Although it may seem that “pure” Silver would be most desirable, that’s not actually the case.
The official designation of “Sterling” to a piece of Silver indicates that it contains at least 92.5% of pure Silver. The remaining 7.5% comprises of other metal alloys, most commonly copper.
Metal alloys with a Silver content of more than 92.5% are too pliable to be used without suffering from dents and damage.
To ensure the metal’s stability and resilience, the second alloy is required.