Oakum is a preparation of tarred fibre 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. It should be understood that this "tar" is not the tar used on streets and roofs, 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 English okum, from Old English Acumba tow, from A- (separative & perfective prefix) + -cumba (akin to Old English camb comb) - literally "off-combings".

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.

References

• This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.

In victorian times in prison for labour they made prisoners pick oakum.

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Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Oakum"