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