Ochre and the Chemistry of Red Pigments: Hematite, reddle and the story of ochre

Summary

In this episode, Chemistry World delves into red ochre pigments and the chemistry of reddle, the dialect term for red ochre. It traces the mineral makeup of ochre, highlighting hematite (iron(III) oxide) and related iron oxides, and explains why these pigments resist sun bleaching, making them valuable for identification on sheep and for durable paints. The narrative connects Hardy's Wessex to archaeological finds, noting that ochre use dates back at least 250,000 years and appears in Neanderthal sites and Blombos Cave. The episode also covers 19th‑century industrial applications, where ochre was boiled with oak bark, tar and tallow to produce rust-preventing coatings for cast iron and even sails, inspiring Red Sails in the Sunset. A teaser for next week’s explosive topic closes the episode.

Overall, the discussion reveals how a simple natural pigment sits at the crossroads of geology, chemistry, engineering and culture.

Introduction to ochre and reddle

The episode begins with a literary vignette from Thomas Hardy's The Return of the Native, describing a reddleman whose cart and clothes are stained red by reddle, a pigment derived from red ochre. The host uses this imagery to introduce ochre pigments and the chemistry behind their color, linking a literary scene to mineralogy and pigment technology.

Ochre pigments are typically iron oxide-based, with hematite (Fe2O3) as a major component. The discussion outlines how different iron oxides, from hematite to magnetite (Fe3O4) and limonite (a hydrated iron oxide), produce a spectrum of reds, yellows and browns. The pigment’s durability under sunlight and its historical use in marking sheep herds (reddle) are explained, along with the mining practices that supplied ochre in Hardy’s Wessex and the geology of the Devon redlands where these pigments were sourced. The section emphasizes the oxidation states and binding to oxygen that give iron oxides their characteristic hues, citing Heinrich Zollinger, a Swiss chemist, as a color science authority.

"Iron oxides cover a large range of hues thanks to differences in oxidation states and the nature of binding to oxygen." - Heinrich Zollinger, Swiss chemist

Ochre in archaeology and art

The narrative then moves through archaeology and art history, noting that ochre pigments appear in some of humanity’s earliest cultural activities. Evidence for ochre use dates back at least 250,000 years, with Neanderthals in the Netherlands site near Maastricht and early cave art at Blombos Cave in South Africa. Finely ground ochre was mixed with charcoal and bone marrow in shells, used for cave drawings and possibly body painting. The discussion highlights ochre’s dual role as a pigment for art and as a durable material for body and object decoration, illustrating the deep cultural roots of this mineral pigment.

"There is evidence that the use of ochre pigments dates back at least 250,000 years." - Heinrich Zollinger, Swiss chemist

Industrial uses and preservation

The episode also covers 19th-century practices when Hardy’s contemporaries boiled mixtures of red ochre, oak bark, tar and tallow to produce a red liquid used to paint cast iron, protecting it from rust. This rust-preventive coating found application on canvas sails and fishing vessels, contributing to maritime heritage such as Red Sails in the Sunset. The discussion ties these industrial uses to the chemistry of iron oxides, showing how ochre pigments transitioned from prehistoric art materials to practical paint and anti-corrosion coatings in the industrial era.

"During the 19th century when Hardy was writing his novels, mixtures of red ochre, oak bark, tar, and tallow were boiled in cauldrons to produce a red liquid that was painted onto cast iron to stop it rusting." - Chemistry World narrator

Taken together, the episode shows how ochre sits at the intersection of geology, chemistry, archaeology and culture, revealing how a pigment shaped both ancient and modern human activities.