Many people characterize the "public domain" as a graveyard for copyrights. That is not precisely true. The public domain is largely made up of of works that are ineligible for copyright protection, or the copyright on those works have expired. Permission is not necessary to use public domain works, because nobody "owns" the work from a copyright perspective.
The four broadest ways in which works enter the public domain are as follows:
For a helpful resource on which materials are under copyright and which are in the public domain please consult Cornell University's Copyright Term and the Public Domain webpage and the Public Domain slider.
This guide is intended to provide information for educational purposes only and does not constitute legal advice. The links to third-party guides and resources on this page are provided to assist you in your investigation. If you have questions about these pages, please seek additional clarity from your subject librarian.
Generally not. "Finders, keepers" does not apply to public domain works. Meaning, if you determine that a work is within the public domain and free of copyright restrictions, you may not claim a new copyright over an unmodified version of the work. However, if you add, new, original, creative content to the work, you may have a copyright interest in the new material that you you have added. Keep in mind, however, that any copyright that you might have over that modification is likely thin and hard to enforce.
This is an example of the question above in action. Jane Austen originally published Pride and Prejudice in 1813. The copyright on the work has expired, and the work is within the public domain. This means that authors who wish to reinterpret the work may do so without seeking permission from the estate of Jane Austen because the estate no longer controls the copyright to the work. This is not to say that the popularity of the work does not still live on. While the copyright may have expired, contemporary audiences are still thirsty for content that reimagines Jane Austen's building blocks. In our lifetime we have seen many fresh interpretations of the work, completed without permission from the estate of Jane Austen. Pride and Prejudice and Zombies is one such reinterpretation. The author did not need to seek permission from the estate of Jane Austen to reimagine her work as a Zombie thriller because the work was out of copyright. Likewise, he did not need to seek permission from the other authors of new works based on the original because, in his estimation, his work is sufficiently different from what others have created based on the same public domain source material. The author may, however, claim copyright over the new expression that he has added to Jane Austen's plot, characters, and other building blocks, provided the new expression is original to him. This means that while he cannot claim copyright over a story about two characters that at first do not appear to like each other because they in fact love each other once they overcome their internal hang-ups, he can claim copyright over the new expression he has added to the underlying plot. Any second-comer who wishes to take Mr. Grahame-Smith's original expression (specifically the expression of the zombie overlay) would need to seek permission until the copyright on the Grahame-Smith work expired.
Translations of public domain works have their own copyright. Languages are not strict equivalents, meaning there is some art and creativity associated with expressing an idea cleanly from one language to another. If a contemporary translator took a public domain work in Greek and translated it to English the new translation would be subject to copyright protection. Second-comers would need to seek permission from the rights-holder to distribute or reproduce the English language translation, even if they would be free to use the original Greek text.
Provided the forward is new, original, and fixed, the forward to a new edition of a public domain work is likely subject to copyright protection.
The Penn Libraries has compiled a number of resources to help you find images for use in your scholarship. Please see the Finding Open Access Images guide as a research launch point.