Costume in early Natal 1850s

This is a beautiful portrait of sitter Charlotte Rose, It is difficult to tell whether it is an ambrotype or a tintype without holding the photograph in one's hand, I would suggest an ambrotype but perhaps a little flat as to colour. She is wearing the undersleeves and dropped shoulder bodice of the fifties. Middle parting another clue as to date. We cannot see much of her skirt but it would be crinoline style..
Checking on dates I find that tintypes did not come into use until about 1863, which would be too late for the costume in this picture - so that decides the ambrotype issue.