Copyright in Glass Negatives

Dear Rich: My great grandfather was among other things a photographer in the late 1800's. We have some of his original glass plate negatives which have never been out of the family's possession. At this time we are considering printing limited edition prints of some of the images. Since we own the negatives can we copyright them? If your great-grandfather died before 1942, and the photos were never published, they are in the public domain. (A photo is "published" when copies are distributed to the public by sale or some other ownership transfer.) If you don't know the year he died, your great-grandfather's unpublished photos are protected for 120 years from creation (that is, anything created before 1892 is public domain). 
Publication. If the photos were published, the rules vary depending on the date of publication and whether the work had to be renewed. You can explore all of these possibilities using this public domain chart.
Exploiting the negatives. We hope you can share and enjoy the negatives in some way. It's true that a few glass negative discoveries bring about a jackpot, but, alas, most glass negatives aren't worth dragging to Antiques Road Show (although they may have some historical value).