Polished brass, an alloy of copper and zinc, will inevitably tarnish over time when exposed to the atmosphere. The act of polishing removes the existing layer of surface compounds, exposing fresh, highly reactive metal to the environment. This causes polished brass to appear to tarnish faster than older, unpolished pieces. Understanding the chemical reactions and environmental factors that drive this process is the first step in effective long-term maintenance.
The Chemical Process of Brass Tarnish
The tarnishing of brass is a form of surface corrosion driven by the copper content in the alloy, unlike rust which is specific to iron. This chemical reaction is known as oxidation, where copper and zinc atoms react with oxygen in the air. The initial result is a thin, dull layer of copper oxide and zinc oxide that obscures the brass’s metallic luster.
A separate, often more aggressive, reaction is sulfidation, which occurs when the brass reacts with sulfur compounds in the atmosphere. Airborne pollutants like hydrogen sulfide or sulfur dioxide readily react with copper to form copper sulfide. This sulfide layer is typically dark gray or black and contributes significantly to the discoloration known as tarnish. Over time, these combined processes form a stable, discolored layer often referred to as a patina.
Environmental Factors Accelerating Tarnish
The rate at which polished brass tarnishes is heavily influenced by the surrounding environmental conditions. High humidity greatly accelerates the oxidation process. Brass located in damp coastal areas or poorly ventilated bathrooms will tarnish much faster than brass in dry climates.
Airborne pollutants, particularly those containing sulfur, dramatically speed up the formation of sulfide-based tarnish. Items near industrial emissions or exposed to combustion products from natural gas appliances are susceptible to rapid discoloration. Furthermore, simple human contact introduces salts and oils from the skin, which act as catalysts to initiate chemical reactions in touched areas.
Safe Methods for Tarnish Removal
Removing tarnish requires a tiered approach, starting with the least aggressive methods to protect the underlying metal. For light surface grime and minor dullness, a simple wash with warm water and a mild, pH-neutral dish soap is often sufficient. This method safely removes dust, oils, and loose surface contaminants without damaging the metal.
For moderate tarnish, a gentle homemade paste can be highly effective, utilizing the mild abrasive action of salt and the acidity of lemon juice or vinegar to dissolve the metal oxides. A common recipe involves mixing salt, vinegar, and flour into a paste, which is applied to the brass and allowed to sit briefly. After using any acidic solution, rinse the item thoroughly with water to halt the chemical action and then dry it completely to prevent immediate re-tarnishing.
Stubborn, heavy tarnish may require a commercial brass polish, which often contains fine abrasives to physically lift the dark layer. When using a commercial product, always apply it with a soft cloth and work in small sections, avoiding steel wool or harsh scrubbers that can scratch the surface. Test any cleaning method, whether homemade or commercial, on a small, inconspicuous area first to ensure it does not cause unwanted discoloration or damage.
Long-Term Protection Against Future Tarnish
Once polished, the bare brass surface needs a protective barrier to prevent the immediate return of tarnish. A clear lacquer forms a durable, transparent coating that seals the metal from oxygen and moisture. Lacquer is ideal for decorative items that are not frequently handled, such as light fixtures or static displays. However, if the lacquer is scratched or chipped, tarnish can form underneath, requiring the complete removal of the remaining lacquer before the item can be repolished.
For items that are frequently touched, like doorknobs or handrails, a protective wax or oil offers a practical, renewable alternative. Applying a thin coat of carnauba wax or mineral oil creates a temporary physical barrier that must be reapplied periodically. This method allows the brass to be easily cleaned and re-waxed as the barrier wears down from handling or cleaning, offering a balance between protection and ease of maintenance.