Overview

Prussian blue is a dark, intense blue pigment and inorganic compound that has been widely used since the early 1700s. It produces a cool, deep blue distinct from natural ultramarine and organic indigo. The pigment is valued for its strong tinting power, relatively low cost and generally good lightfastness compared with many early organic dyes. For a representative color sample and descriptions of its modern digital matches, see reference materials available online.

Chemistry and properties

Chemically, Prussian blue is commonly represented by the formula Fe4[Fe(CN)6]3·xH2O and is described as ferric ferrocyanide or iron(III) ferrocyanide. Its intense blue hue arises from intervalence charge transfer between iron(II) and iron(III) centers in the coordination network. The pigment is an insoluble coordination polymer and is classed as an inorganic pigment. It shows good tinting strength, moderate permanence, and several varieties or shades can be produced by altering preparation conditions; it is discussed in surveys of early synthetic pigments.

History and discovery

Prussian blue was discovered accidentally in Berlin in the early 18th century and is usually associated with Johann Jacob Diesbach and collaborators working in a color-maker's workshop. The serendipitous finding changed color manufacture and rapidly spread through Europe, offering a reliable blue that was cheaper than ultramarine. For historical overviews see introductions to the pigment's origins here and biographical notes on the early workers here. The city of origin and its role in early industrial colour production are discussed in local histories here.

Production and manufacturing

Traditional manufacture involves the controlled reaction of soluble ferrocyanide salts with iron(III) salts, producing a dark precipitate of the pigment that is filtered, washed and milled to the desired particle size. Variations in counter-ions, hydration and particle processing yield different tones and handling properties. The basic chemistry also underlies historical blueprint and cyanotype photographic processes in which iron compounds and light-sensitive iron salts produce Prussian blue images when developed; technical descriptions and procedures are summarised here.

Uses and cultural impact

Prussian blue has found many applications:

  • Art: It became a staple pigment in European and Japanese painting, printmaking and illustration. Its introduction influenced 19th‑century ukiyo‑e prints and the palettes of Western oil painters, offering a stable, economical blue for large areas and deep shadows; see an art context here.
  • Textiles and uniforms: The deep blue was used as a dye for cloth and uniforms, including the dark coats associated with the historical Prussian state; historical textile notes are available here.
  • Industrial and technical: The pigment and related iron–cyanide chemistry have roles in engineering, printing, and corrosion studies; practical and technical references are compiled here and here.
  • Medicine: Certain pharmaceutical-grade forms of Prussian blue are licensed for oral use to help remove specific heavy metals and some radioactive isotopes from the body by binding them in the gut and reducing absorption; regulatory and safety information is available here.

Turnbull's blue is closely related and historically discussed alongside Prussian blue; in many analytical contexts the two are chemically similar or identical in structure. Although the pigment contains cyanide as part of a strongly bound complex, intact Prussian blue pigment is generally regarded as of low toxicity in typical pigment use. Industrial production and handling, however, follow standard chemical safety precautions because the manufacturing steps and soluble precursors can present hazards. Further chemical and safety background is provided here and here.

Identification and conservation

In conservation and scientific study, Prussian blue is identified by its characteristic spectroscopic signatures (UV‑visible, Raman), X‑ray diffraction patterns and elemental analysis. Conservators monitor its stability in artworks; under some conditions the pigment can undergo chemical changes or interact with other materials, so cleaning and restoration require specialist assessment. Practical conservation guidance and case studies are discussed in conservation literature and technical notes here and here.