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

SDStaff VegForLife replies:

While the markup on post cards is surely high enough to warrant some sort of clever ploy to increase sales, Mary, flash photography is prohibited for a different reason.

According to Carl Grimm, head paintings conservator for the DeYoung Museum in San Francisco, the heat and light produced by flash photography speed up the chemical reactions that cause deterioration. Mr. Grimm said:

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.

Mr. Grimm went on to talk of the “light life” of an artwork — that is, how much cumulative exposure to light it can withstand before deteriorating: “If an artwork such as a watercolor were kept in complete darkness it would last much longer, but of course that’s not practical. On the other hand, the ‘light life’ of the artwork can be used up very quickly with intense light exposure.”

I asked what type of chemical reaction occurs when an artwork deteriorates. Referring to the watercolor example above, Mr. Grimm responded:

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.

On the surface, it may seem that the amount of heat and light produced by one flashbulb would be inconsequential. But think of a popular artwork and the annual number of people who see it, many of whom would take a flash picture of it if they could. If all of the flashbulbs from all those visitors were set off at once, well, I imagine the room would get pretty hot. Consider also that some popular artworks have been on display for centuries. I don’t think we really want to do anything that would make Mona Lisa fall apart any more quickly than she apparently already is.

SDStaff VegForLife, Straight Dope Science Advisory Board

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