The author thanks Marco Leona, Scientist-in-Charge of the Department of Scientific Research at the Metropolitan Museum of Art for conducting fluorescence spectrometry on Wheel of Fortune and a valuable discussion of the research, as well as Silvia Centeno, Research Scientist at the Metropolitan Museum of Art, who performed Raman analysis on the watercolors and also contributed her insight. The phenomenon of the phosphorescing lithopone was originally discovered during the author's fellowship in the Sherman Fairchild Center for the Conservation of Works on Paper, funded by the Andrew W. Mellon Foundation. The author thanks all her colleagues for their ideas and support during the research of this paper, and special thanks to Rachel Mustalish for her assistance in editing this work.

The FDA continues to allow for the safe use of titanium dioxide as a color additive in foods generally according to the specifications and conditions, including that the quantity of titanium dioxide does not exceed 1% by weight of the food, the FDA said in a statement to USA TODAY.

They are the only two sunscreen ingredients classified by the FDA as safe and effective. And though titanium dioxide is usually used in mineral sunscreens in the form of nanoparticles, evidence suggests that few, if any, particles penetrate the skin.

Over the last several years, nanoparticles have come under scrutiny for adverse health effects. Nanoparticles are ultrafine particles between 1 to 100 nanometers in diameter. (To put this in perspective, the average human hair is around 80,000 nanometers thick.) Because of their size, which can be engineered and manipulated at the atomic or molecular level, nanoparticles exhibit unique physical, chemical, and biological properties. Titanium dioxide is one of the most commonly produced nanoparticles in the world.

Titanium dioxide can be both safe and unsafe, depending on its use. When inhaled, titanium dioxide is considered possibly carcinogenic to humans. This means that in products that contain powdered titanium dioxide like loose powders, pressed powders, eyeshadows, and blushes in which the makeup is in powder form, titanium dioxide can be inhaled. Titanium dioxide is also an occupational chemical of concern, as workers might inhale titanium dioxide when manufacturing products.

Lithopone has therefore traditionally been used in stoppers and putties, jointing compounds and sealing compounds, primers and undercoats and in road-marking paints.

Acknowledgments