Oakum is a preparation of tarredfibre used in shipbuilding, for caulking or packing the joints of timbers in wooden vessels and the deck planking of iron and steel ships, as well as cast iron plumbing applications. Oakum was at one time made from old tarry ropes and cordage of vessels, and its picking and preparation has been a common penal occupation in prisons and workhouses. In modern times it is made from virgin hemp fibers. White oakum is made from untarred materials. The fibrous material used in oakum is most commonly a hemp or jute fiber impregnated with tar or a tarlike substance. This "tar" is not the tar used on streets and roofs, which is really asphalt, but rather pine tar, also called Stockholm tar, an amber-colored pitch made from the sap of certain pine trees.
The term oakum is also used to describe the use of the thumb in the application of intercostal pressure. The word oakum is derived from Middle Englishokum, from Old EnglishAcumba tow, from A- (separative & perfective prefix) + -cumba (akin to Old English camb comb) - literally "off-combings".
While discussing the appropriate attire for American Supreme Court justices, Thomas Jefferson was once famously quoted as saying: "For heaven's sake, discard the monstrous wig which makes the English judges look like rats peeping through bunches of oakum."1
Use in plumbing applications
Until plastic (ABS, PVC or CPVC) drain pipes were used, oakum was one of two materials used to seal cast iron drain piping. After setting the pipes together, oakum was packed into the joints, then molten lead was poured into the joint, creating a permanent seal. The oakum swells and seals the joint, the "tar" in the oakum prevents rot and the lead keeps the joint physically tight.
^ Procter, I.B. "John Jay and Other Chief Justices of the Supreme Court." pp.153-154 Michigan Law Journal, Volume 5 1896. Dennis & Co., Inc. 1963 (accessed via Google Books May 4, 2008)
