I am one of the new Art+Science fellows, and I working on the light-activated degradation of realgar this summer. But I will begin at the beginning.
This project started 14 months ago, in the “Science of Art Materials” course taught by Susan Roberts-Manganelli from the Cantor Center for the Arts and Curt Frank from the Stanford Department of Chemical Engineering. We had to write a number of research papers, based in part on primary literature. I decided to do one of these essays on realgar, a red molecule with the formula As4S4.
Realgar is a mineral that was used to color paints from ancient Egyptian times to 18th century British and Dutch oils. Before the 20th century and massive use of chromes and other artificial pigments, most pigments were derived from plants or animals: cochineal for red or indigo for purple.
Realgar crystal. Photo by Rob Lavinsky, iRocks.com.
I picked this pigment because it’s colored for a different reason from most other pigments. The color in pigments comes from an energy gap: electrons can sit at an energy higher than this gap, or lower, but not in between. If that gap has the energy equivalent to a particular color, then that color will be absorbed more by the material, reflecting back all the other colors in normal light.
You can build up this gap in a number of ways. Most organic dyes from plants or animals do it with alternating single and double bonds. A single set of one double and one single bond has an energy gap which is very high in energy, absorbing light far away from the visible spectrum. (For more on the different spectra of light, check out our Techniques page). However, if you string a long chain, you can start to get a pigment that absorbs purple light, reflecting yellow light and appearing yellow.
Indigo Structure. Photo by Yikrazuul, wikimedia.org.
Alizarin Chemical Structure. Photo by Calvero wikimedia.org
We can see here that alizarin has a shorter system of alternating single and double bonds than indigo, so it will absorb in the shorter wavelength, or more energetic green part of the spectrum, leaving alizarin red. Likewise, indigo has a much larger system of single and double bonds, so it can absorb in the less energetic yellow part of the spectrum, leaving indigo purple.
Realgar’s color is different from the organic pigments we studied in the Chemical Engineering class. The molecule does not have any alternating single and double bonds — instead, it is made of arsenic and sulfur atoms (four of each). Realgar builds its energy gap from the bonds between these atoms. Its gap absorbs green light, dramatically reflecting red light.
However, while reading into this pigment more closely, I discovered that it decays naturally under light to form another As4S4 molecule, pararealgar. Unfortunately for paintings, this pigment is not a deep red crystal, but a crumbly yellow powder (Bullen et al, Nassau, and Clark).
Fig 6, Burns et al.
The exact mechanics of this degradation are somewhat mysterious. In 1992 it was found that light was necessary for the degradation to occur – when realgar is left in the dark, it’s stable and doesn’t decompose into pararealgar. In later work from 2005, Kyono and coworkers published what has come to be the accepted mechanism for realgar degradation:
- Realgar + Oxygen + Light → Arsenic Oxide + As4S5
- As4S5 → Pararealgar + S
- S + realgar → As4S5
Cycle through 2 and 3 until there’s no realgar left.
What caught my attention is that no one has tested the role of oxygen in this process. What if we eliminate it? Will the reaction still proceed? If Oxygen will stop the degradation, maybe paintings can be saved by simply storing them under nitrogen when they go on display. Somebody should probably test this, and I figured the Art+Science collaboration between the Cantor and the Materials Science Department was a good a place as any to do it.
___________
H Bullen, M Dorko, J Oman, S Garrett. “Valence and core-level binding energy shifts in realgar (As4S4) and pararealgar (As4S4) arsenic sulfides.” Surface Science 531(3) 2003 319-328.
Burns, P and Percival, J “Alcercinite, As4S4: a New Occurrence, New Formula, and Determination of the Crystal Structure” Can Mineral 2001 39(3) 809-818.
Clark, R. “Reflectance spectra.” AGU Reference Shelf 3 1995 178-188.
L Douglas, C Shing, and G Wang. “The Light-Induced Alteration of Realgar to Pararealgar.” American Mineralogist 77 1992 1266-1274.
K Trentelman, L Stodulski, and M Pavolsky. “Characterization of pararealgar and other light-induced transformation products from realgar by Raman microspectroscopy.” Analytical Chemistry 68 10 1996 1755-1761.
Nassau, K. “The origins of color in minerals.” American Mineralogist 63(3-4) (1978): 219-229.
A Kyono, M Kimata, and T Hatta. “Light-induced degradation dynamics in realgar: in situ structural investigation using single-crystal X-ray diffraction study and X-ray photoelectron spectroscopy.” American Mineralogist 90 2005 1563-1570.
Cross-Sections 