Fugitive
A colour that leaves. Fugitive pigments fade, darken or change hue under light, and they do it on a timescale long enough that the painter never sees the result and short enough that we do.
Where the word comes from
From Latin fugere, to flee. It was a trade word before it was a scholarly one: nineteenth-century colourmen's catalogues graded their wares by permanence because customers had begun to demand it, after decades of complaints about watercolours that emptied out within a lifetime. The grading survives as the lightfastness ratings on a modern tube — ASTM I to V, or the Blue Wool scale, which measures a pigment against dyed wool strips of known fading rates.
In the studio
Painters knew, and used them anyway. The fugitive pigments were often the only ones that could do the job: the transparent crimsons were all lakes, the brilliant yellows were arsenic sulphide, the vivid greens were copper compounds that turn brown in contact with sulphur in the air. Turner was warned repeatedly about his materials and continued with them, on the reasonable grounds that a picture has to work now. Some of his watercolours were already visibly altered while he was alive.
Why it matters
It means our taste is partly built on damage. The muted, golden-brown Old Master look that generations of painters imitated is a mixture of intention, discoloured varnish and departed colour — a red-orange that has left a garment, a green that has browned, a blue that has held while everything around it faded. When a museum cleans a painting and the result looks garish to the public, the argument is usually not about whether the cleaning was correct but about which century's version of the picture we have decided is the real one.
Today
The response is environmental rather than chemical: works on paper displayed at around fifty lux, rotated off the wall after a few months, and returned to dark storage for years. It is an admission that the only reliable treatment for fading is not to look. Meanwhile the same problem has moved to inkjet prints and dye-based photographs, whose fading rates are measured in decades and whose owners, mostly, have no idea.
Pigments used: Madder and the red lakes · Orpiment · Sap green
Essays: Why old paintings look brown
See also: Vincent van Gogh · J.M.W. Turner