Duration of Copyright
Many collage artists avoid the issue of copyright by using only source material that is in the public domain. This means either that the copyright has expired, or that the work was never properly copyrighted in the first place.
Since 1978, a work is copyrighted automatically at the moment it is created in a tangible form. (For example, thinking about a short story or plotting it out in your head does not make it copyrighted. Writing it down on paper or your computer does). Before 1978, work had to be published with a copyright notice, and periodicly renewed, to be fully protected by copyright.
It is difficult to determine whether a work was copyrighted without an expensive search of the Copyright Office records. So I always play it safe and assume that all source material was copyrighted to the full extent of the law.
US copyright law was significantly simplifed after 1978. Unfortunately, for the purposes of a collage artist trying to find out whether something is in the public domain, what usually matters is material that was created long before then. Here's a brief summary:
Created before 1978:
As above, the only way to find out whether a work was properly copyrighted and renewed is to search the Copyright Office records (they do it at $20 per hour, or you can do it yourself if you live in DC and have time to kill). I always just assume that source material was renewed, and therefore falls into the public domain 95 years after its original publication.
Created since January 1, 1978:
An added complication is that, from 1978 until 1989, an author/artist was required to attach a copyright notice to the work for it to be copyrighted. If they failed to do so, the work is in the public domain. But after 1989, copyright is automatically assigned to anything that can be copyrighted, from the moment of its creation.
According to intellectual property law expert Ivan Hoffman, "Under the current law, if a work was published on or before December 31, 1922, then it is likely to be in the public domain. But you should never rely solely on this date alone since there may be other factors to consider." You'd better believe that the Gershwin family will file all the necessary papers to keep valuable properties like "Rhapsody in Blue" out of public domain for another twenty years. But if you're wondering about a 1920 cookbook that you found in granny's attic, published by a company that went out of business in 1930, chances are that the extension was not filed. Also, my understanding of the above is that any work at least 95 years old is safely in the public domain, no ifs ands or buts.
I have been told that it is possible to renew copyright registration in perpetuity, so that works of antiquity such as famous Renaissence works of art might still be copyrighted by the museums that own them or the descendants of the artists. Nothing in my research would confirm this. So I am happy to classify this as a myth. However, while a centuries-old work of art is in the public domain, recent photographs of it (such as those found in an art book or museum guide) may be copyrighted. See the "Bridgeman v Corel" page for discussion of a recent court case which affects this issue.
Publish or Perish
Before 1978, copyright law hinged on the date of publication. What does publication mean in this sense? The US Copyright Office defines it as follows:
"Publication" is the distribution of copies or phonorecords of a work to the public by sale or other transfer of ownership, or by rental, lease, or lending. The offering to distribute copies or phonorecords to a group of persons for purposes of further distribution, public performance, or public display constitutes publication. A public performance or display of a work does not of itself constitute publication.
I have heard many times that a work is only published if you sell it to a publisher who mass-markets it. My reading of this statement by the Copyright Office would indicate that any reproduction of a work for others (whether you sell it to them or give it away) would be interpreted as publishing. I'm not entirely sure from reading this whether selling the original would be defined as publishing or not.