Tarnish is a thin layer of corrosion that forms over copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

, brass

Brass

Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...

, silver

Silver

Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

, aluminum, and other similar metals as their outermost layer undergoes a chemical reaction. Tarnish does not always result from the sole effects of oxygen in the air. For example, silver needs hydrogen sulfide

Hydrogen sulfide

Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

 to tarnish; it does not tarnish with only oxygen. It often appears as a dull, gray or black film or coating over metal. Tarnish is a surface phenomenon, that is self-limiting unlike rust

Rust

Rust is a general term for a series of iron oxides. In colloquial usage, the term is applied to red oxides, formed by the reaction of iron and oxygen in the presence of water or air moisture...

. Only the top few layers of the metal react, and the layer of tarnish seals and protects the underlying layers from reacting.

Tarnish actually preserves the underlying metal in outdoor use and is called patina

Patina

Patina is a tarnish that forms on the surface of bronze and similar metals ; a sheen on wooden furniture produced by age, wear, and polishing; or any such acquired change of a surface through age and exposure...

. The formation of patina is necessary in applications such as copper roofing, and outdoor copper, bronze, and brass statues and fittings.

It is the thin layer of aluminum oxide tarnish that protects aluminum in outdoor applications such as screen door

Screen door

A screen door can refer to a hinged storm door or hinged screen door covering an exterior door; or a screened sliding door used with sliding glass doors. In any case, the screen door incorporates screen mesh to block flying insects from entering and pets and small children from exiting interior...

 frames. If the thin layer of aluminium oxide did not form, the aluminium would sizzle when placed in water.

Chemistry

Tarnish is a product of a chemical reaction

Chemical reaction

A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

 between a metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

 and a nonmetal

Nonmetal

Nonmetal, or non-metal, is a term used in chemistry when classifying the chemical elements. On the basis of their general physical and chemical properties, every element in the periodic table can be termed either a metal or a nonmetal...

 compound

Chemical compound

A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

, especially oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 and sulfur dioxide

Sulfur dioxide

Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...

. It is usually a metal oxide

Oxide

An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....

, the product of oxidation. Sometimes it is a metal sulfide. The metal oxide sometimes reacts with water to make the hydroxide, and carbon dioxide to make the carbonate.

Prevention and removal

Using a thin coat of polish

Polishing

Polishing is the process of creating a smooth and shiny surface by rubbing it or using a chemical action, leaving a surface with a significant specular reflection In some materials polishing is also able to reduce diffuse reflection to...

 can prevent tarnish from forming over these metals. Tarnish can be removed by using steel wool

Steel wool

Steel wool, also known as wire wool, is a bundle of strands of very fine soft steel filaments, used in finishing and repairing work to polish wood or metal objects, and for cleaning household cookware....

, sandpaper

Sandpaper

Sandpaper, also known as glasspaper, is a heavy paper with abrasive material attached to its surface.Sandpaper is part of the "coated abrasives" family of abrasive products. It is used to remove small amounts of material from surfaces, either to make them smoother , to remove a layer of material...

, emery paper

Emery paper

Emery paper is a type of paper that can be used for sanding down hard and rough surfaces.It can also be used for resistant technology purposes to give a smooth, shiny finish to manufactured products and is often used in the finishing of high-end watch movements.Similar to sandpaper, it is made by...

, baking soda

Sodium bicarbonate

Sodium bicarbonate or sodium hydrogen carbonate is the chemical compound with the formula Na HCO3. Sodium bicarbonate is a white solid that is crystalline but often appears as a fine powder. It has a slightly salty, alkaline taste resembling that of washing soda . The natural mineral form is...

 or a file

File (tool)

A file is a metalworking and woodworking tool used to cut fine amounts of material from a workpiece. It most commonly refers to the hand tool style, which takes the form of a steel bar with a case hardened surface and a series of sharp, parallel teeth. Most files have a narrow, pointed tang at one...

 to rub or polish

Polishing

Polishing is the process of creating a smooth and shiny surface by rubbing it or using a chemical action, leaving a surface with a significant specular reflection In some materials polishing is also able to reduce diffuse reflection to...

 the metal's dull surface. Fine objects (such as silverware) may have the tarnish electrochemically reversed, or it may be removed with a special polishing compound and a soft cloth. Gentler abrasives, like calcium carbonate

Calcium carbonate

Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

, are often used by museums to clean tarnished silver as they cannot damage or scratch the silver and will not leave unwanted residues.