The complete story behind the newest technique for restoring old artwork
A few weeks ago we ran across an article (http://www.bbc.com/news/science-environment-26024351) in the BBC news that was reporting on a presentation at the annual meeting of the American Association for the Advancement of Science (AAAS). It seems that a group of engineers at Northwestern University had come up with a way of back plotting pigment color changes in old images. They then used the data from their studies to digitally reconstruct these old and faded images to their original color.
Among other works, one of the images they used in their presentation was Renoir’s famous portrait of Madame Valentine Clapisson. Painted over 130 years ago, this work incorporated some interesting materials that are a story itself.
The story behind the pigments used by Renoir in this painting really start sometime in the 1500’s when someone in one of the Conquistador parties in Central America noticed very rich and bright red and blue colors in American Native baskets and fabrics. I suppose questions were asked and some artist or weaver introduce them to a little bug, the cochineal (http://en.wikipedia.org/wiki/Cochineal). This is what happens when you crush the little bug, you get one of the carmine dyes.
A little about the carmine dyes. For those who have studied organic chemistry, you should have learned that the rich colors in these dyes are based on a chemical structure called an anthroquinone (anthraquinone). The relevant part of carmine dye molecules look like this:
This structure is also found in many organic compounds, including several cancer chemotherapy agents.
An interesting side effect of these agents is that the patient should be warned that these drugs are eliminated out of the body intact, without changing color. In other words, patients are (or should be) warned that their urine will turn bright red or blue for a few days. This is because the actual structure of the molecules exclusively reflect red or blue light. The bright color is not due to an additives or coloring agents.
Those experienced in organic chemistry should also have noted by now that the anthroquinone chemical structure is inherently unstable. It desperately wants to do something with those double bonded oxygen’s.
In some cases this tendency can be used to make something useful, like hydrogen peroxide.
Hmmmm, isn’t hydrogen peroxide colorless? Aren’t crystals of the enol and keto forms of this quinone and in fact, most chemical compounds, white or yellow? Of course, the process of chemical breakdown doesn’t happen all at once, but eventually the rich colors fade and are lost. This fact about carmine dyes was well documented by the time Renoir came around so one has a right to question what his intentions were when he chose to use these dyes in his work.
to be continued…..