Question

Why do silver articles become black after some time when exposed to air? What is this process called?

Answer 1

When silver articles exposed to air silver reacts with sulphur present in the atmosphere and form a black layer of silver sulphide on the surface.
the process is called tarnishing

Answer 2

it is a natural phenomenon which is known as tarnishing.
it is because of the fact that silver oxidizes in presence of air and thus tarnish to become black in colour after some interval of time