Radiating Lines

In leaded glass designs it is important to avoid lines radiating from a single point.

Good design will avoid multiple radiating pieces from a single point of origin.

Among the reasons for avoiding radiating lines are:

• It is important to reduce the number of lines that meet in any design to avoid the big bright solder place in a panel.
• It makes for large solder blobs, especially on leaded glass panels, and therefore provides a focus where one may not be wanted or required.
• It also is a point of weakness as the multiple thin or tapering pieces of glass are liable to fracture. 
• Also the difference between the harder solder and softer lead came leads - over time - to cracks in the lead at the edge of the thick solder blob.

There is a difficulty in adapting single radiating points in a drawing to the practicalities of the medium of glass. Examination of older panels (in either copper foil or leaded technique) will show up some of the problems of thin tapering pieces, especially in the Victorian era where tapered pieces where in their glory. Almost always, there will be broken tips. They seemed to compensate for tip stress by making the pieces with narrow tapers smaller than some of the other wider pieces. Even then, there are a lot of broken ones seen. It is the nature of glass, and goes back to knowing how the glass will react to the conditions you create.

Methods of avoiding tapering pieces lie in the design.

You can't use long tapered pieces in larger panels, without modification for the structural reasons outlined above.

Also you can't make a neat termination by joining half a dozen tapers at one point. Your piece will not look exactly what it looks like when you drew it out with a pencil. You can pencil in a termination with six points, ending at one point and it may look good, but try drawing it in with a felt tip pen almost 6mm wide, which shows for what the solder has to  cover, and see what that point looks like then. For copper foil a 2-3mm wide line will demonstrate what will be seen after soldering. You may not be pleased with the large blob at the termination.

No one can teach one easy fix for everything you will encounter, so the answer starts with the design, before you cut and foil, or fit the came to the glass. Art is not about the physical placement of what you see in your mind, as much as it is about the "illusion" you are creating that you want others to see. That starts with the design, and avoiding incorporating something that you know is going to give you a problem. 

Measuring Openings

Measuring rectangular openings

Timber and metal openings can vary in their dimensions. So measure each opening at the top and bottom, and at the left and right as well as the middles of each side and top and bottom.

To be perfectly sure the window has right angles – is square – take the diagonals from opposite corners, e.g., top left to bottom right and top right to bottom left. These measurements should be within 5mm of each other for the window to be considered square.

You can check for ”squareness” with a try square, although that is not completely accurate.

Templates

If it is not possible to tell where the right angles of the opening are, a template is called for. The material to be used for taking templates should be stiff, easy to cut, unaffected by moisture, and relatively inexpensive. This eliminates paper and some cardboards. If you can find stiff corrugated cardboard this works well. Mounting board works well too, but is expensive. Foam board is excellent, but also expensive. Hard board or other thin pressed board is inexpensive but difficult to cut with a knife. Thin plywood is also a good material for templates, especially if the opening is relatively regular. The more complicated the opening, the more cardboard, mounting board, or foam board becomes useful for its ease of shaping to the opening.

What ever material you use, you must mark which is the interior and exterior and for further checks, which is left and right. Fit this template into the opening to make sure it fits into the opening smoothly. This template will form the external extent of the built window when it is installed into an opening with a rebate.

Where the window is to be fitted into a channel, as in stone, you need to make the template of stiff material so you can determine the panel can be installed and that there will be enough of the panel within the stone channels to ensure the stability of the window in the future and still be able to manipulate the leaded panel into the opening.

Encapsulating Glass

Points to note when making panels for encapsulating leaded panels in double glazing units.

In building a new panel, the perimeter should be of “Y” came. This allows the double glazing spacers to be placed on either side of the leg of the “Y”, incorporating it into the structure of the whole unit.

The tolerances for double glazed units are much less than for single glazed wooden units.

If you decide to use mastic, it must not be linseed oil cement, as the oxidisation process produces a condensate that fogs up the interior. Use a butyl putty instead.

It is also possible to finish the panel without any mastic under the leads, as the double glazed unit will provide the structural support. You do need to dress all the flanges of the lead to the glass to avoid light showing around the edges of the glass.

Finishing the leads should be with a polishing brush only to avoid introducing chemicals in a closed atmosphere. The polishing brush will bring up a dark colour on the leads and solder joints with repeated light brushing. This also is an indication that the solder jointing should be as neat as possible with small flat joints.

Bottle slumping

Results

Devitrification is a major problem on bottle slumping. Some means of reducing the problem are

• clean labels off thoroughly
• remove all adhesives
• scrub the whole bottle and polish dry
• apply a devitrification spray before firing.
• Spend as little time in the devitrification range (700C – 750C) as possible
• Vent the kiln up to 540C if you are using fibre papers
• sandblast off the devitrification and fire again if the devit is slight
• in severe cases of devitrification, throw the bottle away.

Champagne and large bottles of sparkling wine split easily due to the differences in thicknesses. Firing of these needs to be very slow until you get past 715C.

Bottles with printed labels need particular care. The enamels used in the printing may contain lead and any other elements in the paint may contaminate your kiln.

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Bottle Slumping

Firing

The amount of slump you want will determine the top temperature combined with the rate of advance.

You have the choice of firing slow and low or fast and high. If you choose the former, you can also choose a lower final temperature. As a starting point for considering your firing schedule you can assume that bottle glass is very similar to float glass. This means that you can start with an annealing temperature of about 548C and a softening point of 720C. The strain point is around 510C, so all the annealing needs to be finished by that temperature.

Bottle glass is fairly robust, so an initial rise of 150C/hr to 600C may be slow enough, considering the differences in thickness that most bottles exhibit and then faster to your top temperature. A little experimentation is required. The minimum temperature required for a slump will be around 720C. The slumping temperature you choose depends on how fast you want to achieve your slump and how flat you want the result to be. Slow slumps can be done at around 720C, but if you want faster or flatter you need to consider temperatures around 770 -790C. You will need to observe to determine what temperatures are best for your desired results.

Annealing needs to be done carefully for two reasons. You do not know how consistently the glass has been made and you have a range of thicknesses involved in the now slumped piece. So it is safest to assume the piece is now about 20 mm thick. This would require a 3 hours soak at 550C; an initial annealing cool rate of 25 C/hr to 495C; a secondary cool rate of 45C/hr to 440; and a final cooling rate of 150C/hr to around 50C. If the bottles are clear or light enough, you should do a stress test on them by placing the bottle between polarised filters and over a light source. This will tell you if you have annealed properly.

You also need to consider whether you can fire bottles from different brands of drink and different parts of the world together. Some say don't try it. Others say there is so little variation in glass the world over that careful annealing will compensate for the differences. Experimentation is the only way that you will be able to tell.

Allow a day after firing before any cleaning or washing of the bottles is done.

Bottle Slumping

Placing

Production made bottles are not evenly thick all the way around. So they roll until the heaviest part is at the bottom.

One way to avoid any rolling in the kiln is to put the bottle on a level surface and let it roll to the heavy side. Mark the up side so that you can place it in the kiln with the heavy side down.

Some times though the seam lines – which will show in the final piece – are in the wrong place. To keep the seam lines to the side, you can place a small piece of 0.5 mm fibre paper against each side of the bottle at the base if you are working on a hard surface.  On a softer surface, the weight of the bottle or a gentle push will sink it into the surface enough to avoid rolling.

The distance to place bottles apart is important to know so they do not become attached when several are slumped together.

You can slump test bottles of various diameters to determine their final width.

However, if you wish to calculate the distances, The width of slumped bottles is approximately 1.6 times the diameter of the original bottle. If you want the base of the bottle to be flat too, the final size will be wider, dependent on the amount of glass in the base.

You also need to leave some additional space than this calculated distance between bottles, as the final size is narrower than the size when hot. You should leave at least an additional 10 mm to each side of the bottle.

So a 50 mm diameter bottle will become 80mm wide. You need an additional 10 mm each side, so the spacing from the centre of one bottle to the next should be at least 100 mm.

The surface that you are slumping onto is important too.

You can place the bottles on a sand base that has been dusted with kiln wash powder. This has the advantage of allowing a gentle push into the sand to prevent rolling. But it imparts a grainy texture to the back.

You can get a smoother surface by using whiting or sifted plaster that has been screeded smooth. This also allows the bottle to be gently pressed into the surface.

You can use fibre papers as separators from the shelf, but they are relatively expensive.

Kiln wash works very well. It can provide a very smooth surface, but if you want more texture, you can sprinkle some kiln wash through a sieve over the shelf.

Bottle Slumping

There are four major considerations in bottle slumping: cleaning, placing, firing, results.

Cleaning

A major consideration in slumping bottles is the cleaning required. This requires a lot of time, as everything has to be clean inside and out.

Labels and the glue attaching them must be cleaned off. The interior needs to be clean. And the bottle needs to be dry before being placed in the kiln.

Soaking first helps the cleaning process. If you are doing large numbers you will need to find large containers that you can leave the bottles in to soak for a couple of days. For small numbers, a few bottles soaking in a bucket of soapy water will do. The bottles should be upright to allow the internal residues to float to the top. After a few days the labels should have fallen off and the internal deposits floated to the top or easily washed out. Screen or plug the sink to be able to remove the residues from the sink before it gets into the drain. Then you can proceed to rinse and clean the bottles.

The bottles will need to be wiped free of the glues used to stick the labels to the bottles.  Any glue residue left on the bottle will show up on the finished piece, usually as devitrification. If the glue or label has not come off with the cold water and soap soak, you can soak them in very hot water. You can then use a variety of solvents to remove the most persistent adhesives.

For the difficult internal deposits, you can use one or two short lengths of small chain and slosh that around with water. The chain used for hanging small stained glass panels is ideal.

As you can see the cleaning process is lengthy and can be time consuming. So you might want to see if you can get new bottles at a reasonable price. Home brew shops may have bottles they are willing to dispose of. Bottling plants may also have supplies of bottles. New bottles will greatly reduce the labour of bottle slumping, although it does not fit the re-cycling ethos that brings people to bottle slumping in the first place.

Hinges

What are hinges in stained glass?

As the glass is the strongest part of a leaded or copper foiled panel, the joining materials (lead, solder) are the places where the panel can flex. If you have long straight or nearly straight lines extending to or toward the edges of the panel (vertical, diagonal  or horizontal), this is the place where the panel can completely bend (hence "hinge"). It provides a danger in handling that the panel will break. Even if the lines do not run all the way to the edge, any significantly long line will put pressure on the glass pieces at the ends of the hinges, such as a series of formal border pieces or narrow central pieces. It will be a weakness in the long term whether it survives the studio processes or not.

However, we all have seen leaded glass windows with single or multiple hinges that survive for many decades, and only as they loose support from firm cement and the ties to the saddle bars break away, does the panel begin to self destruct.

It is important to recognise where these hinges are to be able to place reinforcements on the panel.

But the real solution for making a panel that will last, it is best practice to avoid designing hinges into the finished work.

Silicone Removal

Removing silicone residues from glass is difficult as silicone combines with the surface molecules of the glass.

The best advice seems to use a very sharp flat blade similar to a paint scraper. There is a tool which holds razor blades or Stanley knife blades which is suitable. Keep the blade as near parallel to the glass surface as possible. Take thin slices of the silicone away at each pass. Keep the blade free from nicks. Change the blade frequently. You may be able to remove almost all the silicone, depending on the smoothness of the glass surface, in this way.

However when you have removed as much as possible by the mechanical means, you may need to use a chemical process to get rid of the remaining silicone. Silicone dissolvers are available from most do-it-yourself shops. Make sure you use them in accordance with the instructions as they can be mildly toxic.

Space In Copper Foiled Projects

When cutting for copper foil projects it is important to leave a small gap between the pieces. This is both to allow for the thickness of the foil and for the solder to form a bridge to the other side of the panel.

I prepare the space in the cutting process by scoring at the edge of the pencil thin cartoon line. This leaves the thickness of the line between the pieces. I do not use pattern pieces unless I am using very dense opalescent glass. When I do use pattern pieces I cut them out with a scalpel knife so there is no space between the pattern pieces. I then draw around the pattern pieces and cut to the inside edge of the drawn lines. This also gives a margin for the foil.

Fitting the pieces is still required though. Some people foil one piece and then fit the next to it before foiling it. This is probably the most accurate way of getting a close fit. However, I save up my cut pieces which I have fitted to the cartoon and foil them while watching TV. My cutting and fitting is normally accurate enough that I do not have to take the foil off pieces, grind and re-foil.

While doing this fitting you need to be aware that an additional small gap between pieces is required to allow the solder to join both the front and back of the panel. This forms an “I” beam for strength. The adhesive of the foil is not enough to make a lasting and strong panel. The solder joining the two sides will give the panel the strength to last for a long time.

Stuck Beads

Sometimes beads refuse to come off the mandrel with normal twisting of the bead on the mandrel. In these cases I use two methods.

I lock a small vice grip (locking pliers) on the mandrel near the middle and hold bead with non slip material. Then I twist back and forth to loosen the bead on the mandrel. When it frees from the mandrel, I pull toward the end continuing to twist.

If it does not loosen, or come off the end, I soak the whole in water over night and then repeat the above procedure again.

If these two do not work, I sacrifice the bead, as even using soft jawed vices normally leads to the crushing of the bead.

Thick Uneven Pieces

Occasionally fused pieces come out of the kiln with one side thicker than the other. There are several things that need to be done for the present piece and for the future.


Level

First check how level your kiln is. The best for this is to begin with a check of the bed of the kiln. Check the level in four directions – left-right, front-back and the diagonals. If it is practical, wedge up the legs of the kiln to make the bed of the kiln as level as practical.

Then check how level your shelf is. Put in your shelf supports and then place the shelf on them. Again check with a spirit level the four directions. Place pieces of fibre paper under or on top of the supports to level the shelf. It is only after these checks have been made that you can consider firing your piece to help it return to an even thickness.  As part of your kiln maintenance you should check the level of your shelf at least monthly, if not every time you prepare to fire.

Variation in Thickness
Now that you know the shelf is level, you need to consider what the variation in thickness across the piece may be. The firing schedule needs to be more conservative than just for the thickest part. As the thinner parts will heat through more quickly than the thickest parts, you need to fire less quickly than you normally would for the thickest area. A rule of thumb – not always correct of course – is to add the difference of the thick and thin areas to the thicker and fire for that calculated thicknesses. This will make the firing schedule slower and so allow the thicker part to be the same temperature as the thinner. For example, a piece 6 mm at one side and 10 mm the other would have a difference of 4 mm. Add this 4 mm to the thicker 10 mm and then fire for 14 mm.

Temperature and Soak
You also need to consider the top temperature to use and the length of soak required. Glass flows relatively slowly at kiln forming temperatures. The conservative approach – one that allows further work if necessary – is to use the previous fusing temperature and extend the soak by at least twice the length of time on the previous firing, even perhaps to a couple of hours.

Bubbles
One thing that will happen is that the bubbles that previously were near the surface will rise and burst giving pin holes on this extended soak. So you should consider cleaning the bottom and putting the top face down on a separator between the shelf and the glass.   This will reverse the direction of flow for the bubbles. Few if any will break through the new top and there should be no pin holes when flipped.

Further Firings
When the piece is cool, check it for the even-ness of the piece all around. If it is not even enough, you will need to consider re-firing again. If you decide to do so, you should go no faster than the rate of advance as previously – probably even slower - but consider raising the temperature or extending the soak. Remember that achieving the heat work required at the lowest temperature is the guide line for kiln forming. So an extended soak should be preferred over a higher temperature, unless there are strong indications that a higher temperature is required.

Fire Polishing
Of course, you will now need to throughly clean the face down side and re-fire to fire polish the original top. The rate of advance should be the same or slower than the firing to even the thickness. Once you have achieved about 600C, a soak of about 30 minutes will ensure that the glass is thoroughly heated through. Then you can advance at a quick rate to the fire polish temperature with a soak of no more than a minute. This allows the surface to change without giving the rest of the glass time to begin to move.  Of course, a thorough annealing is required.

This procedure for re-firing  can be used when re-firing pieces for any reason. You only need eliminate the considerations on the uneven thicknesses.

Wire for Hanging

The most common wires used for inclusion in fused objects are copper, brass, nickel/chrome, stainless steel and sterling silver.

The strength of the wires – strongest to weakest - seem to be in the order of stainless steel, nickel/chrome alloy, brass, silver, copper. The metal you choose will be related to the weight of the piece, the available thickness of wire, and aesthetics.

All of these are subject to fire scale or fire stain, a blackened surface on the wire. This can be removed by abrasive cleaning of the exposed metal. The metal within the glass most often takes up the fire scale too. This can be reduced by thorough cleaning of the metal before enclosing it in the glass. Coating the metal with a flux such as borax often reduces the incidence of the fire scale too.

The techniques of cleaning the fire scale from the metal range from scrubbing and polishing to tumbling. The tumbling has the advantage of hardening the softer metals such as copper, and silver.

Copper looses much of its strength in the firing, and often needs gentle working to stiffen it. This is where tumbling is so useful.

Pure silver normally leaves a yellow stain on the glass. Sterling silver - an alloy of copper and silver – is less inclined to do this. However the exposed wire will stain the shelf and any subsequent glass unless well supported by 1 mm or more of fibre paper.

It is common in silversmithing to pickle silver to remove the fire scale after any heat work.

Complex Drapes

A question has been asked that relates to draping over a face mask mould, but this response relates to any drape with compound or multiple shapes.

The rate of advance should be a steady one all the way to the forming temperature. This should be 150C/hr or less. Thicker glass requires a slower rate of advance to allow the glass to heat all the way through. The point is to get the glass all the same temperature by the time it reaches the forming temperature.

Draping over an undulating mould takes more heat or time or both than simple drapes, because there is a much greater variety of form for the glass to conform to.

You have a choice about the top temperature and soak times. You can choose a low temperature with a long soak time, which most often leads to a minimum of marks on the underside of the piece, but requires long vigils at the kiln to determine when the drape is finished. The other strategy is to go for a higher temperature and shorter soak, which leads to more marks, but less time observing the firing. The higher temperature may be as much as 720C.

Note that there are a group of considerations about the size of the drape and the thickness of the glass being draped. 

Weight 

Span 

Complexity of shape 

There are some things you can do to assist the progress of the drape. One, already mentioned, is to increase the forming temperature.

Use a longer time, or as much time as required. Watch the draping progress because it is never certain how long the piece will take to conform to the mould sufficiently.

Use props. Place the props where the glass first starts to form just barely supporting the edge of the glass. As the glass begins to bend, it will slip off the supports - assuming they are well covered in separators, even pieces of kiln paper. This means the folds will start somewhere else than at the nose on a face mould or other high point on any other form.

Manipulate the glass. Reach in with wet wood sticks and push the glass about. One stick will be needed to keep the glass in place while the other pushes the glass about. The sticks do need to be both wet and strong. If you use dry sticks they will mark the glass as well as go on fire. Of course, you need protective gear to avoid burns to your skin and hair if you do this.

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.”

Soldering Zinc

Brass, copper and zinc are heat sinks. That is, the metal conducts the heat rapidly so more heat has to be applied than for lead and tin to keep the soldering site hot enough to accept the solder.

The important elements are:

Use a hot iron. If you use a rheostat, turn it up to full. If you can, change the tip/bit to one rated at 800F – it will have an “8” stamped on the end that goes into the barrel of the iron.

Apply the flux liberally at the soldering point to ensure the area is “wetted”.

Keep the iron in contact with the came for a few seconds to heat the metal. If you are using zinc as your border around a leaded panel, make sure you do not heat up the lead came so that it begins to melt.  Keep the iron on the zinc.  It will transmit heat to the lead came without getting it hot enough to melt.

When the came is hot, apply the solder to the bit. 

Keep the bit on the metal until you see the solder begin to flow on both the zinc and the lead or onto the solder bead (on copper foil), then gently lift directly up.