It is difficult to determine whether a work is in the public domain. Are works made before a certain date a part of the public domain with absolute certainty? Are there any other relevant dates or rules to determine whether a work belongs to the public domain?
There is no certain date for when a work enters the public domain based on when a work was “made.” There is, however, some certainty that works “published” before 1923 are in the public domain. For many works, it can be difficult to determine whether or when a particular work was published. There are several additional factors involved in determining whether a work has entered the public domain and the analysis is fact-specific. For more information about “publication” in copyright law, please see Compendium (Third) Chapter 1900.
Rob Kasunic, Director of Registration Policy and Practices at the U.S. Copyright Office