I woud consider a read of 'The History of Photography' for a review of the time line of when Ambrotype was active as a photographic process, and hwat was involved. I am about half way though that book, and it is a very intersting read. I would put the chance of an ambrotype being the type of image made places the photograph as being made after 1855, and it would be unsual to see them being made after 1870, by which time tintypes were much more often being the medium of choice of travelling photographers.