating to occur, accelerating the aging process.

The next in this series is about how firing practices can affect the life of elements.
Firing Practices

Other relevant posts:
Nature of elements
Maintenance

Kiln wash

When considering how many layers of kiln wash to put on shelves, especially in melts, combing and other high temperature operations, you need to remember what the kiln wash is doing – what its purpose is. 

Kiln and batt wash, shelf and mould primer are all different terms for the same thing – a separator between the glass and the kiln furniture or mould.  The amount needed is enough to completely cover the shelf. This is usually 4 coats - one top to bottom, one side to side, one each diagonal.  If you are spraying the kiln wash, use a coloured kiln wash to help ensure coverage. The shelf is adequately covered when the shelf is a uniform colour although a sense of the original remains while the kiln wash is wet.  Additional coats do not provide additional protection. The disadvantage of thick coatings of kiln wash is that the excess tends to stick to the glass as it is lifted from the shelf or mould.

This post gives guidance about the methods for application of kiln wash.

Multiple firings of kiln wash

Batt Wash Sticking to the Glass

The main reasons that kiln wash sticks to glass are:

1. Firing at too high a temperature. The higher the temperature, the more likely the kiln wash will stick to the glass.

2. Firing with opalescent glass against the shelf. Kiln wash sticks to opalescent glasses more easily than to transparent glass.

3. Re-using kiln washed shelves that have been to fusing temperatures already.

4. Using kiln wash with high amounts of china clay makes for more sticking. Thus some brands stick more frequently than others.

Firing at too high a temperature is probably the worse culprit. The second is using opalescent directly on the kiln shelf.


Strategies to avoid this sticking are:

1. Fire at the lowest temperature you can to get the result you want. This often requires slow rates of advance and extended soaks at the working temperature

2. Use Bullseye kiln wash. It is among the best.

3. Have a transparent glass as the bottom layer.

4. Use iridised glass, with the iridised side down to the shelf, as the iridisation acts as a separator. Do not do this with Thinfire, as it can lead to large cavities in the glass.  Fire onto kiln wash.

There are ways to get the kiln wash off but it's easier to avoid it. Using an iridised sheet on the bottom is probably the most effective prevention.  

Multiple firings of kiln wash

Slip cast moulds

Hard spots in some moulds are the result of the method of creating the moulds. Most of the ceramic moulds we use in kilnforming are slip cast.

This diagram shows the main stages of slip casting

Slip casting is a way of quickly producing multiples from a mould.  The original shape is surrounded by a one - or multiple - part plaster mould.  This mould is used to contain the clay slip which is poured in.  

The plaster absorbs water from the slip, stiffening the clay in contact with the plaster. After a defined time, the remaining slip is poured out of the mould.  The clay remains in the mould a short time until it begins to contract from the plaster mould and is described as leather hard.  

It is then de-moulded, trimmed and cleaned before it is further dried.  When appropriately dry, it is fired.

Some moulds we receive show a spot where the kiln wash does not cover the surface in the same way as the rest.  This is a result of the method of pouring the slip into the mould.  Slip that is hand poured does not fall in the same place for long.  But industrially poured slip often falls in the same place for the whole of the pour.  This creates a hard spot - an area where the slip is more compacted than the rest of the object.

This hard spot does not affect the appearance or performance of the object.  However, it does not absorb the water from the kiln wash as well as the other areas. And this is when the hard spot becomes apparent. It will still have enough separator to keep the glass from sticking, although visually it appears bare. If concerned, you can coat that area more than the rest after the kiln wash has dried a little.  You need to be careful that you do not introduce an unevenness into the kiln washed surface, as that might appear on the slumped surface of the glass.

Smooth Kiln Wash on Shelves

There are a number of ways of applying separators to the kiln shelf.
These go by a variety of names - kiln wash, shelf primer, batt wash, etc. - all are separators to keep the glass from sticking to the shelf. They are all combinations of alumina hydrate and china clay (or kaolin or EPK) in various amounts.  The china clay provides a high temperature binder for the alumina hydrate which does not stick to glass.

These are some examples of glass separators.  The Primo Primer has very little china clay, and is easy to remove.  It is particularly good for small casting moulds.

    

The object in applying the separator is to achieve a smooth surface a possible. Remember there will always be some texture because of the particle size of the wash.  For the smoothest surface, use the finest powder you can find.  You can, if you want to spend the time and effort, put the powder into a rock tumbler with ceramic balls to get an even finer powder.  Avoid shelf primer that is intended for ceramics, as it is coarser than that sold as a separator for glass.

It also is important to prepare the mixture some hours before application to ensure all the particles of the powder are wetted.  Immediate use often leads to a gritty surface.

There are several methods for applying the kiln wash to the shelf.  The two I use are spraying and brushing.  Which I use depends on circumstances - spraying requires more set up time.


Spraying the separator onto the shelves can give an even coating without brush marks, runs or ridges.  In this example a mould is being sprayed.  To ensure an even covering on a shelf, it should be horizontal and leveled so the kiln wash is evenly distributed.  Numerous light passes with the sprayer is best, as in air brushing.

Applying the kiln wash with a very soft brush such as a hake brush in a variety of directions will ensure full coverage. 

The brush should lightly touch the shelf and provide a number of thin layers.  Applying in four directions - horizontal, vertical, and the two diagonals will ensure full even coverage. There may be some residual brush marks.

To reduce the application marks further, you can brush or spray hot water over the still damp kiln wash. This helps to remove brush marks or the stippling that often comes from spraying and brushing.  It is important that the shelf is perfectly level for this operation.


Another way to reduce the texture after the shelf primer dries is to lightly polish the kiln wash with a ball of old nylons or rub a flat piece of paper with the palm of your hand over the shelf.  Be sure to remove the dust that may be left behind from this polishing.


Boron Nitride

Another separator that has become popular in spite of its expense is boron nitride, often referred to by the trade name Zyp.  This is a high temperature lubricant for industrial kiln operations that has been adapted for the generally lower glass forming temperatures.  This is not suitable for kiln shelves, as it completely seals the porous surface of the shelf.  It is difficult to go back to the cheaper kiln wash separator as the water of the kiln wash solution will not be absorbed into the shelf, leaving a patchy coverage of the kiln wash.  Although both separators should be renewed after each firing (above low temperature tack fusing) the boron nitride is much more expensive and cannot provide a smoother surface than the shelf already has.  My recommendation is that boron nitride use should be confined to moulds or other surfaces where the glass may slide or move in the forming process.

Kiln Wash

Kiln forming techniques require separators between the glass and the shelf or mould on which it rests during the heating process. These separators have different generic names – kiln wash and batt wash are two.

There are a number of brands of kiln wash. All of them contain two main ingredients – alumina hydrate (sometimes called slaked alumina) and kaolin (also called china clay). Different producers use these ingredients in various proportions. 

A number of makes also include a colourant that changes when fired above certain temperatures to indicate the wash has been fired.  It also distinguishes between the unfired and the already fired kiln washed shelves.

An important thing to remember is that the kaolin changes its composition once it is fired over 600C/1113F. This change of composition is completed by 900C/1620F.  The change is progressive.  It is so slow that slumping and draping moulds coated with kiln wash will last indefinitely. However this change is great enough by 770C/1419F that the kiln wash sticks to the glass on the next firing. Thus, it is essential to change the kiln wash after every firing that reaches tack fusing temperatures or higher.

It is possible to apply a fresh coat of kiln wash over the old one to save time. However, as soon as the kiln wash flakes you must scrape off all the old kiln wash and apply a new coat to the bare shelf or mould.

Some makers use much less of the binder (china clay) than others which makes them better for the popular casting moulds than those for shelves and slumping moulds as they can be brushed away without abrasion.


In addition, boron nitride is a suitable release from moulds.  It is very stable at reltively high temperatures and so can provide a smooth, "slippery" separator between the glass and its supports, whether shelves, moulds or kiln furniture.  It does seal porous surfaces, meaning that air cannot move through the treated surfaces.  It has to be removed with abrasion and so thought must be given to which surfaces it is applied.

Kiln Wash Removal

There are a variety of ways to remove kiln wash.  Many depend on whether the surface is flat, smooth curves, angles or textured.  Some are applicable to both.

Flat surfaces are the easiest to deal with.

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 as that will, almost certainly, create small depressions in the surface of the shelf.  Sand blasting is only possible on steel moulds.

Wet

Wet methods are applicable if you are concerned about the dustiness of the process.  You can dampen the kiln wash on the shelf and sand or scrape as above.  You will create a paste or slurry in front of the scraper which can be bagged and put in the waste.

You can also use a lot of water and the green scrubby washing up pads.  Unless you use a lot of water, the kiln wash builds up in the scrubbing pads.

Some people use vinegar or chemicals such as lime away with the water. Both are acids – lime away being much the strongest.  I am sure these are used on the basis that kiln wash is based on lime.  However, the material that makes the kiln wash stick to the shelf is china clay which is barely affected by the chemicals.  In addition, the alumina hydrate is impervious to many chemicals available to kiln workers.

One drawback to using wet methods, is that the shelf is wetted and needs drying before use.  The amount of water used in applying kiln wash is minor in relation to washing or soaking the shelf to remove the kiln wash.

Do not be tempted to use pressure washers. Yes, they will remove the kiln wash, but also leave little divots in the shelf which will cause later problems.

Smooth curves

Kiln wash on moulds with smooth curves can be removed with flexible sand papers or the plaster board sanding screens.  Normally, the coating of kiln wash is thin and does not require a lot of pressure or effort.

It is possible to dampen the kiln wash and take it off with scrubbing pads.  Make sure you do not use excessive pressure.  If you have wetted your ceramic mould, you need to dry it very carefully, to avoid having the mould break in the next firing.  This is because trapped water can turn to steam and the pressure will break the ceramic. It is best to let the mould air dry for a week or so before putting it into the kiln to thoroughly dry at about 90°C for a couple of hours.

Do not be tempted to use a pressure washer or water pick, as both can erode the surface of a ceramic mould.

Curves with angles

Moulds with angled areas such as at the bottom or corners of a rectangular mould need a flexible abrasive to clean out the angles.  You can fold a piece of sand paper to use the folded edge to do the final cleaning out of the angles.

The same can be done wet, but all the precautions about wet removal of kiln wash need to be observed.

Textured

Textured moulds require much more care in cleaning the kiln wash away, to avoid damaging the images and textures.  The flat upper surfaces can be dealt with as though it was a flat kiln shelf.  The indentations need to be more carefully treated.  Folded pieces of sand paper can be used to clean the delicate areas.

To ease cleaning of textured moulds it seems best to use kiln washes without china clay as the binder.  These will brush out of the mould with a fibreglass bristled brush.  It is now popular to use boron nitride - often sold as Zyp - as a coating for these moulds.  This needs to be brushed out and renewed with each firing.

Removing kiln wash from glass

Kiln wash stuck to the glass can present greater problems, because you want to avoid marking the glass.  It is best to start with the least aggressive abrasive, such as a green scrubby, and progress toward more aggressive and abrasive methods.  When using the more aggressive methods, try the finest grit first to see if that will work, as it makes for less work cleaning up the grinding marks from the glass.

For flat glass, you can work with a succession of finer loose grits, or a succession of finer diamond hand pads.  

Flexible diamond impregnated sheets can be used for curved surfaces.  Again, this requires a succession of finer grits to get to the polished stage.

You can use small hand held rotary tools with diamond and felt pads to polish out stuck kiln wash.  This helps to remove some of the labour of polishing the glass.

Some people advocate the use of acids to remove the kiln wash.  However, you must remember that glass is an alkaline material and acids will tend to mark the glass.  Vinegar is a mild acid, but prolonged exposure will etch the glass.  Strong chemicals such as lime away or etching cream or hydrofluoric are all strong acids and will mark the glass after brief exposure to them.

High Temperature Wire for Screen Melts

You can use high temperature wire for screen melts. This is variously described as Kanthal or nichrome wire.  It is the same kind of wire used in the heating elements of your kiln.

wire with bent ends

To use the wire, you lay or weave the wire and support it on both ends.  Weaving the wire provides more support, but is not necessary, as the wire is strong enough to support a lot of glass.

first line of wires pushed into board

You need to have the wires as tight as the supporting material will allow. Straightening the wire before beginning to fix them will help, as will thicker wire.

The wires need support at each end, which can be brick, cut up shelves, or strips of tile.  If you do this, you can form a dam or vessel in which to put the glass without fear of it spreading over the edge.

I use fibre board for the support and just bend a right angle into each end of the wire to push into the board.  These can be arranged in any configuration, although for ease of illustration, I have used a rectangular arrangement of wires.

A grid of wires ready for kiln wash

Put the completed screen over a tray or sheet of plastic to collect the excess kiln wash.  Mix the kiln wash very thick and pour over the wires. I put the board with wires into the kiln to heat to about 200°C to help the wash stick.  I repeat a few times.

Make sure you coat the area surrounding the screen to avoid the glass sticking to the supports.

When the kiln wash has dried, knock off the stalactites of wash on the underside of the wire to prevent any excess kiln wash being incorporated into the final piece.

Place on kiln washed supports, and put the glass on top of the screen.

This is a relatively quick and inexpensive means of providing a custom shaped screen.  

One disadvantage of this over stainless steel rods, is that it is difficult to get enough kiln wash to stick to the wires to be able to pull them out easily.