Why do kiln shelves break?

Kiln shelves are made of clay – a very hard clay, sometimes called mullite. So when firing you need to remember that like other ceramic materials it can be heat shocked.

examples of broken shelves

The recommendation is that you put the shelf on supports to keep it above the base of the kiln and allow air to circulate around both the top and bottom of the shelf.

The question remains, why do the shelves break. There are at least two reasons: physical impact and thermal shock. It is possible to knock the shelf while moving it around the studio. This impact does not always cause a break, but sometimes creates a stress point that later can develop into a crack and break. You can sometimes see the start of the crack from the edge of the shelf. In this case, you can either continue to use the shelf with support under the crack or dispose of it immediately, because at some point during a firing it will separate.

The thermal shock that causes the break occurs because (usually) uneven cooling. It seems the shelves are pretty resistant to rapid heating, but less tolerant of rapid or uneven cooling. In general un-dammed fusing and using moulds elevated a little from the shelve do not create that uneven cooling.

However placing a large refractory mould directly on the shelf can promote cracking either immediately or on subsequent firings.

Broken shelf with casting moulds laded to one side 

The main culprit in any breakage seems to be large or heavy and damp refractory moulds directly on the shelf. The mould is giving off water vapour which cools the immediate area around the mould. So as the temperature rises, the covered part of the shelf stays cool, in addition to being shielded from the general heat of the kiln while the uncovered parts of the shelf rise in heat. At some point the temperature differences in the shelf are too great for its strength to resist. The solution is to remove the shelf from the kiln and place the mould, slightly raised, on the floor of the kiln. The bricks, being softer, do not react in the same way as shelves to uneven heating.

For thick fusing with dams all around, it seems best to do this on a shelf that almost fully covered with glass and dams. This promotes more even heating and cooling of the shelf than having a small part of the shelf covered. It does mean having different sized shelves, but then you may already have some of them due to the breaking of other shelves. Just cut the broken shelf to the size you want on a tile cutter.

Small Glass Balls

You can create tiny balls of glass in several ways. You can cut small squares of glass as small as 5mm, place them in the kiln and fire to at least a rounded tack fuse temperature.

You can do the same with frit. Make your own by smashing the glass within some container, which can be as simple as newspaper. Sieve out the finer and larger glass and put the chosen size into the kiln at the same temperature. This will give more irregular pieces than squares, so you may want to include these pieces of frit in a regular fuse firing in the spare spaces.

You can also make balls individually by putting the piece of glass on a graphite block and heating it up with a blow torch. This will round up even a rectangular piece of glass. You can put these into a fibre blanket, but it really is not necessary as the round form will contain a lot of stress that will later be removed by subsequent firing. It is possible to over heat some glass, especially opalescent, so be careful about the amount of heat you apply.

Slumping Etched Glass

In general slumping will not remove the evidence of etching. There will be very little effect on etching on the bottom even at fire polishing temperatures.

When the etching is on the top side exposed to the radiant heat of the elements, you need to be careful to use the lowest practical temperature for slumping. It is possible to achieve a satin finish to a sandblasted surface at 677ºC with a soak of two hours. It depends on the delicacy of the etching texture as to whether the slumping will affect it much.

The more the glass will need to move during slumping the more distortion will be apparent in the finished piece. This can be minimised by using a low heat for considerable time.

If the mould is very detailed, it would indicate that etching should be done before the slumping due to difficulty in attaching the resist to the shaped glass, unless you paint it on. But again, a significantly long soak will be required to achieve the detail of the mould.

If it is a simple and relatively shallow slump it may be easy to etch after shaping. It is a question of how easy it is to get the resist to conform to the curve.

Keeping Copper Inclusions from Oxidising

The colour change in the copper foil is due to oxidisation - if the copper foil is completely deprived of oxygen it stays shiny and copper coloured. If you leave copper exposed at all it will go metallic blue or even bottle green, mostly it turns a lovely burgundy red colour- an intermediate oxidisation stage.

Klyr fire or borax solutions may help the copper stay bright.

Through doing some experiments with art school students, I have found the speed of firing is critical in an electric kiln. In a gas kiln the speed is normally fast anyway and produces better results than an electric kiln. It also is a kiln with a reducing atmosphere rather than oxidising one of an electric kiln.

Summary:

The main elements in keeping copper inclusions (and by extension, other metals) bright is to keep the metal from oxidising. Two elements are important in this:

• Keep oxygen from the metal
• Reduce the time the metal is exposed to high temperatures

Various methods are used to keep the metal from exposure to oxygen. Some of these involve: 

• coating the metal with fluxes to reduce the amount of oxygen in contact with the metal. 
• using a reducing atmosphere, such as a gas kiln. 
• placing an oxygen hungry material in the kiln with the glass and metal. 
• coating the metal with glass powder before encasing it within the glass.

Reducing the heat exposure of the metal also indicates that firing fast would provide better results. This requires very even heating within the kiln to avoid heat shocking the glass.  This is where a gas kiln is most advantageous - it can be fired fast without breaking the glass and it has a reducing atmosphere within it.

In general, it is easier to make use of the effects of the oxidised metal rather than striving for bright metal inclusions.

Fixing Paint for Transport

The very cheapest hair spray works well with glass paint, if you need to transfer your painted glass to another place for firing. Complete the painting and then spray with cheap hair spray as you would to fix a charcoal drawing. This will hold the paint firmly during transport and does not affect the paint during firing.

Tracing on Opalescent Glass

Opalescent and dense glass presents problems as the usual method of tracing the image through the glass is not possible. If you first spray the glass with a cheap hair spray, this gives a “toothed” surface to the glass. Then using carbon paper an image can be transferred. However, the carbon paper leaves a greasy residue, so water based paint will not take, but an oil medium will.

Matting

Oil, and Water and Gum as Media for Matting by Dick Millard [edited from a discussion]

Oil has been used, I believe, since the 16th Century, and certainly up through the 1970's to today. It is used wherever it is determined it should be used, and one is sufficiently informed and facile to use it in a manner of delivering its full and lovely potential.

First of all, oil is not characteristically employed as a matt, out of which, by the negative process, one "takes out lights". In overwhelming instances, with which I am acquainted, it is used as a shading material applied over a pre applied and "worked" under matt of water and gum base.

This provides the required "tooth" to provide both a degree of adherence and ease of application.

So, I would suggest an oil matting, or a shading application over a smooth glass surface, would be generally problematical!

A group of blending brushes

Add a bit more gum to your water under matt which will reduce the necessity to fire that matt, which changes the character of the desired "tooth". The purpose of the "tooth" to receive the oil matt is also to provide "porosity" as an "absorbant", which additionally holds the oil mixed paint to the matt. Otherwise, the oil remains too liquid and does not float in a controlled fashion. It will require a much dryer application of kerosene, or increased absorption by additional blending.

I had a large landscape piece, hills in the back ground, that I matted and applied an alcohol mat too, but I was lifting the water mat trying to cover it with alcohol, so I added more gum to my mat and that did the trick. I also used a very soft Chinese brush. I have found that firing the mat first and looses tooth.

A group of stippling brushes

I have noticed over time that some people seem to have the impression that the less gum used, the better. I advise not to use an excessive amount of gum arabic, as a soft matt, with a soft touch produces a soft look. This is interpreted to mean 'less is better'. That is true, but only up to a point. If too little gum is used, or none, it will come off as if it were flour or mud diluted with water and applied. Too little gum severely jeopardizes any opportunity to produce soft gradation from the highlight to the untouched matt.

Tracing with a Pen

Example of a pen nib

Using paint mixed with essential oil or turpentine and with a fine mapping pen for small lettering works well, as the oil flows better than water. Although with practice, a water based paint can be used with a pen, but it is a little tedious as the pen has to be loaded frequently with a tracing brush and constantly cleaned as the water dries quickly.

Examples of nibs and holders

Radiating Lines

In designs for leaded and copper foiled glass it is important to avoid lines radiating from a single point. Some of these reasons are:

It is important to reduce the number of lines that meet in any design to avoid a big bright solder place in a panel.

Example of pattern with radiating lines

It makes for large solder blobs, especially on leaded glass panels, and therefore provides a focus where one may not be wanted or required.

The difference between the harder solder and softer lead came leads - over time - to cracks in the lead at the edge of the thick solder blob.

In leaded and copper foiled glass it is a point of weakness, as there are likely to be multiple thin or tapering pieces of glass that are liable to fracture early in the life of the panel.

Methods of Avoiding

This umbrella image avoids long narrow pieces by having the ribs and supports crossing to make short narrow pieces

Narrow tapering pieces can be compensated for by making the narrow parts shorter than the wider parts of the taper – although this does add to the density of lead and solder around the termination point. There is a difficulty in adapting single radiating points in a drawing to the practicalities of the medium of glass. Examination of older panels (in either technique) will show some of the problems of thin tapering pieces. It is obvious in older windows, especially in the Victorian Era, when tapered pieces where in their glory. Almost always, the tips are broken. It is the nature of glass, and goes back to knowing how the medium will react to the conditions you create.

The central circle avoids joining all the radiating lines at one point

Good design will avoid multiple radiating pieces from a single point of origin.

It is not possible to make a neat termination by joining half a dozen tapers at one point. The finished piece will not look like it did when it was drawn out with a pencil. You can pencil in a termination with six points, ending at one point and it may look good, but when you draw the design with the width of the led or foil will show the clumsy nature of the design with a large termination point.

As you can see, the answer starts with the design, before you cut and foil, or fit the came to the glass. Art is not about the physical placement of what you see in your mind, as much as it is about the "illusion" you are creating that you want others to see. That starts with the design, and avoiding something that you know is going to give you a problem.

It is not possible to make a neat termination by joining half a dozen tapers at one point. The finished piece will not look like it did when it was drawn out with a pencil. 

Example of a design that will present difficulties at the centre

You can pencil in a termination with six points, ending at one point and it may look good, but when you draw the design with the width of the led or foil will show the clumsy nature of the design with a large termination point. As you can see, the answer starts with the design, before you cut and foil, or fit the came to the glass. Art is not about the physical placement of what you see in your mind, as much as it is about the "illusion" you are creating that you want others to see. That starts with the design, and avoiding something that you know is going to give you a problem.

Aperture Drops Blank Sizes

As the glass drops through the aperture, it stretches, but the whole substance of the piece is drawn toward the hole. If there is not enough spare glass around the hole, the whole piece will be drawn through the aperture.

There is a minimum size of the glass in relation to the size of the drop out hole. Up to some maximum size, the greater the diameter of the hole the greater the amount of spare glass there needs to be.

Also relevant is the depth of the drop. A shallow drop needs only a few centimetres larger than the hole. While a deeper drop needs a greater amount of glass surrounding the hole.

I have found that for a 300mm diameter hole, with a 150mm drop the glass needs to be 35mm larger all around. Thus an aperture of 300mm needs to be at least of 370mm diameter for this 150mm drop. I have done drops with 550mm diameters with only a 650mm diameter blank. This indicates to me that there is an amount of spare glass that will be sufficient even for larger diameter drops, but I have not found it yet. For a large drop with an aperture of 500mm and a depth of 350mm, I used a 100mm margin, giving a disc of 700mm which successfully dropped with the rim moving only about 20mm.    An 80cm/31.5” diameter aperture with a drop of 35cm/ca.13.8” needs a rim of 10cm making a blank a diameter of 100cm./39.4”.

One element that can reduce the size of the blank is to make an inclined collar around the aperture of the drop mould.  This idea is based on the observation that as the glass begins to fall through the aperture, the outer edges of the glass rise from the mould surface so the glass is resting only on the inner edge of the drop out mould. 

This inclined drop out mould will be like a shallow bowl rim, but without a bottom. The glass blank then rests with only its outer edge on the collar.  When the temperature increases to the point that the glass begins to slump, the glass will conform to the slope and so create enough friction to restrict the glass from falling through the aperture, although it is with a smaller than normal rim.  The actual size of the rim for each size and depth will need to be determined by experience. 

More information can be found here

Revised 22.11.24

Temporarily Securing Panels

When installing panels into an window opening or frame, you often need to secure it while you finish puttying or use other forms of weather proofing.

If you use sprigs (headless nails) or even carpet tacks, use them where there are leads meeting the perimeter. There us less chance of a stray hammer strike hitting the glass. It is more likely to strike the lead and so cushion the impact to the panel. I also use a straight putty knife behind the nail or sprig to avoid even the chance of a strike directly on the lead.

These precautions, with suitable modifications, are applicable to copper foiled and fused panels.

Initial Heat Up Rates

Example of a graph of a heat up for industrial purposes

There is quite a lot of information on the annealing cool rates, but not so much on the initial heat up rates. This is probably because the cooling rates are more critical than the heat up. But everyone knows that you can heat the glass up too quickly for its thickness.

My experience leads me to suggest some heat up rates to 50C above the annealing temperature for circular and nearly square full fused pieces. These have worked for me, but of course, may not work in all kilns.

6mm heat up at 160ºC/hr

12mm heat up at 110ºC/hr

19mm heat up at 50ºC/hr

25mm heat up at 30ºC/hr

In general, these heat up rates are no more than twice the initial annealing rate for the relevant thickness.  That is, the initial anneal cool for 6mm is 80C/hr; and for 12mm is 55C/hr.  When you get to 25mm, my initial anneal cooling rate is only 15C/hr.  So you can see how the doubling of the initial anneal cool works.

Tack fused and pointed pieces require much more careful heating because of the differing thicknesses within the piece, or the relative narrowness of one end or area in comparison to other areas. The suggestion is that the heat up for these should be at the rates suitable for items at least twice as thick as the thickest part of the tack fused or tapered piece.

Grinder Bit Height

If your grinder bit is too low or too high the diamond surface will not grind the whole of the glass edge. This can lead to chipping of the surface of the glass at the edges.

Example of top of bit almost too low for the glass

A good practice is to start with the bit as high as possible to allow for differing thicknesses of glass. As high as possible is with the bottom of the diamonds just below the platform of the grinder. This will ensure that you can deal with varying thicknesses of glass without immediate adjustment. You can then lower the bit as it wears.

Example of nicely adjusted bit

Of course, you need to ensure there is adequate water reaching the grinding bit to avoid overheating the glass, and to keep the dust from grinding from getting into the air.

Mobile Glass Storage

Sometimes people consider placing their glass storage onto wheels.  It is better to avoid wheels on glass storage for several reasons.

You would need heavy duty wheels to cope with the weight. Glass is 2.5 times as heavy as water, so it does not take much volume to make a really heavy glass case.

Example of mobile glass storage

You do not need to move all your glass at once - a piece at at time is all that is needed. It will be safer, and in the end easier, to build your work bench nearer the glass.

There are risks breakage while moving. The heavy glass store will vibrate the glass within the storage, and any snags or obstructions while moving the glass will increase the risk of breakage.

The structure can become too heavy to move and so defeat the original intention.

Antiquing Sandblasted Glass

Sometimes a sandblasted area appears too white when finished. One method that can be used to tone down the whiteness is to use low temperature glass stainers' enamel.

The low temperature enamels cure at temperatures between 530C and 580C depending on the type and manufacturer. At this temperature the glass is unlikely to change its shape. The jewellers and ceramics enamels fire at higher temperatures and are not suitable.

Rub the dry powder into the sandblasted area with a cloth or your fingers. The advantage of using the powder dry is that it will not stick to the smooth areas, although you may need to brush it out of any depressions in fused glass.

Fire the glass to the minimum temperature for the enamel, but for S96 or Bullseye try to stay below 540C. This temperature will fix the paint to the glass, but not change the shape of the sandblasting. Float glass will not change if you go to 580C. If you go to higher temperatures, you will go toward a satin effect and finally a smooth surface.

This technique has the advantage of being able to introduce a subtle colour tone to the sandblasted area. This enables you to match older glass that may have a slight colour cast from the glass or materials it has become encrusted with, such as nicotine.

This method requires testing to get the right levels of colour, and the temperature to balance the fixing of the enamel without changing the sandblasted surface beyond your choice. So you need to prepare several samples noting the amounts of enamel and temperatures used.

This has been successful for me when replacing broken sandblasted door panels that need to match the side lights. It removes the excessive whiteness of the new panel and can blend to match the colour of the originals.

Assembly of Circles and Irregular Shapes.

Leading

Circles and ovals as well as irregular shapes significant support as leading often involves sideways pressures to fit the lead to the pieces of glass. Thus there are two main methods of support for the perimeter of the panel.

You can cut supporting pieces of glass to place around the perimeter. These need to be cut to the outside of the perimeter cut line. These supports must be in at least two or more pieces to enable the came to be put in place progressively. You then assemble the perimeter lead into it and continue to lead up as normal. The perimeter support can also be made from thin plywood or similar materials. This can be useful if the shape is to be repeated.

A simple means of supporting irregular shapes while leading is to place a number of nails around the inside edge of the cut line. There need to be enough to support each piece of glass with at least two nails. So you may need to add more nails to the initial set up. Build the panel without the perimeter leads. When the interior is assembled, put the perimeter came around the panel. Ensure the fully leaded panel fits within the dimensions of the opening. Then solder as normal.

Copper foil

The above methods can be used, but are often a bit heavy duty for copper foil processes. Instead of glass, timber or nails you can use more easily cut materials. Such things as stiff double walled cardboard, foam board etc., are suitable for light duty. You can cut the complete shape from these materials, but only good if no pressure is used in fitting the pieces.

If you are likely to repeat the shape and size, you can use plywood or similar materials. Build inside the shape and remove it when the whole is soldered on the first side.

Disguising Joints in Fusing

You can use powder or fine frit to conceal the joints in fusing. This is most easily done before the first firing.

Fine frit can be made from the off cuts from your prepared glass, or you can buy powder of the correct colour. Where two colours meet use the darker or denser colour of frit or powder over the joint. Push the frit into place with a brush or stick to form a regular edge. You should heap the frit over the joint to allow for the reduction in volume when fused. This will not work as well on tack fusing as it does on full fused projects.

Example of frit "painting", the principal of which can be applied to glass pieces

An example of how frit can be brushed around glass pieces

Placement of Pieces for Firing

Placing pieces in the kiln, especially in oval and side fired kilns, is not about filling the kiln completely. Kilns have hot and cold spots, and the arrangement of the elements can have an effect too.

The first thing to determine with a new kiln - and immediately after any alterations to the kiln - is where the hot and cool areas of the kiln are. There is an extensive guide to this on the Bullseye site. In short, the method is to place strips of glass on short kiln furniture all around the kiln at the level(s) you will be firing. These strips should be of equal size and the kiln furniture the same distance apart. Take the temperature slowly up to slumping temperature. Observe when the visible glass pieces begin to slump. Let that continue until they are about half way down. Then proceed to the anneal. When cool you can open the kiln and see the areas where the glass has slumped most – the hotter areas – and where it has slumped least – the cooler areas. This will give you information on areas to avoid if you want an even finish all around the edges.

If your kiln is side fired, you need to consider the shelf placement in relation to the elements. The best arrangement is to have one element below the shelf and the shelf between elements so the radiant heat is not directly onto the edge of the shelf as that may lead to breaks.

Put glass on the shelf as centrally as possible. If the glass must be near the elements, baffle the glass from the direct radiant heat from the side elements.

Glues in Kiln Forming

Glues have two major uses in fusing. One is to stick things together after being fused (cold fusing). The other is to hold things together before fusing.

Holding things together while preparing the piece to be transferred to the kiln is a major use of low tack adhesives and glues. All of these burn off a lot lower than the temperature at which the glass begins to stick together. So, if you are gluing overhanging pieces, for example, they can move after the glue has burned off.  If you are assembling pieces that will not stay in place while you are putting it together, glue will not help get the final result you want.  If you are gluing to keep things stable while you move it to the kiln, you may find everything is ok.

However, glue tends to boil off if the temperature is raised too fast. During this process, the effect of the boiling will move the glass pieces that are most unstable. This also occurs if you use too much glue. You should only use as much as will stick the pieces together. Also too much glue leads to black spots and sometimes bubbles between the layers of glass.

The adhesives commonly used are the Bullseye product “Glastac”, Elmer’s glue, diluted PVA - or school - glue. All of these take varying times to dry and hold the glass pieces in place. So, a popular alternative is hair spray. This is a lacquer which dries almost instantly. It provides a thin film of adhesive and burns off in the kiln with no residue. You should use the varieties with no additives.

Glue most often leads to problems or unexpected results, so several ways have been used to achieve the desired results.

One way to deal with unstable components on small pieces is to make a large piece with a repetition of the design and cut it up after fusing in to the sizes you want.  Clean the pieces very well, and then fire them again to at least fire polish to remove any cutting or grinding marks.

An alternative to using glue, especially at the edges where the pieces are likely to move, is to use dams. My practice is to make the dams slightly taller than the unfired piece and line with fibre paper. I put 3 mm fibre paper against the dam, and thinfire against the glass. Both of these should be 3 mm narrower than the final height of the fused piece will be. This is to allow the glass to make a rounded edge as it will not be able to stick to the fibre as it sinks down to its final height.

Bullseye hot dams as an example of damming

Another alternative to using glue is to fire the piece upside down, so that the pieces do not have to be supported. This does require some planning and forethought. You can draw the design in reverse on thinfire, using different coloured pencils for the various layers to help in building the piece up in reverse. You then cap the assembled pieces with the piece that will become the bottom. Take the whole to a tack fuse. Then clean very well to remove any residues from the shelf. It is possible to sandblast and then clean to make sure there are no residues left. Of course this is not possible if you are using dichroic or iridised glass. Also note that iridised surfaces and thinfire do not get on well – there is extreme pitting in the iridised surface. 

Example of pieces glued and ready for the flip

 Once the piece is cleaned, fire again to get the desired surface texture.

Cleaning a piece after first firing

Bones as Inclusions in Glass

The major components of bones are calcium and organic materials making up the marrow. If the bones are not old and weathered a very bad smell will be produced. The organic material will cause bubbles. Finally, it takes a long time to burn out the marrow, so it is best to use bones that have weathered for a number of years.

Calcium “erodes” during firing, so fine and thin bones will leave a shadow of ash (or a big bubble if there is not a long bubble squeeze. The bone has to be encased or trapped by the glass as it will not stick permanently to the glass on its own.

It can make dramatic shapes if the bones are arranged in novel ways to represent other things. The whole of the bone does not need to be encased, as the thicker parts will be strong enough to support themselves.

Hanging Sun Catchers

Unless you are using some manufactured system or a frame, the most frequent way to provide hanging points is to create a loop from copper wire.

Hangers should originate in a solder bead that goes some way into the piece. The loop's tail should lie a significant distance into the solder line to ensure it does not pull the piece apart. If this is to remain invisible, some planning will be required to allow the small extra space between the foiled glass.

The loops for hanging a piece of any size should not be soldered to the perimeter foil without reference to the solder bead lines, as the adhesive and foil are insufficient to hold the weight of the piece without tearing.

Here the hanging loops could have been moved just a little to engage with the solder joints at the left ear and at the tail to make stronger hanging points

Here the hanging points are at the solder joints giving strong hanging points

Reinforcement of free hanging or projecting elements can be done by placing wire around the piece with a significant excess going along the perimeter in both directions. The supporting wire can go into the solder line, if it is a continuation of an edge of the free hanging piece.

In this case a twisted copper wire around the perimeter gives strong hanging points

The strongest method is to wrap the wire around the whole perimeter of the piece. Choose easily bent copper wire. This will be pretty fine, but when soldered, will be strong enough support the whole piece.

The hanger can be made by leaving a loop of wire free. This way you can hang from any convenient place on the perimeter. This loop can be made by a single 180 degree twist in the wire, or by bending a loop into the perimeter wire. In all cases you will need to tin the wire to blend it with the rest of the piece.

This perimeter wire can be simply butted at the start/finish of the wire. It could be overlapped, but this is unnecessary on any piece where this method is adequate for support. The start can be at the top or bottom, although I prefer the top, so the wire is continuous from loop to loop. The reason for continuing beyond the loops is to provide support to all the edges of the sun catcher.

This single point hanger is at the strong point of the piece

The left hanger is strong, but the right is weaker than if it had been attached to the right of the body

This piece needs wire around the piece, especially to stabilise the tail and ears

Cleaning Magnets

When making frit in steel containers the metal fragments need to be removed using magnets. It can be very difficult to get the fragments off the magnets.

A solution has been suggested. Put the magnet into a small plastic bag before use. After cleaning all the metal from the frit, take the bag to the bin and remove it from the bag. The metal fragments will drop off into the waste bin, leaving a clean magnet.

Single Layer Firing

Preparing a Single Layer for Further Kiln Work

There can be circumstances where you do want to fire a single layer in building up your project. This is more often difficult on rectangular than round pieces.

Some of the considerations are:

Temperature

Heat work

sizing

Cleaning after firing

Firing a 3 mm piece to anything over a laminated tack fuse normally leads to the edges drawing in creating a “dog bone” effect and often leading to bubbles in the interior at higher fusing temperatures. So one approach is to fire at low temperatures and accept relatively sharp edges on the piece.

Diagram of the full fused results of different thicknesses 

However the concept of heat work can help in this situation. Glass reacts to the accumulation of heat, so that slow advances or long soaks can achieve the desired results at a lower temperature without – in this case – getting the “dog bone” effect. This does require a bit of experimentation. Keep good records of all the stages of experimentation as the effects achieved with various combinations of temperature and time will come in useful later.

It is possible that using the concept of heat work will not be sufficient to achieve the desired results. Then you need to consider placing your design in the centre of a larger piece. Fire this to the lowest possible temperature to achieve your results and then cut the fired piece to size. You will need to fire polish or cold work the edges to get a suitable finish on the edges.

The central white piece shows the results of single layer firing that could be altered by the above technique

If you are going to re-fire any of these single-layer pieces, you need to clean them very well. Any dust or other contamination will be incorporated into the final piece. This is especially true if you are combining a flip and fire technique with this single-layer firing.

Slump Point Test

A slump point test is useful when you wish to determine the approximate annealing point of an unknown glass. The methodology follows:

Prepare a strip of the glass 305mm x 25mm. Suspend this strip above the shelf on 25mm pieces of kiln furniture. Leave a 275mm span between the kiln furniture. A piece of kiln furniture also needs to be placed on top of the glass to keep it in place.

Example of an extreme case of testing for slump point

Fire at 200C per hour to ca. 550C, then fire at 50C/hour to about 700C. Observe frequently from 600C. Record the temperature when the middle of span touches the shelf.  This is also the slumping temperature of the glass when fired this way. 

Subtract 40C from the “touch down” temperature for the approximate annealing point temperature.