A fungus made the green in Renaissance intarsia

Fifteenth-century intarsia — pictures assembled from pieces of veneer — contains elements of a blue-green colour that no wood dye of the period can account for. In 1992 Robert Blanchette, Antoine Wilmering and Mechthild Baumeister published an analysis of such material in Holzforschung. The colour turned out to be the work of a fungus in the genus Chlorociboria, not a workshop technique.

The fungus produces xylindein, a pigment that penetrates the wood throughout and stains it evenly. The effect is durable and far more lightfast than most plant dyes of the era, which is why it has survived five hundred years in these objects. Craftsmen did not need to produce it, and could not have: the wood was gathered from the forest already stained, a raw material with its colour built in, chosen much as one chooses a grain pattern.

The fungus itself is inconspicuous. The fruiting bodies of Chlorociboria aeruginascens are cups a few millimetres across that appear rarely and only in the right humidity. For most of the time the only trace of its presence is exactly this — the colour of a decaying branch lying in the leaf litter. The mycelium is visible not as an organism but as a stain on the material it changes.

The same green returned in the nineteenth century in English Tunbridge ware, where stained wood was used in fine mosaic work. The story also continues in laboratories: xylindein is being studied as a natural pigment with unusual resistance to photodegradation. Material that did not fade in an altarpiece for five hundred years is a reasonable place to start.

What the evidence actually shows

The green was not painted onto the surface: craftspeople selected wood previously colonized by Chlorociboria. Researchers microsampled a fifteenth-century panel from the Gubbio studiolo and an intarsia by Fra Giovanni da Verona made around 1500. In the green-stained poplar they found hyphae inside vessels, fibres, and parenchyma, with pigment deposits concentrated near colonized cells. Adjacent oak and walnut pieces lacked those hyphae, while X-ray microanalysis found none of the high metal concentrations expected from a mineral colourant. The pattern matched modern wood stained by Chlorociboria. The persistent blue-green compound is called xylindein. The study establishes fungal causation in these particular works, but pigment-stained historical wood alone usually cannot distinguish confidently between the morphologically similar C. aeruginosa and C. aeruginascens.