Copyright always is global
When you make a new work, you gain copyright everywhere the Berne convention on copyright was signed, to the degree that country provides. That's in all but about 10 countries, among them Iran, Kongo and Somalia.
Now, the runtime of copyright is determined by two things: either the death of the author, in case it is with a natural person, or the publication of the company.
Your plates are well over 120 years old and were most likely company-made. As a result, we have to look back... So let's see... oh, actually it's easy: they were published before 1927. That means they are automatically public domain. Even if they were unpublished, and without known author, they're out of copyright: they were created before 1902, so they are automatically in Public Domain.
Photographies of Plates?
A photograph of the plates creates its own copyright, akin to a translation of a text. However, it is more narrow. Depending on the artistic choices of the photographs, and who made the digital copy, copyright might or might not apply. If copyright applies, then only in the artistic choices of the photographer, e.g. specific lighting or how the pieces are mounted, a text belonging to the picture... Anything that adds to the original work has its own copyright.
As a reasonably safe example: the library of Congress and its employees would not hold copyright in a digital copy, as they are US government actors. Those copies are Public Domain.
It looks entirely different in museums: the MET catalog is copyrighted, despite the items in it being at times thousands of years old. Those pictures are contextualized in the catalog of the collection with notes and such. The MET has a copyright on the collection and its texts.
But... as long as the photos are just mere reproductions with no artistic input no new copyright was created.