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