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 with glass paint due to the risk of removing the glass paint.

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

Initial Rates of Advance

Almost everyone has fired the glass from cold too fast at least once. This provokes the question of how fast it is possible to fire glass. Of course, there are lots of variables which relate to the evenness of heat across the firing surface of the kiln. A number of factors will affect this. Among them are:

• The size and shape of the chamber
• Composition of the chamber – brick, fibre or a combination of both
• Location of the elements - lid, sides or elsewhere
• Distance of elements from the work
• Spacing between the elements

This means that only general guidance can be given for you to use in gaining experience with your kiln, as your kiln is unique, even if it is one of a production series.

Now with all those cautions, some general guidance can be given. This table assumes that you have a top fired kiln and that the glass is about 300mm below the elements and that it is at least 50mm from the sides of the kiln. It also assumes that the glass has not already been fused.

First Segment Heating Times for Top Firing Kilns
by Number of even 3mm Layers and Size

100mm dia: 1 layer - 999C/hr || 2-3 layers - 999 || 4 layers - 750 || 5 layers - 600

200mm dia: 1 layer - 999C/hr || 2-3 layers - 500 || 4 layers - 430 || 5 layers - 330
300mm dia: 1 layer - 999C/hr || 2-3 layers - 335 || 4 layers - 275 || 5 layers - 215
400mm dia: 1 layer - 750C/hr || 2-3 layers - 250 || 4 layers - 200 || 5 layers - 155
500mm dia: 1 layer - 600C/hr || 2-3 layers - 200 || 4 layers - 165 || 5 layers - 120
600mm dia: 1 layer - 500C/hr || 2-3 layers - 165 || 4 layers - 135 || 5 layers - 105

First Segment Heating Times for Top Firing Kilns
by Number of Uneven 3mm Partial Layers and Size

100mm dia: 1+ - 850C/hr || 2+ - 750 || 3+ - 500
200mm dia: 1+ - 500C/hr || 2+ - 430 || 3+ - 335
300mm dia: 1+ - 300C/hr || 2+ - 250 || 3+ - 215
400mm dia: 1+ - 215C/hr || 2+ - 188 || 3+ - 158
500mm dia: 1+ - 177C/hr || 2+ - 162 || 3+ - 137
600mm dia: 1+ - 150C/hr || 2+ - 143 || 3+ - 120

If you are adding decorative elements to the basic disk, you need to consider the effect that the additional pieces have on the whole. Each piece of glass shades the pieces below it from the heat of the elements, so to keep the glass heating at the same rate throughout, you need to slow the rate of heating by using the suggestions for the next higher thickness. So, for example, a two layer piece of 300mm can be fired at ca. 330C/hr, but if you have decorative pieces spread across the base, you need to slow the rate of advance to 250C/hr.

Note that these rates should be taken as the fastest possible rates. They may be too fast for your kiln.

Resists for Large Areas

Avoiding bubbles appearing under the vinyl resist on large sheets of glass when preparing etching or other resist based processes is often difficult.

A trick I learned from the firms that apply advertising vinyl to vehicles is to use a spray bottle filled with water that is just soapy. A few drops per pint will be sufficient.

Their process uses pre-cut vinyl with an adhesive backing. So the first thing to do is to pick out the unwanted pieces. That is the pieces covering the areas that will be etched. Then you need to put a backing onto the picked out vinyl – usually really wide masking tape.

Lay out the vinyl on the glass. Tape one end of the vinyl securely to the glass. This ensures that you get the vinyl correctly aligned over the whole area. Fold the whole piece of combined vinyl and backing back to the taped edge.

Carefully peal back the covering for the adhesive side making sure you do not pull off any of the isolated vinyl pieces. Spray the glass with a mist of soapy water to ensure all the glass is covered, do not have the glass running with liquid, but be generous.

Start the application process by folding the vinyl onto the glass. Use a credit card or better, a large squeegee such as used for grouting mosaics. The tool you use must be smooth to avoid scratching the vinyl. Push the soapy water forward and to the sides as you move along the piece of glass. Keep pulling the protective layer evenly off the adhesive side as you work forward.

When completely attached, remove the backing from the vinyl. This will enable you to see any bubbles you may have left. Work out any bubbles by further pressing the soapy water out from under the vinyl to the edges. Where any remaining bubbles are in the way of the design, puncture them and work out the bubble of moisture through the hole. Cover the puncture with a small piece of vinyl.

Leave for a day for the vinyl to become firmly attached to the glass and then you are ready to do the etching.

Where you are going to cut the vinyl by hand, you do not need the backing. All the rest of the process is the same.

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.