Non-invasive characterization of yellow dyes in tapestries of the 18th century: Influence of composition on degradation

Aubusson (France) has been inscribed on the List of Intangible Cultural Heritage of Humanity by UNESCO since 2009 for its centuries-old tradition for the art of tapestry. The present study concerns one of these “Grand teint” tapestries, a Verdure with Count von Brühl s coat of arms (18th C.), characterized by very high-quality dyeing and fine weaving. The restoration in progress allowed the removal of the lining of the back, which protected the colour from light. Hyperspectral imagery and μ-spectrofluorimetry made on each side of the tapestry showed the influence of natural degradation of dyes. Experimental tests carried out model samples artificially aged, allowed us a better understanding of these observations. The fibers (silk and wool) and dyes (weld, …) were identified and the roles of alum and tartar as mordants were questioned. Our work revealed the action of the recipe on the photodegradation rate and the effect of aging on the spectra s shape for yellow dyes and the disappearance of some compounds, such as chlorophyll “a,” as a marker of the degradation.