As far as I know, restoration is insufficient to claim a derivative copyright in a public domain work because it lacks sufficient originality. To the extent you

As far as I know, restoration is insufficient to claim a derivative copyright in a public domain work because it lacks sufficient originality. To the extent you do a good job, your additions to the underlying public domain work are not your creative authorship. They are a restoration of the original author's authorship. I believe that is why Peter Kuran (Trinity and Beyond) patented a photochemical restoration process for revitalizing the color in films. Because his other restoration work is not protectable.