Science Questions

Why do pictures fade in paintings and books? Is it a chemical reaction? Steve in Flixton

The colours in pictures are made out of chemicals. They tend to be chemicals that interact with light quite well because they have a colour, meaning that they absorb some colours and reflect others. If they are exposed to too much light, especially ultraviolet light from the sun, they break, become damaged and stop being coloured. It depends what dyes you're using. Inorganic dyes that include metals tend to survive a lot better in sunlight than organic ones, but basically the chemicals get damaged and bleached. If the molecule is broken by absorbing lots of UV light, then its absorbency changes. The same thing happens with human hair in summer. You can make the process happen a little bit quicker if you put some lemon juice on. On a similar point, the reason why bleaches work is that dye molecules are sensitive and quite easily damaged. A bleach is something which oxidises something, and goes round and damages things at random. The things that get damaged first tend to be the colour molecules and so things tend to go white.

August 2006