Foil not Sticking on Edge

An enquiry arrived the other day:
I’m working on another irregular shaped suncatcher and I have just completed the soldering. Now I've found one small section the copper foil is not sticking. How can I fix this?

The adhesive on copper foil tape is not a permanent one. It only sticks to the glass long enough to apply the solder to the foil. The heat of soldering often degrades the adhesive so much that it no longer sticks. What holds the solder down is the solder bead. So you probably do not have a full bead on the edge. Placing a bead on the edges of pieces is difficult but you can find a method here.

You can make the edge beading a bit easier by putting thin copper wire around the edge of the piece. This strengthens the whole piece. It allows you to attach a hanger without risk of pulling the whole suncatcher apart. It also allows you to form a bead on the edge more easily.

The bead formed on the edge curves around to the front and back faces allowing the solder to hold the copper tape more firmly to the glass.

Tie Wires

Tie wires for glazing bars are to keep the panel from rebounding due to wind pressures on the window. There also is some pressure created within the house by the opening and closing of doors, although this is minor in comparison to the weather.

The tie wires should be securely soldered to the panel at solder joints. Placing ties elsewhere leads to the tearing of the lead. The soldering of the tie wires requires more heat than simply soldering the lead joints. The tie wire needs to be heated enough to melt the solder of the joint to which it is being attached. Then an additional dot of solder needs to be added so that the wire cannot simply pull out from the joint by being only sweated to the joint.

At installation, when the panel is fully seated in its opening and fastened by nails or sprigs, pull the tie wires out at right angles right at the edge of the solder attachment before twisting the wire. Do not use any more than firm pressure. Then you are ready to cross the wires over the glazing bar. This ensures there is no excessive give in the copper tie.

Do not over tighten the tie wire twist. Only twist until snug against the bar. Then continue to twist the loose ends until you have them a satisfactory length. Cut off the twist rather than the tail ends to provide a neat finish. Then tuck the twist under or over the bar, just as you desire.

Installation of Glazing Bars

There are a few tips that concern the installation of glazing bars into wood frames. An important element to understand is that the purpose of the bars is to protect the panel from horizontal wind pressures on the window, not to lift the panel or in any other way strengthen the panel vertically.

The holes on one side should be at least 5mm deeper than the other. For a really secure attachment one side should be at least 15mm deep and the other 7-10mm. This allows a significant amount of wood to seat the bar. The bar should be at least 10mm longer than the opening is wide.

The hole you drill should be 1mm larger than the bar diameter. This will make moving the bar easier. Additionally, the ends of the bars should be filed to remove any roughness. Also greasing the ends of the bar with tallow or candle wax will ease the movement of the bars.

If the bar is to be installed inside sash windows you can ease the installation by determining the height of the hole to be drilled by presenting the panel to the opening and marking the frame where the bar is to be attached to the panel. Drill the hole so the edge of it is flush with the rebate. This allows you to use a chisel to open the hole enough to allow the bar to be placed in the socket now prepared. In these cases the bar needs to be no longer than the opening.

The installation should be completed by forcing putty into any gaps left between the bar and the hole. This will stiffen and help to firm up the bar’s attachment to the frame.

Cementing Panels

I recently had the occasion to repair a panel made by a friend of the clients several decades ago. It was cemented by pushing commercial putty under the leaves of the leads. It illustrates very well why lead light cement should be brushable to completely fill the space between the glass and the came.

This photo shows how the putty filled the space above and below the glass but not between the glass and the heart of the came.

This photo shows the putty missing from the corners of the glass. There has been a little chipping of the putty in the dismantling process, but not much.

The question may be asked about what is so important about a bit of putty missing from the edges of the glass, it is sealed along the leaves of the came. Yes, this style of cementing will seal the panel from the weather for a time. But had this glass been in a window instead of hung inside, it is questionable whether it would have begun to leak only about 20 years after being made. Certainly as the putty begins to break down, the moisture will rapidly find its way into the inside.

The only way to be certain that the panel is completely weather proofed is to use brushable cement. The cement is pushed under the leaves of the lead with a stiff brush. You know the fill is complete by the cement oozing out of the other side.

It is possible to make up a brushable cement from commercial putty. You simply add some white spirit to the putty. I make a depression in a fistful of putty and add white spirit. Fold over the sides into the well and gradually, the white spirit is mixed into the putty. Continue adding white spirit until you have a very thick molasses that can be pushed around with a brush.

Of course, while you are doing this mixing, you can add a blackening agent - powdered or oil based black pigments are best.

Jewellery-scale Ovals

Rather than trying to perform the difficult task of cutting small ovals, you can use the heat of the kiln to do some of the work for you.

Cut a rectangle the length and width of the oval you want. Then groze the corners to the approximate curve of oval you want. Do not worry about the little inaccuracies of the curve. If it is the curve you want, the heat of a full fuse will even out the edges into oval you want. Clean the glass, assemble and fire to your normal full fuse temperature. The result will be a smooth edged oval of the shape you grozed from the glass. Of course anything less than a full flat fuse will produce a piece with some of the inaccuracies that you grozed into the glass.

If you do not go to a full fuse, or are using only 3mm thickness of glass, this will not work.

Effect of Glass Weight on Slumping

Just as the mould size and shape have effects on slumping temperatures and strategies, so does the weight.

When slumping you are making use of the combined effects of gravity and the increasing softness of the glass. The same thing happens when you have a thick piece of glass as when you have a large span in the mould. As the weight of the glass increases, the temperature at which it will begin to slump is decreased. There is an inverse relationship between the weight and the slump temperature just as there is between increased span and slump temperature.

A 3mm piece will take more time or more heat to fully slump into a mould than a 9mm piece will into the same mould. Observation will give you the information on what the temperature differentials are.

Bowed Glass for Cabinets

This is glass which is slightly convex and normally found in multiple-paned cabinet doors. Glass workers are sometimes asked by antiques dealers to do a replacement.

You can make a mould and do a slump.

However, you should consider doing a drop out or aperture drop. Normally these are thought of as circular, but they can be of any shape you want. The reason for making them as a drop out is that the surface of the bent glass will be completely unmarked.

I have made these several times for antique dealers. To do it, make a rectangle in fibre board about 10mm larger than the glazing size. Place a piece of glass about 40mm larger than the rectangular hole and fire. You need to watch. It will begin to slump at around 520C - or less if it is not float glass. You need to go slowly so the glass does not drop too much.

You will know from the existing pieces how deep a drop is required. Measure that and place a witness to determine when the slump has gone far enough. This can be a piece of kiln furniture with fibre paper over it. It can be a reference point on the far side of the kiln. In my case it normally is a stack of fibre board pieces with fibre paper on top to build it up to the correct height.

When the glass is just about to touch the witness, flash cool the kiln to just above the annealing point and close the kiln. If the temperature rises back into the forming temperature range, flash cool again. Twice should be sufficient to ensure that the glass does not move any further.

Cutting Flashed Glass

Some recommend cutting flashed glass on the clear or non-flashed side. This is based on the idea that the flash is only laminated to the main body of glass. My view is that flashed glass has proved to be very stable over many centuries, and so is firmly a part of the whole sheet.

What is more important is to observe that flashed glass often has a bow. If you place the glass on the bench, you may find that it rocks or sits up from the bench. If you cut the glass on the convex side, that is the side which is not resting on the bench except at the edges, you may find that you break the glass during the scoring, unless you are using the lightest of pressures. It is more certain to get a good break if you score the glass on the concave side - that is where the edges are slightly raised from the bench. So the important element in deciding which side to cut is to score the concave side whether that has the flashed colour or not.

This does not occur with all flashed glasses, and is more important on large sheets than small ones. On the small ones, the curvature is so small as to be immaterial.

Keeping Flashed Glass the Right Side Up

Once you have determined the flashed side on a sheet of glass, mark it with a felt tip or wax marker of some kind so that you will not have to perform this action each time. This should be carried over to each piece as you cut it away from the main piece.

When you have cut a piece from the main sheet, it is easy to turn it over and work on the clear rather than the flashed side. It is essential to know which the flashed side is if you are going to do any etching of any kind. So, as soon as you have cut the piece, mark the flashed side. This will keep you certain that you are working on the flashed side.

Another method to keep track of the flashed side is to mark across the intended score line. After scoring and breaking you will have both pieces of glass marked. All you need to do is make sure you always mark the same side - flashed or clear. Some like to cut on the clear side and some the flashed side. All you have to do is to determine which your practice is.

Distinguishing the Coloured Side of Flashed Glass

On smaller pieces of flashed glass you can determine which the flashed or coloured side is by putting it to the light and viewing it through the edge. If the flash is very thin or you cannot determine which the flashed side is, you can alter the angle a little. If you tip the glass down slightly and the light is coming through the clear side, there will be very little variation in what you see.

If you tip it down and you see the colour very distinctly, then the flash is on the upper side.

Also note that on the left side of the glass you can see the effect of the cutter pressure on the glass.  These little hook like marks are evidence of the stress caused by scoring the glass.  This is the kind of mark you will see on glass that has adequate, but not excessive pressure applied during the scoring.

Now back to the subject of the flash.

On larger pieces this is more difficult, and dangerous to you and the glass, as you risk breakage by holding large sheets horizontally. So you can use your grozers to nip a little glass off the edge. If there is no change in colour of the chipped edge, you have taken glass off the clear side. When you chip off the flash, there will be a little bit of clear showing which the coloured side is. Here are two examples.

Once you have determined which the flashed side is, mark it and all off-cuts with a felt tip or wax marker of some kind so that you will not have to perform this action each time.

Effect of Mould Size on Firing Schedules

The size of the opening of the mould has a significant effect on the schedule you will need to use for slumping. This often referred to as the span of the mould, because the glass spans the mould from one edge to the other. In larger span moulds, the glass drops more easily, because the weight at the centre is effectively more than in smaller span moulds. This means that the glass in large span moulds can be fired at lower temperatures than small span moulds. The difference between a 130mm diameter mould and a 400mm diameter mould can be 40C and 30 minutes - the larger one taking less time and temperature to conform to the mould.

Ball moulds - one of 130 mm and the other of 290 mm dia.

The depth of a mould in relation to its span can have an effect on the schedule required. This is for two reasons: The deeper a mould, the greater the tendency for the sides to become steep, which presents problems as described elsewhere. Deep moulds also require slow careful firings, to help keep the glass from distorting too much from the horizontal and stretching too thin to be robust.

180 mm dia by 75mm deep flared mould

Effect of mould shape on firing schedules

Each time you get a new mould, you should think about the firing schedule that will be needed. The existing schedule you use may need to be changed, so you need to observe the first few firings to be sure you have the correct heating pattern for the mould and the glass.

• Simple curves such as ball mould, square slumper are easiest to slump into, as they have only easy curves to take up. They need only low temperature slumps, and possibly not very long soaks. Although it is best to achieve the slump with approximately a 30 min soak, so that you are using the lowest practical temperature and so minimising mould marks on the glass.

Simple ball mould and slump mould with flat bottom

• Compound curves are those such as an ogee curve that starts in one direction and then moves into another. These require more heat or time than the simple curves. The glass begins to fall into the centre of the mould first, which will be the steepest/deepest part of the mould. The glass will first of all take up a simple curve, and only later conform to the other part of the curve. It is best to start with a low temperature slump and add time (only later increasing temperature) until you find a temperature and time that is practical for the mould.

Moulds with ogee curves and one with an angle at the foot

• The same procedure is needed for moulds with sharp curves or angles. Bowl moulds that have a sharp angle at the foot need much more time than the simple curve. The glass falls to the bottom of the mould first and then has to relax into the sharp angle at the edge of the foot. This takes considerable time. If you add lots of temperature to achieve this relaxation, you run the risk of getting an uprising of the glass near the middle of the bowl. So considerable care is needed to find the right combination of time and temperature for this kind of bowl.

• Draping moulds – those you want the glass to form over rather than into – have other requirements. The mould on which the glass rests forms a heat sink. This means the mould drains heat from the glass in that area while the rest of the glass heats up more quickly. This can lead to breakage. Draping requires more observation to get the forming right than slumping does. Each difference in span of the glass requires a different amount of time to complete the drape even though it is on the same mould. Drape moulds with steep sides require quite different considerations.

Complications in Moulds

Moulds that are easy to slump into are more complicated than they appear. When choosing a mould or making one yourself, there are some things that should be considered.

Steepness, Draft and Undercuts are three elements that can make a mould easy or difficult to use, or make it a one use mould, or a reusable one.

Steepness of the sides or any part of the mould are considerations that make it easy to form the glass to. The steepness of the sides, affect how the glass slides down it. The steeper it is the more likely the glass is likely to hang up on it. This will promote uneven slumps, and needling along the areas where the glass has hung on the mould. The steepness or sharpness of curves within the mould determines how much time and heat is required to allow the glass to conform to the mould. So the steeper the curves, the more time and the less heat is required. For moulds with lots of detail, more time is needed – the amount of heat will be determined by the steepness of the draft of the mould.

Draft relates to the angle of the sides of the mould. A mould with perfectly parallel sides will not release from the mould. In order for the glass to be released from the mould, there must always be an angle making the bottom smaller than the top. The nearer the draft is to parallel the more difficult the piece will be to remove.

Undercuts are the places where the bottom or lower parts of the mould are wider than the upper parts of the mould. This means the mould must be destroyed to allow the glass to be removed. These are therefore single use moulds. If the shape needs to be repeated, a master mould needs to be taken so the mould can be repeated in a material that can be easily broken away from the glass. This is of course, getting into the region of casting moulds.

House Paint on Glass

Windows that have been painted several times over the years often have paint drops or smears on the glass. There are at least two ways of getting it off the glass.
Mechanical means are possible and should be the first trial on unpainted glass. Use a flexible, sharp blade to scrape at the paint. Often there was enough dirt on the glass that the paint will pop off easily. Where you have painted glass – that is glass paint rather than house paint - you need to test how secure the glass paint is. Find an area where any loss of paint will not be noticed and try the mechanical method. If the glass paint does come off, you need to go to a glass conservator who will have a range of chemicals suitable.

The most common chemical removal method is to use an alkaline paint remover. Glass is also an alkaline material, so the paint remover does not affect the glass. Any commercial paint and varnish remover can be used.

Put on a fume mask and rubber gloves. Apply the chemical with a brush and let it work for a while. Agitate the chemical after this pause to see if the paint has been removed. If not, add some more chemical and wait. When the paint has been loosened, rinse with lots of water.

This should not be used on areas of vitreous glass paint due to the risk of removing it.

Growing Panels

What can be done to keep leaded glass panels from growing beyond their original cartoon lines?

I find that most people, who are not used to lead came, cut the crossing pieces too long so the whole panel grows. Each piece of came that is a fraction too long pushes the passing came out, making the glass apparently too large. You can and should make sure that you have pressed the came snugly against the glass. If the next piece of glass you place goes over the line allocated to it, something is wrong with the previous piece. Undo the came and check the size of the glass against the cartoon. If the glass fits inside the lines allocated, the problem is the way you have fitted the came to it.

Another check you can do is to run a felt tip pen at the side of the came onto the glass. Take the glass out and examine the space between the line and the edge of the glass. This will tell you where the glass and came are not fitting equally. A narrow space does not immediately mean the glass is too large, it may mean the calme is not tucked against the glass properly. So check that first, before any grinding.

Nails, push pins or other things that you can push into the work board will keep things stable. If you are working with a rectangle you can use wood battens. If not, multiple close spacing of nails will help. Also you could cut a piece of glass into a shape that will hold the outside of the panel.

Glass Colours

Glass normally has little or no colour because the electrons in the material are tightly bonded so no electronic movement in the energy range of visible light is possible. Glass is given colour by addition of various materials to selectively absorb light in the visible spectrum.

There are three processes: addition of ions of transitional metals; addition of colloidal particles; and addition of coloured crystals.

Ions of transition metals provide electronic excitations in the visible light range. Some of the common ions are:

• Chromium with two positive ions gives a blue, but
• Chromium with three positive ions gives a green.
• Cobalt with two positive ions gives pink.
• Manganese with two positive ions gives an orange.
• Iron with two positive ions gives a blue-green, as can be seen by looking at the edge of much of modern window glass.

Addition of colloidal particles of various sizes causes absorption of some parts of the visible spectrum and reflects the complimentary colours. These are very small particles ranging from 4 to 170 nanometers. For example,

• Gold of 4-10 nanometers will give a pink.
• Changing the size to the range of 10-75 nanometers will produce a ruby.
• As the size of the gold increases to the range 75-110 nanometers a green is produced.
• Between 110 and 170 nanometers browns are produced.

The addition of very small coloured crystals that are dispersed throughout the glass will produce coloured glass.

• The Egyptians made scarlet glass by the addition of red copper oxide. Other examples are
• Lead hexachrome (Pb2CrO6)which produces red, and
• Green is produced with chromium (III) oxide (Cr2O3) crystals, often called viridian.

Based on MIT Solid State Chemistry Notes, p.15-16

Powder Shapes and Clean Up

The crispness of the lines of images made with sprinkled powder depends on the neatness of the edge of the powder. If you are using Bullseye black, you need to use stiff black 000101-0008 rather than the normal which spreads much more than the stiff black does.

There are various ways to create crisp edges, but in some cases it is better to remove the powder than to push it about.

I have adapted a key board cleaning attachment for my vacuum sweeper to clean up the edges of the powder. The narrow head just needs to have a nozzle put in. I used the casing of a ball point pen and filled the remainder of the head with blue tac. Turn the suction of the vacuum all the way down. If you do not have an adjustable power vacuum, make a hole in the hose that you can control the size of to vary the suction.

Manipulation of Frits and Powders

A variety of tools can be used to move frit and powders about to get the shape and edges you want.

One simple tool is a brush. It seems that a soft water colour brush is suitable for very delicate manoeuvring. There are various shapes and sizes for more and less delicate shaping. A stiffer hogs hair brush will move greater volumes.

You can also use a brush to pick up stray pieces of frit. Get the brush damp and touch it to the grains of frit to pick them up. If you do not have excess water on the brush, you will leave no mark behind.

Colour shapers with shaped, rubber tips are good for stroking and pushing frit and powder into place. There are a variety of tip shapes for various uses. Wooden tools as used for shaping clay can be useful in the same way, although they are not flexible.

Another tool that can be used is an adapted keyboard vacuum.

Stencils for Powder Sifting

Use stiff card for the stencil. Make two little holders by sticking tape together in the middle and use the wings to attach it to the card. This makes it easy to lift the stencil straight up from the piece. Do not stick the stencil to the glass. Make the stencil with only enough surrounding card to keep the whole stiff, but ensure you can pick it up easily.

If you want to use multiple stencils on the same piece you need to ensure the stencils are all of the same size to ensure you do not mark the already laid down powder or frit. You also need to make some kind of registration mark on each stencil. Registration marks are used to align subsequent stencils in the same orientation as the first. You can use notches in the stencils and always orient them to 12 o’clock or toward some other indicator. You can also use the notch in combination with a small ink mark on the glass for accurate registration.

Placing Clear on the Top

One effect of placing clear under a coloured glass, especially a dark one, is that the bubbles rising will thin the colour, even to the extent of giving a small clear circle in the midst of the colour. Placing clear on top almost completely eliminates this effect.


An additional effect of placing clear over colour, especially opals, is that it reduces devitrification.

Glass Transition Point

This is the temperature range at which a super cooled liquid becomes a glass. At higher temperatures the molecules are able to reorganise quickly as in a liquid. At temperatures below the transition range, the movement among the molecules virtually ceases and the resulting material is known as a glass.

Two characteristics should be noted here. The temperature range for the transition phase is dependent on the speed of cooling. The slower the cooling, the more time there is for reorganisation and so there is a lower transition temperature. The quicker the cooling of the material through the transition phase, the greater the volume of the material, i.e. it is less dense, although the more slowly cooled glass is still much less dense than the crystalline material.



Based on MIT Solid State Chemistry Notes, 7, pp.7

Formation of Glass

There are a lot of glasses – natural and laboratory created – in addition to the silica based one that we work with. However understanding how glasses in general are created helps to understand “our own”. In general, when the liquid phase of a material is cooled below its freezing temperature it usually transforms into a crystalline solid. But some materials do not crystallise when cooled to their freezing temperatures. Instead they create a rigid network which is known as glass. It is very similar in structure to a liquid – hence super cooled liquid.

At temperatures just above their freezing points, most materials have viscosities that are similar to water at room temperature. They are so fluid that the molecules can rapidly form crystalline structures. But many inorganic silica materials form glasses on cooling because their viscosity at and above their freezing points is very high. There are also high energy bonds between the silicon and oxygen molecules. The viscosity increases very rapidly as the temperature is reduced. These prevent the flow required for crystallisation. In organic glasses, e.g. resin, crystallisation is difficult because of the long chain molecules that the material is composed of, preventing the molecules from sliding past one another, i.e., the difficult structural re-arrangement that would be required to form crystals.


Based on MIT Solid State Chemistry Notes, 7, pp.5-6

Reinforcing Panel Lamp Shades

When constructing large or heavy lamp shades, reinforcement needs to be an integral consideration in the construction. With panel lamps the reinforcement is relatively simple – it can be along the seam lines. In fact, if you do not bevel your panel edges, it can be in the upper seam lines, as the solder filling the open joint will cover the wire. If the panels are bevelled, the wire can just go on the inside along the joint.

The wire should end at the edge of the bottom of the skirt so that it does not extend beyond, but will still be in contact with the edge reinforcement. The upper wire should extend beyond the top of the shade, so that it can be soldered to the vase cap. If there is not one, the wire should be dealt with as for the bottom, and there should be edge reinforcing.

The wire that is easiest to use is single strand copper or brass. It should be of a size to fit at the bottom of the “V” of each joining panel.

Also look at the ways of reinforcing the bottom edges of lamp shades

Flat Bottoms for Bowls

There are at least three ways to achieve flat bottoms to bowls without the use of external supports.


Using drop out rings will enable you to get a flat bottom of whatever diameter you wish depending on how long you let the aperture drop run.

You can put some dry kiln wash into the bottom of the mould, then firmly press it flat with a round piece of glass. You will need to make sure it is horizontal, so the use of a small round levelling bubble can make this easier.

Grind a flat spot on the bottom of the otherwise finished bowl. It is a good idea to use a two way leveling bubble while grinding. The round bubble is easier to use, while the two way bubbles – two leveling bubbles placed at right angles – are more accurate.

Getting Water to the Mini Work Surface of a Glastar G8

Sometimes the water does not rise to the mini work surface. There are a number of things to check. These, in order, are usually the reasons the water does not get to the Mini Work Surface.

• Ensure there is enough water in reservoir, right up to the overflow

• Ensure channel from impeller to the up tube is clear

• Ensure the up tube is clear

• Ensure tap at the top is clear

• Flush the feed lines with a syringe or bulb instrument

• look at the position of the impeller on the shaft. It can move up or down. Repositioning it can improve the flow of water to the top story.

Supports for round bottomed bowls

A number of useful moulds for slumping do not have flat bottoms. There are a number of possibilities to have the bowl sit firmly without grinding the bottom flat. Remember that you do not need to surround the whole bottom to give the bowl stability.

Some of these include things like:

• A rubber “O” ring, although they usually come in black only.

• Thin slices of wide-diameter tubing.

• Wok support rings.

• Plastic tubing with a small joining dowel allows you to make any size. You can then paint it with the appropriate colour.

• Macramé, embroidery and curtain rings can be suitable.

• You can make them using hole saws. Cut out the big ring first so you can use the pilot hole to line up the smaller hole. Then bevel the inside to fit the bowl.

• Use three bumpons on the bottom. Be sure that the bottom of the bowl is perfectly clean, dry and free from oils. Then use some weight pressing on the bumpons for a day or more so that they stick permanently. You can do this by turning the bowl upright and fill it with some heavy objects.

Firing schedules – what are they for?

Firing schedules or programs are the means of controlling the temperature rises, soaks and falls to accommodate the needs of the glass. They consist of a number of segments –or steps - each of which includes: rate of temperature rise, target temperature, and soak time. They vary according to the thickness of the glass and the forming and annealing needs of the glass. Read and understand the Bullseye Technical Note on the way glass behaves at different temperatures. This will give you a good understanding of what happens to the glass at the different temperature ranges and will help you design a suitable schedule for what you want to achieve.


To assist in visualising what the numbers in a kiln programmer do, you can graph the temperature changes indicated by the numbers in the controller. Visualised from the start of the schedule, it appears as a mountain with a steep cliff on the left rising to a ledge. There is then a steeper rise to the top where there is a small plateau. The mountain then has a very steep face on the right, falling to a broad ledge a bit lower than the one on the left. There is a long shallow slope to the right of the ledge that leads to a much steeper drop to the level again. This is the shape – with variations - that you are attempting to achieve in each program/schedule.

The variations have to do with the type of glass being used and thickness of the glass. These variations determine the amount of heat and the speed with which it is put into the glass. It sets the points at which any soaks are introduced to allow the glass and associated moulds or kiln furniture to equalise in heat or to allow air to ease from between sheets of glass. It sets the top temperature and determines the length of soak at that temperature. It controls the temperature fall to the annealing soak - to equalize the temperature throughout the glass. It then controls the rate of fall to anneal the glass – removing the stress and follows up with the fall to room temperature.

A description of each of these stages includes the heat rises and any soaks required, the temperature fall, annealing soak and cool, and the cool to room temperature.

Initial heating rise

In the simplest form, the initial heating is a relatively slow rise to a point about 50C above the annealing point. This allows the glass to gain heat without thermal shock. The initial heating may be achieved in several segments, depending on what you are doing. A thick piece, or one fired many times, might be taken up in a number of stages - initially very slowly (with or without soaks - also known as holds), and then at more rapid increases. A 6mm piece being slumped into a simple curve mould would need only one segment to the top temperature.

Another example of variations required would be a 6mm piece suspended over a cylindrical mould for a drape. My experience has shown that there is a requirement for multiple segments. This starts with an initial rise of 50C/hr to 100C with a 10min soak, then 100C/hr to 250C, 10 mins, then 150C/hr to 500C, with 10mins and finally 200C/hr to forming temperature - in the region of 630C - 677C with an appropriate soak to achieve the effect desired - peeking is required to determine the length of this soak. The point being that some circumstances require much more complicated arrangements. Here it is because the mould drains the heat away from the centre of the glass while the edges heat up.

Final heating rise

Above the annealing plus 50C temperature is when the rise can be much faster up to the working/top temperature. This speed should not be as fast as possible, because it has a number of drawbacks. The speed of this rise is influenced by the amount of heat work you wish to put into the glass. This in turn will influence the top temperature and length of soak at that point.

You most often want to insert a bubble squeeze in this rise to avoid large bubbles due to trapped air.

Cooling phases

The cooling phases are several: fast drop to annealing soak, annealing cool, cool to room temperature.

Fast drop

Once the soak at top temperature is finished the requirement is to cool the glass and kiln as fast as the kiln will allow. This is to avoid the devitrification that can occur in the range of 650C to 760C.

Annealing soak

This soak at the annealing point is to allow the glass to reach the same temperature throughout from side to side and top to bottom. The length of this soak will depend on the thickness of the glass. More information on annealing is here.

Annealing phase

The slow steady cool from the annealing point to about 55C below the annealing point is where the annealing of the glass is done. What is required is a gradual, but steady decline in temperature to allow the glass to reduce in temperature evenly throughout its thickness. This even reduction in temperature should continue to the strain point and slightly below. So this phase must not be done quickly. For a 6mm piece 80C/hour is usually adequate. More on the annealing phase is available here.

Cooling to room temperature

Cooling to room temperature should be done at an even rate, although faster than the annealing cool. Too fast a cool below the strain point can cause thermal shock and therefore breakage. Typically the cool to room temperature from the strain point can be two to three times faster than the annealing cool. It is a good idea to control this cool to at least 100C. If your kiln cools more slowly than this, it will not be using any electricity, but it does protect against too rapid cooling if you open the lid or door.

Ceramic Mould Repairs

Most moulds have a long but limited life due to cracks appearing and accidents. However the life of moulds can be extended with repairs. Most moulds can be repaired, unless shattered.

Cracks can often simply be ignored. If the glass is not getting marked by the crack, then you can keep using it until it widens or goes completely across the mould. If you feel the need to protect the mould before it completely fails, you can add a layer of cement on the back of the mould to support it.

The cement can be a high temperature product like “Sairset” or any other high temperature ceramic cement. The one I like is cement fondu. It comes as a powder – often from sculptural suppliers – which you mix with water to a paste. Wet the mould well to ensure it does not pull the water out of the cement, causing it to fail. Then apply the cement liberally to the back of the mould over the crack.

If you feel the need, you can fill the crack from the front also. Again insure the mould is wet and then press the cement into the crack. Wipe the excess cement off immediately or it will stick leaving blemishes on the mould. Use a wet cloth to do this. You can smooth the filler by using a wet finger to run along the filled crack. These notes apply to which ever kind of cement you use.

Divots or little chips from the surface of the mould can be ignored, if there is no effect on the glass at your operating temperatures. If they need to be filled, you can use a temporary patch by making a paste of batt/kiln wash and smoothing it over the divot. This will last a couple of firings probably. A more permanent repair is to use cements. Prepare as above and smooth into the depression. When cured, particular attention will need to be paid to getting a good coating of batt wash, because the cement surface will reject the water carrying the powder more than the ceramic surface does.



If the mould has broken you will need to stick it all back together. Do not attempt to smooth the edges, they are needed to make as close a match as possible to each other. The rough edges provide a key to location as well. Soak the mould pieces very well. Prepare the cement and apply a little to one edge of the matching pieces. Press together firmly and then apply a backing of the cement as for a crack. Clean off the face of the mould with a wet sponge or cloth until it is smooth and level with the working surface of the mould. Bind this as tightly as the shape permits and leave for several days.

Curing requirements

When using refractory cements, it is best if you can give it a wet cure for a day. This is often easiest to achieve by putting the cemented mould in a plastic bag. After the one day wet cure, it needs to dry for several days. Finally, it needs to have a permanent cure by firing to a temperature of about 25C above the operating temperature for the mould.

Making Powder Designs Crisp

Tidying up powder designs is often a time consuming process using brushes. One way of cleaning the edges of lines and the bottoms of furrows in the midst of the powder designs is to use a modified keyboard vacuum.

I use a Miele vacuum sweeper –it has a variable suction - with a keyboard cleaning attachment.

I have modified the finest nozzle by putting the end of a ball point pen in it and filling in the remainder of the rectangle with blutac or a similar material. Turn the suction on the vacuum down to minimum and you can be very accurate about the amount of powder you remove to achieve crisp lines.

Creating your own Iridescence

Often iridised surface details are created by using iridised sheet glass and then masking and sandblasting off the unwanted portions. But you can make your own iridised surface detail much more cheaply by using pearlised  mica powder.

One way to apply the mica in areas of detail is to make a stencil from stiff card and sift a smooth relatively thin layer of mica onto the area of glass you want to be iridised.

A second is to mix the mica and powdered clear glass in equal amounts and sift that onto the glass through the stencil. This can help more of the mica to stick to the surface. 

A third is to sift clear powder on first and then a coat of mica. This works less well for me than the other two.

It does not matter if you put too much mica on, as the excess will not stick and can be brushed back into your container for future use. The firing should be at full fuse temperatures to allow the mica to sink into the surface of the glass. When you have poured the excess powder off you are left with an iridised surface where the mica has sunk into the glass. You can, of course, use any of the coloured micas for this purpose.

Cutting Bottles

Cutting bottles seems to have a fascination for many people. There seem to be three methods – heat and cold, scoring, sawing.

There are various ways to apply heat and cold to assist with breaking the bottles.

- A string tied around the bottle and soaked in a flammable liquid is a common way to apply heat. As soon as the flame has gone out, you immerse the bottle in cold water; the temperature differential should crack the glass where the string was.

- Filling the bottle with water to the level where the break is wanted and then applying gentle heat with a torch flame at that level should promote a crack.

- Alternatively, the bottle can be scored and put into the freezer for a while and then into hot water.

Scoring is the common method to start a crack.

- This is followed by tapping from inside the bottle with tools from a purchased kit or home-made tappers – a metal ball on the end of a curved piece of metal.

- The score line can also be the preliminary step in the application of heat or cold.

These provide the cleanest edges to the cuts. However there is quite a high failure rate using these methods.

Sawing is method that provides a higher success rate, but is wet, and leaves rough edges to the cut, requiring further cold work.

- Band saws designed for glass can be used, but usually do not have a high enough throat to allow the thickness of the bottle to pass through.

- Most tile saws cut from underneath, so rotating the bottle can lead to a cut completely around. This requires a lot of skill to do free hand, so you need a jig to keep the bottle at right angles to the blade and the bottom the same distance from the blade while rotating the bottle all the way around.

Float Glass Characteristics in Relation to Kiln Forming

A reported 90% of the world's flat glass is produced by the float glass process invented in the 1950's by Sir Alastair Pilkington of Pilkington Glass. Molten glass is “floated” onto one end of a molten tin bath. The glass is supported by the tin, and levels out as it spreads along the bath, giving a smooth face to both sides. The glass cools as it travels over the molten tin and leaves the tin bath in a continuous ribbon. The glass is then annealed by cooling in a lehr. The finished product has near-perfect parallel surfaces.

An important characteristic of the glass is that a very small amount of the tin is embedded into the glass on the side it touched. The tin side is easier to make into a mirror and is softer and easier to scratch. It also becomes apparent when compressed.
Float glass is produced in standard metric thicknesses of 2, 3, 4, 5, 6, 8, 10, 12, 15, 19 and 22 mm. Molten glass floating on tin in a nitrogen/hydrogen atmosphere will spread out to a thickness of about 6 mm and stop due to surface tension. Thinner glass is made by stretching the glass while it floats on the tin and cools. Similarly, thicker glass is pushed back and not permitted to expand as it cools on the tin.

The heat characteristics of Float glass depend in large part on which company manufactures the glass being used, so the temperature characteristics are given in ranges.

The softening point is around 760C

The annealing point is around 560—540C

The strain point is around 525-505C. The strain point being the temperature below which no further annealing can occur, but the glass can still be thermally shocked below this range.

The characteristic of float glass having a molecular level of tin left on the “tin side” but not the “air side” is important to distinguish. If any forming of the glass is planed after fusing, the tin side in compression will show a “tin bloom” similar to devitrification.

The fact that there are many manufacturers of float glass means that they are not all made to the same specifications. It is not advisable to fuse float glass from different suppliers in kiln forming, so the best advice is to fuse only from one sheet for each piece.

Due to the robustness of float glass, it can be fired with a quicker initial temperature rise than glasses formulated for kiln forming. The down side is that it devitrifies very easily and very badly. Rarely can you perform more than two firings before the devitrification begins to become troublesome.

Application of devitrification solutions

Smooth and complete coverage of the piece is the aim when applying devitrification solutions. A soft brush, an air brush, a mouth atomiser are some of the ways to apply the solution. Some even use a sponge - all these application methods will do the job.
It is a pretty simple process, but requires concentration to ensure the piece is evenly covered. If it isn't, there will be areas of devitrification left after firing.

Longevity of Borax as a devitrification agent

It is true that Borax is water soluable. However, the borax has done its job by preventing the devitrification, so it does not matter whether it has or has not disolved, nor whether it is inside or outside.

Borax as a flux for paint in excessive quantities has the effect of corrosion on the paint or enamel it is mixed with. It is not actual corrosion, just that its effects are like that. The borax expands when wet. The expansion is very little, but over time "pops" off the paint - the time scale is 50-80 years. This happens on the inside of windows where the paint is. So it is not an inside/outside issue, just one of moisture.

But this irrelevant in kiln forming applications when attempting to prevent devitrification, or even to correct existing devitrification. The subsequent possible disappearance of the borax will not matter to the appearance of the piece. It has been reported that borax covered sushi dishes going through dishwasher cycles in a restaurant for years show no devitrification after the presumed disappearance of the borax. In fact, the proprietary devitrification solutions that contain lead would not be applicable in this food containing situation.

Other references to devitrification are:
Homemade devitrification solution
Description of devitrification
Temperature range

Devitrification Prone Glasses

"Are there specific glasses that are more prone to devitrification, and knowing that, what steps can you take to try to avoid it?"


Glasses that are formulated and tested compatible for kiln forming are less likely to devitrify than other art glasses.

Opalescent glasses even if tested compatible for kiln forming are more likely to devitrify than their compatible transparent counterparts.

Yes, you can fuse some of the transparent glass made by a single manufacturer - Spectrum transparent and especially the water glasses are most often compatible within certain limits. But you will find that the edges show devitrification almost always. When using glass untested for compatibility, capping with clear glass often helps in reducing or preventing devitrification, as the clears seem less prone to devitrification than coloured glasses

You can clean very well and hope for the best, or you can clean and then use a devitrification agent - normally a flux or low firing glass in suspension - and spray or brush it on. If it is one of the low firing glasses in suspension, make sure you put it on before taking it to the kiln, as it will stick to other things when fired.

Another method is to avoid staying in the devitrification range of temperatures very long - both during temperature rise and cooling.

A description of what devitrification is


The temperature range in which devitrification occurs


A homemade devitrification solution

Annealing

Stress is induced into glass during cooling through the outsides of the glass cooling more quickly than the interior. This contraction causes residual stress. Annealing is the process to relieve that stress. The annealing soak temperature is determined by a number of factors, of which coefficient of expansion, viscosity, exposed surface, and thickness are some. “The relief from stress happens because of a process of viscous flow. At the annealing point it can take place within a few minutes whilst at the lower annealing temperature…. It can take a few hours.” (Dictionary of Glass, Charles Bray, p.27)

The above statement is applicable to glass of a single colour from one manufacturer. When combining colours in kiln forming, the colours absorb and give off heat at different rates and so you need to allow more time for the annealing – relieving of heat induced stress – to occur. 

The annealing soak has the purpose of allowing all the glass to be the same temperature (within 5°C) from top to bottom, and side to side. The annealing occurs during the slow cool past the lower strain point.  The manufacturers give annealing and strain points for their glass. These should be observed, rather than anything pre-programmed into your kiln’s controller.

Note that the stress of incompatible glass cannot be relieved by annealing.

Also, each time the glass is taken to a temperature above the annealing point, it must be annealed again.  There is no short cut to this.

There are more notes on annealing here.

Achieving a Matte Finish by Cold Working

Although sandblasting and then firing a piece can achieve a matte finish, there are several other ways to improve the quality of the final finish.

One of these involves the use of manual sanding after sandblasting in order to smooth out uneven spots and achieve a better final finish.

• Start with a 400 mesh diamond hand pad. It shouldn't be necessary to start out with a lower mesh (coarser) pad.

• Alternatively use wet/dry silicon carbide sandpaper. A combination of 400 mesh paper, followed by 600 mesh paper will work well.

• If you're using sandpaper, place a sponge between the paper and your hand for improved comfort and to improve the evenness of the final finish.

• An alternative to hand sanding is to use a electric sander or grinder, but be careful with the pressure you use, as it is possible to grind into the surface with a rapidly spinning surface. You also need to keep the surface wet to avoid heat build-ups.



You can also use a lathe with appropriately shaped wheels to give decorative effects to the object.

Firing for a Matte Finish

Glass can be fired to take on a satin appearance that is both appealing to the eye and pleasing to touch.

The first step toward the matte finish is to sandblast the piece after fusing, then fire to a temperature between 600C and 675C. A short soak - or no soak at all - is all that is needed.

The exact temperature needed depends on a number of factors, including:

• The specific glass being used. A soft glass such as black generally needs to be fired to a lower temperature than glasses that do not absorb the heat so easily. Every colour and type of glass will behave a bit differently, so experimentation and record keeping is critical.

• The grit and type of sandblasting medium. Generally, a grit from 120 to 200 is preferred, with aluminium oxide performing a bit better than silicon carbide – which can often lead toward some devitrification.

• The particular kiln being used. Your kiln is a bit different from any other one. Start with a temperature in the middle of the 600-650C range and adjust depending on the results you achieve.

• The finish you want will vary with only a few degrees difference. This means that you have to observe the firing. Make sure you keep good records of the specific firing schedule used so that you can make adjustments if needed for future firings.



Some variations can provide distinctive elements to the finished piece.

• Masking certain elements before sandblasting can provide contrasts of texture within the piece.

• Firing at a lower temperature for longer can give the results you want, without any additional marking on the bottom of the piece.

• To keep the matte texture, any subsequent slumping of the piece should be done at as low a temperature as possible.

Preventing Chipping When Using a Tile Saw to Cut Glass

One of the most common problems in using a tile saw to cut glass is the tendency for the saw to chip the edge of the glass as it completes the cut. This occurs when the blade of the saw has less glass to cut through. Excessive and uneven pressure and the lack of support cause this break-out.

It's possible to improve the quality of the cut by slowing down and pushing the glass through the blade more gently, but this seldom solves the problem completely. Pushing equally on both sides of the cut is also important to minimise the break-out.

One solution that does work is to provide support for the end of the bar. This adopts a woodworking method for preventing splintering at the ends of cuts.
Use a scrap length of pattern bar or other thick glass. Place it against the glass being cut. As the blade emerges from the glass being cut, hold the two pieces firmly together and continue cutting. The blade should immediately engage the second piece of glass. Once the saw blade entirely clears the first piece, you can turn off the saw and remove a chip-free slice from the pattern bar.

You'll need to trim off the ends of the scrap piece from time to time, but you can use the scrap over and over until it becomes too small to do the job.

This works best with a tile saw where the blade is below the cutting surface. When you use an overhead saw, the breakout is much rarer.

Dams for Pattern Bars

Once you have cut and arranged the glass for your pattern bar, you need to dam the bars in the kiln to prevent the glass spreading.

The materials required for forming the sides of the dam can be made from anything that is rigid and can withstand the heat of the kiln, e.g., cut up kiln shelves, rigidised fibre board, vermiculite board. The material being used to dam must be over 13mm and preferably around 25mm thick. It should be capable of standing vertically on its edge without support. Cut the dam material into strips at least as long as the pattern bars you're damming, and at least as wide as the bars are tall.

You also need fibre paper for lining the edges of the dam and keeping the glass from sticking to the dam. Cut the strips of fibre paper to line the walls of the dam and keep the glass from sticking to the dam material when you fire the kiln. Three millimetre fibre paper works best.

The width of the fibre paper should be 3mm narrower than the pattern bars are high. By cutting the strips shorter than the pattern bars you allow the bars to round perfectly on top and help prevent needling.

The fibre paper should go around all sides without gaps. They should have straight edges so the glass of the pattern bar does not leak between or underneath the fibre paper. The use of iridised glass on bottom and sides will provide a smooth release from the fibre paper and is a second option. It is also possible to line the fibre paper with thinfire paper to provide a smooth release, although it is more time consuming than using the iridised side of glass against the fibre. But do not combine thinfire and iridised glass. There is a reaction that leaves holes and craters in the glass.

Pattern Bar Box

Making a box for a pattern bar design that involves frit or lots of small pieces is necessary and simple.

Let's assume you want to make a pattern bar that's 25mm by 25mm by 200mm long. Start by cutting three strips of glass, each 25mm wide and 200mm long. Also cut two 25mm squares of glass. You can use any colour, but remember that the colour you choose will make up the outside of your pattern bar.

Assemble the three strips and two squares to make a small box, with one piece on the bottom and the others attached to form sides and ends. To do the attaching, use a hot melt glue gun.

The advantage of using hot melt glue to make a pattern bar box is that after the box is assembled, it can be filled with frit and other scrap outside the kiln, then easily carried into the kiln and dammed as usual. The glue will burn off during the firing. You can finish the box with a final strip of glass laid on top, but this isn't essential.

You then dam the box as for any other pattern bar.

Designing a Pattern Bar

Assuming that you are not going to just dump your scrap glass in a random pattern to form a pattern bar, you need to spend some time designing it.

The simplest kind of bar is composed of strips of glass which are stacked or assembled in the kiln, but there are many other more elaborate configurations.

Because of the additional annealing time required for larger and thicker items, most pattern bars range from 1" by 1" to no larger than 2" by 2". The length of the pattern bar can be any length, up to the maximum that will fit in your kiln.

The design process begins by thinking about the cross section of the bar. This is what will appear when cut and assembled. As a simple exercise, assume you are making a diamond pattern in the bar. You can draw this out using 3mm as the thickness (or 1.5mm if you are using thin glass). Rough out the pattern and then begin using 3mm as the grid. Remember that you will need to cut your strips 4mm or wider to obtain a clean break. As you plan it out you will see that you need one length at the base one half of the space remaining after you have laid down the first, central piece for the diamond. The next layer will have two strips for the diamond, giving a requirement for one strip to fill the space between the two for the diamond shape and two strips each one half the remaining space. This process goes on until the area is filled.

Pattern Bars

A pattern bar is a thick bundle of glass that has been fused together. These can be in the shape of a rectangle, or can be a thick pot melt – whether a disc or a rectangle. The length of the individual bars can be as long as your kiln allows, but needs to be practical to handle when cutting.


The basic steps involved in making a pattern bar include deciding on a design –whether controlled or random, cutting glass for the bar, assembling the cut glass into the desired bar shape, then firing to a full fuse. Once fired, pattern bars can be cut into slices with a saw - tile, glass, lapidary, or stone – which uses water for cooling and lubrication. The individual slices are then assembled and re-fused to make bowls, platters, and similar shapes. They can also be used as accents in any number of applications.

There is a caution about using pattern bar pieces. As the glass in the bars has been fired to a relatively high temperature, some of the characteristics may have changed. So you need to do a compatibility test before doing the main piece.

Designing Pattern Bars
Boxes for Pattern Bars
Dams for Pattern Bars

Float Glass in the Kiln

An important characteristic of float glass is that a very small amount of the tin is embedded into the glass on the side it touched. The tin side is easier to make into a mirror and is softer and easier to scratch than the air side. The characteristic of float glass having a molecular level of tin left on the “tin side” but not the “air side” is important to distinguish. There are short wave UV light sources to help determine this. The tin side gives a whiter glow than the air side. If any forming of the glass is planed after fusing, the tin side needs to be on the side being stretched, as when in compression the tin side will show a “tin bloom” similar to devitrification.

If the tin side is down on both sheets, and it is slumped into a mould there will be no tin bloom because the tin layer is stretched. If the tin side is up on both sheets and it is slumped into a mould there will be tin bloom because the tin layer is compressed. If you have placed the tin sides together, or on both the top and bottom, one of the tin surfaces will be in compression and so will show tin bloom. This is often mistaken for devitrification, and no amount of any devitrification solution will help.

A borax solution can help with the devitrification on float glass in some circumstances. It is not a perfect solution. This is because tin bloom and devitrification are often not distinguished correctly. But a high level of cleanliness and polishing the glass until squeaky clean is the best start.

The heat characteristics of Float glass depend in large part on which company manufactures the glass being used, so the temperature characteristics are given in ranges.

The softening point is around 760C

The annealing point is around 560—540C

The strain point is around 515-495C. The strain point being the temperature below which no further annealing occurs, although the glass can still be thermally shocked below this range.

Due to the robustness of float glass, it can be fired with a quicker initial temperature rise than glasses formulated for kiln forming. The down side is that it devitrifies very easily and very badly. Rarely can you perform more than two firings before the devitrification begins to become troublesome.

All window glass now seems to be referred to as float glass. However, the float glass process was invented in the 1950’s. Prior to that time, window glass was drawn. Float glass can use more iron in its composition, because it does not have to be drawn up out of a molten vat of glass as the drawn glass did and still does. Float glass is formulated to be stiffer at forming temperatures, whereas the drawn glass has to be flexible due to the mechanical stresses it is put under during the drawing. Except for low iron glass, the float glass has a distinct blue green colour when viewed through the edge. Drawn glass has a variation in thickness and is much paler when viewed through the edge. These visual differences can help distinguish the two kinds of glass, but are not foolproof.

More information on the general characteristics of float glass can be found here.

Stainless Steel Moulds

Stainless steel is sometimes called the almost the perfect mould material. It is lightweight, difficult to deform, and durable for a very many firings. Simple bowl forms are relatively inexpensive to buy — you can even use cheap stainless steel bowls. All you need to do is drill three or four small 1.5mm holes in the bottom for air to escape.
It often is a good idea to fire the mould to working temperature (say 650C) before attempting to kiln wash the form. This burns off the protective oils from the steel. Alternatively, sandblast the mould to clean it and give a small tooth for the kiln wash.

Stainless steel moulds do need to be covered with kiln wash. This is difficult to do when the mould is at room temperature, but it can easily be accomplished by heating the mould to around 150C, then brushing or spraying on the kiln wash while the mould is hot. The water in the wash will evaporate rapidly, leaving the protective elements behind. If you heat the mould too high the water will boil off, leaving gaps.

Also, it’s important to realize that steel contracts more than the glass. This is the opposite of ceramic, which contracts less than the glass. As a result, slumping on the outside of a steep stainless steel form generally works better than slumping on the inside.

Still, you can get away with slumping inside gentle bowl forms; just make certain it’s well covered in kiln wash. A sprinkle of a little kiln wash powder inside can also be considered. Be aware that slumping inside deeper forms may not work.

Draping over steel

Steel absorbs heat much faster than glass, so the glass suspended on the steel is cooler than the suspended perimeter during the heating and cooling cycles of the firing. This does not apply to slumping when the glass is supported on the edges, as so little of the glass is touching the mould at the start.

The fact that the steel “bleeds” the supported glass of heat while the unsupported parts heat up, requires slow heating with or without periodic soaks on the way up to ensure the glass and steel are the same temperature up to about 540C or the upper strain point of the glass.

I tend to be very cautious, and for 6mm pieces heat up approximately like the following:

100C/hr to 100C, soak 20

150C/hr to 200C, soak 20

200/hr to process temperature



When cooling, the steel is in closer contact with the glass, no special considerations are needed, so the normal annealing soak and cool are used.

Removing Glass from Kiln Shelf

Care is needed when removing glass that is stuck to the shelf. You need to protect your hands with thick gloves, as any slip will cut your hands deeply.

For mullite and other ceramic shelves you can use a variety of tools:

If there is a small amount of glass in one or more spots, you can use a scraper or lead knife. The wider the blade is, the less chance there is of creating a big divot beside the stuck glass.

If the stuck glass is large or thick, you can use a hammer and chisel. Care is needed to avoid creating a bigger hole in the shelf. Use very shallow angle, almost parallel to the surface of the shelf to chip out the glass.

Diamond hand pads are useful to get the last bits smoothed out. You need to be careful of creating a low spot by working only in a concentrated area. One way of avoiding that is to use a slurry of grit and grind with large sheet of float glass. The area being covered is large and so reduces the danger of creating low spots. Remember you can get away with smoothing the shelf, not all the glass has to come out of the shelf. If the bits of glass are only small, it will not reduce the life of the shelf much, although glass tends to be corrosive to kiln brick and ceramic that it is in contact with.

If removing the glass has taken a significant amount of the shelf surface off, you can repair it. A temporary repair is to fill the divot with dry kiln wash and smooth it with a plasterer’s float or a piece of float glass. A more permanent repair is to mix a small amount of cement fondue with or without a little vermiculite. Smooth this level with the rest of the shelf while wet, as it is very hard after curing, which occurs at about 600C. If the mix is of cement fondue only, it will tend to reject the kiln wash, as it is more dense than the shelf.


Removing glass from fibre shelves in some ways is much easier, as the shelf material comes away with the glass. This does mean that repairs are always necessary. This can be done with a temporary fill of dry kiln wash or more permanently with a mix of 1 part cement fondue to about 6-7 parts vermiculite. This makes a less dense filler than cement fondue on its own, which would be too hard for fitting with the fibre shelf.

Silver Foil Inclusions

Silver foil is better for inclusions than silver leaf as there is more substance and so less likelihood that it will burn away.

Sterling silver – not that common in foils – will darken easily on exposure to temperature and air.

Silver foil will also react with some glass colours. A good guide to this can be obtained from Bullseye’s chart on glass interactions. This shows which glasses react with silver. To minimize the reactions, minimize the amount of time spent above 600C.

Only a short bubble squeeze is required with foil as it is not so stiff that it will resist the weight of the glass on top, so creating bubbles. A short squeeze to allow the air out is still a good idea.

Venting moulds

Raising the mould from the shelf to provide ways for the air to get out of the mould is as important as providing holes in the mould itself. Often people recommend placing the mould on kiln furniture, but it is very easy to have a number of pieces fibre paper to stack in three places under the edge of the mould to provide space for the air to be expelled from the kiln by the dropping glass. Only a little space is required for the air to escape.

Vertical Kiln Formed Holes

For vertical holes in frit-cast reliefs you can fill a drinking straw with plaster, cut while still wet and build the frit or cullet around it.

Already fused pieces can have the holes made much neater and smoother by using the above method in pre drilled holes. Another method is to wrap a thin strip of fibre paper around a pencil or end of a paint brush. Then push this circle of fibre paper into the hole. If this is the same height or a little less than the glass, it provides a clean fire polished hole, if the glass is taken to the high end of fire polishing temperatures.