Mitred Corners

There are various ways to determine where and at what angle to cut the lead came, especially for panels with more than four corners. But the most common is a four cornered panel. The following method works for that circumstance very well.

My preference is to use the battens surrounding the cartoon as a guide for the placing of the mitres on the first two cames. The battens are placed around the cartoon to suit the came width. The vertical came is placed in contact with the bottom came.

A short piece of came – ca. 50 mm – is placed on the external cartoon line almost in contact with the vertical lead. A second short piece is placed on top of the short piece and extended over the top of the side lead.

Mark the lead lightly with a nail or pencil. Cut the angle from the inside to the outside corner. This will form a 45 degree angle.

You can do the same for the bottom came, by removing the vertical came for the time being and doing the same operation on the horizontal came. Then they can be place back together for the next operation.

The finished mitre

To determine the length of the came which is already mitred on one end, put the came in place on the cartoon.

Then place a short piece of lead on the cut line which is at right angles to the came to be measured. Then place a short piece over the two cames as for the first mitre cut.

A nail or other pointed implement is used to scratch a line on each side of the overlapping lead. When this overlapping lead is removed, a diagonal is drawn from the inside mark to the outside mark. Cut along this diagonal.

Ready for cutting

There are other methods for panels with more than four right angles.

Firing Prices

A number of us with kilns are sometimes asked to fire a piece or several for someone. The question of how much to charge – if anything – quickly comes to the fore.

There are several considerations if you decide to charge - and I think you should. As I go through the cost elements, I include a worked example.

Replacement cost

This is one of the elements least considered in costing a firing. You already have bought the kiln and so it seems like you do not need to consider that cost any longer. At the least, though it is a variable cost – the cost is related to the frequency of use. Fewer uses leads to higher costs for each firing and vice versa.

You can use a discounted cost or a replacement cost for your calculations.

I take the cost of the kin and discount if over five years. Divide the purchase cost by the number of firings per year and you have the cost of each firing spread over five years.

So if the kiln cost you £5,000 – enough for a metre square kiln - and you fire on average 3 times per week this gives 156 firings per year and 780 over the discounting period. This means the depreciation cost of each firing is £6.41 per firing. Of course, if you fire more frequently, the cost per firing comes down.

You can also consider the replacement value after 5 years. If you assume the cost of kilns will increase by 5% per year then your replacement cost will be £6077. So the replacement cost of each firing would be £7.79. This replacement cost is of course a kind of unseen cost of using your kiln. It needs to be accounted for so you can buy a new kiln when you need it.

These considerations show the use costs of the kiln are between £6.41 and £7.79 per firing even before you consider the material costs. For ease of the example, lets round this to £7.10.

Energy Costs

To the replacement cost you need to add the electricity cost per firing. A 30kwh three phase kiln can use around 50kwh for a full fuse firing. If the electricity costs 0.15/kwh, the electricity cost of firing is £7.50. This increases the cost to £14.60

Material costs

If you are covering the shelf with Thinfire, the additional cost will be in the region of another £7. This brings the cumulative charge to £21.60 per firing. Even if you don't use Thinfire or other ceramic fibre paper, you should add 10% for the materials used, but are too small to be accounted for separately – £2.16 – giving £23.76.

Your Time

Then add time that you spend. If you are placing the material in the kiln, or programming the controller, the time that takes needs to be added. If you charge your time out at £20 per hour, you may use a large part of that hour just assisting the person. If you are placing things for the person, the time used will increase. Assume all you have to do is prepare the kiln and shelf and programme the controller – this will take the best part of an hour, so add £20 to the charge. You now are up to £43.76 in costs.

Opportunity Cost

Then add opportunity cost - the price you put on the time you couldn't use the kiln. If it is not putting you out, the opportunity cost is £0. If however, you could have been doing something else in it then you need to charge for the disruption. This is flexible, but might be the cost of one kiln firing. If it is the cost of one firing you add £23.76 giving £67.52.

The firing charge

The minimum you should be charging for others to use your kiln is £43.76. If you consider the disruption and opportunity costs, the price should be at least £67.52 in this example.

What this means for the pricing of your work

These considerations make for an apparently high charge, but shows you what your firings cost you and what should be factored into your charges for kiln formed materials. The minimum you should be adding in this example is £23.76 for each firing for your own work. If you could get four pieces in your kiln and need a fuse and slump firing, each piece has a firing cost of £11.88 (23.76 * 2 / 4). Then you need to add glass costs, time and profit to get the wholesale price. Double that for the retail price.

Fusing is not cheap.

Pickling

Pickling Silver

This term relates to the removal of firescale from silver by use of chemicals, often slightly warmed.

When heated, silver blackens on the surface. It is common in silversmithing to pickle the object, bringing the shine back.

There are several methods.

Hydrochloric acid is the most common chemical used. It normally is used in concentrations of 10% or less and often is slightly warmed in a soup warmer or other similar temperature controlled container

Hydrogen peroxide (sparex) solutions can be used, but are a bit slower. This also is used in a soup warmer.

Acetic acid, available from most chemists and home-brew suppliers, can be used but is so much slower that significantly long soaks are required.

The best solution for this is a 5% solution of citric acid or similar concentration of tri-sodium citrate.  This latter is best for glass, as it chelates the corrosion or stuck kiln wash, but does not etch the glass even after 48 hours soaking.

Revised 6.1.2022

Fibre Blanket Moulds

Fibre blanket moulds are good for free form moulds. The blanket can be cut into shapes or crumpled. It does not have binders as the papers do, so kiln wash is not necessary. Still, I have always sprinkled alumina hydrate powder over the mould. You can then support the high spots with kiln furniture – existing or custom made.

Pre-wetted fibre blanket is available - Moist Pack is one brand name.

Or you can make the mould yourself from fibre blanket and hardener. You need:

• ceramic fibre blanket. It should be 3 mm or thicker, but 25 mm needs to be compressed when wet. It is possible to use two layers of 3 mm fibre blanket, but they do not stick together well unless thoroughly wetted.
• colloidal silica - often is called mould hardener or rigidiser. Paint this onto the fibre blanket liberally, both sides if possible.


The rigidiser can be brushed on or sprayed on. Some people soak the blanket in the rigidiser and then squeeze out the excess.

You must protect the master with cling film, Vaseline, or other waterproof separator. Be sure about whether you want a draping or slumping mould, as the inside needs to be smoothest for a slumping mould and the outside smoothest for a draping mould.

Press the wet fibre blanket to the master. Then let it dry for a couple of days to become stiff enough to remove from the master. Let the negative dry for another period when out of the mould.

The drying method for rigidised fibre mould depends a bit on the structure from which you are taking the shape. If the shape is a piece of glass you can heat slowly to about 300C, but you have to be careful not to go much above that temperature to avoid the mould sticking to the glass. When cool you can carefully remove the mould from the glass and fire it to about 720C to cure it.

Other materials should be able to withstand at least 400C if you are drying in the kiln.

Materials that cannot be subjected to heat should be air dried. This will take a long time, possibly a week or more. The master should be coated with petroleum jelly or cling film to ensure the drying of the mould does not also cause it to stick to the master.

When the mould is dry, put into the kiln and fire to around 760C to cure the mould. You can fire fast, and after 10 minutes at 760C, you can just turn the kiln off, as there is no possibility of thermal shocking the mould. The point is to get the glass which has been in suspension to soften and stick together. Upon cooling the mould will be hard, as it is held together by the glass structure within the fibre blanket.

Once rigidised, you can sand the mould to refine the shapes. But you must use dust mask as the dust and fibres are dangerous to your health.  Do it out doors if possible. Otherwise a well ventilated room is necessary. You can sand down the high spots and generally smooth the mould to obtain a finer texture. Usually 100 grit sandpaper does the job quickly and leaves a relatively fine surface texture. If unhardened blanket is exposed during the sanding process, You can add a mixture of the rigidiser and the "dust" from the sanding to any holes, dimples or exposed unhardened fiber in the mould Then re-apply rigidiser to the sanded areas, and cure the mould at 760C again.

If you are rigidising, you need a separator – kiln wash – either powder or in a solution brushed on. A rigidiser does not burn off; it fuses to itself within the mould material and makes it harder. The resulting mould material will also be more brittle and should be handled with some care. I.e., never pick up the mould by the edges or with a piece of glass on top.

When you are satisfied with the shape and texture, you apply the kiln wash and fire.

The rate of heating the kiln and the soak will depend on the complexity of the shape of the mould and the thickness of the glass but there are no concerns about the mould as it is not subject to thermal shock.

With delicate treatment, the mould can be reused many times.

Two examples are shown here:

Lamp shade panel form

This is a "free form" mould made to give the glass sheet the appearance of crumpled paper

Cutting Small Diameter Circles

It is possible to cut regular, small diameter circles without buying a lens cutter. It can be done with the assistance of a Lazy Susan or cake decorating turntable.

Draw the circle of appropriate diameter on the turntable with a compass. Place the glass on top of the turntable, and position your cutter above the drawn circle. Press on the cutter with one hand and rotate the turntable with the glass on it, with the other.

Steady your hand with the cutter by keeping your elbow tight against your side. This enables you to make a very good, if not perfect, circle without buying an expensive small circle cutter.

If the glass is too dark or opalescent to see the line, make a template and put it onto the glass. Cut beside the template or use the template to mark the glass. Then place the marked glass on to the turntable and cut as with transparent glass.

You will not be able to run the score by turning the glass upside down and pressing as you can with larger circles. You will need to make a number of relieving cuts to the tangent of the circle and break them away one by one. Yes, this does leave a rough edge at various places around the circle, so grozing or grinding will be necessary.

Removing Cement

Sometimes life gets in the way and a partially cemented panel is left for days. When you come back to it the lead light cement is hard. Removal requires a material hard enough to shift the cement but not cut into or damage the lead or glass.

The best tool is a rectangular stick of hard wood. It should be at least 6 mm thick to stand up to the pressures of cleaning, but not much more than 12 mm - 15 mm wide to enable you to get into corners. It should be 200 mm – 250 mm long for ease of handling. Shape one end in a chisel or wedge shape. I prefer the wedge shape, but the chisel shape can be re-formed more quickly than the wedge because there is only one edge.

Use the stick by running it along the lead with some downward force, but remember you can break the glass with too much pressure. This should break the adhesion between the cement and the glass. To get all the cement off the glass, you will need to use the stick in localised areas almost as a kind of pick. This is the kind of tool that I use in conjunction with a stiff brush for the final clean up of each panel before polishing.

Stress Testing

You should be testing for stress in any new set up. This includes new processes, different layering, different colour combinations, and any other variation that you make in your basic processes.

You can buy kits called stressometers. The devices called stressometer are not actually meters. They are battery powered light sources with two pieces of polarized film in frames. This is very good for small kiln formed pieces. For larger pieces you can use your light table with larger pieces of polarized film. A description of how to use these is given here.

Stress appears as a “halo” of light around the stressed areas. The more light that appears in, or at the edges of the piece, the greater stress is being indicated. The amount of acceptable stress is given in a Bullseye Technical Note.

However, if you test only the combination of glass you propose to use, you will not know if the stress is from incompatibility or from annealing – the appearance is the same for both. This means that you need to place an additional test into the kiln, to determine the adequacy of annealing. This is especially important when tack fusing and doing thick work. The process for doing this is given here.

Bending Wide Cames

The way to bend larger leads such as flat outside leads around pieces of tracery or other curved shapes in window panels is to lay the lead upon the bench, and use a curved, preferably wooden, lathekin.

Progressively manipulate the lead into the curve. Hold the lead steady by keeping your fingers spread on the top and back of lead and manipulate the curve between your extended fingers. Gently push the curved lathekin along the heart of the lead with small, smooth, circular strokes. Smooth the lead flanges by pressing down on the flanges on the inside of the curve as you go. If you try to do it too quickly the lead will probably buckle.

Frequently turn the lead over, applying the process to both sides.

If the flange crimps or buckles, put smooth jawed pliers inside the lead and squash the flange flat. The pliers can be used to flatten any kinks that develop in the lead.

The key is to handle the lead gently and in stages, gently flattening the complete lead and not flattening completely one spot before moving on to the next.

The advantage of round over flat in this circumstance is that round came of the same size can be bent into smaller curves that the flat came of the same width.

The technique for finishing a curve around a single piece of glass can be seen in the tip “Leading Small Circles”

Glass Stains

A lot of people want to use old window glass – for recycling reasons, for the character of the older glass or because it is cheap or free. This glass often has stains or the appearance of corrosion or etching from age or storage conditions. To remove these blemishes you should start from the least aggressive method and work your way through the more aggressive methods until you find one that achieves the cleanliness you desire.


When using these methods appropriate breathing and hand protection are a requirement. 

Surface deposits

The first method is to wash the glass in water with a bit of crème cleaner. Place the glass flat on a work bench and scrub it with a brush containing the water and cleaner. Rinse and dry. This may be all that is needed.

More persistent stains require chemicals. Use rubber gloves for this kind of process. Soak the glass in a dilute (5-10%) solution of lye. This is also known as caustic soda. Chemically it is sodium hydroxide (NaOH). After the soak wipe with a disposable towel and dip it into a dilute solution of hydrochloric acid (5-10% again) or vinegar to neutralise the caustic soda. Wipe the glass with a towel and rinse with plain water and polish dry.

Metallic stains or iridescence and etched surfaces

If the blemishes are more than surface deposits, more aggressive methods are required. The metallic-like stains and iridescence are usually evidence of the corrosion of the surface of the glass. These and any etching require grinding and polishing.

You should start with a polishing process to determine if that will be sufficient to bring clarity back to the glass. The use of an optical polishing material such as cerium oxide paste and a felt polishing head is often enough. Ensure that you do not let the cerium oxide get any drier than a paste to avoid localised heating and therefore breakage of the glass. Rinse the glass in clean water and polish dry.

If this does not remove enough of the surface to provide the desired clarity of the glass you can try either using pumice or jeweller's rouge, or dilute hydrofluoric acid.

Hydrofluoric acid used in a 10% or less solution will remove the surface of the glass, so eliminating the evidence of corrosion. It will even out, but not eliminate, the evidence of any etching. The disadvantage to this method is the risks associated with such a dangerous chemical. There is within this note on acid etching a door panel some advice on safety.

If you decide to avoid the hydrofluoric acid method, you can use mild abrasives such as jeweller's rouge or pumice with water and a felt polishing head. Once you have ground the whole of the surface, you need to wash it very thoroughly in clean water. Then change the felt head and go to cerium oxide to provide the optical finish.

Note:

All the dilutions and solutions suggested here are with water.

This note assumes the glass has had any paint already removed. For a method of removing house paint see this tip.

Negative assembly

To get a crisp design finish to a fused piece especially with thin elements like stringers it is often recommended to fire upside-down. This means that the bubbles move toward the final bottom of the piece rather than rising and disturbing the design on the top.

Assemble the piece on your workbench as usual on top, but use a toothpick or small brush to dab some GlasTac or other glue near the two ends of each piece. Let it dry overnight and then carefully flip the whole arrangement upside down onto the kiln shelf.

If you don't like the glue and flip over technique you can try another. Draw your design onto Thinfire shelf paper in negative with a graphite pencil. If you have a strong light source you can draw on the reverse (printed) side and trace the negative onto the upper face of the Thinfire. Then assemble your pieces upside-down on the shelf. You can assemble the whole in the kiln and there is no need for glue.

When fired, clean the piece thoroughly and decide whether you want that texture or a smoother surface. If you want the smoother surface, put in the kiln for a fire polish. This is often known as “flip and fire.”