Scientists have discovered why some of celebrated artist Vincent Van Gogh’s paintings have changed colour over the years. The striking discoloration is the result of a chemical reaction between the paint and the lead-based varnish used to protect it.
Originally yellow, the flowers in Flowers in a Blue Vase have turned orange-grey over time, leading researchers to chemically analyze the paint.
A new paper by Geert Van der Snickt and colleagues, published in an upcoming issue of Analytical Chemistry, reveals that a coat of varnish applied years after the painting was finished to protect the artwork against degradation was in fact causing degradation.
Van Gogh used a yellow pigment in many of his works derived from cadmium sulphide (CdS), which oxidizes when exposed to air, forming cadmium sulphate (CdSO4). The oxidized form of the pigment loses much of its colour and intensity.
Samples of the orange-grey crust, smaller than a millimetre wide, were removed from the painting for chemical analysis. Microscope images of the samples found yellow pigment below a crust of varnish, with the orange-grey colouring appearing at the boundary between the two materials.
The composition of the orange-grey crust was analyzed through X-rays, which showed that the negative sulphate ions from the paint had reacted with the lead ions in the varnish, forming anglesite (PbSO4).
The discovery of this orange-grey crust’s composition and reaction mechanism will generate new efforts to restore Van Gogh’s paintings. Removing the varnish layer without damaging the painting will be a challenge for conservators of the future.
Source: Science Daily
