Tarnish, the dark, dull coating that appears on silver, is often mistakenly attributed to water. Highly purified water alone does not chemically react with silver to create tarnish; the metal is stable when exposed only to \(H_2O\). However, contaminants and trace elements found in virtually all water sources—especially tap water and high-humidity environments—are the true catalysts that accelerate the discoloration process. The presence of water transforms a slow chemical process into a rapid one, making it seem as if the water itself is the cause.
The Chemical Reaction Behind Tarnish
Tarnish is fundamentally a form of corrosion that occurs when silver atoms bond with sulfur compounds in the surrounding environment. This chemical interaction is known as sulfidation, not oxidation, which is the reaction that causes rust on iron. The resulting black layer is silver sulfide (\(Ag_2S\)), a non-reflective compound that adheres tightly to the metal’s surface and is responsible for the characteristic darkening.
The reaction involves silver metal (Ag) reacting with airborne hydrogen sulfide (\(H_2S\)) in the presence of oxygen (\(O_2\)) to form \(Ag_2S\) and water. Sulfur is the primary reactant, with oxygen acting as a necessary co-reactant. Silver sulfide is a microscopically thin, opaque layer that completely masks the bright, reflective surface of the underlying metal. Even trace amounts of sulfur compounds initiate the reaction that causes the tarnish layer to form.
How Water Impurities Accelerate Tarnish
Water, particularly tap water or moisture in the air, acts as an electrolyte solution that dramatically speeds up the tarnishing reaction. Impurities dissolved in water break down into charged particles, or ions, providing a pathway for the chemical exchange to occur more efficiently. High humidity is a major factor because water molecules adsorb onto the silver surface, which lowers the energy required for the sulfidation reaction to proceed. This is why silver stored in a damp environment, like a bathroom, tarnishes much faster than silver stored in a dry location.
Many common water sources contain trace elements that actively participate in the corrosion process, such as chlorine, a common disinfectant in municipal water supplies. Chlorine can react with silver to form silver chloride (\(AgCl\)), a compound that contributes to the dark, dull appearance. Dissolved salts and minerals often found in hard water or seawater also increase the water’s conductivity, essentially creating a small electrochemical cell on the silver’s surface.
Protecting Silver from Environmental Exposure
Because tarnish is accelerated by moisture and environmental contaminants, prevention involves minimizing exposure to both. After washing silver, immediately and thoroughly dry the pieces with a soft cloth. Allowing silver to air-dry leaves behind a concentrated residue of salts and minerals that quickly initiate tarnish. Avoid contact with personal care products, such as perfumes, lotions, and hairspray, which often contain sulfur and other reactive chemicals.
Proper storage is the most effective long-term defense against environmental exposure. Silver items should be kept in a cool, dry place with a relative humidity below 50% to inhibit the moisture-accelerated reaction. Storing silver in airtight containers or specialized anti-tarnish bags minimizes contact with airborne sulfur compounds. These protective bags are made from treated cloth that absorbs sulfur gases, creating a micro-environment free of the primary tarnish-causing agent.
Certain storage materials, such as rubber, wool, and some types of cardboard, contain sulfur compounds that can off-gas and cause rapid discoloration. Placing small packets of silica gel within the storage container helps to absorb excess moisture, further reducing the reaction rate. For routine maintenance, silver should be gently wiped with a soft, non-abrasive cloth after wearing or handling to remove skin oils and salts.