Ochre
The oldest pigment in human use, by an enormous margin, and still one of the most used. Ochre is iron oxide mixed with clay, it is lying on the ground on every continent, and people were collecting it before our species existed in its current form.
Origin
Ochre is weathered iron ore: goethite, the hydrated form, gives yellow; haematite, the anhydrous form, gives red. Crucially, one converts to the other with heat. Roast yellow ochre in a fire and it turns red, permanently — a chemical transformation that any hearth can perform, and almost certainly the first deliberate chemistry our species carried out. Deposits are everywhere, but some were famous and traded over long distances: the Roussillon ochres of Provence, the ochres of Cyprus and Sinope, and the red ochre of the Kimberley, moved along Aboriginal trade routes across Australia for thousands of years.
In art history
Ochre use is documented at Blombos Cave in South Africa around 100,000 years ago as a processed, stored, workshop-made material, and pieces engraved with geometric patterns from the same site are among the earliest evidence of symbolic behaviour anywhere. It is the colour of Lascaux, Altamira and Chauvet, of Egyptian tomb painting, of Roman Pompeii, of Aboriginal Australian painting continuously to the present day, and of every European painting ever made. In this atlas yellow ochre appears in twenty separate eras — tied with lead white for the most transversal material in the entire database — and red ochre in eleven more. There is no period of art history in which someone was not using it. Rembrandt built entire pictures from ochre, umber and lead white with almost nothing else; the restricted "earth palette" attributed to Apelles and revived repeatedly since is essentially ochre plus black plus white.
In the studio
Ochres are the most reliable pigments that exist. They are completely lightfast, chemically inert, compatible with every binder and every other pigment, non-toxic, and usable in fresco, oil, tempera, watercolour and acrylic without modification. They are opaque, dry at a moderate rate in oil, and have a body and tooth that make them pleasant to handle. Their limitation is chromatic: they are dull. An ochre cannot produce a saturated yellow, and the entire history of yellow pigment is a search for the brilliance that ochre lacks — a search that produced orpiment, lead-tin yellow, Naples yellow, chrome yellow and cadmium yellow in turn, each of them more poisonous or less permanent than the earth it was meant to replace.
Afterlife
Natural ochres are still mined and sold, and synthetic iron oxides — chemically identical, more uniform, marketed as Mars yellow and Mars red — have taken over industrial use since the nineteenth century. The synthetic versions are stronger and cleaner; the natural ones retain the clay and silica impurities that give each deposit its particular character, which is why painters and conservators still buy ochre by place-name. Roussillon ochre is not the same colour as Italian.
Where it appears
Every era and artist in this atlas whose palette includes this pigment.