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BackgroundThe important trecento Florentine artist Giotto (c. 1266-1337) is renowned for his naturalistic and realistic works in tempera and fresco. His innovative paintng style involved painting expressive, emotive faces and use of pictorial devices for depicting space. This report focuses on the analysis of the materials and methods used in a panel in the collection of the National Gallery of Art, Madonna and Child (1310/1315).ResultsGiotto used inky washes under thin layers of egg tempera paint. Yellow iron earth and lead tin yellow are present in the paint used to depict the lining of the Virgin’s mantle. SEM-EDX of one of the yellow pigments confirmed it is lead tin yellow type II, PbSn1-xSixO3. The ratio of colorant to the glassy phase indicates this material was produced for use as a pigment rather than as a glass. Ultramarine was not used in this painting, azurite is the blue pigment. The azurite used here does not contain elemental impurities, however malachite and the rare green–blue mineral mixite, BiCu6(OH)6(AsO4)3(H2O)3, are found in the blue paint.ConclusionsWe do not know if Giotto intentionally used mixite as a green–blue pigment or if it was adventitious with the azurite, though given the rarity of the mineral, the latter is more probabale. Nevertheless, the artist’s choice of color was deliberate and the presence of mixite and the clear-hued lead tin yellow pigment attest to his coloristic decisions, in this case opting for cool hues rather than the warm blue of ultramarine which we typically associate with paintings of the Madonna from this time period.Graphical abstractA microscopic sample from Giotto’s Madonna and Child (c. 1310/15) shows the paint layer structure and range of pigment colors. SEM-EDX reveals the chemical diversity of the phases, and BSED proves the presence of the rare mineral mixite, BiCu6(OH)6(AsO4)3H2O

Unusual pigments found in a painting by Giotto (c. 1266-1337) reveal diversity of materials used by medieval artists