Legal Question in Intellectual Property in Texas

In this case, you are probably safe to use the paintings. In the 50's and 60's works of art published without copyright notice were considered donated to the public. Also, copyrights lasted 28 years from registration and were renewable for 28 more years. The law changed in 1978 to expand the renewal period to 47 years, for a total of 75 years to make the term equal to what the 1978 law provided for new company owned works. However, works that were unregistered did not gain a renewal as they had previously been dedicated to the public and it was thought unfair to then remove something public from the public as people relied on the fact that dedicated works were public domain.

If an artist or descendant shows up it will be irrelevant since the works will have been public domain for a half century.

Having said that, you should set aside some portion of the income (say 10%) to cover the possibility that you are incorrect on the copyright issue. For example, perhaps your paintings are forgeries and the authentic work does have a copyright notice. In that event your paintings would be infringing and any copies thereof would be infringing. Also, if a challenge arises that you win, you will nevertheless have legal expenses to pay to defeat the challenge.

After a reasonable time, say three years or so, you might begin to draw down the reserve in reliance on the lack of any copyright infringement allegation.

This is, of course, general comment and not specific legal advice since you are not a client and not paying for this. If you want specific legal advice you need to go to me or some other legal expert in this field for such advice with the specifics of your paintings so reliable research can be done to best ascertain your true risk. That might actually be cost effective if it results in better certainty so that you do not need to have as large a reserve for possible challenges. It is your business and your call whether that makes sense.