Question: I wrote a note (in black ink) and stuck it on my wall opposite my window. I has been a while but now the writing is distinctly maroon in colour. Why did this happen? Also, why do water colour paintings always fade to a blueish tint?

Susan Cartwright answered on 19 Jun 2015:

This happens because some dyes are unstable in sunlight. Sunlight, especially the blue and ultraviolet part, can provoke chemical reactions which cause the dye molecules to break down, which changes their colour (they may become colourless, or they may change colour). Your black ink probably contained several different dyes, and one of them – probably a greenish one – has been broken down by sunlight, causing the maroon colour.

To see what dyes your ink contains, put a blob of ink near the bottom of a strip of filter paper, and suspend it from the edge of a jamjar so that just the very bottom of the filter paper is in the water. As the water rises up the filter paper, it will carry the dyes up with it, but different dyes are different sized molecules and will travel different distances. You should see your black ink separate into stripes of different colours. (If this doesn’t work, try different liquids instead of water. Paint thinner is a good possibility.)

The fading of water colours is the same process. Artists know that some pigments are “fugitive” – they don’t stand up long to sunlight. This is also why some ancient paintings discovered by archaeologists, e.g. on the walls of Pompeii, are no longer clearly visible as they were when first uncovered.