Oakum is a preparation of tarred fibre used to seal gaps. Its main traditional applications were in shipbuilding, for caulking or packing the joints of timbers in wooden vessels and the deck planking of iron and steel ships; in plumbing, for sealing joints in cast iron pipe; and in log cabins for chinking. In ship caulking, it was forced between the seams using a hammer and a caulking iron, then sealed into place with hot pitch.
Oakum was at one time recycled from old tarry ropes and cordage, which were painstakingly unravelled and taken apart into fibre. This task of picking and preparation was a common occupation in prisons and workhouses, where the old and infirm were often put to work unpicking oakum when they could no longer perform heavier labour. Sailors undergoing naval punishment were also frequently sentenced to unpick oakum, with each man made to unpick 1 pound (450 g) of oakum a day. The work was tedious, slow and taxing on the worker's thumbs and fingers.
In modern times, the fibrous material used in oakum comes from virgin hemp or jute. The fibers are impregnated with tar or a tar-like substance, traditionally pine tar (also called 'Stockholm tar'), an amber-coloured pitch made from pine sap. Tar-like petroleum by-products can also be used for modern oakum. White oakum is made from untarred material.
Picking oakum was a common occupation in Victorian times in British prisons and workhouses. In 1862, girls under 16 at Tothill Fields Bridewell had to pick 1 pound (0.45 kg) a day, and boys under 16 had to pick 1 1⁄2 pounds (0.68 kg). Over the age of 16, girls and boys had to pick 1 1⁄2 and 2 pounds (0.68 and 0.91 kg) per day respectively. The oakum was sold for £4 10s (equivalent to £406 in 2016 in modern money) per hundredweight (100–112 lb, 45–51 kg). At Coldbath Fields Prison, the men's counterpart to Tothill Fields, prisoners had to pick 2 lb (0.91 kg) per day unless sentenced to hard labour, in which case they had to pick between 3 and 6 lb (1.4 and 2.7 kg) of oakum per day.
Oakum can be used to seal cast iron pipe drains. After setting the pipes together, workers pack oakum into the joints, then pour molten lead into the joint to create 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. Today, modern methods, such as rubber seals (for example, gaskets or o-rings) are more common.
Charles Dickens's novel Oliver Twist mentions the extraction of oakum by orphaned children in the workhouse. The oakum extracted was for use on navy ships, and the instructor said that they were serving the country.
The Innocents Abroad, a travel book by Mark Twain, also mentions in chapter 37 a "Baker's Boy/Famine Breeder" who eats soap and oakum, but prefers oakum, which makes his breath foul and teeth stuck up with tar.
We tore the tarry rope to shreds
With blunt and bleeding nails;
We rubbed the doors, and scrubbed the floors,
And cleaned the shining rails:
And, rank by rank, we soaped the plank,
And clattered with the pails.
- Chisholm, Hugh, ed. (1911). "Oakum". Encyclopædia Britannica. 19 (11th ed.). Cambridge University Press. p. 935.
- Kemp, Peter (1979). The Oxford Companion to Ships & the Sea. Oxford University Press. p. 807. ISBN 978-0-586-08308-6.
- Mayhew, Henry; Binny, John (1862). The Criminal Prisons of London, and Scenes of Prison Life. Volume 3 of The Great Metropolis. London: Griffin, Bohn, and Company. p. 477.
- Mayhew & Binny (1862) p. 312.
- Yates, David (1 February 2005). "The Lost Art of Making Lead Joints". Contractormag.com. Retrieved 6 July 2011.