I’m guessing you are using the 20g sheet? From the sound of it, you
are not getting the metal hot enough. I don’t know what a goss torch
is…I use a Smith acetylene/air torch, and have no trouble with the
18k/925 bimetal. I’ll bet you don’t need higher temperature, but the
ability to put out enough energy per second to overcome the high
heat capacity of silver, and overcome the bleeding away of heat from
the edges and corners. Try heating up the corners and outer edges
first, before you ever address the area where the bezel is.
When soldering items that have a lot of surface area, I put a
support* under it, so torch heat can easily flow all around,
including underneath. With thicker gauges, it allows me to address
both top and bottom of the sheet. Sounds like you need to be more
vigorous in heating the sheet - you not only have a lot of surface
area but (if it is 20g), considerable thickness. If I’m working with
a large and/or thick piece, I’ll use the #2 head on my torch. Try a
bigger torch tip if you have one.
Most of the time, the support I use is a flattened wad of 26g iron
binding wire. I take a looooong length, wrap lightly around my
fingers a few times, then cross-wrap until I have sort of a bulging
oval. Then I put it between two steel bench blocks and mallet it a
few times until it’s flat. This gives me a very porous surface with
plenty of support for my piece. It doesn’t act too much like a heat
sink, and prevents the work from sagging.
hope this helps,