Peeking Without a Vent

What can I do if my kiln does not have a plug?

To understand thoroughly what is happening to your glass while firing, observation is key.  This means that an observation port is an ideal feature of any kiln.

However, many kilns are made without ventilation or observation ports.  This means that several possibilities need to be considered.

The easiest is simply to open the door or lid a small amount to make a brief observation.  This means that you have to set up the piece to be fired in such a place it can be seen from a small opening of the door/lid.  This brief opening of a small space will not normally cause any problem to the glass or kiln.  At the higher temperatures, you need to take personal safety precautions against the heat and light from the kiln.

It is possible to be more radical and drill an observation port through the metal casing and brick or fibre lining of the kiln.  This is then filled with a piece of fire brick or roll of fibre blanket.  This is sufficient to insulate the heat from the external part of the kiln.  This port should be about 50mm diameter to give a decent field of view.

A further refinement is to place a quartz viewing window in the hole you have drilled.  This viewing piece will become very hot, but not visibly red.  So, you must provide some insulating cover over the window.

But best of all, is to purchase a kiln with a viewing port in the first place.

Powders Burn Away - Kiln Forming Myths 21

Glass powders will burn off at high temperatures.

No.  The powder is glass.  Glass does not evaporate or otherwise combust at kiln forming temperatures.

The appearance of glass powders fading at fusing temperatures is related to the different appearance before and after firing.  Before firing, the powder looks both denser and paler than the final colour.  The initial experience with glass powder always is to put less on than needed. 

You need to remember that a thin film of powder is a tiny fraction of the thickness of the glass it is made from, so the colour will be much fainter.  A considerable amount of powder is required to give the colour shown by the colour charts – as much as 2mm for paler and transparent colours.  Opalescent colours show a little better with thin applications, but still require significant amounts.

This shows the application of powder on a piece where the powder provides almost all the colour for the piece.

The best procedure is to make test tiles with varying amounts of the powder to determine the thickness required for your desired result.  This gives a visual reference and experience in laying down the powder in appropriate thicknesses.

The appearance of the glass powder burning off, is merely the application of too little powder.

Organic and Mineral Inclusions

Encasing organic materials adds a new level of complexity to inclusions.  In addition to bubble formation, you need to consider how to eliminate the combustion gases from the vegetable matter.  On the other hand you don’t need to worry about expansion differences.

Some examples of metal and glass inclusions

The requirement is to burn out all of the vegetable matter to avoid creating big bubbles from the burn off of the material.  There are two elements to this burnout.  One is the amount of moisture contained in the object and the second is the volume of dry material that has to burn out.

Drying

Unless you have dried the material before including it, you will need to leave enough soak time before the glass begins to move to ensure all the water is removed.  It is also advisable to place small shards of glass at the corners of the piece, to allow easy ventilation both for the moisture to evaporate and the vegetation to burn easily.  You can estimate the time required and then put a witness piece of glass or better mirror above the vent or peep hole to see if there is any fogging on the glass from escaping moisture.  You need to continue soaking until there is no fogging.

Burnout

The second element is to give the vegetable matter enough time to burn out.  The burn out should occur at about 400°C.  This is high enough to ignite carbon based materials, but not so high that an extended soak will allow the glass to sufficiently deform to seal the un-burnt material inside.  If you have a really good sense of smell you can tell when the carbon has burned away by the absence of the smell.  For the rest of us, we need to open the peep hole and use a strong light to tell how much is left to burn away.  The burning is much more like a smouldering with very little light coming from it.

An incompletely burned out leaf in a large trivet with felt feet at the corners. 150mm square

The length of time you need to soak below the softening point of the glass is directly related to both the water content and the amount of combustible material you have included.  The burning will not begin until everything is dry.  If the material is not dry, the time for this needs to be added to the burnout time.  The length of soak for burnout is much more difficult to determine and needs periodic observation beyond the time when the smoke stops coming out of the kiln ports.

Bubble squeeze
Once the drying and burnout are completed, you need to advance to the bubble squeeze.  This will need to be longer or slower than usual to ensure all the combustion gasses are out for organic materials.  Minerals will normally be thicker than the organic materials and so need long bubble squeezes. These can be at or just below slumping temperature, or a slow rate of rise, taking an hour or more, from about 50C below the slumping point.

It is possible to include other minerals such as bone, or ash, or other inert particles that will not stick to the glass. Materials that contain silica are not suitable, as they stick to the glass and cause breakages.  So most stone, which contain silica, however thinly sliced will not be suitable as an inclusion.

Crash Cooling - Kiln Forming Myths 20

Crash cooling will harm your kiln or break your glass.

Crash or flash cooling was often a requirement in the early days of fusing to avoid devitrification. The kilns used were ceramic ones that did not lose heat very quickly.  The glass also was more subject to devitrification than the glass being made now.  Since those early days, kiln design has advanced so the kilns lose heat more quickly, although still well insulated; and the glass is more resistant to devitrification.  Thus, crash cooling is no longer advised.

If you have a brick lined kiln, crash cooling is hard on the bricks.  The cold air causes rapid shrinking of the brick.  The more rapidly the brick heats and cools, the more fractures will develop in the brick.  This effect will take place over many firings before there is any noticeable damage to the structure of the brick.  However, if you have brick tops or lids, there is the increasing likely development of crumbs of brick falling onto your work.  Brick lids and tops should be vacuumed frequently to remove the crumbs as they form.

Crash or flash cooling from top temperature toward annealing temperature is unlikely to break any glass other than thick glass pieces.  However, when using glass formulated for kiln forming, you do not need to crash cool. The crash cooling may be more useful when using glass that is not formulated for kiln forming.  The purpose in this case would be the same as that for the early fusing – avoiding devitrification by moving as quickly as possible through the devitrification range.

Sometimes flash/crash cooling is required to fix a free drop in place.  If allowed to cool on its own, the glass will continue to move for a while.  If the extent of the drop is critical, crash cooling is required.  This should be to a point below the slumping but above the annealing temperature.  The flash cooling will cool the outer portions of the glass, stopping any further movement. Meanwhile the inner portions are still hot.  This sets up significant stresses.  By stopping the cooling just below the slumping temperature, you allow the internal and external temperatures of the glass to approach one another before going into the anneal soak where the temperature equalises throughout if the differentials are not too great from the flash cooling.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Metal Inclusions in Glass

There are a number of reasons to include metals in glass, not least colour.  However there are some things of which you should be aware.

Coefficient of Linear Expansion of some metals and glass is very different.  This listing gives some of the characteristics:

(All numbers given as 10^-7)

Aluminium          230

Glass              ca. 85

Brass                 180

Bronze               190

Copper              170

Borosilicate glass   33

Gold                  140

Iron                  116

Lead                  280

Nickel                130

Platinum              90

Quartz               7.7 to 14

Silver                195

Stainless steel     100 to 170

Mica                    30

Porcelain              65

Clay tile               59

Stainless steel     (in general) 100 to 170

Stainless steel     (418 series)          99

Stainless steel     (310 series)        144

Stainless steel     (316 series)        160

Tin                    234

Zinc                  297

Titanium              86

From this you can see there is little that is similar in expansion coefficient to glass.  Those that are, are expensive.  The implications of this difference in expansion are that the metals upon cooling contract more than the glass and so these are the effects you need to watch for:

• ·         Metals create strain when fused within the glass. 
• ·         Thin section is required to reduce the strength of the metals. 
• ·         The tensile strength of the metal may be more important than the CoLE
• ·         The amount of the metal should not be great or concentrated in one spot
• ·         Where thick sections of metal are required, a space should be created for later insertion of the metal.

In addition to expansion characteristics, the strength of the metal should be considered. Numbers are MPa (approximately equivalent to one atmosphere pressure)

Aluminium          40-50

Glass (float)        55-138

Brass                 250

Bronze               172

Copper              210

Gold                  120

Iron                  350

Lead                    12

Nickel                140-195

Platinum            125-240

Quartz               48.3 (and borosilicate glass)

Silver                170

Mica                  250-300

Porcelain            110-160

Stainless steel     (in general) 860

Tin                    15-200

Zinc                  110-200

Titanium            200

The greater the strength of the metal, the thinner the pieces should be to avoid excessive stress.

Melting temperatures are also a factor in including metals in glass

(°C)

Aluminium          660  

Brass                 930-1000

Bronze               913

Copper              1084

Gold                  1064

Iron                  1149

Lead                  328

Nickel                1453

Platinum            1770

Quartz               1670

Silver                961

Stainless steel     1510

Mica                  600-900

Tin                    232

Zinc                  420

Titanium            1670

This shows that aluminium, lead, tin and zinc are not good inclusions as their melting temperatures are below the fusing temperatures of glass. This means they will not retain their structure when fired.  It can of course provide a “frozen” liquid appearance.

Finally, the oxidisation characteristics should be considered.  The following metals tend toward the description after the arrow “>”

Aluminium    > brown

Brass   > some browning

Bronze  > sometimes a red cast

Copper > from red oxidising to green in the presence of soda or chloride

Iron  > black

Nickel  > retains its colour well

Platinum > > retains its colour well

Silver > reacts with sulphur to form a yellow

Stainless steel > blackens

Mica  > retains its natural colour, although some is low temperature coloured and so blackens, others have high temperature colours

Titanium  >  oxidises to white

Gold  > generally retains its colour except in leaf form when it becomes silver in colour

These are not exhaustive descriptions of oxidisation characteristics of metals in glass. They are a good starting point though.

Stainless Steel Magnetism

Is all stainless steel non-magnetic?  The answer is “No”. 

The scientific answer is of this nature:

“The magnetic behaviour of stainless steels varies considerably, ranging from the paramagnetic (nonmagnetic) in fully austenitic grades to hard or permanent magnetic behaviour in the hardened martensitic grades.

“All austenitic stainless steels are paramagnetic … [but the magnetic] permeability increases with cold work due to deformation-induced martensite, a ferromagnetic phase.  For certain grades such as types 302 and 304, the increase in magnetic permeability can be appreciable, resulting in these grades being weakly ferromagnetic in the heavily cold-worked condition.  The susceptibility of a particular grade to becoming ferromagnetic when heavily cold-worked depends on the stability of the austenite, which, in turn, depends on chemical composition and homogeneity.”

From https://www.cartech.com/techarticles.aspx?id=1476

The layman’s explanation is of this nature:

Stainless Steels are identified by series numbers ranging from 100 to 600.  Each series is organised by the alloy and grain structure.  The ones we are interested are mainly the 300 and 400 series.  The 300 series have an austenitic grain structure and the 400 series has a martensitic structure. 

"In the martensitic structure at the atomic level all the iron atoms are acting as mini magnets aligned in the same direction.  Cumulatively they are all adding to the overall magnetisation of the material, this is known as ferromagnetism.  However the addition of nickel disrupts this regular structure giving it an austenitic nature, so inhibiting the magnetism of the stainless steel."  

This makes the 300 series in its un-worked state non-magnetic and the 400 series magnetic.

A relatively general characteristic of the 300 series is the approximate 18% chromium and 8% nickel (among other alloying elements) content.  There is an interplay between the chromium and nickel content.  Chromium allows the stainless steel to have a magnetic structure, while nickel reduces the magnetic properties of the steel.  The 300 series mostly contain enough nickel to make them non- magnetic or only weakly magnetic. 

However, cold working to form the sheets into vessels or other objects can break down the non-magnetic structure of the steel by aligning the atoms together.  So some highly worked 300 series steels can become magnetic, although their corrosion resistance does not change.  The 316 stainless steel contains higher amounts of nickel than others and exhibits almost no magnetism in its cold worked state.  But 304 with less nickel does become mildly magnetic. Another advantage of high nickel content is that assists the chromium to form a passive surface layer, so resisting corrosion. This is of assistance to kiln formers, as it reduces spalling.  But note that magnetic response is a function of the metallic structure, not the corrosion resistance formed by the chromium and nickel (as well other trace metals) content.

Based in large part on: Magnetism and Other Properties of Stainless Steel, by Gregg V. Summers, P.E. Director of Product Development, Peninsula Components Technical Bulletin http://www.pencomsf.com/wp-content/uploads/2012/08/TB_MAG_SS.pdf 

The addition of nickel in the 300 series eases the workability and welding of stainless steel over the 400 series. You are more likely to find the 300 series in worked vessels and other kitchen equipment.  But the stainless steel knives are much more likely to be of the 400 series.  Both of these stainless steels have a high degree of corrosion resistance.

The Effect of Glass Temperature on Cutting

There are many opinions on how glass cuts when cold.  Some report cutting outdoors in sub-freezing temperatures, others that only warm glass cuts well.  I decided to see what scientific information there may be on this idea.

The Science

The scientific literature mostly concentrates on the effects at higher temperatures than we are concerned with.  However, there are some things that are applicable, and some of these effects of temperature are outlined below.

·         High humidity results in loss of strength. 

·         The strength of glass is reduced by 25% at 100°C compared to 0°C.

·         Glass needs several days to be at an even temperature throughout.

·         Variance in temperature across the glass causes unwanted breakages.

·         Colder glass becomes more brittle due to loss of elasticity.

·         Hardness of glass increases with decreasing temperature.

The terms of strength, hardness and brittleness have scientific definitions that are hard to apply to the everyday glass cutting that we do.  Strength may or may not have applicability to glass cutting.  Elasticity may or may not be an important factor in cutting.  Surface hardness may play a part in cutting while cold.

Applicability of the Science

However some things seem to apply. 

High humidity results in loss of strength.  This may be a factor in low temperature cutting.  The humidity in a relatively closed environment increases with the reduction in temperature.  Breaking glass is about the creation of a weakness in the glass along the score line.  In so far as strength is a factor in the break running along the score line, this may be an element in cold glass cutting.  If the whole glass is weaker, the difference in strength at the score line is less and so promotes unwanted breaks.

Variance of the temperature of the glass throughout the substance of the glass promotes unwanted breakages.  Perhaps the cold glass that is difficult to cut is not equally cold throughout.  Certainly a number of people report that they store their large glass outdoors and can still score and break the glass during the winter perfectly well before bringing it into the studio. 

Glass becomes more brittle with decreasing temperature, and it also becomes harder.  Perhaps these two elements are a factor in controlling breakages.  If the glass is both harder and more brittle, a different scoring method is required. 

The way in which glass at any temperature breaks is related to the force of the score, the speed of the score and the angle of the cutting wheel.  If the glass is both harder (at the surface) and more brittle it requires less scoring force or a blunter wheel angle.  The more blunt the wheel on a thicker (i.e. stronger) glass, the more vertical the stress lines are created in the glass.  So in a cold and harder glass, a blunter wheel angle seems appropriate, even though the glass is not thicker.

It is not usual for people to have cutting wheels of different angles, so an easier, although more skilled, approach is to reduce the scoring force in cold conditions.  Reducing the force in scoring a hard and brittle glass causes the stress lines to be more vertical than increased forces do.  Increased forces cause lateral lines of stress to be created, leading to unwanted breakages.

Secondly, the glass being more brittle, less force in breaking stress is required.  As the glass becomes colder, the less elastic it is.  This elasticity is an important element in breaking the glass at room temperatures. The score needs to be run gently to counteract the loss of elasticity and the consequent increase in the brittle strength of the glass.

Conclusions

My conclusion, after the reading I’ve done, is that cold glass becomes slightly stronger and more brittle than room temperature glass, and so requires a slightly different method of cutting. This difference is to reduce the pressure of scoring and the force of breaking (applying stress to the glass).  

Of course you can warm the glass up before scoring it, but the research seems to indicate that significantly long times are required to equalise the temperature throughout.

Firing AFAP - Kiln Forming Myths 19

Firing as fast as possible harms your kiln, or at least will wear out the kiln elements.

I believe this comes from the days when ceramic kilns were commonly used.  Certainly this is still the mantra among ceramicists today.

A number of people fire their kilns as fast as they can, especially small ones, all the time.  Refractory fibre kilns are not affected at all by rapid changes in temperature. 

There might possibly be some small damage to the light weight refractory brick used in glass kilns in that the frequent expansion and contraction may cause crumbs to fall from the brick.  But this will happen anyway as the brick expands and contracts the same amount every time it is fired.  There is no definitive information on whether rapid increases in temperature have any greater effect on brick than slower increases. 

Any rapid change in temperature is unlikely to affect the kiln elements.  Attempting to bend the elements while cold is very likely to break them, as a compound is formed on the surface which makes them brittle when cold.  But this is very different from rapid changes in temperature.  As an analogy, the elements in electric fires are made of the same material and are always heated as fast as possible from cold.  They have a long life, so there should be no difference in effect on kiln elements, which are generally thicker and less exposed to drafts and rapid temperature changes once hot.

It could be said that firing as fast as possible would reduce the stress on the relays in the controller, as they will be closed for the whole of the temperature rise, with no opening and closing.  Thus, the number of firings will be increased without equally increasing the number of cycles the relays have to perform.

However rapid rises in temperature affect the kiln is secondary to how it affects the glass.  Except for small pieces, extremely rapid rises in temperature increase the likelihood of the glass breaking.  This is the more important consideration when thinking about afap firings.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Rapid Rates of Advance to Avoid Devitrification - Kiln Forming Myths 18

Firing as fast as possible, or at least, very fast above annealing point will avoid devitrification.

Of course, this is true in one sense. Moving quickly through the devitrification range will reduce the time the glass has to crystallise – the action we call devitrification.

It will not on its own prevent devitrification.  Nowadays fusing compatible glass is formulated to resist devitrification during the firing.  However, devitrification still occurs during prolonged soaks at high temperatures, and slow rises or falls in the temperature range of 720°C to 760°C.  So you should always be trying to fire quickly through this range, whether up or down.

The contaminants that can form nucleation points for crystal growth can be oils from fingers, or cutters, residue from glass cleaner or refractory fibre papers, or even dust. 

This means the first line of defence against devitrification is cleaning.  Cleanliness is next to perfect results in kiln forming.  Use glass cleaners without additives.  In the UK, Bhole produce excellent glass cleaners.  In the USA, Spartan glass cleaner is recommended by Bullseye.  These may be better than clean water if your water supply contains a lot of minerals or additives for health purposes.

If you feel the need to make your own cleaning fluid do not use denatured alcohols such as rubbing alcohol.  They contain additives which may leave residues.  Use something like isopropyl alcohol and distilled water.

The drying of the glass should be accomplished with a thorough buffing to squeaky clean with plain paper towels or lint free cloths that have been washed without softeners in the washing.

The burn off of organic binders in fibre papers can produce enough residue to affect your glass, so it is best to keep your kiln vented until the burnout has completed – around 400°C.

To prevent dust settling on your pieces, clean and place into the kiln immediately.  If this is not possible, make sure the surface is well covered until placing in the kiln.

Hand Finishing to a Flat Edge.

Hand finishing an edge does not require expensive electrical tools, although they do make the process quicker.  This is a note on how to get good-looking edges without expensive equipment. Only a few materials are required.

• ·         A thick sheet of float glass for the grinding plate
• ·         Aluminium oxide or silicon carbide grit in approximately 80, 180, 400, and 600 grits to act as the abrasive.
• ·         Wet and dry sandpaper of approximately 1200 and 2400 grits
• ·         Paint pens (white and gold work well)
• ·         Paper towels for drying
• ·         Water for rinsing
• ·         Large bucket or basin to collect the rinsing water

The thick flat float glass acts as the grinding plate.  It is flat and smooth, making the grinding and polishing flat. 

If you have a lot of glass to take off to get a straight edge use 60 or 80 grit.  If there is not much to take off, start with 120 or 180 grit.  There is no need to make deep scratches on the edge that will take time to eliminate, if a finer grit will do the job.

Put 80 (or finer) grit aluminium oxide or silicon carbide on the glass grinding plate and make slurry with water. 

Slurry mixed and the circular motions of grinding can  be seen
photo: hisglassworks

Move the edge firmly in circular or figure of eight motion over the grinding plate until flat. If the slurry becomes pasty, add more water as you do not want a thick grinding mix. Maintain the same angle of the glass piece to the grinding plate at all times so you have only one plane of glass to take to a polish.

When the edge is flat, clean and dry the glass, and especially the ground surface to remove all traces of the coarser grit.  Set the piece aside to dry.

While the piece is drying, clean off the grinding plate. Scrape off the slurry into a pot set aside for that grit for further use, or into newspaper or other temporary container and then into a waste bin, not the drains.  It is a heavy material and will block drains. Rinse the plate off in a basin of water to ensure there are no coarse grains on the glass.  The residue will settle to the bottom and you can decant the water off once it clears. These grits are not very expensive so repeated use is not essential, just economical.

The next step is to paint the now dry glass edge with a white paint pen.  This will allow you to see when you are ready for the next step, by the disappearance of the paint from the scratches.  Of course, if you are grinding a white or other pale glass, a gold paint pen will be better to see those scratches.

While the paint is drying, make a slurry of the next finest grit. Then begin grinding. The first element in each grinding stage is to give an arris to the edge of the glass.  This prevents chipping the sharp edges.

 

www pavingxxpert.com

When the white paint is gone from the edge, you can progress to the next grit.

At each stage of grinding you can reduce the grit size by half (double the number). This is the generally accepted reduction of grit size to make the removal of the scratches of the previous grit least time consuming. You can reduce the grit size by more than half if you want. Most often reducing grit by large amounts means more time is spent at each stage.  Experience will show you how much you can reduce the grit sizes beyond the accepted intervals.

Stopping at 600 or 800 grit will enable an edge to be fire polished with ease and minimum heat.

At each stage you need to clean the glass and grinding plate as for the first change of the grit size. This repeated cleaning usually means that the artist either has separate grinding plates for each grit, or the grinding is saved up until there are a few pieces that need the same treatment.

A piece of wet and dry sandpaper fixed to a glass plate

After 800 grit, you may wish to progress to wet and dry sandpaper for the finer polishing, using 1200, 2400 and, if you want. 6000 grit. Fix the paper to a glass plate.  Often, simply folding two edges under the glass will be enough.  Add water and proceed as for loose grit.  Hand finishing to this level will eliminate the need for fire polishing. 

Of course, for smaller areas, you may wish to use diamond hand pads.  The need to use water and rinse between grits still applies.  The diamond hand pads are usually most suitable for short straight edges. The longer ones need the kind of treatment outlined above.

Borax Characteristics

Borax is a glass making flux used to reduce the melting temperature of glass. 

It is almost colourless - grey, white, or yellowish; seldom bluish or greenish; and colourless in transmitted light.

The chemical composition of Borax is:  Na₂(B₄O₅)(OH)₄ · 8H₂O

It has a hardness rating of 2 – 2.5, about half that of glass at approximately 5.5.

The melting point is 878°C. At this temperature borax dissolves numerous metal oxides. In spite of this high melting temperature, it acts as a flux reducing the softening point at the surface of the glass at kiln forming temperatures.

The specific gravity of borax is approximately 1.7, considerably ligther than glass at ca.2.5.

Borax is sparingly soluble in cold water, although readily soluble in boiling water. It is insoluble in ethanol.

Description of devitrification
Temperature range

Devitrification spray

Does Wider Foil Give Greater Strength

The strongest part of a stained glass panel, whether leaded or copper foiled, is the glass.  The weaker points are the matrix that holds the panel together.

Of the matrix, the solder is the strong part.  The copper foil is weaker (and much thinner) and the adhesive is the weakest part of all.

A wider bead gives more apparent strength, but on the surface. It provides a broad line to grasp the glass.  But wide beads are often not what is visually desirable, nor practical.  And the wider the bead, the more solder will be used.

The most important part of a panel is the thin fin of solder between the top and bottom of the solder beads.  This is the connector between front and back. The strength of the whole panel depends on that fin.  So, it could be argued that very closely fitting foiled pieces lead to a weaker panel than loosely fitting ones.  I would not argue that, but it is important to have that connector of solder between the surface beads for a panel to be strong.

The solder connects the two solder beads together and forms the matrix which holds the panel together.

Here the fin of solder is a little thinner, so the matrix is marginally weaker

For larger panels, reinforcement will be required, either between the glass pieces, or on the surface.  The fact that reinforcement is so often used in the gap between the pieces, is confirmation that the fin of solder between the front and back is very important.

Cutting Fused Glass

The same principles of glass cutting are applied to fused glass as to the glass used to make the fused piece.  The differences relate to thickness and variations in thickness.

You still score on the smooth side.  More pressure is not required.  For glass thicker than 6mm, you may wish to use a wheel with a more blunt angle. as shown in this illustration.

www.oaklanddiamondtools.com

The main reason that people may feel it is difficult to cut fused glass relates to the additional thickness.  Just as breaking 4mm glass requires more force than 2mm, breaking a 6mm piece requires more force than a 3mm piece.  

floridastainedglass.net

Properly adjusted metal cut running pliers can do the job, but a cut runner designed for thicker glass can be a boon.  They are designed to provide greater leverage and so more force to the glass breaking.  With these the glass breaks along the score line cleanly.

www.glass-tool.com

The breaking of glass that is uneven in surface levels, as in tack fused pieces, can be more difficult.  One is that running the cutter over glass with distinctly different thicknesses can be difficult.  Maintaining consistent pressure and speed over the bumps of the tack fused pieces is difficult.  The second is that the running of the score will not always follow the score line.  For example, if the score line runs close to the edge of a thick piece, the break is likely to skirt around the thick piece, and possibly off to the edge of the piece, rather than continuing to follow the score line. Planning the score line on tack fused glass is important to avoid trying to break near the edges of thick pieces.

One possibility is to score the glass on the shelf side.  This is certainly possible, even though the surface is rougher.  It does avoid scoring across different levels and makes the break along the score line more probable.

Baffles in Side-Fired Kilns

The object of using baffles in side fired kilns is to keep the direct radiant heat from the edges of the piece(s) being fired.  If the edges receive direct radiant heat, they increase in temperature more rapidly than the interior of the piece.  This means the edges become sticky and seal before upper layer of the interior begins to conform to the lower layer.  This seals air into the piece.

fusedglass.org

The materials and placing of the baffles is important.

Baffles can be made from almost anything that can withstand the heat of the firing.  There is an argument that light-weight materials such as fibre board, vermiculite board, or fibre paper should be used to reduce electricity costs.  Heavier pieces such as brick and kiln shelf pieces require more energy to heat them up.  They then of course, store heat that needs to be released on cooling, so slowing the cool down and increasing the risk of devitrification.

The placing of the baffles is important too.  Baffles protect the glass edges from radiant heat until the general heat of the kiln can come into effect over the whole of the piece.  This means that if the baffles are placed against the elements at shelf level, the element above can still give radiant heat to the edges.  Therefore, baffles placed near the glass are better.  They protect the edges from radiant heat at whatever level the side elements are placed.  This is more important for pieces that are further from the edge of the shelf, than those nearer the edge, as the centrally placed glass can “see” the radiant heat from the upper elements. 

Anneal according to the Number of Firings - Kiln Forming Myths 17

You should anneal longer each time a piece is fired.

It seems the idea is that each time you do something to a piece the risk of poor annealing increases. So, an increase in annealing soak will reduce that risk.

In fact, you should anneal for the thickness of the piece plus any additional complicating factors introduced.  No further annealing based purely on the number of times fired is required.

If you have merely added a layer of powder to the glass, you only need to anneal for the thickness of the base.  It is when you begin to add significant amounts of frit or other glass pieces that you need to increase the annealing soak and also slow down the annealing cool.  If you go on to complicate matters by only tacking those elements to the base glass, you will need to slow down even more.  This blog post gives some indicators on how much you need increase the soak and slow the cool.

Score coated glass on the back - Kiln Forming Myths 16

Cut iridised glass on the back

The idea seems to be to get a more even score and avoid chipping of the iridised surface.

First, the iridised surface is almost microscopic in thickness.  It is put onto the surface as a mist of metallic oxides as it begins its run through the annealing lehr.  This thickness will not affect your scoring.

The back is usually rougher side of the glass and so will be more difficult to get a smooth, even score than the front iridised surface.

Chipping of the iridised surface is caused by too much pressure during the scoring.  Reduce your pressure and review your scoring practice.

Scoring the iridised surface with appropriate pressure will produce a clean break without chipping the surface.  These comments apply to dichroic and flashed glass too.