Tarnishing is a natural chemical process that affects the surface of certain metals, causing them to darken, discolor, or lose their original luster. This phenomenon is a superficial change, meaning it only impacts the outermost layer of the metal rather than its entire structure. It manifests as a dull, often dark film that obscures the metal’s natural shine. This surface alteration occurs due to reactions with specific elements or compounds present in the environment.
Understanding the Chemical Process
Tarnishing is a form of corrosion, specifically a chemical reaction where the metal surface interacts with airborne substances. The primary culprits in this process are oxygen and sulfur compounds. When a metal comes into contact with these reactive elements, it forms a thin layer of a new compound on its surface. For instance, in the presence of oxygen, a metal can undergo oxidation, creating a metal oxide layer.
Similarly, sulfurization occurs when metals react with sulfur-containing gases, such as hydrogen sulfide, forming metal sulfides. This newly formed compound layer, whether an oxide or a sulfide, is what we observe as tarnish. Unlike rust, which significantly degrades iron, tarnish remains a surface-level alteration that can protect the underlying metal.
Metals Prone to Tarnishing
Several common metals are susceptible to tarnishing due to their reactivity with environmental agents. Silver, for example, readily tarnishes when exposed to sulfur compounds in the air, forming black silver sulfide. This reaction is accelerated by humidity and pollutants. Copper and its alloys, such as brass and bronze, also tarnish over time, developing a greenish or brownish patina. This coloration results from the metal reacting with oxygen and moisture, forming copper oxides or carbonates.
Aluminum, while corrosion-resistant, can develop a thin, dull oxide layer when exposed to air. This aluminum oxide layer serves as a protective barrier, preventing further reaction.
Strategies for Prevention
Preventing or slowing down the tarnishing process primarily involves limiting the metal’s exposure to reactive elements in its environment. Proper storage is a highly effective method; storing items in airtight containers or sealed plastic bags can significantly reduce contact with oxygen and sulfur compounds. Including anti-tarnish strips or silica gel packets within these containers can further absorb moisture and reactive gases, creating a more controlled microenvironment.
Controlling humidity levels in storage areas also helps, as moisture accelerates tarnishing reactions. Minimizing contact with certain reactive chemicals found in common items can prevent tarnish. This includes avoiding direct contact with rubber products, some foods like eggs or onions, and air pollutants, all of which can contain sulfur or other reactive substances.
Methods for Removal
Once tarnish has formed, various methods can effectively remove it, restoring the metal’s original appearance. Commercial metal polishes are specifically formulated to gently abrade or chemically dissolve the tarnish layer, and they are available for different types of metals. For silver, a common home remedy involves a chemical reaction using hot water, baking soda, and aluminum foil. This method uses an electrochemical process to transfer the sulfur from the silver to the aluminum, effectively reversing the tarnishing.
For copper and brass, a simple mixture of lemon juice and salt can act as a mild acid abrasive to remove the tarnish. Apply these methods with soft cloths and gentle rubbing motions to avoid scratching the metal surface. After cleaning, rinse thoroughly with water and dry completely to prevent water spots and slow future tarnishing. Always choose a method appropriate for the specific metal to prevent damage.