Silver is a precious metal, white and very reflective. The fact that it is 925, means that it is 92.5% pure. That means, it is 92.5% pure silver and 7.5% copper.
Copper is required for the silver’s durability and resistance. If it would be 100% pure silver, it would be too soft to be worked on. Silver has a marvelous neutral color.
It is important to acknowledge this, for silver and copper have different properties and they react and pollute by different elements.
SILVER PROPERTIES
Silver is a very ductile and tractable. Silver has a white metallic shine, susceptible to polishing. It preserves in water and air. Although the surface tarnishes in the presence of ozone, hydrogen sulfide or sulfur air. Silver dissolves in concentrated nitric or sulphuric acid.
COPPER PROPERTIES
The red salmon color turns red-violet when it is air exposed, due to the compound of cuprous oxide, and it eventually turns black because of the compound of copper oxide. When exposed to wet air for a long time, it compounds an adherent, water-resistant coat of green and toxic basic carbonate (cupric carbonate).
Halogens easily damage copper, especially in the presence of moisture.
Oxoacids damage copper, that is why they are used as paint strippers (sulphuric acid) and polishers (nitric acid).
Citric acid dissolves copper oxide, which is why it is applied to clean copper surface, polishing metal and compounding copper citrate.