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This depends on a number of factors including whether it was published, and if it was, the date and location of first publication. For works created after January 1, 1978, copyright protection last for 70 years past the life of the author. Anonymous works, pseudonymous works, or works for hire have a copyright duration of 95 years from the date of first publication or 120 years from the date of creation, whichever expires first.  

For works published before 1978, copyright duration varies depending on several factors including whether the copyright registration was renewed. To help to you determine the copyright status of these works, Cornell University has an excellent resource on its web site that should be of help to you. It is available at http://copyright.cornell.edu/resources/publicdomain.cfm.