Why does planks turn gray after you leave them outside for some time?
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A lot of the color in a material is determined by the chemicals it’s made from – called pigments. The sun’s rays contain a form of radiation (light) called Ultraviolet light, or UV for short. UV can strongly interact with the bonds between atoms in molecules so things left in direct sunlight often change color with time due to the exposure (because the UV light changes the chemicals in the material by breaking the atomic bonds).
In the case of wood the chemical being broken down is called “lignin”, a common chemical in the cells the wood was made from. The result is often that the “changed” lignin becomes a grayish/slate color.
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