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.