Silver solder--what wattage soldering iron?

Try using some flux which is 'acid based' to clean the metals. It is made just for soldering. I haven't soldered or brazed with silver for about 35 years but without the flux it will never make a decent solder/braze joint.

40 watts is MORE than enough. Are you heating the wire first, then adding the solder? Make sure to use alligator clips for heat sinks so as to not melt the dielectric. If your out of practice, practice on some spare wire of the same (or close to the same) gauge.

I do power cord soldering w/40 watts.

I would get a better iron. There are ones with variable power available now that I wish had been around when I could see well enough to use them. I also used Weller, usually 25 watts.

When I learned electronics in the 60's, acid flux was a no-no for electronics. It would cause corrosion in the weeks following the solder operation. Acid was for plumbing only.

I struggled with the same problem when I had a 40 watt Radio Shack soldering iron. A 25 watt Weller made an enormous difference. It seemed hotter despite a lower watt rating. If you plan to do more projects, get a better soldering iron. You'll save yourself a lot of time and cursing. I've heard excellent comments about this $15 station from Parts Express:

Stahl Tools Variable Temperature Soldering Station