Hi Roy,
This is actually 100% pure silver leaf and because of the patination over time has acquired a gold tint, which many people, myself included, find appealing. I will post some more notes about this tomorrow.
The silver leaf is fused into the surface of the glass while hot, but while one side is fused to the glass, the other side is effectively open to the elements, which is why it can patinate. The silver is very, very thin - just a few microns. Silver leaf is very soft and unfortunately the adhesive from labels or sellotape will adhere to it and remove it.
Sadly, it is not simple to restore. Cold gilding with silver leaf looks completely different. The heat process changes the state of the silver leaf and allows it to be burnished onto the glass.
A number of glassmakers, including myself, who use silver leaf, are concerned that once work reaches the secondary market, there is a risk of damage from incidents such as these. Thank you very much for raising this issue, Roy, as it may prevent others from befalling similar mishaps.
Many dealers know that they should not put any adhesive labels or tape on silver or gold leaf, but it is important that collectors are also aware of this as well. Obviously not all volunteers in charity shops would be aware of this, which is understandable, though a pity.
For future reference, the best way to remove such adhesive labels or tape is generally to soak it off - a bit like soaking stamps off envelopes.