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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 pipe plumbing applications. 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 penal occupation in prisons and workhouses.
In modern times, the fibrous material used in oakum is derived 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. Petroleum by-products can be utilized for a tar-like substance that 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 a day, and boys under 16 had to pick 1.5 pounds. Over the age of 16, girls and boys had to pick 1.5 pounds and 2 pounds per day respectively. The oakum was sold for £4 10s (£386 in modern money) per hundredweight (100-112 lb). At Coldbath Fields Prison, the men's counterpart to Tothill Fields, prisoners had to pick 2 pounds per day unless sentenced to hard labour, in which case they had to pick between 3 and 6 pounds of oakum per day.
Charles Dickens's novel Oliver Twist mentions the extraction of oakum by orphaned children in the workhouse. The oakum extracted was to be used by navy ships, and the instructor said that they were serving the country.
The Innocents Abroad, a novel 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.
While discussing the appropriate attire for American Supreme Court justices, Thomas Jefferson was once famously quoted as saying, in reference to traditional court dress: "For heaven's sake, discard the monstrous wig which makes the English judges look like rats peeping through bunches of oakum."
Oakum can be used to seal cast iron pipe drains. After setting the pipes together, oakum is packed into the joints, then molten lead is 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. Currently other methods, such as rubber seals, are more commonly used.