Tarnish is a common chemical process that affects the surface of many metals, causing them to lose their original luster and shine. This phenomenon begins when the outermost layer of the metal reacts with nonmetal compounds in the atmosphere, resulting in a thin film of discoloration. While tarnish can appear in various colors, the distinct darkening to a dull, gray, or black shade is a specific chemical signature linked to certain reactive elements. This particular form of surface change provides a clear indication of the chemical interactions occurring between the metal and its environment.
Understanding Tarnish Versus Oxidation
Tarnish is technically a mild form of corrosion, but it differs fundamentally from more destructive processes like the rusting of iron. True corrosion, such as the formation of reddish-brown rust, involves the deeper, structural degradation of the metal, often resulting in flaking and eventual material failure. This degradation occurs because the corrosive layer is porous and allows reactive elements to penetrate further into the material.
In contrast, tarnish is a superficial layer, typically measured in nanometers, that adheres tightly to the metal’s surface. This thin film, whether a metal oxide or a metal sulfide, often becomes self-limiting. Once the layer forms, it acts as a protective barrier, slowing or stopping further reaction with the underlying metal. Tarnish, while aesthetically undesirable, does not structurally compromise the metal object itself.
The Chemical Mechanism That Causes Black Tarnish
The primary cause of black tarnish is a process known as sulfidation, which is a reaction between the metal and airborne sulfur compounds. While general tarnish can involve simple oxidation (reaction with oxygen), the dark black color specifically points to the formation of metal sulfides. The most common culprit is hydrogen sulfide (\(\text{H}_2\text{S}\)) gas, which is naturally present in trace amounts in the atmosphere from sources like air pollution, industrial emissions, and certain foods.
When a susceptible metal surface encounters hydrogen sulfide, a reaction occurs that strips the sulfur atom from the gas molecule and binds it directly to the metal atoms. For instance, this produces silver sulfide (\(\text{Ag}_2\text{S}\)), which is characterized by its dark gray to black color. The reaction is accelerated by high humidity, as moisture acts as an electrolyte to facilitate the transfer of electrons required for the chemical change. Even at low concentrations, sulfur compounds can initiate and sustain the tarnishing process.
Specific Metals Susceptible to Black Tarnish
Silver is the metal most famously affected by black tarnish, driven by its high affinity for sulfur. Sterling silver, an alloy of 92.5% silver and 7.5% copper, is particularly vulnerable because the copper content also readily reacts with sulfur compounds. The resulting black film is silver sulfide, which forms a dense layer that quickly dulls the metal’s reflective surface.
Copper and its alloys, such as brass and bronze, can also exhibit a dark tarnish, though their final patina is often green or blue-green. Initially, copper reacts with oxygen to form a reddish-brown cuprous oxide layer, which then reacts with sulfur to form dark copper sulfides (\(\text{Cu}_2\text{S}\)). In alloys like brass, the copper component drives the initial darkening reaction.
Other metals can tarnish black under specific sulfidation conditions, including lead, which reacts with hydrogen sulfide to form black lead(II) sulfide (\(\text{PbS}\)). While pure gold and platinum are highly resistant, lower-karat gold jewelry will tarnish dark because it contains reactive metals like copper and silver in the alloy mixture. The tarnish on these alloys is due to the non-gold metals forming sulfides on the surface.
Practical Prevention and Cleaning Strategies
Preventing black tarnish requires minimizing the exposure of susceptible metals to moisture and sulfur compounds. Storing items in a cool, dry environment is important, and using specialized anti-tarnish cloths or strips that absorb airborne sulfur is highly effective. These measures work by intercepting the reactive gas molecules before they can reach the metal surface. Avoiding contact with common household items like rubber bands, wool, latex gloves, and certain paints, all of which contain sulfur, also helps slow the process.
Once tarnish has formed, it can be removed through either abrasive or chemical means. Mechanical removal involves gentle polishing with a soft cloth and a non-abrasive metal polish to physically lift the sulfide layer. For a less abrasive, chemical approach, an electrochemical reaction can be used. This process involves placing the tarnished item in contact with aluminum foil in a hot solution of baking soda and water. The aluminum acts as a sacrificial metal, causing the sulfur atoms to transfer from the metal sulfide back to the aluminum, reversing the sulfidation and restoring the metal’s shine.