Lapis lazuli is a complex rock composed mainly of lazurite, a sulfur-containing sodium-aluminium silicate, with common inclusions of calcite (white impurities) and pyrite (gold like impurities). The principal historical source was the mines of Badakhshan in present-day Afghanistan, though deposits also existed in Chile, Siberia, and Myanmar. Simple grinding of the raw stone produces only a dull, grayish blue, because the white calcite and other impurities dilute the color of the lazurite.
By the 13th century a refined extraction method was developed, described in detail by Cennini as the pastello process. The finely ground mineral was mixed into a dough of melted wax, pine resin, and linseed oil, then kneaded repeatedly under warm lye. The pure lazurite particles, being heavier and less bound by the wax matrix, washed out first, yielding the deepest and most brilliant ultramarine. Successive washes produced progressively weaker grades, with the final extractions yielding ultramarine ash (cenere di lapis lazzuli), a pale grayish blue of little value. The best grades rivaled or exceeded the cost of gold, and contracts for important commissions often specified the quality and quantity of ultramarine to be used.
Due to its extraordinary cost and labor-intensive preparation, natural ultramarine was reserved for the most important passages in a painting, above all for the mantle of the Virgin Mary. It was frequently applied over an underpaint of the far cheaper azurite, which provided body and depth while minimizing the amount of precious ultramarine needed for the final surface layers. In manuscript illumination it was the most prized blue, used for the finest initials and miniatures.
Unlike azurite, natural ultramarine is chemically stable and does not discolor over time, which is why the blue passages in paintings executed with ultramarine have retained their brilliance while those painted with azurite have often darkened or turned green. Synthetic ultramarine, chemically identical to natural lazurite, was first produced in France in 1828 by Jean-Baptiste Guimet and rapidly replaced the natural pigment at a fraction of its cost.
Usage Timeline:
| Pigment | Prehistory <3000 BCE | Antiquity 3000 BCE–500 CE | Medieval 500–1400 CE | Early Modern 1400–1800 CE | Industrial 1800–1950 CE | Present 1950–today |
|---|---|---|---|---|---|---|
| Lapis Lazuli |
Key Properties:
| Properties: | Brilliant deep blue with high chroma; Semi-transparent; Stable and permanent; Color intensity depends on grade of extraction |
| Usage: | Antiquity to present (natural); synthetic form widely used since early 19th century |
| Source: | Natural/Mineral |
| Toxicity: | Non-toxic |
| Fresco: | Yes |
| Tempera: | Yes |
| Oil: | Yes |
| Chemical description: | Primarily lazurite, Na₆–₈Al₆Si₆O₂₄S₂–₄ |
| Visualization: | Visible microscopy |
| Identification: | Raman, FTIR, SEM-EDS |