From Google, describing silver...
"It does not react with oxygen in the air under normal circumstances. It does react slowly with sulfur compounds in the air, however. The product of this reaction is silver sulfide (Ag 2 S), a black compound. The tarnish that develops over time on silverware and other silver-plated objects is silver sulfide."
I wonder if the silver foil is coated to prevent this?
Also, silver nitrate is photosensitive, and darkens when exposed to light.
They should have used gold foil.