Diagnosis of Cutting

If your scoring and breaking of your glass is not going well, you need to diagnose the reasons.  There are always a lot of suggestions that warming the glass will solve the problem. Yes, warming glass may help. A discussion of the effect is here. But it will not overcome any faults in the basic skills of scoring.

A lot of images, shown on the internet, of straight line scores failing to break along the score, indicate some possible elements in scoring that lead to these unwanted break-outs. 

One possibility is you are using too much pressure. A discussion of the amount of pressure required is here.  You should be scoring to the pressure required, rather than any sound that may come from scoring.  This is emphasised when cutting opalescent glass.  The correct scoring pressure makes almost no sound or only a gentle rumble as it cutter moves over the undulations of the glass.  The most frequent reason for more difficulty in breaking opalescent glass is excessive pressure while attempting to get the same sound as from transparent glass.  There are even a few transparent glasses that make little or no sound when being scored with the correct pressure.

Another common problem in scoring is keeping an even pressure throughout the score.  It can be difficult to keep the pressure even on complicated cuts.  When the cartoon has multiple curves or deep concave lines, it can be difficult to keep the pressure even as you move your body around to follow the line.  One piece of advice I received early on in my learning was to rehearse the score allowing the cutter wheel to move along the score line with virtually no pressure.  This shows how the piece of glass needs to be oriented to ease your movement around the glass to make the score.

Slowing the cutting speed can help to keep the pressure evenly distributed along the score.  Straight lines are often scored quickly.  But, even on straight lines, slowing the speed can make the pressure more even throughout the score.  It can also avoid variable speed during the scoring, which leads to different forces being placed on the glass.  The pressure may be consistent, but the effective pressure is greater when slow than when fast scoring is used.  If the speed is variable, the effective pressure differs along the score line.

A fourth thing that may be happening on straight lines is that the cutter wheel is at an oblique angle to the direction of the score.  This will often be heard as a scratching sound as you move along the score line.  This can be overcome by a gentle pressure against the straight edge you are using to align your score.  Of course, the straight edge needs to be held firmly to avoid having it move.  Allowing the head of the cutter to have a little freedom of movement also helps keep it parallel to the straight edge.

All this is merely speculation about your scoring practice.

You need to get someone to observe you scoring.  They do not need to be experts, nor other glass artists.  They just need to be observant. Tell them what you are looking for in each of the four elements of scoring and have them observe only one thing at a time.

First get scales that you can zero when you have a small piece of glass on it. Score without touching the glass. Have the observer tell you if the pressure was consistent throughout the score, and if you are in USA, whether the pressure was above 7 pounds or below 4 pounds. (For the rest of the world 3kg to 1.8kg). Practice until you can score consistently at about 2.2kg (ca. 5 pounds).

Second, have the observer stand a little distance from you. Score toward the observer. They need to observe whether your cutter is perpendicular to the glass while scoring and if there is any variation.

Next, they need to tell you if your head was directly above the cutter all the way through the score. They will be able to see whether your eye is directly above the cutter

Is your body behind the cutter, or do you use your arm to direct the cutter?  The observer will be able to tell that when you are scoring curves. The most consistent speed and pressure is delivered when the cutter is steered from your torso, rather than your arm and wrist.  It slows the scoring action, gives smoother curves, and more even pressure.

The last element, you can do yourself.  Once you are doing all the things above, you will be able to hear any scratching noise, rather than the gentle creaking noise of an even score with adequate pressure.  If the scratching noise is intermittent or only at one point, the likelihood is that you are twisting the cutter head, so the wheel is not in line with the score line.

Glass Overhanging Moulds

Glass Overhanging Moulds

Glass that overhangs moulds by too great an amount is likely to break upon cooling.  Even during slumping,with its lower than fusing temperature, glass expands.  Unless the overhanging glass is drawn into the mould while slumping, there will be some draped over the edge of the mould. When cooling, it begins to contract. 

This post shows the way in which glass behaves during the slump.  This may help with determining how much of an overlap is allowable.  If the mould has a broad, nearly horizontal rim, the glass will not rise and slip down into the mould enough to avoid  an overhang during the firing.  However, a mould with a small rim can accommodate an overhang.  Circumstances vary, but generally a 6mm overhang on a narrow rimmed mould will be safe.  For broad rimmed moulds, no overhang is safe.

The risk of breakage is not so great on steel, where the metal is contracting more than the glass.  However, all of us normally use ceramic moulds which expand and contract less than the glass.  This means the glass will trap the ceramic until the stress is relieved by breaking a part of the overhanging glass.

There are methods to support the overhanging glass during the slumping, as described in this blog post, which will eliminate the risk of the glass trapping the mould.

Glass on Drop Rings

When glass drops through a ring, you need to check on some things relating to the placement and firing.

When thinking about the relationship between the size of the flat glass and the size of the aperture, you need to remember how the glass behaves as it heats up toward the drop temperature.

Glass behaviour

The glass begins to sag at the middle of the aperture, however the glass is still relatively stiff.  The weight of the rim is not enough to keep it from rising from the ring. The rim of the disc maintains the angle from the centre of the drop to the edge, until it gets hot enough for the weight of the rim to allow the edge of the disc to settle back down onto the ring.  This is the source of a lot of the stretch marks at the shoulder of drops.

Rim width

To avoid the glass dropping through, you need to have an adequately sized rim.  The width of the rim sitting on the ring, needs to be related to the size of the hole.  

The consequence of an inadequate rim

I have found that for apertures up to 300mm diameter there needs to be at least 35mm on the rim.  The consequence of this is that your blank diameter needs to be 70mm more than the hole diameter.  For larger apertures – up to 500mm – you need 50mm, or 100mm added to the diameter of the hole.  I do not have the experience to say how much more is required for larger diameter drop rings.  There is more discussion on blank sizes here. 

Heat

The rate at which you heat the glass and the top temperature both have effects on the possible drop through.  

High temperatures. The higher temperature you perform the drop out, the more likely you will need larger rims or other devices to reduce the drop through possibilities.  It also promotes excessive thinning below the shoulder. 

Fast rates. The surface will become hotter than the bottom, but at different rates.  The glass over the hole is heating from both top and (to a lesser extent) bottom.  The rim is sitting on the ring and so heats only from the top.  The differential in heat may cause a break.

Weight. The thickness of the glass effects when the drop will begin.  The heavier the glass and larger the hole, the effective weight will be greater.  In these cases, you can use a lower temperature for the drop.

Additional methods.  You can use other methods to reduce the chance of a drop through.  Two of them are:

Weights. You can put kiln furniture on the glass rim to keep it from rising during the initial stages of the drop.  These must be placed symmetrically. Four or six pieces of kiln washed props or small dams would be sufficient up to 300mm diameter.  More would be required for larger apertures.  Of course, these will mark the rim, meaning that it must be cut off.

Inclined rings. Another possibility is to use an inclined ring, with the glass resting on the upward incline, so the glass is held above the aperture and is heating evenly until the drop begins.

Dichroic coatings

Description

“Dichroic glass is a multi-layer coating placed on glass by using a … vacuum deposition process. Quartz crystal and metal oxides [such as titanium, chromium, aluminium, zirconium, or magnesium] are vaporized with an electron beam gun in …[a] vacuum chamber and the vapor then floats upward and … condenses on the surface of the glass in the form of a crystal structure….  [As] many as 30 layers of these materials [are applied] yet the thickness of the total coating is approximately 35 millionths of an inch.”  http://www.cbs-dichroic.com/faq.asp

“This coating that we commonly call dichroic glass today, is actually an “interference filter” permanently adhered to the surface of a piece of glass. The technology used to manufacture the optical interference filter has been in existence for many years. It is known as vacuum thin film deposition“  Howard Sandberg.  http://www.cbs-dichroic.com/fyi.asp

“The total light that hits the dichroic layer equals the wavelengths reflected plus the wavelengths passing through the dichroic layer.  A plate of dichroic glass can be fused with other glass in multiple firings. Due to variations in the firing process, individual results can never be exactly predicted, so each piece of fused dichroic glass is unique.”  Wikipedia

Care in Use

Dichoric glass can be used in stained glass as well as kilnforming.  There are some precautions to be observed when handling dichoric coated glass.

Determining Coated Side

The coating is a thin film that can be damaged easily. So, the first thing is to determine which is the coated side when the film is on a clear base.  One way is to look at the glass at a very acute angle.  If you see the colour above the clear, the coating is on the top.  If the clear is above the film, the coating is down.  Another way is to put a sharp point in contact with the glass.  View at a sharp angle.  If the point appears to touch the surface, the coating is up.  If there appears to be a small space between the point and the surface, the coating is on the bottom.  It is normal to check both sides to confirm the first impression.  Of course, if the dichoric is on black, the coated side is obvious. A more complete description of the method is describe here.

Scratches

The dichoric film is strong, but very thin.  This means that anything that could scratch the glass will also scratch the coating.  Avoid the use of abrasives when cleaning the coating.  This means that steel wool and harsh abrasive cleaners should not be used.

Scoring and Breaking

As the film is very thin, it is best to cut on the non-coated side.  This avoids any chipping as you score the glass, and provides a clean break.

Grinding

Also, when grinding the edge, you should use a fine grit to avoid chipping off the dichoric.

Fusing Notes

The dichroic coating is a strong thin film that does not expand and contract to the same extent as the glass being fused.  

Avoid movement

When there is a lot of movement in the glass, the coating can split. If the dichoric is on a clear base, you can fire it facing down to reduce the fracture of the film. You can also fire it with clear glass above to reduce the stretching and tearing of the dichoric film.

Over firing

Firing too hot causes additional movement in the glass, so you can think about reducing the temperature to avoid that over firing, which causes lots of movement of the glass.  You should also think about the volume.  If there is more than 6mm of glass, it will begin to spread to reach that thickness.  The spread causes a stretching stress in the dichoric film that can cause it to break apart.

Orientation

You should not fire with the dichoric faces together.  The films do not fuse together, so the glass bases and tops will act as single layers and pull in, creating multiple fractures in the coatings.

Frit

In addition to dichoric coated sheet glass, there is also dichoric coated frit from CBS.  They have designed a proprietary process that allows the frit to be coated on approximately 80% of the surface area of the frit. Due to this high ratio of coating versus glass, the dichroic frit responds very differently under heating/hot working conditions.  Based on: http://www.cbs-dichroic.com/faq.asp

First Kiln Selection

Glass fusing works best in top fired kilns.  Glass casting and some tall work are best with side or bottom elements too.  Compromises can be made of course.  The comparison of glass and ceramics kilns is important to understand.  

Kinds

Most of the following types of kilns are available for glass purposes.

Front loading.  These are good basic, multi-purpose kilns with good viewing properties.

Bell kiln.  This is where the whole of the heating chamber lifts up from the firing bed.  This is more common with very large kilns and is usually combined with lifting gear.

Clamshell kiln.  This is where the firing chamber is hinged, usually on the long side.  This kiln provides access from three sides. It can become too large to reach to the back of the kiln, so these tend to be rectangular.  The lid can also become too heavy for ease of movement and support.

Top loading.  Often called a coffin kiln, there are very good for casting or deep work, but are hard on your back while loading.  They need peep holes at appropriate levels to be able to monitor progress of the firing. These tend to have smaller floor areas than the clamshell.

Car kilns.  These are those where the firing chamber lifts like a bell kiln, but has the firing base on rails or tracks to move multiple firing bases under the firing chamber.

Modular kilns.  These are normally rounded kilns where each ring is controlled separately and can be placed on top of one another.  This is good for large heavy castings, as the refractory and glass reservoir can be placed on the base and the rings built up around the work.

All these kilns come in a variety of sizes.

Choose a kiln relevant to your current work.

The first thing you must decide is the kind and scale of work you intend to do in the near future.  It is too difficult to predict how your work might progress based on experiences with your current work.  It is better to by a smaller kiln that is ideal for the current work and then move to a different kiln, if necessary, or a kiln for different styles or scales of work.

The general advice is to buy as large a kiln as your budget and space and electrical installation will allow.  This remains the case with some precautions.  Think about how often you will fire - daily, a few times a week or a few times a month.   Think about how long it will take to fill the kiln.  A large kiln can take days or even a week to fill with small works. This would really limit the variety of things you could do in that period.  You would have to wait to slump until you had enough things fused to fill the space.  Indeed, you would need to have more moulds than if you had a smaller kiln.

I’m sure you can envisage a time when you will want to work larger than at present, but your first kiln will not become redundant.  It will continue to be useful throughout its long life.

Factors in the choice

Size. As already alluded, the size needs to fit with your current style and scale of work. 

Access.  How big a kiln can you get through the doorways?  How much bigger than actual external dimensions will the packaging make it upon delivery?  It is no use buying a kiln that must be taken apart, or all the packaging removed, to get it into your studio.  Of course, the wider the entrance(s) to your studio the easier it will be to get a larger kiln.  If you really need to have a large kiln, you might have to alter or move your studio space.  You also need to think about the kind of access to the studio.  Does the kiln have to come along the side of the house? Is the path paved or gravel? Stairs? Lift size? Parking for the delivery vehicle?

Space. The kiln also needs to fit into the space you have.  You will need about 15cm all around the outside dimensions for safety purposes.  This applies to ceramic kilns also, even though they routinely reach higher temperatures. The skin of the kiln does get hotter than is comfortable for your hand, but normally not hot enough to burn paper. You can reduce the front to back storage space by putting the kiln on wheels.  But the 15cm saved is not worth the time required to once again ensure that the kiln and shelves are level each time you move it. 

Accommodation also needs to consider access around the kiln to place work in the kiln, especially if you build elements in place on the shelf. 

Location within the studio is important, as the kiln needs to be near a power supply and in a place where it is away from the movement within the studio.

Power supply.  The nature of your power supply will also determine what size of kiln you should buy.  Note both the wattage and amperage required for the kiln and determine whether your household supply can cope with the energy requirements.  Usually a kiln can be run on household supply until it reaches the 1 metre² size, where three-phase power is required to have efficient use of the electricity.

Wattage. Kilns below the 1 metre² (approximately 1 square yard) in size have a need for at least 0.6 -1.2 watts per cm², or 4-8 watts per inch².  Once the kiln is larger, more power is required per area to accommodate the greater mass of the kiln.

Insulation.  All kilns require insulation.  This can be fibre or light weight brick, or a combination of the two.  These insulating bricks can be red hot internally, but only warm to the touch on the outside.  Generally, the refractory fibre – whether board or blanket – requires less energy to heat and cools more quickly in the critical devitrification range.  Most often the kiln floor will be made of brick to provide a firm base to support the kiln furniture.

Features

All kilns come with a range of features, many of them relevant to the size, but not all have the same ones, or the ones important to kiln forming.

Viewing ports.  These are variously called vents, ports, bung holes, etc.  Their importance is at least three-fold. 

·        These provide an opening(s) for you to view the progress of the firing, so you can add more time or heat, or skip to the next segment when adequate heat work has been completed earlier than expected.

·        They provide a means of venting the kiln.  This is important in the burn out of any fibre paper binders, and in allowing enough air to promote the oxidisation and maturation of the hot enamel colours.

·        These openings allow the kiln to safely cool more quickly at lower temperatures, say 300°C, but lower for thicker or more delicate pieces.

Opening.  The way the kiln opens is an important consideration.  Some kilns do not allow the kiln to be opened at all during firing.  This is not a desirable feature on a glass kiln.  It is important to have a switch that will turn the kiln off after a certain degree of opening, so that no contact can be made with a live element. 

·        A front opening kiln allows maximum flexibility to view the progress of slumps, drapes, tack and full fuse kilnforming.  It should have a switch to turn off the power to the elements after a certain degree of opening.

·        A top loading kiln allows you to add glass during a casting process, but is not suitable for working the glass during firings – E.g. combing, manipulation of a slump or drape.  This type of kiln occasionally has no allowance to open the top without turning off all the power to both the controller and the elements.  Avoid this, or have it changed.

·        A clamshell or bell kiln allows maximum accessibility during the loading phase and the forming stages.  Although a lot of heat is dumped forward, it is the easiest to use for combing and other manipulation of the glass during the firing. Again, this kiln needs a lid operated switch to cut the power to the elements when opened beyond a certain point.

Controller.  Although essential, controllers are often given as options, especially on smaller kilns.  There are at least two reasons for this.  There are a variety of controller styles and costs.  The buyer may already have their own controller, or wishes to specify the kind.  Controllers are significant costs involved in smaller kilns – sometimes being at least one-sixth of the price.  In general, the more features a controller has, the more it costs.

Controllers are often classified as “three-key”, or as full number pad.

·        The three-key controller – even if they have many more than three keys – is one where the numbers must be cycled through by holding an up or down arrow to change the numerical information.  This includes the programme number, segment number, time, rate, temperature, and sometimes other information. 

·        The full number pad controller will allow direct entry of numbers at each segment of the programming.  It will often have additional features, such as calculating the firing cost or kilowatts used, elapsed time, additional capacity for more saved programs, ability to control different areas of the kiln heating, etc.

Extras 

There are often things which will be worth considering purchase along with the kiln, but are not usually included in the base price.

Stands.  Smaller kilns range from table top - which do not need stands at all – through medium sized – which have optional stands – to larger ones that come with the stand integral to the whole kiln. Unless you intend to move your kiln about, it is not necessary to buy one of the metal stands. Even so, most of these stands come without wheels, so check that they do have wheels already attached.  If you will not be moving the kiln, you can use a wooden table with a refractory fibre board between the stub legs of the kiln and the table surface.  If the kiln does not have stub legs, you can set it on 4 house bricks. 

Kiln furniture. This consists of the refractory props and dams that will be needed in kilnforming.  The most essential are short (2.5cm) kiln posts to support the shelf.

Shelves.  Most shelves require a mullite/cordierite shelf to fire on.  This is a robust shelf that does not have the quartz/crystobalite inversions that ceramic shelves and tiles used for shelves have.  It is a good idea to buy one of these to fit your kiln at the time of purchase. Smaller kilns can use fibre board or vermiculite board as the shelf.  These can be purchased later.

Extractor fans.  These are available on many kilns. They are unnecessary on smaller kilns as they cool quickly anyway.  Larger kilns in a production environment may need quicker cooling, and these arrangements are very useful in those circumstances, but not others, as most kilns will cool in 8 – 16 hours without drawing air through the kiln.

There are a lot of other considerations in buying a kiln, but these are among the important ones, especially in selecting the first one.

Multi Stage Slumping

Deep slumps cannot be done in one slump. Usually, multiple slumps are required to get an even rim with even thickness along the sides.

Special three stage moulds have been developed for deep slumps. The set is expensive even if you have the shallow starting mould already.

When deep slumps are tried in a single stage, uneven sides, hang ups at the edge, needling at rim, and distortion of the image are common in addition to some thinning and significant distortion.

Do it yourself

This leads to investigating whether it is possible or reasonable to try do it yourself methods.

The DIY process involves using two moulds and filling the deep mould with powdered separator.

·        First stage – slump the glass blank into a shallow shape first.  The starting diameter of the blank will need to be about one third larger than that of the finished vessel.  This can be determined by measuring the diameter of the deep mould and adding one third. This means that if your deep mould is 300mm, you will need a 400mm diameter starting disc and an equivalent size of mould. Fire this slump at your standard slumping schedule for large shallow pieces.

·        Second stage – Add powdered kiln wash or whiting to the deep mould.  Fill the mould to half or two thirds of the volume.  Smooth a shallow depression in the powder.  It should rise to meet the curve of the mould shoulder, even if it does not fully match it. This firing is probably the most critical in the DIY process.  The shallow shape will be considerably larger than the diameter of the mould on which you are placing it.  This means that you must fire slowly and you should peek frequently.  As the glass begins to slump, the outer edge will begin to rise at first.  As soon as the outer edge begins to relax, you must advance to the annealing segment.  If you allow the rim to sag, it will not sit very well in the mould at the next stage.

·        Third Stage – This may require more than one firing to achieve the intermediate shape.  In preparation, remove about half of the powder from the previous firing. Shape the remaining powder to a smooth curve. Fire the glass, again watching and advancing to the anneal when the rim begins to flatten.  If the glass has not touched the powder at the bottom, you will need to do another firing.

·        Fourth stage – Remove all the powder from the deep mould. Place the glass and fire.

Keep the kiln wash powder for future use. Its composition will not have changed as you have not fired it to tack fusing temperatures.  Dispose of the whiting, if you used it.  It may be fine for further use, but since it is cheap, it is not worth the risk of it sticking to the glass in subsequent firings.

Remember that long – low and slow – slumps are required at all stages of creating a deep slump.  As a comparison, think about the hours required for a free drop to form and still keep the glass at the shoulder thick.  Deep vessels require long hours of watching just as aperture drops do.

Fire Polishing

Polishing of glass can be done in the flame, in the kiln, by acids or by grinding with successively fine abrasives depending on the nature of the piece and the equipment available.  Fire polishing in the kiln is widely popular as it utilises existing equipment, avoiding purchasing additional cold working equipment. This post indicates some elements about fire polishing in the kiln. 


Fire polishing is the technique most often available to kiln formers. This is the process of heating the glass to less than a full fuse to achieve a smoother texture on the glass. It is often used after sandblasting or hand sanding a piece in order to give a smooth shiny surface to the glass without extensive cold working with successively finer grits to get a polish. It also can be used to give a variety of textures from a sealed but almost unchanged sandblasted surface, through a satin-like finish to a very subtle difference between full polish and slightly textured surfaces in the same piece.


Fire polishing range

The temperature range that this occurs between slumping and tack fusing. The normal range is 650C to 750C depending on the glass, the soak time and the speed of advance.  The purpose of this kind of firing is to get the surface of the glass hot enough to form the desired surface without soaking long at higher temperatures, as this is also the devitrification range (700C - 760C).  Normally there would be a minimal or no soak at the top of the temperature range.

When to fire polish As this temperature range is above the slumping temperature, fire polishing should be done after fusing and before slumping. As this will be the last operation before forming, you also should do any work to shape the edges and deal with any other imperfections, before fire polishing. After doing any grinding or other work on the edges or surface of the piece, thoroughly wash and polish the piece dry.

Methods
You can take the fused piece that has been treated to remove the devitrification up at the same rate as for slumping the piece to the tack fuse temperature.  The higher you go, the less soak time is required. Of course, the higher you go, the longer you are in the devitrification zone.  

Some people advocate a quick fire polish.  This is achieved by firing at a relatively slow rate until a low slump temperature is achieved.  Then fire very quickly to the tack fuse temperature with no soak and return to annealing temperature as quickly as possible.

The quick fire polish does achieve a minimum of time in the devitrification zone, but it eliminates all subtely in the surface.  A long soak of up to 90 minutes at a moderate slumping temperature will give a satin appearance to an abraded or sandblasted surface.  A shorter soak will seal sandblasted work without eliminating the texture of the sandblasted image.

In all the cases of fire polishing you need to peek at intervals to determine when the desired surface has been achieved.  This requires careful placement in relation to the place from which you will be able to peek at the surface.  For a fully polished piece, you will see the reflections of the elements.  For more subtle textures, you need to think about what you want to see, peek, close the lid or observation port and think about what you saw.  If it is not yet what you want, peek at another interval in the same way, until you observe the surface you want.

Combining fire polish and slumping It is sometimes possible to fire polish and slump at the same time, but this is a risky technique often leading to changes in shape or an uprising of the glass at the bottom of the mould. It is possible to fire polish glass as low as 630 with a long soak – 60 minutes or more. If you are determined to fire polish and slump at the same time, it's essential that you watch the piece very carefully to prevent over-firing.

Fire polishing already slumped items Similarly, re-firing already slumped items to a fire polish rarely succeeds. Distortion of the piece is more likely than achieving a fire polish on an already slumped item.

Again, in these more difficult circumstances, you must observe at intervals to ensure you do not over fire and distort your piece.

Schedules
The reason that no indicative schedules are given is that different glasses, and different lay ups require different firing conditions.  These are dealt with elsewhere in the blog.

Alternatives Alternatives to fire polishing include acid polishing, which can present a health hazard, and is normally an industrial process. The other common method of polishing is to cold work the piece. This often requires specialized equipment, but can be done by hand if you have the time.

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.

Adding Colour to Slumped Pieces

Sometimes an already fused and slumped clear piece needs colour for interest, definition, etc. The problem is how to do it without altering the fused piece.

You can use cold paints. Both acrylic and oven baked paints can be applied, but often they are unsubtle, harsh colours.  These are not permanent.

You can apply enamels.  These can be the ones produced for glass fusing, if fired carefully. The curing temperature of 780°C means that you need to fire slowly to about 600°C and then quickly to 780°C with no soak and AFAP to annealing.  This fast rate of advance is to preserve the shape as much as possible at temperatures above that required for slumping. This will need to be done in the mould, of course.

You can more safely use traditional glass stainer colours, which are also called enamels, although they are slightly different from the traditional ones.  These cure between 520°C and 580°C so can be fired as normal in one steady climb to the top temperature with no soak and quickly down to annealing. To be sure the shape is retained, the glass should be in the mould during the firing.

Use of frits and powders requires the higher temperatures that will distort the piece unless fired in the mould. When firing to tack fuse in a mould, you need to be careful that you do not damage the mould during the higher temperature firing, nor get the separator incorporated into the powder.  If you can place the powder or frit on top of the glass, you will get a better result at a lower temperature as the frit will heat and spread more easily on top than on the bottom. 

In general, it is not recommended to add colour to slumped pieces with frits and powders.  It is hard on the mould, and risks the glass sticking to the mould. Even if successful, the slumping mould will have to have the existing kiln wash removed and new added to avoid the kiln wash sticking to the next piece to be fired.  

It is better to slump the piece to flat, if possible, and then add the frits and powders before fusing.  Then slump again.



These notes show that it is important to assess the flat piece critically before proceeding to the slump.  This can mean setting the piece aside for a few days to review your impression of the fusing result.  This little time elapse can give you a fresh view of what the piece requires, if anything.