Foiling Space

There are a lot of views on what amount of space is required between copper foiled glass pieces.  Some say the pieces should be tight, others that a consistent space is needed, and some who say that variable spaces are fine.

It is necessary to consider what holds a foiled panel together.

Adhesive

The foil is supplied with an impact adhesive which helps keep the foil attached to the glass before soldering.  However, the heat of soldering deteriorates the adhesion of the glue.  If you must take a foiled piece apart you will find that the adhesive is sticky rather than firm. Also, the adhesive will continue to degrade during the life of the object.

Solder

The solder bead is significant in creating the matrix required to hold the panel in one piece.  The bead on each side holds the glass in place and resists deformation away from a single plane. This resistance is significantly reduced if there is not a fin of solder connecting the two beads.  The beads and the fin of solder form an “I” beam which together resists movement of the glass.

Strength

To form that “I” beam there does need to be space between the foiled pieces. It does not need to be wide, but it does need to be enough to wiggle the pieces.  This will allow the solder to flow from one bead to the one on the other side, forming a strong “I” beam.

In vertical panels, the glass is the strong element.  The solder lines serve to hold the matrix together.  Where people indicate the strong border will keep the whole panel from falling apart, they are correct in part. But, if there is not a sufficient “I” beam between each piece, the whole panel is subject bowing, either from wind pressure, vibration or mechanical pressure from handling.  Therefore, you cannot rely on the border to make your panel strong and long lasting.

Dissent

Some take the view that there will be enough unintentional spaces created between pieces to allow the fin form between beads intermittently.  But the gaps in the “I” beam due to tight fitting pieces will make it much weaker than a continuous bridge between beads.  The existence of gaps puts greater pressure on the solder that does bridge between beads.

An example was provided for me in a lamp brought in by client which spontaneously fell apart one evening.  (Not made by me, I add). The upper band of glass remained attached to the vase cap, but separated from the rest of the shade.  Fortunately, it fell straight down and only a little of the bottom edge was broken.  Investigation showed there was very little solder between pieces, although there was a good bead on each side of the lamp.  The lamp pieces separated, in different places, at the foil-glass interface and elsewhere at the foil to foil interface.  This indicates there was little or no solder where the foil remained on the glass, as the adhesive is much weaker than even a thin fin of solder running between the inner and outer beads. This case is an example of the need for a fin of solder to be formed between the beads on either side to provide a strong, long lasting object.

Heat Cracks

There is sometimes a fear expressed that tight fitting of foiled pieces can lead to heat fractures when soldering due to expansion.  Yes, when soldering pieces with a lot of variation in width, you do need to move reasonably quickly. Come back later to improve a bead if you need, to avoid overheating the glass.  Even the thin copper foil can transmit heat along its length, which reduces direct heat transfer to the glass.  Mostly, breaks occur from dwelling too long in one place with the soldering iron. It may be better to tin the foil all around the suspect piece just before running the bead.  This will warm the glass around the edges in preparation for the greater heat of laying down the bead.

The main point is that the solder needs to connect the beads on either side of the glass to provide a stable, strong and long-lasting piece.

revised 28.12.24

Foiling Tight Curves

Foiling tight curves often leads to splits in the foil.  This note gives information on how to generally avoid the splits and a repair method when they do occur.

Make sure the piece of glass is free of dust and oil and that there are no sharp ledges. Find the flattest part of the glass piece to begin the foiling.

To get the foil to stick to the curve and fold over onto the glass with the minimum of tears, use your fingers first to burnish the sides of the foil onto the piece of glass beginning on the outside curve.

As you move to the inside curved portion gently and progressively ease the edge of the foil with your finger toward the surface of the glass. Easing and burnishing one side of the curve at a time will give you better results toward getting the foil to stretch onto the glass without tearing the foil.

Finish by burnishing the foil down using a fid. This helps keep the foil firmly adhered to glass through the soldering.

Dealing with splits
Tears in the foil line happen. Clean up the broken foil lines with a craft knife and the solder line will look nicer when project is finished. You can also patch the tears by placing a small section of foil over the broken foil.  Place the scrap on one side, burnish it, and fold it onto the edge.  Ease the foil onto the second surface and burnish all the surfaces again.  Then trim the scraps that extend beyond the rest of the foil with a craft knife.

Foiling and Soldering Small Pieces

There are several approaches to dealing with small pieces in copperfoiling:

No-foil approach
One approach is to have some of the pieces held in place by over-beaded solder without foil on the tiny piece, but it is patchy at best and likely to lose pieces in the long term.

Bevel approach
A very good and strong approach is to partially 'bevel' the edges of each piece on both faces. Grind at 45 degrees until the very edge is only 1 mm thick. Then use foil that is 4 mm wide for 3mm thick glass. For 4 mm glass, you will use 5.4 mm foil. Make sure that the foil covers only the bevelled edges and does not extend outside them.

Solder into the 'V' formed by the bevelled edges. Don't over-fill the joints as you don't want solder outside the 'V'. It also is best if the panel is supported underneath the area being soldered by a wet sponge to more quickly cool the solder.

With the solder contained by the 'V', the solder lines will be of constant width throughout the piece. Best to practice this technique on some scraps before you start the main job.

This approach will minimise the amount of light blocked by the foil - important with tiny pieces - while still providing the strength of fully foiled pieces.

Triming approach
If you have to have really small pieces, just foil them as you would any other piece, and burnish it as normal. Then take a very sharp craft knife (Exacto or similar) and trim the foil so that just a little tiny bit of foil is on the front and back of the piece.

No glass approach
Tiny pieces are really tedious to work with. So if the piece is going to be black or really dark, for example a small hummingbird's beak, or a bird’s eye, don't bother with glass but just fill the space with foil and solder.

Silver Foil Inclusions

Silver foil is better for inclusions than silver leaf as there is more substance and so less likelihood that it will burn away.

Sterling silver – not that common in foils – will darken easily on exposure to temperature and air.

Silver foil will also react with some glass colours. A good guide to this can be obtained from Bullseye’s chart on glass interactions. This shows which glasses react with silver. To minimize the reactions, minimize the amount of time spent above 600C.

Only a short bubble squeeze is required with foil as it is not so stiff that it will resist the weight of the glass on top, so creating bubbles. A short squeeze to allow the air out is still a good idea.

Copper Foil Oxidisation

Protection of foiled pieces from oxidisation

If foiled pieces are going to sit a while before soldering, put them in a sealed plastic bag with the air squeezed out. This will prolong the time before the oxidization becomes a problem for the soldering process.

Another possibility is to tin all the pieces before putting them away in the plastic bag. Solder oxidizes more slowly than copper does.

Copper Foil Splits

Copper foil splits often occur at tight inside curves. This is caused by the inability of the copper to stretch enough at the edges to make it around in tact.

There are at least two ways to deal with copper foil splits: avoid them or repair them.

Avoidance
One way to avoid splits at tight inside curves is to hand foil pieces that have these curves. When folding the foil from the edge to the sides of the glass, gently and progressively ease the foil toward the sides of the glass with your fingers rather than a hard tool. Do not try to make the fold all at once. Give the foil the chance to stretch into the curve through gentle persuasion.

Another way to avoid the splits is to use several short strips of foil at the tight inside curve. Apply each strip from one side around the edge to the other side of the glass. Overlap each strip until the inside curve is covered. Then you can cover the whole curve in the normal way with the copper foil. When the piece is foiled, you can then take a craft knife and trim the overlapped foils to match the edge of the long strip of foil. Or you can just trim the overlapped foil pieces to match the rest of the foil. Be aware that this increases the space required between the glass pieces as the foil is now effectively twice the thickness of regularly applied foil.

Repair

To repair splits in the foil at tight inside curves that have been foiled along with the rest of the piece, you take small pieces of foil and overlap at the splits.

Then trim the overlaps to match the line of the main foil. You can do this only on the surfaces or by going from one surface around the edge to the other surface.

When tinning these repairs, you need to take care that the heat from the iron does not loosen the adhesive so much that the foil pieces move.

Copper Foil Adhesive

The only technical purpose the adhesive on copper foil serves is to hold the foil on the glass until it is soldered. It is not intended to, and will not hold the piece together. You shouldn't ever rely on it to hold the pieces together. The heat from the soldering breaks down the adhesive, as well as time and chemicals like glass cleaner and flux. If you think that burnishing it harder will seal it, you are mistaken.If your foil is pulling away from an edge, you need to build up the solder on the edge, add wire if necessary, or a perimeter metal of some sort.

If you have a piece of glass that is "slipping" out, you may be trying to hold up too much weight by the foil. Put a perimeter metal on and hang the piece from a solder joint that extends into the design.

Foil Pulling Away from Edges

If foil is pulling away from the glass on the perimeter of pieces, there are several things to remember.

Clean all the edges and surfaces just before foiling. This ensures there are no oils to interfere with the contact adhesive of the foil.

Avoid hand creams just before foiling as this increases the amount of oils getting onto the glass.

Remember that lots of heat breaks down the adhesive. So do not remain in one place too long. However the adhesive is not the element that keeps the foil attached to the glass in the long term.

Instead, think about whether the bead on the edge is thick enough to provide the rigidity required without relying on the adhesive of the foil.

Finally, think about whether an edging came would provide better support and finish to the piece.

Choosing Copper Foil Widths

Width of the foil relates to:

Glass thicknessThin glass requires thinner foil to maintain a neat solder line of about 3mm. Thicker glass requires thicker foil to maintain the same width of solder line. So 2mm glass would require 4mm foil, and 3mm glass would need 5mm foil.

Glass textureHeavily textured glass usually needs thicker foil, as the thickest part of the glass is greater than the thickness of average glass. This requires a little experimentation to get the one you like best, but usually is one step up in thickness from your usual. Remember you are looking at only one half of the resulting solder line thickness on each piece of glass.

Desired width of solder lineThe desired width of the solder line will also affect the choice of foil. If you like a thin line of solder, you should choose foil that is only 1 or 2mm wider than the thickness of the glass. This will give a solder line of 1 or 2mm wide. Be careful when choosing a thinner width. You still need enough foil on your glass to maintain the strength of the solder bead. If you like wider lines, a foil that is 3 or 4mm wider than the glass thickness will give a 3 to 4mm wide solder line.


Of course is possible to trim the copper foil to be thinner after foiling.  This can be to even up the line, or to thin it.  Gentle pressure with a sharp craft knife will cut the foil to the width you want. You may want to do this when all the pieces are assembled before soldering.  This gives you the opportunity to see how the width of the resulting solder line will be.  It is also the time when you can see what the line of the solder bead will be and make adjustments before begining the the soldering.

Choosing copper foil Thickness

Foil comes in different thicknesses as well as widths. Thinner is easier to form round curves, but tears more easily. Thicker is more robust, but crinkles up more on inside curves and so needs more burnishing. Mostly it is personal choice on what is easiest to work with.

Choosing copper foil backing

The question most people want to know is why there are different colour backs to the foils. The answer has to do with the finished piece, and only matters if you are using transparent glass. If you are using opaque glass that you cannot see through it doesn’t matter which type you use.

The backing should be of the same colour as finish to the solder lines. Since you can see through transparent glass, you will be able to see bits of the back of the copper foil, especially at certain angles. It would look peculiar to have a piece with a beautiful black patina, only to catch glimpses of copper when you are looking at it.

Therefore, if you are planning to patina your piece black, use black-backed copper foil. If you are planning on leaving it silver, use silver-backed copper foil. If you are planning to patina it copper, use copper backed copper foil.

Applying Foil by Hand

There are a number of tools and machines to assist centering the foil on the edge of the glass pieces.  Sometimes, though, you want to do foiling where use of the machines is not convenient.  This describes a method of doing foiling by hand without centering tools.


Clean the edges of the glass of any powders, or dust from grinding and oils from handling to ensure the foil sticks tightly to the glass. You do not have to be particularly careful about marks on the surface of the glass.

Keep your hands dry and clean while foiling, as oil or moisture on your hands will prevent the foil from sticking to the glass.

Start foiling on a straight length of glass. If the glass is only curves, begin the foiling on an outside curve. If the end of the foil wrap doesn't meet perfectly with the beginning, you can trim off the uneven overlap with a sharp craft knife. Only light pressure is required to trim the foil. Be careful not to scratch the glass, which can happen if your blade is dull, or you apply too much pressure.

To centre the foil on the edge of the glass, hold the piece vertically and look on both sides of the glass while you apply foil to the bottom edge with the sticky side of the foil facing you. This enables you to judge the evenness of the application.  It may take a bit of practice to look down both sides of the glass as it requires overcoming the habit of using the dominant eye.  To practice, you can look down the side that matches your less dominant eye and judge the amount of foil that overlaps the edge.  You can switch your concentration from side to side to determine the equalness of the foil overlap on each side.

Burnishing the foil onto the edges and then on the upper and lower surface with a fid will help the foil adhere firmly to the glass throughout the soldering.