Kiln Characteristics Investigation

Many people ask about the best kiln to buy.  Sometimes they mean the cheapest, but mostly they mean the best for their favoured processes. To get the best from your proposed kiln, you should be aware of its characteristics and how it fits your proposed kilnforming practice.  There are a range of factors that interact to give the special conditions of your kiln.  They range from the purpose, the materials of construction, the placement of heating elements, how it opens, and its shape.  All these can affect the degree of even heating of the kiln bed or shelf.

Kiln types

There kilns for many purposes. Some of them are powder coating of metals, enameling of metals, vitreous painting of glass, glass forming, ceramics, casting of glass and metals, lehrs for annealing, and furnaces among many others.  

Large powder coating kiln

Large enameling kiln

Jewellery enameling kiln

Electric glass painting kiln with multiple shelves

Example of a sheet glass annealing lehr

For our purposes we are concerned with the glass and ceramics kilns.

Ceramics vs Glass Kilns

In general ceramics kilns are made to lose heat slowly, while glass ones are designed to lose heat relatively quickly.  There are many glass kilns based on ceramic ones.  You should be aware of the differences between kilns designed exclusively for glass and those based on ceramics kiln designs.

Small ceramic kiln

Small glass kiln

Construction Materials 

The materials used in constructing kilns are refractory insulation and a steel structure of a design to hold all the refractory materials together. 

Refractory bricks for glass kilns are light weight and usually designed for temperatures under 1200°C (dense bricks rated much higher are normally used in ceramic kilns). 

Light weight refractory brick

Bricks tend to be used in most glass kilns on the floor as well as the walls (some smaller ones use only refractory fibre).

Small fibre kiln

Kilns derived from ceramics tend to have brick walls and lids.  Most kilns designed for kilnforming have fibre walls and lids.  In the cases of top hat opening kilns, fibre is a necessity to reduce the weight of the lid.

Fibre board and fibre blanket are used widely.  The floor tends to have a floor consisting of steel, fibre board on top and brick on top of the board. Fibre blanket tends to be used on the walls and ceilings of rectangular glass kilns. Oval and circular ones tend to have brick walls and ceilings.  The use of fibre board and blanket walls and ceilings leads to a more rapid cooling than those with brick ones.  This will affect the scheduling of the kiln firings.

The steel used to contain and support the refractory materials is important.  Many kilns use mild steel in sheet form to fill the spaces between the heavier structural support steel.  The higher quality kilns use stainless steel sheet, even though they may use mild steel for structural support.  The stainless steel lasts much longer than mild steel, especially when there is liable to be moisture involved in the kiln processes, such as pate de verre or casting.

Opening Method

This post gives a description of the common methods of opening the kiln.  

The purposes for which you want to use the kiln relate to the firing characteristics needed.

Top opening

Top opening kilns have the advantage of depth, normally with elements around the sides.  This makes them good for casting, but not so good for processes that need observation or manipulation.  The depth is most useful in casting  and deep slumping work, but requires a lot of experimentation to make use of multiple shelves in one firing.

Front opening kilns have the advantage of being able to observe the whole depth of the firing, if you protect yourself from the heat that will be dumped from the kiln.  They often have elements on the sides which is an advantage for drops and melts (when observation is necessary).

Top hat opening kilns are those that have the whole heating chamber hinged at the shelf level.  These are very good for placing of work, as you can work directly above the pieces.  These are one of the best types of kiln for combing or any other manipulation of the glass during the firing. You can also observe by opening the kiln a little during the firing.

A range of top hat and a bell kiln

Bell kilns are those where the whole of the heating chamber lifts above the bed.  These are often equipped with two bases which can be wheeled in turn under the chamber which is lowered before firing.  These tend to be very large kilns.

Small gas fired kiln

Heat source

Most kilns are heated with electrically powered elements, either exposed or in quartz tubes.  The quartz tube contained elements provide more even heating than the exposed ones.  The most even heat is provided in gas fired kilns, although these are generally more expensive and less widely available.

Element Placing  

The location of the heating elements can have a significant influence on the way you fire your glass.

·        Top fired kilns are generally the easiest to use as the glass is most affected by radiant heat.

·        Side fired kilns provide the radiant heat to the edges of the glass first, before the air temperature can begin to affect the surface of the glass.  This means more caution is required in the heat up of the glass.  However, side elements are very useful in drops and casting processes.

·        Some kilns have both top and side heating elements.  This provides flexibility in heating up and in cooling evenly.

·        A few kilns have elements around the sides but below the shelf.  This promotes even cooling of glass from both the top and bottom. It is most useful in dealing with the cooling of thick slabs.

Kiln sizes and shapes

Kiln sizes have an effect on the behaviour of the kiln.  Smaller kilns (depending on the refractory materials) generally heat and cool quicker than large ones.  The mass of a larger kiln takes more energy to heat up and more time to release the heat than smaller ones do.  This will influence the scheduling for different sized kilns.

 

The shape of the interior of the kiln affects the distribution of heat within the chamber.  Rectangular kilns tend to have cooler corners than circular ones (as there are no corners).  Oval kilns tend to give space for longer pieces and reduce the cool corners.

 

The height of the kiln also affects the heat distribution within the kiln.  Taller kilns are cooler at the bottom than the top, even with side elements.  They are especially good for casting and drop processes.  Deeper kilns, even if rectangular, require more energy to complete any given process, because of the distance between the radiating elements and the glass.

Hot and cold spots can be tested for by using this method.  The actual operating temperatures can be tested by the use Orton cones to measure heat work. This depends on the speed used to get to the process temperature.

There are many factors that make up the characteristics of kilns. The main ones are style, construction materials, opening method, shape and depth. These need to be considered in relation to the kind of kilnforming you intend doing, to make the selection optimum for your practice.

More information is available in "Your New kiln" from Etsy shop VerrierStudio: https://www.etsy.com/uk/shop/VerrierStudio

or direct from stephen.richard43@gmail.com

Large Tiles for Kiln Shelves

Pizza stone in use

People frequently wonder if other materials than mullite can be used for kiln shelves.  Mullite is used for its strength and very small expansion, even at high temperatures, as used in ceramics firing.  There are other materials that can be used in kilnforming of glass such as refractory fibre board, and ceramic pizza stones,  the best of which are made from mullite.  This post is about using ceramic floor tiles.

An unglazed floor tile, 11 x 11 inches

Structural Soundness

A major element in obtaining and using a floor tile is how sound it is.  Tapping the tile to determine whether the sound is a low toned ring or a dull thud is important.  There may be invisible cracks within the tile.  A dull thud is an indication that the whole tile has one or more cracks in it, or that it has not been fired high enough to completely vitrify the clay. A low frequency tone indicates there are no cracks and that it has been fired sufficiently high.

Flatness

The first thing you need to do is make sure the ceramic tile is flat and without undulations before using it. To test this, get a straight edge and move it along the tile to look for any slivers of light coming through underneath the straight edge. Any light or variation in the amount indicates depressions that can produce bubbles during the firings. Do this test at least twice at right angles to each other.  Take note of the depressed areas (or even possibly high areas) to know where these uneven areas are to work them out of the tile. 

You can do the above test in the showroom.  Another more accurate means of checking is more difficult to do in a sales area.  Place a line of dark powder, say black glass powder, and with a straight edge held vertical to the shelf, drag the powder across the shelf.  Where there are dark patches is an indication of depressions.  The area and depth can be seen from the spread of the visible powder and to some extent the density of the colour.

Making Shelf Flat

If you buy two of these large tiles, you can rub them together face to face in circular motions. The abrasion marks will show the high spots, with the low spots clear of those marks.  This will indicate the amount of work needed to get the whole surface even.  The smaller the unmarked areas, the less grinding will be required. You can add an abrasive with some water to form a slurry and continue to grind until everything is even. The use of water with the abrasives is important to eliminate dust which might be harmful, and to ease the grinding process.

The above is a manual process.  If you have a large enough flat lap, you can mechanise the flattening process.  Using decreasing grit sizes, you can grind the shelves level with a high degree of smoothness. You do not have to use a grit of less than 200, as the tile structure is even more coarse than that.

If you can't find unglazed floor tiles, you need to look at the back of the tiles.  Many floor tiles have a grid pattern on the back to ensure sufficient adhesive is used.  This makes getting the back, unglazed side flat more difficult or time consuming, because they will need to have the grid ground down to the lower surface.  In this case, it may be that you need to sandblast the glazed side before making sure it is flat.  The sandblasting can make a flat tile uneven by unequal times spend on various parts of the tile, so you have to check after sandblasting for the flatness.

Bubbles on Drop-out Rims

Sometimes people doing dropouts get bubbles or unevenness on the rims of their pieces.  This means that it is not suitable to leave the rim on the piece.  Most times, this does not matter, as you intend to cut the rim away. But if you do want to have a rim these uneven surfaces are unsightly and not suitable for high quality pieces.

One person has indicated that they used a schedule of 250°C per hour to 520°C for a 30-minute soak and then proceeded at 330°C to the top temperature of 710°C.  This is probably too fast a heat up at the second segment.  Slower rates of advance are advisable.

One of the advantageous methods of scheduling for dropouts is to put the heat into the glass steadily.  I suggest there are two problems with the rates of advance and soaks in the above (partial) schedule.

The soak at 520°C would be more useful if it were at around 600°C.  This would allow the heat to be distributed throughout the glass before it begins to droop significantly.

The rapid advance of 330°C is much faster than needed, or desirable.  This rapid rate of advance allows the glass to move into the aperture, before the rim is plastic enough to stay on the supporting ring.

These rough drawings show how the rim initially rises from the ring, pivoting on the edge of the aperture.  This happens on all moulds (drops or slumps) where there is a rim.

With a rapid rise in temperature the raised rim edge gets more heat than the depressed middle, as it is closer to the elements.  This additional heat allows the edge of the rim to curve downwards forming air pockets as the edge touches back to the supporting ring.

Some people use fibre paper between the ring and the glass to prevent bubbles. This addition allows a passage of air from under the glass and reduces bubble formation.

Others have developed sloped drop out rings that eliminate the rising of the glass from a flat ring.  The glass is suspended above the aperture and only touches the edge of it as the glass softens. These crude drawings show the process.

To be certain of avoiding air bubbles under the rim of dropouts whatever the style of ring, you should use moderate rates of advance, with a possible soak at around 600°C which is before the glass begins any significant movement. The moderate rate of heating should be continued after this soak, rather than increased.

Drying kiln wash

“Dry your kiln wash between coats and before firing.” 

This is a frequent statement when talking about renewing kiln wash on shelves and moulds.  The main reason given seems to be that there will be less risk of creating bubbles by evaporating moisture.  The air drying will reduce moisture in kiln is a second reason.

There are some difficulties with this statement and reasons.

Drying between coats of kiln wash means you are applying liquid over powder. This can promote clumping and streaking through a too rapid absorption of water by the dry kiln wash. Also, it makes applying kiln wash a lengthy process.  It is not like painting a door or even a floor, where you must allow drying to avoid streaks. 

Credit: Ceramicartsnetwork.org

Applying kiln wash by brushing is smoothest if all coats are done at once.  This is what happens if you spray kiln wash on your shelves and that gives a smooth surface.  If it were otherwise, drying between coats would apply to spraying too.  Drying between coats promotes streaks in the applied kiln wash that needs to be smoothed before use.  This of course, does need to be done after the kiln wash has dried.

Drying before using the shelf or mould is unnecessary. The evidence I have to offer is that I frequently fire within an hour of applying fresh kiln wash to a cleaned shelf. I have had no problems with creating bubbles or glass picking up the kiln wash. The shelf dries, with a moderate rate of advance, long before the glass settles into the texture of the surface.  It is only as the glass settles into the contours of the kiln wash that it seals air, or any other material, under the glass.

The pigment in most kiln washes is to tell you which shelves have not yet been used.  If they are fired dry at even moderate temperatures, the pigment disappears.  Then you have removed that indicator of freshly prepared shelves or moulds.

Drying of kiln wash before use in not necessary.  If you wish to be cautious, air drying will be enough to avoid any problems with moisture.

What are enamels?

Not all enamels are equal

Enamel paints

This description refers to a paint that air dries (or with minimal heat) to a hard finish (usually gloss). Most commercially available enamel paints are significantly softer than either vitreous enamel or heat cured synthetic resins. The term "enamel paint" generally is used to describe oil-based covering products, usually with a significant gloss finish. Many latex or water-based paints have adopted the term.

Enamel paint has come to mean a "hard surfaced paint" and usually is in reference to paint brands of higher quality, floor coatings of a high gloss finish. Most enamel paints are alkyd resin based. Some enamel paints have been made by adding varnish to oil-based paint. Enamels paint can also refer to nitro-cellulose based paints. Nitro-cellulose enamels are also commonly known as modern lacquers.  These have been largely replaced by synthetic coatings like alkyd, acrylic and vinyl.

More information at: https://en.wikipedia.org/wiki/Enamel_paint

Enamel paints are used for coating surfaces that are outdoors or otherwise subject to hard wear, or variations in temperature.  A widespread application is in paints for cars. It is also used frequently to decorate or label bottles due to the low curing temperatures of some formulations.

Vitreous enamels 

Vitreous enamels are used in a variety of circumstances.  Metal signs are most frequently enamel coated; they are used in ceramics as over glazes;  and they are used on glass in many circumstances.

Vitreous Enamel is simply a thin layer of glass fused at high temperature on to the surface of a metal or glass. Vitreous Enamel can be defined as a material which is a vitreous solid obtained by smelting or fritting a mixture of inorganic materials.  The word enamel comes from the High German word ‘smelzan’ and from the Old French ‘esmail’.

The key ingredient of vitreous enamel is finely ground glass frit. Colour in enamel is obtained by the addition of various minerals and metal oxides. 

Vitreous enamel is made by smelting naturally occurring minerals, such as sand, feldspar, borax, soda ash, and sodium fluoride at temperatures between 1200°C and 1350°C  until all the raw materials have dissolved. The molten glass which is formed is either quenched into water or through water-cooled rollers. This rapid cooling prevents crystallisation. The resulting frit is ground in a rotating ball mill either to produce a water-based slurry or a powder.

At the milling stage, other minerals are added to give the properties and colour required of the final enamel. Different enamel colours can be mixed to make a new colour, in the manner of paint. Enamel can be transparent, opaque or opalescent.

More information at: 

https://www.vea.org.uk/what-is-enamel/

https://en.wikipedia.org/wiki/Vitreous_enamel

https://www.iom3.org/vitreous-enamellers-society/vitreous-enamel-information

Metal enamelling

Modern frit for enamelling steel is typically an alkali borosilicate glass with a thermal expansion and glass temperature suitable for coating steel and other metals. Raw materials are smelted together between 1,150 and 1,450°C (2,100 and 2,650°F) into a liquid glass that is directed out of the furnace and thermal shocked with either water or steel rollers into frit. Vitreous enamel is often applied as a powder or paste and then fired at high temperature. This process gives vitreous enamel its unique combination of properties. The smooth glass-like surface is hard; it is scratch, chemical and fire resistant. It is easy to clean and hygienic.  It all started 3500 years ago in Cyprus. Since 1500 BC, enamelling has been a durable, attractive and reliable material.

More information at: 

https://en.wikipedia.org/wiki/Vitreous_enamel

https://www.vea.org.uk/what-is-enamel/ 

Enamels in Ceramics

Overglaze decoration, overglaze enamelling or on-glaze decoration are all names for the method of decorating pottery, where the coloured decoration is applied on top of the already fired and glazed surface, and then fixed in a second firing at a relatively low temperature.  The colours fuse on to the glaze, so the decoration becomes durable. This decorative firing is usually done at a lower temperature which allows for a varied and vivid palette of colours, using pigments which will not colour correctly at the high temperature necessary to fire the clay body.

More information at:  https://en.wikipedia.org/wiki/Overglaze_decoration

Glass Enamels

Glass enamels are produced in the same way as enamels for metals and ceramics.  The frit characteristics are adjusted for various applications and temperatures.  This combination of finely ground frit and metals for colouring are often combined with a binder or carrier medium.  It is similar to vitreous enamel on metal surfaces, but the supporting surface is glass. It is also close to "enamelled" overglaze decoration on pottery, especially on porcelain, and it is thought likely that the technique passed from metal to glass (probably in the Islamic world), and then in the Renaissance from glass to pottery (perhaps in Bohemia or Germany). 

Glass may be enamelled by sprinkling a loose powder on a flat surface, painting or printing a slurry, or painting or stamping a binder and then sprinkling it with powder, which will adhere.  The powdered frit can be in the ceramic on-glaze composition suitable for fusing or casting temperatures, or it can be adjusted for slumping temperatures as in the traditional glass stainers’ enamels. It can produce brilliant and long-lasting colours, and be transparent, translucent or opaque. Generally, the desired colours only appear when the piece is fired, adding to the artist's difficulties.

More information at:  https://en.wikipedia.org/wiki/Enamelled_glass

The term enamel is applied to a wide variety of coating materials.  The range of usage is indicated, and the manufacture and applications of vitreous enamels is indicated.  The term enamel is not properly applied to finely ground coloured glass in a medium.

Materials for making dams

Rectangular or straight sided shapes

Broken shelves

Accidents happen to mullite shelves causing breaks or cracks.  Rather than throwing them out, you can cut them into rectangles or 50mm strips with a tile saw.  The resulting shapes need to be kiln washed to keep glass from sticking.  They can be used flat or stood on their edges with supports on the outside.

Thick ceramic tiles can be used in much the same way.  You do need to remove the glaze from the tile to make sure they don’t stick to the glass.  Or you could use the unglazed side toward the glass. Again, the tiles need to be kiln washed.

Stainless steel can be used as a dam.  It will need treatment with a separator such as boron nitride or kiln wash.  In addition, it needs to be lined with refractory fibre paper to cushion the force of the greater contraction of steel than glass.

These materials cannot easily be adjusted in length to fit the size of the glass piece being dammed.  Instead, arrange them in a swastika like formation. 

This photo also shows how shorter lengths can be incorporated to make the whole dam.

Vermiculite board is a refractory material that can be used to form dams by cutting with a wood working saw.  The saw you use to cut the vermiculite will be dulled and only be useful for cutting vermiculite in the future.  Do not use any expensive cutting equipment!  

Credit: Bullseye Glass Company

Refractory fibre board is available in many thicknesses.  It can be cut with craft knives even though it dulls the blades quickly.  The thicker boards can be used without rigidising.  This avoids the need to kiln wash and allows adjustments in length.  If you do rigidise fibre board, you must coat it with a separator such as kiln wash or boron nitride.

Weighted fibre paper can be used.  It is sometimes the quickest and easiest to use, as there normally is a stash of scraps around the studio.  It is easily cut with a craft knife.  You can build up the thickness of the dam by layering pieces on top of one another.  Sometimes people put metal wire or pins in the layers to ensure there is no movement between the layers. I’ve found that if weighted, the fibres interlock enough that the layers do not shift.  But you need to line the layered fibre paper dams with vertical strips of fibre paper, so the glass does not take up the layered dam profile on its edge.

Note that you need to use breathing protection when cutting all these materials.

Curved and circular pieces

Many times, the shape to be dammed is not formed of straight lines.  Different materials need to be used in these cases.

Formed stainless steel is a good durable and reusable material.  You need to line the shape with fibre paper if it completely contains the shape, because it contracts more than the glass and can crush the piece.  It is expensive to have made and so needs to have multiple uses to justify the cost.  A cheaper alternative is to make your own shape using stainless steel strapping as used for shipping crates and pallets. 

Fibre paper is an excellent material for damming irregular shapes.  It can be cut into complicated shapes, and it can be layered to attain the required height. You can weight it if you are taking things to a high temperature and fear that the glass will flow under the fibre paper.

You can also use the thicker fibre papers upright by backing up with multiple pieces of kiln furniture to maintain the shape you desire.

Vermiculite board is a good material for making shapes, although not as complicated ones as possible in fibre paper.  Vermiculite can be shaped with wood working materials, but cheap ones should be used as they are quickly dulled. You can rough out a shape with a jigsaw and refine it with various wood working tools, including coarse sandpaper.  Because it is a relatively rigid material, a lot of inventiveness can be used in forming the edges by altering angles from the vertical, incising designs into the edge, etc.  Be certain that you have adequately kiln washed or put other separator on the board, as it will stick to the glass if left bare.

Fibre board is a less rigid material than vermiculite, but is easier to work with simple craft tools.  It is simple to use for a unique one-off shape. It only needs smoothing and does not have to have a separator applied because it does not stick to the glass.  If you create a shape that you will want re-use, you can rigidise the board after shaping, but it will require separators then.

Note that when working with refractory materials, you need to wear respiratory protection and clean surfaces with a HEPA vacuum or by dampening dusty surfaces and wiping them clean.  Dispose of cleaning materials safely.

Colour Dilution of Powders

Sometimes you do not have a tone or shade of a colour you need for your project.  Other times you want to have a gradation of shade across a piece.  There is the obvious solution of mixing a colour with clear to produce lighter shades.  But there is a difficulty when mixing clear with powders to fuse. The result is often a pointillist effect with points of light coming through the colour. There are several approaches to this difficulty.

One way is to use a powder made from a tint of the colour.  But sometimes there is not a tint made. Sometimes you do not have that tint in stock. So, you must look to other solutions.

Credit: www.warm-glass.co.uk

An alternative is to use clear powder to mix with the intense colour you want to dilute.  You will need to test varying proportions of clear to colour to get the tone you need.  You may be surprised at the amount of clear needed.  And there still is the slight possibility of points of light coming through the clear.

Another possibility is to use one of the less dense white powders to mix with the colour.  White powders such as the Bullseye 000243, translucent white, or the 000113, dense white are possible.  The very dense or lacy whites are not as suitable. One is too opaque, the other is uneven in colour. Again, testing will be required, and you may be surprised at how little is required to alter the tone.

One other way I have used is to mix fine frit with the powder.  This has less control than the other methods but can provide significant dilution of the intense colours.  If you want to see if this is suitable, you can follow this process. 

Add a few drops of water to the clear frit in a small container. Close it and shake to get all the frit coated with a film of water. If after shaking the frit is not “clumping” you can add a little more. Too much water will create a slurry which is not suitable.  So, add only a small amount of water at a time until the frit is like damp sand on the beach. Any excess water must be poured off. 

Add powder to the damp frit, and shake well again to coat the frit with powder. If the frit does not seem to be fully coated, add a little more powder.  The film of water on the frit allows the powder to adhere temporarily to the frit.  

This mixture can then be applied to the surface and smoothed with a pallet knife. This will not guarantee there are no clear pinpoints, but it will reduce them to a minimum. You will not have the subtle differences in tone that sifting powder can give you, but it is a cost-effective way of diluting intense powder colours that can have advantages over mixing powders.

Of course, the various methods of diluting colour described here can be used to combine powders to produce new colours.

Sharp points on rectangles

At the conclusion of firing pieces with right angles or sharper shapes you often find very sharp needle points at the corners.

This is a result of the expansion of the glass as it heats up.  At top temperature, the glass piece is larger on the shelf than when you put it in cold.  The amount of this expansion is related to the thickness of the piece and the temperature it has been fired at.

As the glass cools, it contracts.  The contraction at corners and points has greater effects on the glass than at the sides.  The corners are contracting from two sides rather than only one.  This makes them a little more resistant to contract and often leaves a little of the glass stuck at the furthermost point of expansion as it contracts.

I have found the best prevention of sharp points on the corners of rectangular pieces, and those with even sharper angles, is to nip off the tiniest bit of the corners. This very slight blunting of the corners allows the glass to expand and then retract without the corner or point catching on the separator and so creating the sharp needles.

Further information is available in the e-book: Low Temperature Kilnforming.

Soaks Below the Softening Point

There are frequent suggestions that holds in the rise of temperature for glass are required.  Various justifications are given.  A few notes before getting to the explanation of why they are uncessary.

A note is required about the softening point sometimes called the upper strain point. There is a reasonable amount of discussion about the lower strain point.  So much that it is often simply referred to as the strain point.    Below the lower strain point, the glass becomes so stiff and brittle that no further annealing can occur.  Thermal shock can happen though, so the cooling needs to be controlled.

There also is an upper point at which the behaviour of the glass is different.  Above this temperature, no annealing can occur either, because the glass has become plastic and the molecules randomly arranged.  It is only just pliable, of course, but its molecules are no longer strongly bound to one another.  This is the temperature at which much of slumping is done.

It is disputed whether such a point exists.  Still, in practical terms it is where the glass becomes so plastic that it cannot be temperature shocked.  The temperature of this “point” is approximately 45°C above the annealing point, rather than the temperature equalisation soak. 

Note that the temperature at which Bullseye recommends that the annealing soak should occur is a temperature equalisation point, which is about 33°C below the glass transition temperature - the point at which glass can be most quickly annealed.  The average glass transition point for Bullseye is 516°C.  Most other fusing glasses use the glass transition (Tg) point as the annealing temperature for the soak.  They or you could employ the Bullseye technique on thicker slabs of the glass by setting the temperature equalisation point 33°C below the annealing point, and soaking for the same kinds of time used in the Bullseye chart for annealing thick slabs.  In fact, this is what Wissmach has recently done with its W90 and W96 fusing glass ranges.  They now recommend 482C (900F) as the anneal soak temperature.

Now to the point of the post.

The soaks that are often put into schedules on the rise in temperature are justified as allowing the glass to equalise in temperature.  Glass in its brittle phase is an excellent insulator.  This means that heat does not travel quickly through the glass.  Consequently glass behaves best with steady and even rises in temperature (and correspondingly on the reduction in temperature).  Rapid rates risk breaking the glass on the temperature rise, no matter how many or how long the holds are.  

This means a slower rate of advance will accomplish the heating of the glass in the same amount of time, and in a safer manner, than rapid rises with short soaks/dwells/holds.  The slower rate of temperature increase allows the glass to absorb and distribute the heat more evenly.  This slow heating is most obviously required in tack fusing where there are different thicknesses of glass.  

This means that it is possible for thin areas of glass to heat up much more quickly than glass covered by different thicknesses of glass.  It also applies to strongly contrasting colours such as black and white, because they absorb the heat differently - black more quickly than white.

There are, of course, circumstances where soaks at intervals are required – usually because of mould characteristics, in slumping, and in pate de verre.

Sometimes people add a soak at the annealing temperature on the way up in their schedules.  This is unnecessary.  If the glass has survived up to this point without breaking, it is highly unlikely it will break with a further increase in the rate of advance unless it is very fast.  The temperature after all, is above the strain point meaning the glass is no longer in the brittle phase.

Many people add a soak at around 540°C (ca. 1000°F) into their schedule on the increase in temperature, before their rapid rate of advance to the top temperature.  The choice of this temperature relates to the lower strain point.  This also is unnecessary, except possibly for very thick pieces. By this time the glass has reached its plastic stage and if it hasn’t broken by then, it won’t with a rapid rise in temperature either.

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

Soaks at various temperatures during the advance to the upper strain points of glass are not necessary.  What is necessary is a knowledge of when the glass becomes plastic in its behaviour, and an understanding of how soaks can overcome characteristics of moulds, or how to achieve specific results and appearances of the glass.