A Mudlark is someone who scavenges in river mud for items of value, especially in London during the Industrial Revolution. Poor peasants would scavenge in the River Thames during low tide, searching for anything of value.
During the Industrial Revolution, mudlarks were usually young children or widowed women. Becoming a mudlark was a cry of desperation as it is considered one of the worst "jobs" in history. At the time of the Industrial Revolution, excrement and waste would wash onto the shores from the raw sewage which wasn't treated. The corpses of humans, cats and dogs would also wash up. Mudlarks would be lucky if they made a penny a day selling what they had found during low tide, which was the only time people could scavenge along the shores of the rivers.
Henry Mayhew in his book, London Labour and the London Poor; Extra Volume 1851 provides this detailed description of mudlarks:
A modern organisation founded in 1980, the Society of Thames Mudlarks, has a special license to search the Thames mud for treasure and historical artifacts http://news.nationalgeographic.com/news/2004/05/0503_040503_mudlarks.html.
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