If the filigree was really dirty to start with try hitting it with plenty of flux while it is really hot so that the flux boils. I always tin the pieces before I attach them to the glass. Thanks for your help. As usual most of my problems can be solved very simply. I don't remember how they did it or what was used.
Tinning is the name for coating any solderable metal with a thin coat of solder.
On copper or brass tinning is very easy. Just apply flux and rub a hot soldery iron tip across the metal and it quickly turns silver in color, it is tinned. Tinning an iron tip is often more difficult. Many iron tips are plated with steel or other metals that don't tin easily and getting it tinned takes something with more oomph than plain flux. To get the tip to take a tin the best thing is a sal-ammoniac block.