Why there is a black coating on silver?

Tarnishing:

• Tarnish is a thin layer of corrosion that occurs on the surface of copper, brass, aluminium, magnesium, neodymium, and other similar metals as their outermost layer reacts chemically.
• Tarnish is not usually caused only by the effects of oxygen in the air.

The black coating on silver:

• Blacking coating on silver is due to the formation of silver sulphide (${\mathrm{Ag}}_2\mathrm{S}$).
• Silver when exposed to the atmosphere, reacts with Sulfur and Sulfur compounds present, to give silver sulphide (${\mathrm{Ag}}_2\mathrm{S}$) which is black.
• exposed silver produces ${\mathrm{Ag}}_2\mathrm{S}$ when it reacts with ${\mathrm{H}}_2\mathrm{S}\left(\mathrm{g}\right)$ which comes into the atmosphere due to many industrial processes.
• The chemical reaction for the tarnishing of silver is as follows:
• $4\mathrm{Ag}\left(\mathrm{s}\right)+2{\mathrm{H}}_2\mathrm{S}\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2{\mathrm{Ag}}_2\mathrm{S}\left(\mathrm{s}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)$