Not being a copyright attorney, please consider my reply for what it's
worth (free).

As I understand US copyright law, copyright can never be assumed, and
always remains in the legal possession of the creator (artist, in your
case), unless assigned to someone else by the work's creator.

Some artists never assign copyright, although they might give a letter
to an institution  saying their work can be reproduced for publicity
non-profit puposes only, with prior written approval of the proposed
reproduction for quality control reasons.

Some artists or their heirs give away (or sell) their copyright
ownership for limited use in creation of products for sale in, say,
museum giftshops or one-time museum events.

Copyright law varies from country to country, and there are excellent
websites for each country's copyright offices.

If you're in America, the US Copyright Office in DC gives excellent
analysis of complicated issues.

Always engage a copyright attorney to protect your position in this
changing, and murky, area.

Very truly yours,

Timothy George Hare, R.A., M.A.

Internationally-Published Architect
and Architectural Historian

Specializing in Architectural Portraits
in Pen & Ink with Watercolor Embellishment

667 Ferry St.  Easton, PA USA 18042-4423
Telephone: 610-258-0834
E-Mail: [log in to unmask]

WebSites: http://www.silo.com/HARE
http://geocities.com/SoHo/7708/