Natural · Pre-Columbian period to present
The colourant is carminic acid, an anthraquinone glycoside made by the female scale insect Dactylopius coccus, which accumulates it in quantity. On its own it is water-soluble and changes colour with acidity, from orange through red to violet. Bound to aluminium (with calcium) it forms carmine, an insoluble lake; on textiles, mordants such as alum or tin fix it to the fibre and set the shade, tin giving the brilliant scarlets of early modern Europe.
Cochineal insects live on prickly-pear cacti, and in Mexico and the Andes they were cultivated and harvested for dye long before the Spanish conquest. After 1521 dried cochineal became one of the most valuable exports of New Spain, displacing the Old World kermes and Armenian and Polish cochineals. Painters' carmine and crimson lakes were prepared by colourmen by precipitating the dye onto a mineral base; today the insects are still farmed, mainly in Peru and the Canary Islands.
The dye behaves very differently in cloth and in paint. On well-mordanted wool it is reasonably fast, but as a lake in thin paint it is fugitive: carmine glazes and watercolours fade in light, sometimes leaving flesh tones and skies cooler or paler than the artist intended. Reynolds himself called carmine treacherous yet kept using it, and SERS has identified carmine in faded flesh tones in portraits by Reynolds and Feke. In watercolours, including Turner's, faded cochineal is a recurring problem.
Carmine contains no heavy element that XRF can recognise — only the aluminium of its substrate — so identification is molecular. On textiles, extraction followed by HPLC measures carminic acid and its minor companions, which also distinguish American cochineal from kermes and Old World species. On paintings, where samples are tiny, SERS detects the dye directly, even in glazes that have faded almost to nothing.
In paint, alizarin crimson from the 1870s, then quinacridones and other synthetic reds replaced carmine for lasting crimson glazes, being much more lightfast. Cochineal itself remains in production as a colourant for food, drinks and cosmetics (listed as E120 in the EU), where it competes with synthetic reds and other natural colours.
FactStrong evidence
Cochineal pigments in paintings tend to fade when exposed to light, especially in watercolour, as in some of J.M.W. Turner's landscapes.
FactStrong evidence
SERS has identified carmine lake in faded flesh tones of portraits by Robert Feke and Joshua Reynolds, who called carmine 'treacherous' because of its fading yet kept using it.
Caveat: A handful of paintings; it shows the pattern exists, not how often it occurs.
FactStrong evidence
Cochineal was used by the Aztecs for dyeing and painting and reached Europe after the Spanish conquest, becoming one of the most valuable imports after gold and silver.
Source: Pigments through the Ages — Carmine lake; The Use of Insect Pigment in Art Works