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The story goes like this. During exile on Saint Helena, Napoleon lived in a lodge lined with a particular kind of green wallpaper. The dye used in the wallpaper contained arsenic which reacted to the humid conditions of the island and contributed to the illness which eventually killed him.
The story is false. The amount of arsenic in the wallpaper was comparatively low by contemporary standards. In 2008, researchers analysed samples of Napoleon’s hair from throughout his life and found levels had remained relatively high since childhood. It is more likely that he died of a combination of a peptic ulcer and gastric cancer.
The story was told to me by Dr Gerald Smith, not in an attempt to convince me of its substance, but to demonstrate the ways in which materials can react in certain atmospheres. Dr Smith directs the master’s programme in Heritage Materials Science at Victoria University, which teaches students to identify fundamental chemical processes in the degradation of cultural artefacts, and the ways in which materials can be stabilised and objects preserved.
Dr Smith, whose research focus is on the chemical makeup of dyes and pigments, initially got involved with heritage sciences after an invitation from a colleague at the British Museum who was preparing to mount an exhibition of taonga.
“I started out there looking at the degradation of Māori flax that had been dyed with a traditional black dye,’ he tells me, ‘whenever this dye has been used, it degrades the substrate: in this case, the flax fibres.”
The degradation, he discovered, was the result of the production of acetic acid in the mud-based dye.
Some things we know by sight: van Gogh’s reds are faded, van Dyck’s browns are smudged, Rembrandt’s whites are darkened and cracked.
Understanding the chemical processes behind these degradations can aid conservators in treatment. Van Gogh, for instance, used a red lake dye, derived from the roots of madder plant. The pigment is translucent and, when used in combination with a darker, more opaque pigment, creates a deep, rich tone. Lake pigments are also unstable under light. The longer they’re exposed to light, the higher the likelihood the molecules in the pigment will break apart, resulting in the fading of colour.
“There’s a famous pigment called van Dyck brown,” Dr Smith explains, “which is obtained by charcoal, but it also contains other substances that prevent the drying oil from drying, so those brown pigments tend to bleed.”
As for Rembrandt, on the surface of The Anatomy Lesson of Dr. Nicolaes Tulp, there appeared a number of red-orange protrusions. These protrusions are called lead soaps, and are caused by the reaction between the lead white paint, linseed-oil binder, and sulfur in the atmosphere.
The ethical considerations of conservation are fraught. The question of cleaning lead soaps, for instance, is incredibly contentious. “When you’ve got protrusions like that occurring, abrasion during cleaning could easily upset [the surface].”
Another issue that emerges is the authenticity of artworks. Our conversation returns to Rembrandt. In 1968, a group of art historians embarked on a 40-year project to assess the 420 known works attributed to Rembrandt across various public and private collections. The aim was to apply exactly the same methods and scrutiny to each work to assess whether they were the product of the artist himself. The plan was contentious: some public and private institutions were reluctant to present their works for analysis. A removal of attribution could knock tens of millions of dollars off the painting’s value.
One of the most obvious ways chemical analysis of pigments aids in the authentication of works is in its ability to help date the work. Dr Smith tells me the story of the Vinland map:
“It’s purported to be a map of a pre-Columbian map of North America, and it was bought for a considerable amount of money… but there have been analyses done on some of the pigment, and it’s claimed they’ve used a substance that is synthetic, and was only available post-1950.”
Authentication is not always so simple. Using synthetic materials is an obvious trap to fall into, he tells me. For works produced in Rembrandt’s studio, their proximity, in terms of their intimacy with the artist and the distance of 300 years, makes authentication a difficult task. “I’ve heard it said,” Smith says, “that it’s impossible to actually prove something’s authentic: the best you can do is to say sometimes that it’s not.”
The conservator operates in the realm of uncertainty. The decision to intervene means navigating not just the degradation of the material but a range of economic (Is funding for ongoing restoration available? How will restoration affect the monetary value of a work?), ethical (How to ensure alterations are reversible?) and practical considerations of restoration. Analysis of the chemical makeup of the materials can never amount to the elimination of doubt; it can, however, reduce it.