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Carbon paper (originally carbonic paper) is paper coated on one side with a layer of a loosely bound dry ink or pigmented coating, usually bound with wax. It is used for making one or more copies simultaneous with the creation of an original document when using a typewriter or a ballpoint pen. Manufacturing of carbon paper was formerly the largest consumer of montan wax.
Carbon paper is placed between the original and a second sheet to be copied onto. As the user writes or types on the original, the pressure from the typebar or pen deposits the ink on the blank sheet, thus creating a "carbon copy" of the original document. This technique is generally limited to four or five copies.
As the ink is transferred from the carbon paper to the underlying paper, an impression of the corresponding text is left on the "carbon" where the ink was removed. A single piece of carbon paper can be repeatedly reused until the impression grows too light.
Fuel Cell Application 
Currently, carbon paper is being widely used in fuel cell applications to help as an electrode that facilitates diffusion of reactants across the catalyst layered membrane portion of membrane electrode assembly . Companies such as FuelCellsEtc, FuelCellStore, and many others make the carbon paper in to gas diffusion electrodes to serve as a more conducive electrode.
Other Uses 
Carbon paper has been mostly superseded by electronic means such as photocopying, and by carbonless copy paper in situations where instant copies of written documents are needed. Examples of this are receipts at point of sale (though they have mostly been relegated to being backups for when electronic POS devices fail) or for on-the-spot fine notices, duplicate, checks, and some money orders (though the United States Postal Service has recently converted to an electronic format), and tracking slips for various expedited mail services requiring multiple copies. As of 2013, in Canada only one eight-person company still manufactures carbon paper, in the United Kingdom one company and in the United States only two small companies. 
There have been some experimental uses of carbon paper in art (as a surface for painting) and mail art (to decorate envelopes).
See also 
- Wissinger, R. R. (1950). Carbon Papers and Other Duplicating Papers. In Mosher, R. H. (ed), Specialty Papers, Their Properties and Applications (pp. 335–367). Brooklyn, N.Y.: Remsen Press.
- Westerheim, Daniel. "What is the Difference Between Carbon Paper and Carbon Cloth Based Gas Diffusion Layers (GDL)?". Fuel Cells Etc - Tech Article.
- "Inside the UK's last carbon paper factory". BBC News. 2013-05-15. Retrieved 2013-05-15.
- Morgan Campbell (2013-02-08). "Copy this: North York’s Form-Mate is the last supplier of carbon paper in Canada". Toronto Star. Retrieved 2013-02-09.
- Carbon Paper Being Used in Fuel Cells
- The Exciting History of Carbon Paper!
- How Carbon Paper is Made