Heat transfers to the glass during soldering. Normally this does not produce any difficulties. However with slender pieces, deep curves, or band saw cuts, the heat generated by soldering can crack/break the glass. This means that you need to ensure that you do not linger for a long time on the solder beads along these kinds of pieces.
You can do several things:
Solder roughly at first, and then continue soldering somewhere else on your piece, to let the heat of the solder dissipate before finishing soldering by filling the gaps in the bead.
Create the bead in a single relatively swift pass. It has to be slow enough to produce a bead, but not linger in any area. The bead should not be so large as to turn over on itself. It should be similar to a quarter or at most a third of a circle.
Build the bead up with a series of “pats” along the copper foil joint. This involves putting a dot of solder to the copper foil tape and resting long enough for the solder to spread to its natural dimensions, and then place another dot at the leading edge of the first and so on until you reach the end of the line. This provides a relatively cool method of soldering. Its disadvantage is that it leaves a number of “tide” marks at the cool end of the bead. These can be changed to a single tide mark by re-melting the solder at that end.