Malachite is a natural basic copper carbonate occurring in the oxidation zones of copper deposits. Known since antiquity and widely used from ancient Egypt through the Renaissance. Prepared by grinding the mineral and washing to remove impurities; particle size affected color intensity, with coarser particles yielding deeper greens. Common in European painting until the 16th century, especially in tempera and fresco applied a secco. Sensitive to acids and may darken in contact with sulfur-containing pigments. Azurite can convert to malachite in humid conditions, explaining green discoloration in some paintings. Gradually replaced by more stable synthetic greens in the 17th-18th centuries.
In humid conditions on a wall, azurite can also slowly convert back to malachite, a transformation facilitated by their close chemical relationship: both are basic copper carbonates, differing only in the ratio of carbonate to copper. Azurite (2CuCO₃·Cu(OH)₂) contains two carbonate groups per formula unit, while malachite (CuCO₃·Cu(OH)₂) contains one. The loss of a single CO₂ molecule is sufficient to drive the conversion. This phenomenon explains the green discoloration visible in many medieval frescoes originally painted with azurite blue, most famously in the ceiling of the Upper Basilica of San Francesco in Assisi, where extensive areas of sky intended as deep blue now appear green. Conversely, malachite and azurite are so closely associated in nature that pure samples of either mineral are rare, and medieval painters working with one inevitably encountered traces of the other.
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 |
|---|---|---|---|---|---|---|
| Malachite |
Key Properties:
| Properties: | Bright to pale green depending on particle size; Opaque; Moderate tinting strength |
| Usage: | Antiquity to 18th century, limited thereafter |
| Source: | Natural/Mineral |
| Toxicity: | Moderate toxicity (copper) |
| Fresco: | Limited (a secco) |
| Tempera: | Yes |
| Oil: | Yes |
| Chemical description: | Basic copper carbonate, Cu₂(CO₃)(OH)₂ |
| Visualization: | Visible microscopy |
| Identification: | XRF (Cu), Raman, FTIR, SEM-EDS |