Kiln Cleanliness

Problems with finished pieces can be caused by an untidy or dusty kiln interior.  Pieces can be affected by devitrification or specks of refractory material in or on the fired project.

Dust is a common problem.  Kiln wash, fibre papers and combustion products all produce particles that are collectively referred to as dust.

Vacuuming the kiln regularly is a good practice to keep the dust down.  It is best if the vacuum has a variable suction control to avoid damage to the refractory materials which make up the kiln.  It is best to use a brush attachment rather than the bare hose.

Dust on the brick or fibre board floor of kilns can be vacuumed easily if you remove the shelf.  Usually you need to use the most suction available to pick up heavier particles such as glass frit along with the dust that accumulates on the floor.

Dust also accumulates on the sides and top of the kiln too.  If you have brick sides and tops, you can continue to use the high suction.  You need to be careful around the elements so that you do not bump them.  This is where the brush attachment is most useful, as you can gently brush out any accumulated dust and any loose particles from the brick.

If you have fibre sides or top, the high suction setting on the vacuum will pull fibres from the refractory material.  You need to use a low setting to avoid damaging the insulating materials.  Gently pass the brush attachment along the insulating fibre and along the elements.

This vacuuming of the kiln does not need to be done on every firing, only at regular intervals.  It is also a good time to check the condition of the elements and condition of the interior of the kiln.  Any element tail connections can be checked for tightness.  The condition of the bricks can be checked as you vacuum. 

Of course, if you are going to fire an important piece, it is a good idea to make sure the kiln is clean before you start. But daily cleaning is not required.

It is not only the interior structure of the kiln that needs to be clean.  You should be checking the cleanliness of your kiln furniture too.  Make sure you keep the shelves dust free and regularly kiln wash them.  Check the kiln posts for flaking kiln wash and dust.  Clean off any dust or loose material and re-coat as necessary.  

And while you are doing all this cleaning, you could vacuum the outside of the kiln too.

Firing Bullseye and Oceanside Together

Is it possible to fire Oceanside (formerly Spectrum) and Bullseye at the same time?

Yes, it is possible to fire pieces made of Oceanside and pieces made of Bullseye in the same firing – as long as the glass is not mixed in one piece.

There will be differences in profile as the temperatures for Spectrum are a little less than for Bullseye at all stages.  A rounded tack for Spectrum will be a much sharper edged tack for the Bullseye, etc.  If you can accommodate those differences you can continue to fire.

It is a bit easier on slumping operations as you can use the lower slumping temperature for Spectrum and extend the soak for the Bullseye glass.  Or, choose a mould for the Bullseye that requires less time than the Spectrum, so they complete the slump at the same time.

The annealing points are different, of course.  But not by much – Spectrum is 510°C and Bullseye 516°C (for any but thick pieces).  These are not far away from each other.

There are two main approaches to annealing different glass in the same firing.

One is to use a shotgun approach.  This means that you choose your upper anneal soak – in this case 516°C – and hold the temperature for the required amount of time.  Then proceed more slowly than usual – say 50°C /hour rather than 80C/hour – until about 55°C below the lower anneal point.  Then proceed to the rest of the cooling.

The other approach is to anneal soak at both annealing points before proceeding to the anneal cool.  This approach is probably best with thicker than 6mm pieces than the shotgun method.  It is also required if you use the Bullseye lower annealing point of 482C.  You would anneal at 510°C and again at 482°C and soak at each point for the required time for thickness.  This doubles the annealing time, thus reducing the advantage of one over two firings.

There is a third approach for pieces less than 9mm that will eliminate the double anneal soak.  Choose a single annealing temperature.  The two annealing points for Bullseye and Spectrum are so close (510°C and 516°C) that you could chose a mid-point between them (say 513°C) and soak there before proceeding to the anneal cool.  

It might be even better to choose a temperature midway between 510°C and 482°C (say 499°C) and soak both glasses for a longer period to ensure the temperature is equalised before proceeding to a slow rate of anneal cool.  This will be especially applicable for tack fused pieces, which require more care than full fused pieces.  Remember that you should be soaking at the temperature equalisation hold for at least twice the thickness of the thickest part of the piece.  Then reduce the temperature at the rate recommended for the thickness indicated.  Look at the Bullseye chart for annealing thick slabs for the rates. 

The reason that you can anneal at different temperatures is that annealing occurs over a range of temperature.   The annealing point is the temperature at which annealing can most quickly occur.  There are several of physical changes that are affected by temperature and rates of cooling. 

If you cool too quickly after the anneal soak, you will induce stress and probable breakage.  The cooling after the anneal soak is an essential part of the whole annealing process.  Annealing at a lower temperature requires more certainty that the glass is all equal in temperature.  This means a longer anneal (or temperature equalisation) soak is required.  It is also a good bet to slow the anneal cool to be less than you would use for a single glass.

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

Diagnosis of Fractures

Knowing what has happened to your piece when it is broken or cracked is important to developing your skills as a kilnformer.  Most of the knowledge about diagnosis comes from looking carefully at the cracks and the shapes apparent in the flawed piece.

Breaks in the Kiln

Breaks in fusing at tack or full fusing levels in the kiln are generally of four kinds.

Breaks with hooked ends

Breaks that go across the whole piece, with a hook or significant curve at each end, usually indicate an annealing problem. The slight hook seems to result from inadequate annealing. The break will have sharp edges as it occurs as the glass is entering the brittle stage.

Multiple breaks in a crazed pattern

Crazed glass – similar to the cracks in ceramic glazes - usually indicates the glass has stuck to the supporting materials. These materials can be shelves or moulds. It is a sign there was not enough separator present between the two surfaces.

Breaks following the edge of glass pieces

Breaks that skirt around colours or pieces of glass almost always indicate a compatibility problem with the glass pieces chosen.  In severe cases the crack will be all around the incompatible pieces of glass as though it is trying to escape the base layer.  Sometimes the break will be from side to side, but skirting the incompatible glass.  These breaks will have sharp edges as the compatibility problem only becomes apparent on the cool.

Breaks from side to side following the line of glass pieces is not an infallible indicator of incompatibility, though.  Glass which has varying levels or thicknesses can break alongside the thicker areas, even though the glass is compatible. Often the break will be rounded due to temperature differentials in the glass on the heat up.  As the glass continues to get hotter, glass pieces on top - or strongly contrasting colours - can heat as such different rates that the stress overcomes the strength of the glass.

Of course, this kind of break can be sharp because the break occurred during cooling.  In effect, this appears to be an annealing problem when it really is a problem in matching the scheduling with the annealing requirements of a complex piece.  You need much longer soaks and slower cooling on tack fused pieces than on flat fused ones.

These two contrasting causes of a break means that you need to think about how the glass is layered.  One is to do with compatibility and the other to inadequate annealing due to the complexities of the layup.  They also tie up with the fourth cause of breaks.

Breaks can also follow the edges of inclusions.  This of course, indicates incompatibility.  All metals are incompatible, but if thin and not excessively large in relation to the piece, the glass is strong enough to contain the stress.  When the metal or other inclusion is too large, strong, or thick, the glass will break or show cracks around the inclusions.

Broken and separated lower layers

Sometimes people will open the kiln to find the lower layer of a multi-layer piece has broken and separated a small distance.  This is the fourth kind of break. This break will most often be a nearly straight break from edge to edge.  The broken edge will be rounded but the top layer(s) will have the expected profile.   This is an indication that the heat up was too fast not allowing the lower layer to achieve the same temperature as the top. 

This most often happens where there is an exposed lower layer (which gets hot) along with areas on top that get equally hot, but not the glass underneath.  Glass is a poor conductor of heat, so the upper layers "shade" the heat from the glass below.  The temperature difference between the two can be great enough to break the base glass apart but leave the top intact.  You know this was on the heat up because the layers of glass could move independently when the base broke and moved under the upper layers.  The glass was not hot enough to be sticky yet, so it had not reached lamination temperatures before the break.

Rounded vs. sharp edges

In addition to the location of the breaks, the condition of the edges is important in diagnosis of the cause of the problem. The accepted rule is that rounded edges mean the break occurred during the heat up.  Sharp edges occur during the cooling.  This is most often the case (but see the conditions for slumping). For flat pieces breaks that occur on the heat up will be rounded to some extent.  In a full fuse, usually the edges of the break will be rounded similar to the outside edge.


Cracks on the bottom surface

Sometimes the broken pieces will recombine either partially or all along the line.  There may even be a full recombination leaving only a crack like appearance on the bottom.  This recombination also will be the case where there was where only a partial break or crack in the early stages of firing. It leaves a smooth top surface, but a visible crack on the bottom. That means there is only a marginal reduction required in the scheduling of the initial rate of advance, as the temperature differentials were not great enough to break the piece completely across.

Force of Breaks

The space between the broken pieces shows the relative force that caused the break.  Greater space is related to more stress; lesser space or only partial cracks indicate a lower amount of stress. The amount of space indicates the degree of change required in scheduling. A small parting of the glass requires only a little (maybe 10% - 15%) reduction in the rate of advance.  Large spaces indicate that much slower rates of advance are required, and possibly a complete rethink in the scheduling of the firing.

Slumping breaks

Breaks in slumps are usually caused by a too rapid rate of advance. But this is not always the case.  The usual check of a sharp or rounded edge to tell when the break occurred does not work well at slumping temperatures.  The edge will be sharp whether it occurred on the heat up or the cool down because the temperature is not high enough to significantly round the edges.  The test must be different on slumps than that of sharp edges.  The test is related to the shape of the pieces. Take the pieces out of the mould.  If you can fit them together exactly, the break occurred on the cool down.  This usually will mean the anneal soak was too short and the anneal cool too fast.

Most slumping breaks occur on the advance in temperature.  The means of determining when the break occurred can be tested by putting the broken pieces together.  If they do not match exactly, the break occurred during the heat up.  This is based on the observation that broken pieces separated slightly in the mould by the force of the break on the heat up, and so will slump in the mould in slightly different ways from each other due to their positions.

Remember the blank for slumping is thicker than the original un-fused pieces.  This thickness requires a slower heat up than the original blank consisting of separate pieces.  In addition, the glass is supported at the edges of the mould which can allow the central area of the glass to heat faster than the edges, so further slowing the rate of advance is required.  These two factors of thickness and supports explain most of the breaks during slumping.

Splits in slumps

Sometimes the upper surface of the slump appears fine.  It is the bottom that exhibits a split or tear that does not go all the way to the upper surface of the glass. This is similar to the cracks on the bottom of a flat piece described above. It indicates the rate of advance was too - but only just - too fast.  The rate of advance has been quick enough to get the top heated and become plastic. But the lower surface is still cold enough that it is brittle. The weight of the upper softened glass begins to push down before the bottom has become hot enough to be plastic.  The force of the weight of the upper portion of the glass can be enough to cause the glass to separate because it is brittle, rather than move as the surface does. This split on the bottom but not the top indicates a slightly slower rate of advance for the thickness of the glass is required.

Breaks out of the Kiln

Breaks after the piece is cool

Breaks that occur days, weeks, months after a piece is cool can be impact damage, annealing or compatibility problems. 

Impacts

Impact breaks will be obvious in handling or moving other pieces near to the affected piece.  Usually there is evidence of impact by a small chip removed from the glass at the origin. The piece may or may not have been stressed to allow an easy break rather than a chip.  It is not possible to be sure of the secondary cause after the primary impact damage has occurred.

Breaks in warm glass

If the break occurs shortly after having been removed from the warm kiln, it is probable that the thermal shock to the glass has a contributory factor to incompatibility or inadequate annealing.  The diagnosis of the cause is the same as for breaks in the kiln - hooked for annealing and straight or following colours or inclusions for compatibility.

Breaks in cold glass

If the glass has been sitting undisturbed in a shaded place and suddenly breaks, the reason can be there was an incompatibility or that the annealing was inadequate.  There usually is not much difference in the breaks in a piece that has been cold for a long time.  If the break distinctly follows colours or pieces of glass, that would indicate a compatibility problem.  If the break crosses colours and thicknesses it is more likely to be an annealing issue.  But, as you can see, there is no certainty in this distinction as to the causes of breaks a considerable time after removing from the kiln.

Glass in strong light

Glass placed in strong sunlight that breaks can be incompatibility or simply contrasting colours being heated unevenly by the sunlight.  It is difficult to tell with certainty whether it is compatibility, annealing, or heat differentials that have caused the breakage.

Problem Solving

The essential purpose of problem solving is to prevent the same thing happening again. To solve the breakage problem, you need to think about the interrelationships between the various parameters – firing rates, soaks, cooling rates; and the ways in which the glass was set up.

Rounded edges

If the break is shown to be in the early stages of the firing, they most generally are caused by thermal shock.  They will generally be straight on an evenly thick piece.  If the piece is with variations in thicknesses, the line of the break may follow the thicker pieces. In both cases, you need to think about the rates of advance you are using.  If the separation of the edges is small enough that they have begun to recombine later in the firing, the rate of advance was only a little too fast.  If there is considerable space – say more than a finger width – the rate of advance was significantly too fast.

Sometimes the condition of the upper glass can give an indication of when in the firing the break occurred.  On a first firing, if the upper piece has broken together with the lower one, the break occurred after the pieces became sticky. This would mean the break occurred at or higher than laminating temperatures.  This is rare during the heat up.

If the break has moved small top pieces, it indicates the break occurred early in the heat up.  Sometimes the break will occur under the top piece.  Later it slumps and fuses into the space created by the break.  This also indicates a break early in the firing.  All these conditions indicate that the initial rate of advance needs to be slowed to avoid the thermal shock.  It does not indicate that soaks should be added at various stages up to the softening point of the glass.  Glass generally behaves better with steady, gradual inputs of heat rather than quick rises with soaks (although there are exceptions).

Sharp edged breaks

These occur generally on the cool down or after the piece is out of the kiln for a while.  If the break has occurred in the kiln, you should look at it carefully before moving it.  The relative location of the pieces can tell you some things about why.

Crazed glass normally indicates the glass has stuck to the supporting material – shelf, moulds, or other rigid materials.  This crazing may all still be in one piece, or slightly separated, sharp edged chunks.  These effects indicate there was not enough, or appropriate, separator for the process used.

The distinction between annealing and compatibility breaks is given above. 

Breaks all around a piece or pieces – looking as though they were trying to escape the base - clearly indicate an incompatibility problem.  You need to identify that glass and separate your stock of it from the rest of your fusing glass. 

Cracks that skirt pieces of glass can be incompatibility.  This is easiest to determine on flat pieces which have been full fused, or nearly so.  There is not a variation in thickness to complicate matters.  In full fusing, if the break skirts around a piece or pieces of glass along its path, it is likely caused by incompatibility between pieces and their base.

Breaks skirting pieces can also indicate problems with thickness, especially in tack fusing.  The more angular the tack fusing is, or the greater the difference in thickness, the greater the potential for an annealing break.  The annealing soak for tack fusing needs to be significantly longer than for a flat fused piece of even thickness.  Recommendations vary, but the anneal soak time needs to be at least twice the thickest part.  The anneal cool rate also needs to be half that for the the thickest area.

Breaks or cracks across the piece with hooked ends indicate inadequate annealing.  This will require some consideration to come to the appropriate length of soak and rate of the anneal cooling.  The anneal soak is about getting all the glass to the same temperature - top to bottom, side to side.  The soak is about temperature equalisation not just annealing.   This is shown by the Bullseye research on annealing thick slabs.  They discovered that a longer soak at a lower temperature can provide as good a base for the anneal cool as a higher temperature. The differences are that the soak at the annealing point can be shorter, but the annealing cool is much longer.

Annealing continues below the anneal soak - whether you chose the annealing point or a temperature below.  Bullseye uses a temperature about 30C below the annealing point.  This can apply to any glass.  Because the glass is cooler, a longer temperature equalisation soak is needed. But the anneal cooling range is shorter, making for a reduction in cooling time for thick slabs.

The point of this discussion is that when considering the solution to annealing breaks, you need to have a relation between the temperature equalisation soak and the rate of the anneal cooling.  If you have decided you need a longer soak, then you also need to reduce the rate of the anneal cool.  If you do not, you will still have annealing breaks or even thermal shock breaks, even with long soaks at or below the annealing point.

Breaks of slumped pieces

Breaks in slumping almost always appear to be sharp edged, unless you look carefully at the edge.  Fitting the pieces back together will give an indication of when the break happened.  If they fit, the break occurred upon cooling.  The anneal may have been inadequate, or the cooling too fast.  Unfortunately, in a formed piece, the curved hook of an inadequately annealed piece does not often show up.

If the break occurred early in the firing, the piece may still have sharp edges, unless you were firing at the upper end of the slumping range.  Here again the test of trying to put all the pieces back together is important.  If the pieces do not fit exactly together, the break occurred during the heat up.  This will mean that you need to slow the rate of advance for subsequent pieces.

“It hasn’t happened before” Scenario

Often people experience breaks even though the set up was very similar and the schedule was the same over several pieces.  There are two responses to this – “what did you change for the firing of this piece that broke”, and “you have been skating on the edge of disaster for a while.”  Glass behaviour is predictable. Since the break occurred when the setup was very similar, and the schedule was the same, something has changed.

The first thing to do is to test for stress. This means test before the piece is broken, as once the piece has broken most, if not all, the stress has been relieved.  You will need to construct another piece in the same way as the successful or the broken one – whichever you prefer.  Test the flat fired piece for stress. Remember to include an annealing test, so you can determine if the stress is compatibility or annealing related.  If there is stress in the flat piece, but not in the annealing test, you need to consider whether all the glass is compatible, or you need to slow the annealing cool for the larger test piece.

Next you need to consider what was different.  Review the differences in set up of the piece – colours, arrangement, thickness, volume of material used – everything that might be different at each stage of the layup.  Note these differences and review them one by one.  Could have any one element been sufficient to make the firing conditions different?  Could a combination of these differences have been significant?

Are there any differences in the firing schedule?  Have you made any little tweaks in the schedule? What is different?  Different times of the day, different power supply, plugs in or out, venting, peeking, different shelves (or none) – any small thing that could have introduced a variable in the firing conditions.

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

Conclusion

Although breaks generally have only three causes – thermal shock, incompatibility and inadequate annealing – the diagnosis of which it is and how it was promoted is complex.  All three are forms of stress.  To problem solve, first attempt to determine the type of stress that induced the break.  Then attempt to determine the cause of that stress.

It is important in the early stages of a new kind of piece, or early in your fusing career to test for stress after each firing (although I fail in this often).  This will give you the information to progress to the next firing or to revise the conditions – glass or schedules – to remove the stress for this or subsequent pieces.

Firing Practices that Affect Kiln Elements

The way that you fire glass and other materials in your kiln affect the longevity of the kiln elements.  Some things you can do and avoid are given here.

Venting

Even if you have the best aluminium oxide coating, the fumes that emit from glazes, paints, organics, inclusions and devitrification solutions can still attack the element through cracks in the coating. Downdraft vents are your best defence against potentially harmful fumes. Downdraft vents pull the fumes from the kiln chamber before they have a chance to damage the elements.

If you do not have a downdraft vent your next best option is to prop the lid a couple of inches until the kiln reaches 540°C to allow the fumes out of the chamber. You should also consider leaving at least one peephole out during the entire firing for the fumes that escape above 540°C.

This presents a dilemma, as the recommendation is to keep the kiln closed from 540°C upwards to protect the glass from cold air drafts.   Those who rarely fire above 800°C do not have the same problem as those who regularly fire at 850°C and above for casting, combing, and melts.  The higher the temperature, the greater the effect of fumes on the elements.  At fusing and below temperatures the effect on the elements is not as great.  Thus, low temperature firings can follow the standard practice of closing the kiln above 540°C.  Those going higher, should consider venting the kiln all the way to the top temperature to reduce the wear on the elements.

Maintain an Oxidising Atmosphere

Elements need an oxidising atmosphere to provide a long dependable service.  Subjecting elements to reducing atmospheres will age the elements quickly.  This is be done by introducing organics or oils into the kiln without venting.  Among the things that will attack the aluminium oxide coating of the elements are

• ·        Carbon - this includes materials made from carbon and plant-based inclusions.   
• ·        wax burnout – it is best to steam wax out of moulds to eliminate most of the wax before any burnout, as the fumes are largely carbon.
• ·        halogens (such as chlorine or fluorine) 
• ·        molten metals (such as zinc, aluminium).  This is a more important reason for avoiding the use of zinc and aluminium in kilnforming than the possibility of health problems.
• ·        lead bearing paints and glazes – lead is a common component of paints, enamels and glazes.
• ·        alkaline metals – the main one we come across in kilnforming is magnesium which produces an amethyst colour of varying intensities.  This has a melting point of 650°C and boils at 1090°C, so some fumes can develop during firings and affect the elements.
• ·        borax compounds – used in enamel glazes and some devitrification sprays. 

If you use these materials in the kiln, you need to ensure that the kiln is well vented while these are in the kiln.

When you do have to use these elements - even when you vent - it is good practice to follow this firing by one without materials corrosive to the coating.  This allows the coating to re-form around the element surfaces after a corrosive firing.

Trying to do reduction firings in your kiln will greatly limit their useful life and is definitely not recommended.

Avoid Contaminants

Contaminants such as silica which is contained in kiln wash and some glazes attack the aluminium oxide coating of the wire.

Powders, paints and kiln wash accidentally touching the elements cause rapid corrosion of the elements if not cleaned off before firing.

Placing

Firing close to the elements allows any fumes from materials being used to affect the elements more than allowing some space between the glass and the elements.  This provides another reason to keep the glass away from the edges of the kiln in addition to the possible uneven heating of the glass.

High Temperature Firings

High temperatures with very long soak times will accelerate an increase in element resistance through the differential expansion of the inner wire and the coating. The higher the temperature, the longer the soak, the sooner the element will decrease in life. Usually short soaks work much better for the longevity of the element.  This is not such a big factor for glass kilns as it is for ceramic kilns.

The next part in this series deals with the maintenance of the elements.


Earlier relevant posts
Element Description

Aging

Kiln Elements - Aging

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

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

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

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

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

Other relevant posts:
Nature of elements
Maintenance

Kiln Elements

Questions arise on whether old elements can become inefficient or hold a “memory” of previous firings. Old elements increase in resistance so decreasing amperage and consequently reducing the temperature that can be achieved or speed with which the kiln can reach the top temperature required. It not a "memory" of heat or temperature.

This series of blog posts will look at the nature of elements, their aging, effects of firing practices, and maintenance of the elements.

Nature of elements

A kiln element is a wire designed to have considerable resistance to electricity passing along it so creating heat.  The measurement of the amount of resistance is in units named ohms.

Most electric kiln elements are made of Kanthal A-1, a trade name of Sandvik.  This is an alloy of iron, chrome and aluminium. During the first firings the wire develops an aluminium oxide layer on its surface.  This helps protect the rest of the substance of the wire from further corrosion.

It is critical to the life of the elements to develop this aluminium oxide coating and is the reason people are told to fire their kiln clean and empty.  Any contaminants, including dust can inhibit the formation of the protective oxide locally.

The new kiln elements begin to achieve their protective coating when they reach 1000°C.  The kiln does not have to reach that temperature, just the element.  But it is advisable to fire toward 950°C at a moderate rate of about 250°C per hour and soak there for half an hour to ensure the coating is firmly in place. Your kiln may never be fired as hot again, but you will be sure the elements are properly prepared.

It is important to remember that the elements are flexible when heated initially, but after a few firings become stiff and brittle.  After the initial firings, you need to ensure the elements are still well seated in their groves.

A well-designed kiln will have the largest diameter element wire and the largest distance between the coils (runs cooler). The thicker the elements, the greater the stability and the longer the life.  When elements get above 925C they become very soft.  As they soften, the coils begin to collapse, causing the distance between the coil turns to lessen. When the distance between coil turns is small the element will overheat in those areas.

The next in this series is about the aging of elements.
Aging of elements
Firing Practices
maintenance

Slumping Breaks

“Why does my full fused disc break when I slump it?”

There are several possibilities. The two main ones are annealing and ramp speeds.

Inadequate annealing in the fusing stage can lead to a very fragile piece when being re-heated.  If there is significant residual stress in the fused piece, it is much more sensitive to heat changes during subsequent firings whether full, tack, or slumping/draping. It is important to thoroughly anneal any piece at every firing.  If you are firing a different layup or contrasting colours and styles, you should check for stress using polarising filters.  

The slump – or drape – firing needs to be much slower in temperature rise than the fuse firing.  You now have a thicker piece which takes longer to absorb the heat evenly. 

If your piece is tack fused, it needs an even more slow rate of advance.  Sometimes this needs to be as though the piece were two to four times the actual thickness of the piece.  The more angular and pointed the tack fused elements, the greater the reduction in firing speed.  This post gives guidance on how the piece is designed and its thickness affects the rates and soaks in tack fusing. 

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

Observation

The importance of Observation (and recording)

Observing what you, or the kiln, is doing whether you are using a cartoon, or a schedule obtained from elsewhere – including the kiln manufacturer – means that you will learn much more quickly as you progress.  You will be able to alter things as you go.  This applies to all stages of the piece from design to removing the piece from the kiln.

The Design

Once you have made your design – whether as a drawing or a mock-up – look at it.  Really look at it.  Look at it from a distance, climb a ladder if you can’t pin it on a wall and look down on it.  Look at it from the sides so you have an oblique angle view.  Turn it upside down to confuse your expectations and so see what is really there.  Look at it, using a mirror to see if it still looks good. Make the alterations you need as you go along to get the look you want and then repeat the process until you are happy.

Assembly

Observe how you have put the piece together.  Do the pieces fit?

Is everything in the right place? Are the colours right? Does it match your vision – symmetrical or asymmetrical?   Do these things as you progress, so you become aware of the process and its rhythm.

Firing

Once you have determined your schedule, you cannot just leave the piece. If you are new to fusing, you need to observe the stages of firings to begin to understand what is happening to the glass at various temperatures and rates of advance. You would not put a cake into the oven and leave it without checking on it from time to time. Why would you fail to observe a much more expensive process?

Even when you are experienced  - observation of new layups, new processes and anything you haven’t done several times before - you need to know how things are progressing during the firing.

Observing a firing is relatively simple.  You need to check on two things:

·   Check for a too rapid rate of advance.  Peek into the kiln at around 540C to see if the piece is still whole.  If not, you can abort the firing and progress to fixing or move on to another project.

·   Check to see when the desired shape has been achieved.  Peeking to see if the slump is complete or needs more time is important to getting the shape right.  Peek to determine if the tack fuse has been achieved.  When it has, advance to the next segment to avoid over firing.  If it hasn’t, add time to the schedule to get it right.

Recording

It is not enough to simply observe.  You need to record what you intended and the results you achieved.  That includes what you did to get things right as well as wrong.  What did you do to correct elements?  These are all things that you will need to refer to in the future.

The key to rapid learning is observation and recording what you see.

Kiln wash

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

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

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

Multiple firings of kiln wash

Textured Side

There is a little concern about whether the textured side of the glass pieces in leaded and copper foiled glass should be towards the inside or outside.

The traditional advice is to have the textured side toward the inside.  This is based on the piece being used as a window. It is easier to keep the weather side clean if the smooth side is on the outside. The same thinking leads to the recommendation to allow the cemented panel to rest with the smooth (outside) down.  This minimises the thickness of the putty and so allows less water to collect on the outside horizontal leads.

If the window is not primary glazing, it does not matter which side, nor how consistent you are in placing the glass.  It becomes a matter of aesthetics – which ever way you prefer is fine if it gives you the effect you want.

There is a small visual effect if you are using transparent glass.  There is slightly more dispersion of light if the textured side is outwards. 

Placing the textured side inwards can be useful if you wish to indicate a rough surface contrasting with a smoother one.

These considerations show that the placing of the textured side is largely determined by the function of the panel and the aesthetics applied.