Fade to ... blue?

Back when the canoe was purple.

Fading: it sure can make things look old. Red patio umbrellas fade to pink; posters fade to monochrome; and twenty years after I bought my purple canoe, I found it was still shiny, but now it was blue! It totally changed colour. How is that possible?

Not all colours fade at the same speed. Red is the least stable, and blue seems to last longest. Why is that?

What Makes Colour & What Makes it Fade

Colour comes from pigment either naturally or by adding it. (Tulip colour is natural and paint has colour pigment added to it.) Air and sunlight break down the chemicals in that pigment colour, though, and so can the other substances the pigment is mixed with, like plastic. Sometimes a reaction creates oxygen, and that bleaches the colour too. Heat and humidity can speed this up; they tend to speed up all reactions.

Colour Wavelengths Matter

The longer the wavelength of absorbed light is (which causes the colour we see), the faster it will break down. Even the longest colour wave is stupendously small; you could fit almost 1600 wavelengths of red light into the width of a human hair.

Blue is in the 400 nm range of wavelength and red has much longer, weaker waves up in the 700 nm range. Red fades much faster than blue.

Grab your crayons or your paints and you will find that mixing blue and red makes purple. That is what my canoe builder did; she told me she mixed red and blue paint to create purple that could not be bought. Since the red part of the mixed paint degraded faster than the blue part, it resulted in a 20-year-old canoe that is blue.

The red faded from my purple canoe, leaving behind only the blue in the mixture after 20 years. The purple underneath peeks out after sanding.

Colour to Last 

If you were making something to sit outside in the sun for a long time, what colour would you make it? Look around you: Does it look like makers tried that? What colour are tarps? How about tents or sail covers?

To preserve cave paintings, we limit their exposure to light, heat, and humidity by limiting how many people can visit them. People bring heat with their bodies and humidity with their breath, as well as lights to view the paintings.

Colour to Go 

What if you wanted to get rid of colour? How could you use what you know about light and oxygen to bleach something?

How are colour fading facts being used to get this laundry bright white?