Lightfastness

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Clay earth pigments such as burnt sienna often have a high lightfastness

Lightfastness is a property of a pigment or paint that describes how resistant to fading it is when exposed to light.[1] Light striking a painted surface can alter or break the chemical bonds of the pigment, causing the colors to bleach or change, in a process known as photodegradation.[2] Materials that resist this effect are said to be lightfast.

Standards[edit]

Some organizations publish standards for rating the lightfastness of pigments and materials. Testing is typically done by controlled exposure to sunlight, or to artificial light generated by a xenon arc lamp.[3] Watercolors, inks, pastels, and colored pencils are particularly susceptible to fading over time, so choosing lightfast pigments is especially important in these media.[1]

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