Chemical Characterization of Pigments in Art

Why does light appear to be “colorless?”  White light can be dispersed by a prism because the relative speed of different wavelengths changes in a material such as glass. When all the wavelengths arrive at our eye, we perceive no color, just white.  However, if an object is made from  molecules that  absorb blue and green light, then we will perceive that object as red.

This is how color is understood by artists. It is a material entity and not thought of in terms of wavelength at all. 

Artists' colorants come from many sources: animal, mineral, vegetable, and synthetic.

The artist lives in the material world where yellow and blue make green, because those pigments mixed together create that hue.  Scientists more often work in the red, blue, and green world of additive color.  However, additive color from a perceptual viewpoint appears to contradict what people see in the world around them.

In some organic compounds with delocalized electrons the energy difference between the bonding and non-bonding pi and n orbitals and the anti-bonding pi star orbitals is in the visible region of the electromagnetic spectrum. When some photons are absorbed from "white light," we perceive color.

Several other analytical instruments can be used to obtain information about the chemical nature of pigments in works of art—these include Fourier transform infrared spectroscopy, near-infrared imaging, and Raman spectroscopy.

Other nondestructive methods of collecting valuable research data include the instruments illustrated here. These camera-like devices capture information that a detector can analyze to determine the composition of a pigment used by an artist. In some instances, very minute samples are taken from existing cracks or the edges of a painting. Amazingly, a vast amount of information can be obtained from a tiny sample, smaller than a head of a pin.

The sample comes from a loss. A scientist mounts a sample in a plastic block. Most samples are very small—but when placed under a microscope and photographed, the sample can be seen rather easily. When scientists look at the sample using a microscope, they can identify all the layers of the painting. In this case, there is a white chalk ground, a line of drawing, and two or three layers of paint that blend together, each mixed from lead white and azurite—indicating that the artist worked from light to dark.

Scientists can examine the paint sample using scanning electron microscopy along with energy dispersive X-ray analysis to identify and characterize the inorganic colorants.