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.