Silver articles become black on prolonged exposure to air. This is due to the formation of

Silver on exposure to air reacts with ${\text{H}}_{\text{2}}\text{S}$  and forms the black color substance of ${\text{Ag}}_2\text{S}$ .

$2\text{Ag}+{\text{H}}_{\text{2}}\text{S}\to {\text{Ag}}_{\text{2}}\text{S}+{\text{H}}_{\text{2}}\uparrow$