Van Gogh Research of Underlying Paint
What do x-rays and Infrared thermography reveal in a painting? All that exists beneath the top layer of paint.
We know that Vincent painted over several of his paintings, primarily due to a lack of funds. When the Van Gogh museum in Amsterdam x-rayed 130 of Vincent’s paintings – they determined that 15% of said works had been painted over.
Still life with two sunflowers (F 376 JH 1331) is painted over a portrait of a man wearing a straw hat. (Most likely a self-portrait).
Cottage at nightfall (‘La Chaumière’) (F 83 JH 777) Painted in Nuenen 1885 shows a shepherd with his flock underneath, as does Two baskets of potatoes (F 107 JH 933); also, a Nuenen completed work in 1885. This painting additionally reveals a woman at the spinning wheel underneath.
Glass with Yellow Rose (F 218 JH II44) is painted over a plaster study… to name just a few.
Underlying paint (and nail holes) from the painting in question
It has been determined from sheer scrutiny using magnification that the painting in question was created using several layers of paint. Of course, this is precisely how Van Gogh applied his oils.
He started with a white ground, like Calcium carbonite white, as his first layer and added a second layer of lead white, zinc white, or other pigments. Zinc white yielded a higher impasto, though it was also more susceptible to flaking.
When Vincent reused canvases, meaning he painted over existing paintings, he employed various methods of concealing the underlying artwork. Sometimes he would scrape down the paint or use dark-blocking paint before applying fresh ground. Other times he would apply a combination of pigments directly on top to obscure the original, and more often, the choice of pigment nearly always included emerald green.
Over the past decade, chemists and scientists have carried out several studies to determine the degradation and instability of the pigments cadmium yellow, chrome yellow, red lake, and other colors used by Van Gogh.
In many of his paintings, Vincent used his favorite color, yellow, and would employ three different pigments; chrome yellow, yellow ocher, and cadmium yellow; Chrome yellow contained sulfate groups, and was susceptible to discoloration by turning a brownish-green when exposed to light. We can see this occurring in the Sunflowers series (and possibly the thatched roof of the painting in question).
By analyzing a selection of Vincent’s work using x-ray spectromicroscopy and related relevant methods, we now know that Van Gogh’s Irises which appear blue, were once vibrant violet. The walls of Vincent’s “The Bedroom” were also violet, though they are now blue, and the floor now brown from pink. This is due to the red lead turning white and essentially appearing faded.
An international team of researchers led by Koen Janssens at the University of Antwerp identified Plumbonacrite (3 PbCO3∙Pb(OH)2∙PbO) as the missing chemical link responsible for the degradation of Red Lead.