I use this on the back of my paintings. Got the idea from another painter.
Certificate of Authenticity
This is an original oil painting, done en plein air by artist Linda Blondheim. There are no reproductions of this painting for sale. The artist uses reproductions for promotional materials only. The artist owns the copyright on this and her other paintings. www.lindablondheim.com
Obviously, I don't use prints of my paintings and want my clients to know they have the only original. That is a selling point with the clientele that I market too. It would be a different certificate for those who sell reproductions or real hand pulled prints.
Hope this is helpful.