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 fiting 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.
