Dark solder line on fine silver bezel

Hello! I hope that someone will be able to help me.

I am a self-taught silversmith and I have been making jewellery for about 4 years, but recently I have noticed that once I have polished a piece up to a mirror finish the solder line is visibly darker than the rest of the piece. My joints are flush and closed without any light shining through before soldering with hard solder from Cooksons, and are cleaned and put in safety pickle. I hadn't ever noticed this colour difference in any of my pieces after polishing until about a month ago, on a fine silver bezel, so worrying it was something in my workspace contaminating the process I cleaned all of my equipment thoroughly, changed my pickle, tried a new flux (I had been using a borax cone, so I bought some tenacity flux) and bought new hard solder but alas, the same problem occured! The fine silver I used was pre-cut bezel strip (5mm wide 0.3mm thick) from Cooksons. I was worried i was overheating the piece, so tried again, heating very, very slowly on a charcoal block. The silver was had no fire scale, and I hadn't applied any direct heat to the solder join until the piece was hot enough for the solder to run, no excess solder, the join was flush and lovely but when it was polished up there is still the tiniest dark line.

I'm completely stumped. I've shown the pieces in question to my husband and a few other people and they really have to look to see the line, but I know it's there and it bothers me no end! The weirdest thing is that I soldered another piece with 3mm fine bezel wire and this seems perfectly fine, and I don't seem to be having any problems with medium or easy solder, or with sterling silver, but surely it can't be a problem with fine silver bezel wire can it?? Do you have any suggestions of ways I can check this? It's so frustrating when you finish a lovely piece and then you see this horrible dark line peeking through... Any help would be greatly appreciated! Thank you so much.

There was an old thread with a very similar problem, but there didn't seem to be a resolution so maybe someone may know something new...

Ella

Hi Ella,
The silver hallmarking solders are alloys designed to melt at lower temperatures and formulated to match sterling in colour. It is possible that a new batch of solder is darker, because Cookson do make changes without notice.
Fine silver is as white as it gets, so if you are good with colours, you will see a difference.

If you are seriously bothered, and most of us aren't, you can complain to Cookson, minimise the effect by having very tight joints and judging the amount of solder closely, or connecting by fusing, which is a right pain but does not involve solder. Dennis.