article_snippet: Samples from Van Goghpaintings have been found to contain the lead-based mineral plumbonacrite that reacts with carbon dioxide in the air.
article_snippet: The researchers, whose work is published in the journal Angewandte Chemie, used a technique known as x-ray powder diffraction tomography to study the paint fleck.
article_snippet: The researchers, whose work is published in the journal Angewandte Chemie found that at the centre of the fleck, the paint was still a vivid red colour where the original red lead pigment had been protected from the air while the colour changed closer to the surface.
article_snippet: Red lead was thought by much of the art world to be a relatively stable pigment, so the idea that it has been breaking down has surprised many.
article_snippet: Experts believe that sunlight has caused the original red pigment to be converted into plumbonacrite, which in turn reacts with carbon dioxide in the air to form the white crystals.
article_snippet: Professor Janssens and his colleagues propose that impurities in the original red lead pigment, also known as minium, may have triggered the degradation in the first place.
article_snippet: Professor Janssens and his colleagues say a compound known as litharge, a natural lead oxide mineral, is often a remnant of the process of producing red lead pigment.
article_snippet: This mineral is more reactive than the other minerals in red lead and so may have triggered the production of plumbonacrite.
article_snippet: There are some indication in the literature that the residual amount of PbO (litharge) is also due to the starting material.
article_snippet: In our case, the presence of this carbonate-poor lead compound in between the red lead and the carbonate-rich lead white layer strongly suggests that plumbonacrite is present as an intermediate degradation product formed during the whitening of minium.'
highlights_snippet: Impurities in the paint may have triggered a chemical reaction in sunlight .
  
highlights_snippet: Scientists found a rare lead-based mineral has been forming in the paint .
  
highlights_snippet: This then reacts with carbon dioxide from the air to create white crystals .
  