White lignin-free papers and their response to light: from research to conservation practice.

White papers show a variety of hues that often provide key information about their condition and previous storage history. Since destructive analysis is not possible for papers from fine art collections, visual analysis remains the key approach. We strive to explain visual phenomena on fine art papers, backing them with analytical evidence gained from defined lab papers and selected mock-up prints following aging protocols, using color measurements and, for lab papers, iron ion content, molar mass and carbonyl groups. Lab papers dominated by iron ions and gelatin sizing changed most significantly, which corresponds to typical color change of naturally aged papers. Our data support the existence of post-irradiation effects and damage caused by light on exposed rectos compared to their non-exposed versos. Our results aid the characterization of color change on a selection of original artworks, where the possible causes of observed changes are described using levels of certainty.