Oh, sorry! Figured everyone knew ahead of me. :-) From OED: "The name of ‘Pure-finders’. . . has been applied to the men engaged in collecting dogs'-dung from the . . . streets."
And: "The occupation of collecting dog faeces for sale to tanneries (which used it as a siccative for bookbinding leather). Undertaken by old women in Britain in the 18th century. (Reference: Robert Hughes, The Fatal Shore, 1987.)"
Apparently the tanning process was called puring; hence "pure" in a not-so-pure practice.
"It is August 1854, and London is a city of scavengers. Just the names alone read now like some kind of exotic zoological catalogue: bone-pickers, rag-gatherers, pure-finders, dredgermen, mudlarks, sewer-hunters, dustmen, night-soil men, bunters, toshers, shoremen."
Opening sentences of "The Ghost Map" by Steven Johnson.