The relative order of kiln forming events

When preparing for multiple firings of elements onto a prepared piece, you need to consider the order and temperatures of events so that you do not harm an earlier stage of the project.  This blog entry will not give definitive temperatures, as that varies by glass and by kiln.  Instead, it indicates what happens in progression from highest to lowest temperatures in approximate Celsius degrees.  

ca. 1300C  -  Approximate liquid temperature 

ca. 850 – 1000C  -  Glass blowing working temperature

ca. 950C  -  Raking and combing

ca. 850C  -  Casting

ca. 810C  -  Full fuse

ca. 790C  -  Large bubble formation

ca. 770C  -  High tack, low contour fuse

ca. 760C  -  Tack fuse

ca. 750C  -  Fire polish

ca. 700C – 760C  -  Devitrification range

ca. 700C  -  Lamination tack

ca. 600C – 680C  -  Slump and drape

ca. 650C  -  Vitreous paint curing temperature

ca. 600C  -  No risk of thermal shock above this temperature 

ca. 540 – 580C  -  Glass stainers enamel curing temperature

ca. 520 – 550C  -  Silver stain firing temperature

ca. 550C  -  Glass surface beginning to soften

Slow rates of advance needed from room temperature to ca. 500C

These temperatures are of course, affected by the soak times. The longer the soak time, the lower temperature required. The rate at which you achieve the temperature also affects the effective temperature.  Slower rates of advance require lower temperatures, than fast rises in temperature.  These illustrate the effect of heat work.

The table shows for example you need to do all the flat operations and firings before slumping or draping.  It also shows you can use vitreous glass paints at the same time as slumping and draping.  This emphasises that the standard practice is to plan the kind of firings you will need for the piece and do them in the order of highest temperature first, lowest last.

In general, you do need to do the highest temperature operation first and lowest last.  But there are some things you can do with heat work.  For example, if you needed to sandblast a tack fused piece, but did not want to risk reducing the differences in height there things you can do.  From the list above, you can see the glass surface begins to soften around 500C.  It is possible to soak the glass for a long time around 500C to give it a fire polish, instead of going to a much higher temperature.  You will need to experiment to find the right combination of temperature and soak length, but it can be done.

This article is to show that knowledge of what is happening to the glass at different temperatures, can help in “fooling” the glass into giving you the results you want without always following the “rules”.  This may also be what it is to be a maverick glass worker.  Use the behaviour of glass to your advantage.

Repairs to a Vermiculite Mould

Occasionally, during the demoulding of a form, the mould will break.  Not all is lost.  It can be repaired. 

In this example, the mould is not yet fully cured and is damp.  But this can be applied to fully cured and dried moulds too. Notes will be included where the practice varies for the dried mould.

The first stage is to make up a paste of the ciment fondue for the edge to edge repair.  This should be the consistency of pancake batter or slightly wetter.  The mixed cement is shown at the top of the picture in a small plastic tub.

Wet the edges of the mould pieces thoroughly.  This is to prevent the mould from sucking too much water from the cement, which would give a weak adhesion.  On dried moulds, you may have to do this several times to thoroughly wet the mould and the broken piece.

Then begin applying the wet cement thinly to all the edges.  Do not put it on thickly, as you want the pieces to fit back together smoothly. 

Place the pieces together with gentle pressure. 

Then begin to smooth the wet ciment fondue into the cracks between the broken pieces and the main body.  Be careful to smooth the ciment fondu immediately, as it is very difficult to change once cured.

Continue to work the ciment fondue into any cracks that appear as the mould is wetted.

Make sure the cement is smoothed into the cracks so there are no proud areas above or around the cracks.

This photo shows the smoothed ciment fondu on the interior.

Continue smoothing the cement into the cracks at the edges.

Fill the cracks from the outside also

When the application of the cement is completed, make up a mixture of 1:4 ciment fondue to vermiculite. 

The purpose of this is to strengthen the mould in the weak area.  It is not wise to rely entirely on the strength of the edge bonding of the ciment fondue.

You will need to estimate the total volume required, but it is better to mix too much rather than too little.  Make this mix a little wetter than for the original mould.  Water should not be standing in the mix, but you will be able to squeeze water from the ball of mix easily. 

This is especially important for moulds which have already been cured.  You should also put water on the surface that you are going to back up.

It is important to put a water proof material on the workbench to avoid the mould sticking to the bench, or water dripping over other things.

Having wetted the mould exterior again, begin applying the mix to the outside of the mould.

Continue building up the mixture in thin layers.  This allows the best adhesion of the material to the mould and to each layer.  It is easier to compact a small amount of material than a large amount all at one time.

In this photo, you see some of the water being forced out of the mixture by the compaction of the mix onto the mould.

Continue building around the broken area until you have applied sufficient material to the mould to strengthen it.

When you have finished, one area of the mould may be a little larger than the rest.  This is not a problem in its use, as it does not thermal shock, and it does not keep one part of the glass hotter than the glass touching the rest of the mould.

You can now loosely wrap the water proof material around the mould.  Do not seal it completely.  Place the mould in a plastic bag to cure for a day or more, just as for the original mould.

You can then unwrap the mould and fire it to cure it just as the original. The method for curing vermiculite moulds is given here.

Schedules for Steep Drapes

I have been asked for a schedule for draping in the context of a tip on steep straight sided drapes.

What you are trying to do with a steep drape is two things. One is to compensate for the heat sink that the glass is supported by, and the second is to compensate for the relative lack of weight at the outer edge of the glass.


The supported glass transmits its heat to the support, leaving it colder than the unsupported glass. This often leads to breakage due to heat shock at much lower temperatures and slower rates of increase than glass supported at its edges. My experience has shown that - contrary to what I recommend for other kinds of firings - a slow rise with short soaks at intervals up to the working temperature works best. The reason for these slow rises and soaks is to try to get the support and the glass to be as nearly as possible at the same temperature throughout the rise in temperature. The soaks help ensure the mould is gaining heat without taking it from the glass.

The other problem with steep drapes is that the edges of the glass begin to drop more quickly than the area between the support and the edge. This leads to the development of an arc that touches the mould side near the bottom before the glass between the edge and the and the support. Extended soak times are required to allow the glass to stretch out and flatten. If this is done at high temperatures, the glass will thin - possibly to the extent of separating.

So the requirements for a firing schedule on this kind of drape are slow increases in temperature with soaks to avoid thermal shock, and an extended soak at the forming temperature.

Whether using steel or ceramic moulds, I use a slow rise in temperature to 100C with a soak of 15 minutes. I then increase the rate of rise by 50% for the next 100C and give a 15 minute soak there. For the next 200C I raise the temperature at twice the original temperature rise, again with a 15 minute soak. The glass and mould should now be at 400C. This is still at the point where the glass could be heat shocked, so I only increase to 2.5 times the original rise rate but this time all the way to forming temperature.

Each kiln has its own characteristics, so giving schedules is problematic. A side fired kiln will need slower heat rises than a top fired one. The closer the glass is to the elements, the slower the rate of increase needs to be. The kind of energy input - electric or gas - has an effect. The thickness of the glass is also a factor in considering what rate to use. The size of the glass in relation to the size of the support is important - the greater the differential, the slower the heat rise should be. So in making a suggestion on heat rises, it is only a starting point to think about what you are doing and why you are doing in this way.

I have usually done this kind of draping in top fired electric kilns where the elements are about 250mm above the shelf, and about 120mm apart. In the case of a 6mm thick piece about three times the size of the support area, I use 50C/hr as my starting point. This is one third of my usual rate of temperature rise. However you must watch to see what is happening, so that you can make adjustments. You should observe at each of the soaks, so you know how the glass is behaving. It will also help you to pinpoint the temperature range or rate of advance that may be leading to any breakages.

On steep slumps, the temptation is to use a high temperature to complete the drape. This is a mistake as the glass will be more heavily marked and tends toward excessive stretching and thinning. What you really need is a slow rate of advance to a relatively low temperature. If you normally slump at about 677C, then you want to do this steep, straight sided drape at 630C or less. It will need a long soak - maybe up to an hour. It will also need frequent observation to determine how the drape is progressing. So plan the time to make yourself available during this forming soak.

Annealing is done as normal, since the mould and glass are more closely together and will cool at the same rate.

The original tip on the set up of a steep straight sided slump is here.

Tapping Glass Scores

Many people tap the underside of the glass after scoring.  The purpose of this is to run the score.

However, this tapping is often unnecessary.  Running the score can be done in a variety of ways, some more suitable for one kind of score line than another.

Straight score lines can be run in several ways.

• ·        Move the line to the edge of the bench or cutting surface and use a controlled downward force on the glass off the edge while holding the remainder firm.  Works best if at least a third is being broken off.
• ·        You can place a small object, such as the end of your cutter or a match stick, directly under the score and place your hands on either side and press firmly, but not sharply, down on each side at the same time.  This is good for breaking pieces off from half to a quarter of the full sheet.
• ·        Make your hands into fists with the thumbs on top of the glass and the fingers below.  Turn your wrists outwards to run the score. Works best if the glass is approximately half to be kept and half to be broken off.
• ·        Take the glass off the cutting surface, hold in front of your knee at about 45 degrees and raise you knee quickly to the glass.  This will break the glass cleanly, but is only useful for moderate sized sheets and where you are breaking off about half of the sheet.
• ·        Use cut running pliers to run the score.  Be sure the jaws are adjusted for the thickness of the glass, and do not apply excessive pressure.  If the score does not run all the way, turn the glass around and run the score from the opposite end. Best where there are approximately equal thin parts to be broken away from each other and when the score line is no less than an oblique angle to the edge. It does not work very well for thin pieces or acute angles.
• ·        Use two grozing pliers nose to nose and flat side up at the score line and bend them down and away.  This works best on thin and or pointed pieces.
• ·        Breaking pliers can be used at intervals along the score. This is most useful on long thin pieces.

Curved score lines, of course require a bit more care but generally employ the same methods.

• ·        Gentle curves can be dealt with as though they are straight lines, although the breaking at the edge of the cutting surface is a bit risky. This means the two-fist, running pliers, two grozing pliers and breaking plier methods can be used.
• ·        Lines with multiple curves usually require cut running pliers to start the run at each end of the score.
• ·        Deep curved scores may require the running pliers whose angle can be adjusted to be at right angles to the score.  The ones I know are Silberschnitt, made by Bohle, although the ring pliers by Glastar work in the same way. This usually requires that the edge of the glass is not more than 5 cm from the score.  This blog gives information on a variety of cut running pliers. 

Tapping

After trying all these methods to run the score, sometimes the score is so complicated or deep into the glass that you cannot simply run the score.  Tapping may then be required, but it is a last resort.

Tapping, to be effective, must be accurately directed to places directly under the score line.  The tapping cannot be at random places under the glass. Each tap must be controlled – to be direct and to be firm. 

The impact needs to be directly under the score. 

• ·        Taps that are either side of the line will either not be effective, or will promote breakage other than along the score line. 
• ·        Tapping to either side of the score also promotes shells to either side of the score line.  These are not only dangerous when handling, but also require further work to remove these ledges of glass.

The impact also needs to be firm. Random impacts to the glass promotes breakage other than along the score line.

• ·        The taps need to be firm – neither light nor hard.
• ·        Each tap should be at the end of the run begun by the previous one.  This promotes a smoother run of the score with less opportunity to start a run off the score line. 
• ·        To avoid the incomplete running of the score that leaves parts of the score untouched you need care. As the glass begins to break along the score line, place the next impact at the end of that start to continue the run. 

Tapping the glass under the score should be a last rather than first resort in running a score.

Firing for 3mm Channels

A question has arisen on how to put together a design of pieces for a lamp, but only one layer thick, because 3mm is as thick as the fittings will accept.

The design has no overlaps, so it is a series of butted 3mm thick pieces.  Damming has not been successful in keeping the parts from retreating from one another.  This means that making the design as a single layer will not be successful.

The problem is how to make a two-layer piece that will be able to fit into 3mm fixings.

Design on oversize 3mm base

One way to overcome the fixings’ limitations is to make the bottom layer larger than the top.  The bottom can be any colour you choose.  Make the design on top of that. 

The designed pieces will need to fit snugly beside the fittings. However, the bottom needs to be cut larger than the final size, as it will retreat and become smaller during the firing.  About 20mm larger all around will usually be enough extra for ease of cutting down. If you fire with a larger base piece, you can cut it to size after firing, so it will fit the width of the opening and still fit inside the 3mm fitting space.  This will make your design proud of the fittings.  This may, or may not, be possible for the lamp’s fitting design.

An alternative

Maybe that is not the only way to look at the problem. There is another way.  It is essentially the opposite of the first approach. 

Make the top layer larger than bottom. The design will be on the top still, but with larger than final dimensions to accommodate the reduction in size of the single layer.  The bottom layer will need to be small enough to fit within the space between the fittings of the lamp.

To keep the unsupported parts of the upper layer in one plane, support the larger upper layer with 3mm fibre paper. Coat the fiber paper with boron nitride or cover with powdered kiln wash, Thinfire or Papyros to get a smoother back.  When fired, cut the piece to size.  If you like to score on the smoothest side, you can support the edge with the fibre paper or other 3mm substance.  If you are confident, you can score on the back with no special support.

These are two approaches to making a piece to fit in a 3mm channel.  This will apply to insertions of fused glass leaded glass panels, as the came is designed to accommodate 3 mm glass.

What Cartoon Lines Represent

A frequently asked question by novice glass workers is whether to score at one side of the line or in the middle.  This question revolves around the meaning of the cartoon lines.  What do the lines of a cartoon represent?

Meaning of Cartoon Lines

The lines on a cut line cartoon represent the space required between pieces of glass.  This will vary, depending on the style in which you are working.  In most glass working, a matrix of lead or foil is used.  The space required by these materials needs to be represented in the cut line cartoon. You may have other cartoons for other purposes – painting, came width, foil width, etc., but the lines in the cut line cartoon are there to represent the space required between pieces of glass.

An example of a cartoon for painting

Lead Came

In general, a 1.2mm line is required for standard lead came. This is close to the line made by a new bullet pointed felt tipped marker. If you are working with high heart cames, you will need a 2.8mm wide line. Some chisel point markers, if used on the sharp edge have this approximate width.

The glass is scored at the inside edge of the cartoon line.  This can be done by scoring directly on top of the cartoon, often with a light underneath.  You can make pattern pieces when the glass is too dense for enough light to come through.  If you must, you can draw the score line on the glass. You can score around pattern pieces, but if your scoring wheel goes over the pattern in any place, the scoring pressure will not be delivered to the glass.

Example of came varieties

Copper Foil

In copper foil, a much thinner line is used as the space between pieces of glass needs only be approximately 0.4mm. This is approximately the width of a sharpened pencil or ball point pen line.

The scoring is at the edge of the line as for lead came.  Also, you can score directly over the cartoon, draw on the glass, or make pattern pieces as for lead came projects.

Fusing Cartoons

When preparing a cartoon for fusing, the lines need to be as fine as possible.  The pieces of glass require no space, as they will be butted against each other.  However, unless cutting by computer controlled instruments, the cutting cannot be completely accurate, so the same size of line as for copper foil will do.

As you are going to try to butt the glass pieces together in fusing projects, you score along the middle of the cartoon lines.  As much as possible, cutting over the cartoon will give the best result.  Of course, there are many times when the light is not good enough and pattern pieces will be required. 

Another approach is also possible. Having scored and broken the first piece, you can place it on top of the glass to be cut for the adjoining one.  With a very fine felt tip or fountain pen, trace the edge of the first piece. Score down the middle of that line to create the best fitting second piece.  And so on through the whole project where the glass is not too dense to use a light box.

Conclusion

The line widths in a cartoon are determined by the space required between pieces by the assembly method.  The thicker the matrix material, the thicker the line and vice versa. 

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