There is a thorough explanation of copyright and fair use here, at Stanford University libraries, which includes this passage:
"The term "public domain" refers to creative materials that are not protected by intellectual property laws such as copyright, trademark or patent laws. The public owns these works, not an individual author or artist. Anyone can use a public domain work without obtaining permission, but no one can ever own it.
An important wrinkle to understand about public domain material is that collections of it may be protected by copyright. If, for example, someone has collected public domain images in a book or at a website, the collection as a whole may be protectible, even though individual images are not protected. You are free to copy and use individual images but copying and distributing the complete collection may infringe what is known as the "collective works" copyright. Collections of public domain material will be protected if the person who created it has used creativity in the choices and organization of the public domain material. This usually involves some unique selection process, for example, a poetry scholar compiling a book, The Greatest Poems of e.e. cummings.
There are four common ways that works arrive in the public domain:
expiration of copyright: the copyright has expired.
failure to renew copyright: the owner failed to follow copyright renewal rules.
dedication: the owner deliberately places it in the public domain.
no copyright protection available: copyright law does not protect this type of work.
Let's look at each of these routes into the public domain more closely.
1. Expired Copyright
Copyright has expired for all works published in the United States before 1923. In other words, if the work was published in the U.S. before January 1, 1923, you are free to use it in the U.S. without permission. As an example, the graphic illustration of the man with mustache was published sometime in the 19th Century and is in the public domain, so no permission is required to include it within this book. These rules and dates apply regardless of whether the work was created by an individual author, a group of authors or by an employee (the latter sometimes referred to as a "work made for hire.")
Because of legislation passed in 1998, no new works will fall into the public domain until 2019 when works published in 1923 will expire. In 2020, works published in 1924 will expire and so forth. If a work was written by a single author and published after 1977, the copyright will not expire until 70 years after the author's death. If a work was written by several authors and published after 1977, it will not expire until 70 years after the last surviving author dies. ..."
And so on. If you have questions about a specific work, please let me know.