Pitch is aviscoelasticpolymer which can be natural or manufactured, derived frompetroleum,coal tar,[1][2] or plants. Pitch produced from petroleum may be calledbitumen orasphalt, while plant-derived pitch, aresin, is known asrosin in its solid form.Tar is sometimes used interchangeably with pitch, but generally refers to a more liquid substance derived from coal production, includingcoal tar, or from plants, as inpine tar.[3]
Pitch is the principal product ofcoal tar, at least from the economic perspective. Coal tar is produced by thermolysis of coal, usually with the objective of obtainingcoke, which is heavily used in the production of iron andsteel. The thermolysis is conducted at >700 °C, yielding about 75% of coke, the rest being tar and gases. The tar is further processed, generating some chemicals, some fuel (for the thermolysis and distillation), and a soft pitch. Further treatment of the soft pitch yields pitch.[4]
Pitch sometimes refers to materials obtained fromconifers, i.e.naval stores. Somewhat like the processing of coal to coke and tar, tree exudates (rosins) are processed (solvent extraction or distillation) to obtain organic compounds. The pitch is obtained from fractions with a highboiling point or isundistillable.[5]
Pitch, a traditionalnaval store, was traditionally used to helpcaulk the seams of wooden sailing vessels (seeshipbuilding). Other important historic uses included coating earthenware vessels for the preservation of wine,waterproofing wooden containers, and makingtorches. It was also used to makepatent fuel from coal slack around the turn of the 19th century.[6] Petroleum-derived pitch is black in colour, hence the adjectival phrase "pitch-black".[7]
Theviscoelastic properties of pitch make it well suited for the polishing of high-quality optical lenses and mirrors. In use, the pitch is formed into a lap or polishing surface, which is charged with iron oxide (Jewelers' rouge) or cerium oxide. The surface to be polished is pressed into the pitch, then rubbed against the surface so formed. The ability of pitch to flow, albeit slowly, keeps it in constant uniform contact with the optical surface.
Chaser's pitch is a combination of pitch and other substances, used in jewelry making.
Naturally occurringasphalt/bitumen, a type of pitch, is aviscoelasticpolymer. This means that even though it seems to be solid at room temperature and can be shattered with a hard impact, it is actually fluid and will flow over time, but extremely slowly. Thepitch drop experiment taking place atUniversity of Queensland is along-term experiment which demonstrates the flow of a piece of pitch over many years. For the experiment, pitch was put in a glass funnel and allowed to slowly drip out. Since the pitch was allowed to start dripping in 1930, only nine drops have fallen. It was calculated in the 1980s that the pitch in the experiment has aviscosity approximately 100billion (1011) times that of water.[8] The eighth drop fell on 28 November 2000, and the ninth drop fell on 17 April 2014.[9] Another experiment was started by a colleague of Nobel Prize winnerErnest Walton in the physics department ofTrinity College inIreland in 1944. Over the years, the pitch had produced several drops, but none had been recorded. On July 11, 2013, scientists at Trinity College caught pitch dripping from a funnel on camera for the first time.[10]
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