Hi Linda! This is my first reply to the forum, therefore I have to
ask You to excuse:
-
My english, which has gotten a little “rusty” over the years
-
My very limited knowledge in the field of goldsmithing.
As an M.D. and (from the end of next week on - finals are almost
over! ;-)) dentist, thought, I can provide some knowledge in the
field of glueing, since dentists do that a whole lot! 
Glueing
itself has only very seldom to do with effective covalent binding,
but - in the vast majority of cases - is mere mechanical retention.
Like flies walking around on windowpanes, glueing uses the
microscopic irregularities in order to hold on to them. It’s quite
amazing, if one imagines what forces can be built up through mere
"undercuts" on the microscopic level, if You think of e.g. wings of
airplanes being glued to the body before being fixed a little more
reliably.
I don’t know the type of glue You’re using, but I suppose that it
works on an acrylate basis. Most fast-setting glues are. These reach
very fast setting times and extraordinary bonding strengths, but the
all are susceptible to solvents like acetone. If You’re aiming at a
"fool-proof" bond, I’d rather try with some clear epoxy resin - but
beware! - over time these might turn yellowish, therefore, as a
general rule in glueing: Use strictly as little glue as necessary. In
fact, the weakest link of the “glueing chain” is the glue itself! If
You apply too much glue, the object might come off - not, because the
glue let go of the objects to be bonded (…since it still has a dead
grip on it, even after your object has come off…), but because the
layer of glue has torn in itself. Try to keep that weak link the
thinnest and therefore strongest possible, ok?
The reason Your fine silver works bond better with the glue than
Your sterling parts most probably is the porosity derived from the
clay-process. When the clay sinters, the matrix around the silver
metal will evaporate, leaving microscopic gaps, which only partially
will be filled with the (almost) molten silver. It is exactly this
porosity, which the sterling sheet lacks, since forged or compressed,
which provides the “super-retention” You have described.
So how can You glue to Sterling, when not using the clay method? In
dentistry, we use various acids in order to create a retentive
surface. I would suggest doing that as well in Your case. You could
even slightly etch the glass with F-acid (I don’t know how it’s
called in English, it’s the acid formed of fluor) in a low, I repeat!
- l-o-w, concentration. 2-4% will do. Apply it only gingerly, and
leave it on for 60 sec., after which you spray it off with abundant
water and dry it thoroughly. (obviously, drying is another very
important step in glueing, since virtually every glue used in
commerce is hydrophobic - therefore won’t bond with wet surfaces.
it’s a question of surface-tension…)
For etching the sterling, You’ll have to ask the experts, which are
generously represented here at orchid. Most probably it’s enough if
You etch the copper in the sterling a little, e.g. with sulfuric
acid. You don’t have to achieve a visual etching effect, notabene,
You’re aiming at micro-retention, and for that just a little (couple
of minutes or so) is plenty.
In any case, etching will give You by far the better retentive
surface than any kind of grinding - an alternative, though, might be
to treat the area to be glued with a sandblaster. That’s quite
retentive too, but not as good as etching.
So long, I hope that my babble has been somewhat conclusive for You
and wish You lots and lots of inspiration and success,
Yours sincerely,
Emanuele Campagna