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descale

British  
/ ˌdiːˈskeɪl /

verb

  1. to remove the hard deposit formed by chemicals in water from (a kettle, pipe, etc)

"Collins English Dictionary — Complete & Unabridged" 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012