A question of preservation

“For artists, the effect isn’t simply cosmetic, because they intentionally use these colors for their visual impact,” said Sarah Schmidtke Sobeck, a photochemist at The College of Wooster who spoke at the ACS meeting.

Sobeck and Gregory Smith, a conservation scientist at the Indianapolis Museum of Art, have been collaborating on researching such materials for the last decade after meeting at a cultural heritage science conference. Their partnership has already produced a two-part study on the composition, spectral properties, and light stability of daylight fluorescence artists’ pigments.

That earlier work showed that the optical brightener compounds that give fluorescent pigments their brightness degrade more quickly than the pigment dyes. Thus, even when there is no degradation in the pigments, the colors can appear to darken. That poses a unique challenge for conservationists responsible for long-term restoration of these works. Not only must they match colors under both visible and UV light, but the pigments will also change subtly over time. Gaining a better understanding of the unique photochemistry of such materials is critical for this purpose.

During her talk, Sobeck reported on the group’s most recent work, extending their research to glow-in-the-dark phosphorescence pigments, with particular focus on the museum’s Sprouse collection. Among the early findings: Such materials have a markedly different composition from fluorescent colorants, which typically use organic dyes. The phosphorescent materials contain mineral-based pigments and inorganic compounds.

Also, humidity turns out to be a significant factor in the breakdown of phosphorescent pigments—as much if not more so than prolonged exposure to light, which will help the museum improve its storage and exhibition practices for such artworks. The next step in this ongoing project will focus on the white pigment powder lithopone, testing how long phosphorescent materials containing that pigment will glow after each charge.