“Van Gogh loved the vibrant lead pigment colors, and the red in “Wheat Stack” turned out to contain a rare mineral lead called plumbonacrite that through light exposure was gradually coated in carbonates that were causing the discoloration. Or, in less science speak, the paint particles are now like if you had a gobstopper with the red core inside and a light blue layer and then gray layer on the outside of the particle mass.”