Copyright laws, use of early 1900s folk art for greeting cards
2 attorney answers
The following is not legal advice and should not be relied on to take or refrain from taking any action.
There are intricate and convoluted rules that control how long copyright protection lasts for various works. One bedrock rule that seems to apply to your situation, however, is that ALL works that were published in the U.S. before 1923 are now in the public domain -- and are, therefore, free for all to use as they wish (with some limitations on paintings). The lone wrinkle, in what seems to be an unusually certain answer to your question, is that the postcards must have been "published' in the U.S. for the rule to apply. "Published" means, essentially, that they were publicly available in the U.S.. If that criteria is met (i.e., you didn't gather them from some non-public source or from someone overseas) then it appears that you're quite free to use the postcards to create your art.
FYI - a very useful work for artists is "The Public Domain" by Stephen Fishman which provides a wealth of information about what pre-existing works can be used as fodder to create new works.
You have two defenses: public domain and the first sale doctrine.
The "first sale" doctrine will likely protect you in this instance. The "first sale" doctrine says that a person who buys a legally produced copyrighted work may "sell or otherwise dispose" of the work as he sees fit, subject to some important conditions and exceptions which don't seem to apply here. THe doctrine is codified in section 109 of the Copoyright Law. If you legally buy a book or a postcard, "first sale" gives you the right to loan or re-sell that book or postcard. Libraries heavily depend on the first sale doctrine to lend books and other items to patrons. What is not protected is if you are making multiple copies of the same postcard and re-selling the copies. The first sale doctrine would only apply to the one actual item - you buy it use it, then re-sell just the one that you bought. You are safe even if you copy the cards under your example because due to the age of the postcard it is in the public domain at this point.
This is also a good time to talk about the length of copyright:
1. Works published from 1909 through 1921.
The initial copyrighted term of the work was 28 years from the date of publication. If the copyright was renewed during the 28th year, the copyright was extended for an additional 28-year period.
2. Works published from 1922 through 1963.
The initial copyrighted term of the work was 28 years from the date of publication. If the copyright was renewed during the 28th year, the copyright was extended for an additional 67-year period.
3. Works published from 1964 through 1978.
The initial copyrighted term of the work was 28 years from the date of publication, with an automatic renewal of an additional 67 years.
4. Works created on or after January 1, 1978.
The following rules apply to published and unpublished works:
• For one author, the work is copyright-protected for the life of the author plus 70 years.
• For joint authors, the work is protected for the life of the surviving author plus 70 years.
• For works made for hire, the work is protected for 95 years from the first publication or 120 years from the date of its creation, whichever is less.
• For anonymous and pseudonymous works, the work is protected for 95 years from the first publication or 120 years from the date of its creation, whichever is less. (However, if the author's name is disclosed to the U.S. Copyright Office, the work is protected for the life of the author plus 70 years.)