Silver is prized for its bright, reflective luster, but a common problem is its tendency to change color, or tarnish. While most people are familiar with the dark, black discoloration, the appearance of a green hue on silver items is often confusing. Understanding the distinction between these two forms of discoloration reveals that the silver itself is only partly to blame for the color change. The chemistry behind these reactions is distinct, and knowing the true source of the green color is the first step in restoring and protecting your silver pieces.
Standard Silver Tarnish: The Black Reaction
The most common form of discoloration is a black or dark gray layer, which is the result of a chemical reaction known as tarnishing. This process involves a reaction with sulfur compounds present in the environment, unlike rust which involves oxygen. The primary culprit is hydrogen sulfide gas (\(\text{H}_2\text{S}\)), a trace component in the air that comes from sources like pollution, rubber, and certain foods.
When silver (Ag) comes into contact with hydrogen sulfide, a thin surface layer of silver sulfide (\(\text{Ag}_2\text{S}\)) forms. This compound is naturally black, creating the familiar dark patina that dulls the metal’s shine. High humidity, moisture, and air pollution accelerate this chemical process, making regular cleaning necessary to maintain the silver’s brilliance.
The True Source of the Green Color
Pure silver does not turn green; the green discoloration is a clear sign that the alloying metal used to strengthen the silver is reacting with its environment. Sterling silver, the most common alloy for jewelry and flatware, is typically composed of 92.5% silver and 7.5% copper. It is this copper component that is responsible for the green color.
Copper is more reactive than silver and readily undergoes oxidation when exposed to moisture, oxygen, and certain chemicals. This reaction forms copper compounds, often referred to as verdigris, which is a blue-green patina. The green hue may be copper oxide, copper hydroxide, or basic copper carbonate, depending on the specific environmental factors at play.
The process is often accelerated by contact with the wearer’s skin, a reaction commonly known as “body chemistry.” Sweat, which contains salts and organic acids, and residues from cosmetics, lotions, or perfumes can act as catalysts for this copper oxidation. The acids in these substances react with the copper, forming soluble copper salts that appear as a noticeable green residue on the skin or the silver piece.
Removing and Preventing Discoloration
To remove the green discoloration, you must target the copper compounds without harming the silver itself. A simple and safe method for removing both black tarnish and green residue involves using a mild soap and warm water with a soft cloth or brush. For more stubborn black tarnish, an electrochemical cleaning method using aluminum foil, hot water, and baking soda can reverse the silver sulfide reaction. This method should be avoided for pieces with gemstones or other non-metal materials.
Preventing discoloration requires reducing the exposure of the copper alloy to moisture and chemicals. Always remove silver jewelry before swimming, showering, or applying personal care products like lotions and perfumes. Proper storage is equally important; keep silver in a dry, low-humidity environment, ideally in an airtight container or a cloth-lined box with anti-tarnish strips. Regular, gentle wiping with a polishing cloth after wearing removes body oils and moisture before they initiate the chemical reaction.