Why isn’t flash photography permitted in museums?

Dear Straight Dope: Can you explain the rationale behind the prohibition of flash photography in many museums? Is there some dangerous “wave” or “ray” produced which could harm the painting or statue? Or . . . is this a ploy to get naive tourists to buy more post cards in the gift shop? Mary Colpi

In general, a 10-degree F increase in temperature doubles the speed of chemical reactions, so any increase in heat — even brief — speeds up deterioration. Heat is produced just beyond the red end of the visible light spectrum in the invisible, longer wavelengths known as infrared. The short, high-energy wavelengths of visible light at the other (blue) end of the spectrum, and especially the invisible ultraviolet radiation that is just beyond visible light, are very effective at breaking chemical bonds, which also produces deterioration. You can see this effect very quickly in newsprint that has been lying in the sun — it begins to turn yellow and brittle, eventually turning to dust. Flash photography produces a burst of light that contains both long and short wavelength radiation that injures the artwork. That’s why we request that photography be done using existing light.

Light hitting the paper — and there’s often very much exposed paper in a watercolor — causes breakage in the paper fibers. These fibers are made up of cellulose, in the form of long chains of cellulose molecules. High energy radiation, such as ultraviolet light, causes a long chain of cellulose to break into two parts. At the point of breakage there is produced a molecule of sulfuric acid, which in turn can react with other cellulose to cause another break, and so on, in a chain reaction. As the cellulose breakes into smaller and smaller particles, the paper becomes yellow-brown and brittle; often it smells sour (from the acids) and can be powdered into dust with your fingertips when the deterioration is advanced. Light also can cause fading in the colors. Pigments come from many different sources, and some are not completely light stable — that is, they change their chemical structure with the absorption of high energy light into chemical structures that are not colored or are of a different color.