This is most likely corrosion. When pure silver reacts with sulfur chemicals it forms a black coat tarnish (silver sulfide)
The sulfur comes from hydrogen sulfude from air pollution this may result in an increase in mass or you can say: When iron or any other metal oxidizes, oxygen from the air combines with the iron to make iron oxide or rust. So the weight increases, due to the increased weight of oxygen which has combined with the iron either one they are both correct.
Hope this helped ya