I have two pieces, both made by me 30 years ago; one is a copper-foil window and the solder is as shiny as the day it was made. The other is a lamp with zinc came soldered together and it is grey and dull, both zinc aznd solder alike. Any ideas why the difference?
The lead in the solder can react to the zinc. It's called galvanic reaction. It's caused by moisture, perhaps humidity, bridging from the solder (which has the lead in it) to the zinc. Zinc over time will also get dull just by it's self. You should be able shine it up with some #0000 steel wool.
InspirePoint website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.