Neck collar
A metal collar was placed around the necks of captured Africans for months at a time to remind them of their wrongdoing. Such collars were thick and heavy; they often had protruding spikes that made fieldwork difficult and prevented the wearer from resting at night. It often took one hour of filing to remove the collar.
http://hitchcock.itc.virginia.edu/Slavery/details.php?categorynum=16&categoryName=Physical%20Punishment,%20Rebellion,%20Running%20Away&theRecord=25&recordCount=87
