If low karat solder was used on a repair or assembly there are two remedies. The first option is less likely to help. Hopefully, the tarnished area can be sanded and polished where just a small seam might retarnish.
Most likely the area that is contaminated will have to be removed or the surface area where the low karat solder is on will need to be sanded and polished to remove all of the bad solder. On a chain the darkened area will have to be cut out and reassembled. In the case of a head, the head will need to be removed and the area sanded to remove any of the old solder on the surface and then have a new head installed. (Most likely you will not be able to remove all the old solder from the head.)
Atlas Jewelers Bench Talk 