Ultramarine (natural)

What it is

Ultramarine's colouring mineral is lazurite, a sodium aluminosilicate of the sodalite group whose open cage structure traps sulfur. The blue comes from the trisulfur radical anion, S3−, held inside those cages: its unpaired electron absorbs strongly in the orange-yellow, leaving a deep, slightly violet blue. The cage protects the radical from light and alkali, which is why ultramarine is so permanent — but acid opens the cage and destroys the colour, releasing hydrogen sulfide.

How it was obtained

Lapis lazuli came overwhelmingly from mines in Badakhshan, in present-day Afghanistan, and travelled west along long trade routes — the name means 'from beyond the sea'. Simply grinding the stone gives a greyish blue, because lazurite is mixed with calcite, pyrite and other minerals. Medieval painters' handbooks, notably Cennino Cennini's around 1400, describe separating the blue from the rock with a waxy, resinous dough, yielding a first, richest grade and then paler, greyer 'ashes'. The labour and distance made it one of the costliest materials a painter could buy.

How it ages

In most paintings natural ultramarine keeps its colour for centuries. Its known failure is 'ultramarine sickness', a greyish, blanched or mottled surface on some oil paintings. Proposed causes include acidity in an oil or oleo-resinous medium, sulfur dioxide and moisture in the air, and slow drying; much of the visual change is thought to be the surrounding medium degrading around the particles rather than loss of the pigment itself. Coarse natural particles also sit in thick, dark-looking layers once the oil around them yellows.

How it is identified today

XRF finds sodium, aluminium, silicon and sulfur but cannot tell natural from synthetic. Microscopy and spectroscopy do: natural ultramarine has angular, uneven particles with accompanying minerals such as calcite, diopside and pyrite, whereas synthetic ultramarine is fine, rounded and uniform. Raman spectra of natural lazurite can show extra features from associated sulfur species, and X-ray diffraction identifies the accompanying minerals.

What replaced it

A prize offered in France in 1824 led to synthetic ultramarine in the later 1820s, and 'French ultramarine' soon replaced the natural pigment. It is the same colouring structure, but its particles are smaller and more uniform, so it is a more vivid, even blue with fewer impurities — and far cheaper. Natural lapis ultramarine is still made in small quantities for restorers and painters who want its granular, slightly greyer character.

What the evidence says

• FactModerate evidence

  Ultramarine's blue comes from the S3− radical anion held in the lazurite structure; the pigment resists heat and alkali but decomposes in even dilute acid, releasing hydrogen sulfide.

  Source: Ultramarine (natural) — ColourLex pigment page; Ultramarine — Wikipedia

• FactStrong evidence

  Natural ultramarine can be separated from synthetic by its accompanying minerals and Raman features; synthetic material may instead carry residues of its industrial raw materials.

  Caveat: Based on one Roman site and reference samples; mineral assemblages vary between lapis sources.

  Source: Characterization of ultramarine blue in Roman wall paintings: case study from Volsinii (Bolsena, Italy)

• FactLimited evidence

  Proposed causes of 'ultramarine sickness' in oil paintings include acidity of the medium, atmospheric sulfur dioxide with moisture, and slow drying of the oil.

  Caveat: These are proposed causes rather than an established mechanism; the phenomenon is still studied case by case.

  Source: Ultramarine — Wikipedia

• FactModerate evidence

  Synthetic ultramarine is a more vivid blue than the natural pigment because its particles are smaller and more uniform.

  Source: Ultramarine — Wikipedia