Natural · Prehistory to present
Ochre is an earth: clay and quartz coloured by iron oxides. In red ochre the colouring mineral is haematite, an anhydrous iron(III) oxide; in yellow ochre it is goethite, a hydrated iron oxide-hydroxide. Both absorb blue and violet light strongly through electronic transitions involving iron, and the difference between them — water bound into the crystal or not — is enough to move the colour from yellow-brown to red. Browns such as umber add manganese oxides to the same iron base.
Ochre was dug wherever iron-rich deposits came to the surface, which is almost everywhere, so most communities had a local source. Archaeological work traces people selecting particular deposits and carrying ochre over long distances, and a site in present-day Eswatini is dated as the oldest known intensive ochre mine. Later, colourmen sorted earths by shade and washed out coarse grit. Because yellow ochre turns red when it is heated, reds could also be obtained from yellows — a transformation noted from antiquity onward.
Iron oxides are among the most permanent colourants known: they do not fade in light and survive in rock art made many thousands of years ago. What changes in ochre-painted objects is usually the surroundings rather than the pigment — a binder that has decayed, a surface that has eroded, soot or mineral crusts deposited on top. Ochres are attacked by acids, so polluted or acidic environments can etch them, and a fire can redden yellow ochre on a wall by the same dehydration that turns it red deliberately.
Iron shows up immediately in X-ray fluorescence, but iron is present in many materials, so analysts confirm the mineral: Raman spectroscopy separates haematite from goethite, and reflectance spectroscopy picks up their distinct absorption edges. Microscopy shows the mixed, clay-rich texture typical of a natural earth, as opposed to the uniform particles of a manufactured iron oxide. In archaeology, trace-element geochemistry is used to match ochre to the deposit it came from.
Natural ochres are still sold, but most iron-oxide colour today is synthetic — the 'Mars' colours and industrial iron oxides used in paint, concrete and plastics. They are chemically the same family, but manufactured to a controlled particle size and purity, so they are stronger, more consistent and cleaner in hue than a dug earth, and lack the clay and quartz that give natural ochres their softer, more varied character.
FactModerate evidence
The red of red ochre comes from haematite, and yellow ochre can be turned red by heating, which converts its iron mineral.
FactStrong evidence
Lion Cavern in Eswatini, dated to about 48,000 years ago, is the oldest known evidence of intensive ochre mining, and compositional analysis shows ochre being moved between sites.
Caveat: A regional study; 'oldest known' is as of the publication and may change.
FactModerate evidence
Iron oxide pigments are stable at high temperatures and to light but are not resistant to acids.