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.