1. Did all Roman slaves wear collars? (If not, for what reasons might they not have one?)
2. Would a slave keep the same collar when they were sold to a different owner, or was this something that the owner had to provide?
3. Were they generally plain, or inscribed with "I belong to [whoever]"-type messages?
4. What kind of material were they usually made from? (The pictures I've found suggest metal, but I'm not sure what kind...)
ETA: Thanks a lot, people! :-)