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2019 Issue 3

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How to look at a Rembrandt like a conservator

How to look at a Rembrandt like a conservator

Heidi Sobol explores the techniques – and the chemistry – behind the masterpieces.

[photo of Heidi Sobol at the Royal Ontario Museum]
Tina Weltz

Heidi Sobol at the Royal Ontario Museum's exhibition "In the Age of Rembrandt: Dutch Paintings from the Museum of Fine Arts, Boston."

Behind her, Rembrandt van Rijn's Portrait of Aeltje Uylenburgh, 1632. Oil on panel. 73.7 x 55.8 cm. Promised gift of Rose-Marie and Eijk van Otterloo, in support of the Center for Netherlandish Art.

"When I look at a Rembrandt,” says Heidi Sobol, MAC’00, Senior Paintings Conservator at the Royal Ontario Museum, “I tend to go straight for the nose.” As part of the team responsible for the ROM’s current exhibition, “In the Age of Rembrandt: Dutch Paintings from the Museum of Fine Arts, Boston,” she examined many 17th-century Dutch masterworks recently. “It was amazing to work with them. I saw the paintings during the condition-reporting stage, and when they were being hung.”

When she’s face to face with, say, Rembrandt’s Portrait of Aeltje Uylenburgh, why look first at the nose? “One reason is you can see Rembrandt’s most notable ways of painting there – scumbling, paint on paint that creates an impasto of pure colour,” she says. “Rembrandt’s noses are a sculpture unto themselves. What he did with the paint application, especially on the faces, was a great leap forward.” As well, she’s fascinated by a technical quality: durability. “The wonderful thing about Rembrandt’s flesh tones is the significant amounts of lead white, a strong and stable colour. Lead paint tends to be robust, so the noses withstand the ravages of time a lot better than with other paints,” Ms. Sobol explains. That the work is well-made is important for a professional conservator: strong materials increase the chance of a successful treatment, removing a varnish perhaps, or cleaning dirt from a painting.

The way that Heidi Sobol looks at a Rembrandt – for both its aesthetic and technical aspects – hints at the conservator’s twin areas of expertise. A graduate of the Queen’s Master of Art Conservation program, she is trained in art history and organic chemistry. For a conservator, these disciplines come together to confront the tricky challenges of cleaning, conserving, and restoring paintings. Chemistry has always been essential to both the making and the conservation of art, she says.

Multiple reactions are going on all the time: pollution, for example, can cause a crust to form on a painting as it reacts with surface layers, and there are continuous reactions within the painting too. Knowing how substances interact is vital for conservation. “As a scientist, chemistry is the backbone of my work,” Ms. Sobol says. She has to know the chemical composition of paints to restore damaged areas and prevent fragile materials from degrading. Works of art are not only valuable but also irreplaceable, so a measured scientific approach is required for removing or adding anything, with the goal of preserving paintings for future generations.

She also draws on chemistry knowledge for mixing substances, such as the tailored waters she often uses. “I build cleaning solutions that are specific to the kind of dirt and the kind of painting.” She needs to know how to use a variety of unusual adhesives, from fish bladders to funori (a seaweed-based material), and how they will react with other materials. As well as cleaning works, a conservator may also provide more structure to a painting so it will last, or work on authenticating a painting and placing it within a specific artist’s oeuvre. She draws on both art and science to create the complex materials and to solve the complex problems she encounters.

A beautiful exercise of the mind

Heidi Sobol’s chemist is always in dialogue with her artist. “Conservation is a beautiful exercise of the mind, because you are constantly going between the brain’s left hemisphere – the rational, analytical, objective, chemistry side – and the right hemisphere, the creative side, which asks different questions: ‘What’s the aesthetic?’ ‘How will this be interpreted?’ ‘Have I understood what the artist was trying to do?’”She needs to have expertise in the painter’s techniques, palettes, and recipes (which combine pigments with specific binders, such as linseed oil). This ensures any treatments are in harmony with the artist’s style and intentions.

Ms. Sobol's painstaking work involves many hours in front of an easel, uncovering what’s beneath, layer by methodical layer. Whether it’s cleaning decades of dirt to reveal the original colour or reconstructing damaged or lost parts of a painting, treatments today tend to be reversible, she says. “We work in small areas, slowly and cautiously, so in the event that something goes wrong, we can arrest it quickly.” Art conservation has become “less intervening over the years – all of us have come across restoration treatments that were too permanent. We’ve all sat up nights cursing the restorer, even though they had their reasons, wondering, ‘What adhesive did they use? It's so intractable and not responding to typical removal strategies!’”

Conservation is also specialized work, so instead of developing new technologies, innovations from other industries are often “poached” and turned to a conservator’s purposes. “Laser cleaning is something we’ve adapted for cleaning specific kinds of artwork,” Ms. Sobol says. The lasers help destroy dirt, remove old varnishes, or even correct previous restorations. A century ago, an intentional technique on a Rembrandt was mistaken for damage and covered up, but luckily the cover-up was removable.

Unlocking the secrets of Rembrandt's palette

Non-destructive testing can give a conservator important information about a work of art. “We take a sample of the paint layer no bigger than a period at the end of a sentence,” Ms. Sobol explains. With Rembrandt’s work, microsampling has solved some mysteries. “We’re using chemistry to reverse-engineer, to go back in time to understand how Rembrandt constructed his paintings. That’s giving us his secrets, because his paintings were so uniquely constructed compared to what his contemporaries were doing.

Testing may include taking a paint sample no bigger than the period at the end of this sentence.

"We’ve determined that what we think are very complex paintings actually use a limited palette of 12 to 16 colours, We’re finding out the ways in which he uses his few colours, the manipulation of the media, to create these fantastical works.”

Asked what we can learn from Rembrandt’s palette, Ms. Sobol says she’s most interested in the skilful, “counter-intuitive” way he got his effects. “His light colours are these wonderful complex opaque creations – the scumble, the layering – while the dark parts, the shadows, are often very thin, consist of few colours, and tend to be translucent and layered repeatedly. Rembrandt took what most artists would do and turned it on its head.”

[photo of Rembrandt's Head of an Old Man with Curly Hair]
In his portraits, Rembrandt often used scumbling, a technique in which light layers of paint are dry-brushed over darker layers, creating highlights on a face, for instance, while allowing the darker tones to shine through.

 

In her work of connecting with the past through what remains, does it ever feel like the forces of deterioration are winning? “Sometimes,” she admits. “But you just have to live with it. All paintings age.” Ironically, just as science has provided a fairly good understanding of Rembrandt’s methods, other, less welcome discoveries have been made. We are finding lead soaps, which, in a complex chain reaction, are causing craters, or pustules, in the paint over a long period of time. And there’s little we can do about it,” she laments. Plumbonacrite, a lead compound recently discovered in some of Rembrandt’s work, is also problematic. “Much like the lead soaps, it results in deterioration over a long period of time, caused by the irradiation of light and absorption of CO2 from the air, resulting in a blanching appearance of the paint.”

"Lead soaps" are actually carboxylate salts, which are formed when heavy metal-containing pigment, like lead white, reacts with the fatty acids in a binding medium or varnish.

Despite the challenges of a conservator’s quest for permanence, on the 350th anniversary of Rembrandt’s death, we’re still enjoying his paintings. “It’s like when you go to an antique car show, and you wonder, ‘How can this thing be running so well after so much time, when a five-year-old car is a complete clunker?’ You have to ask yourself, ‘Well, how is it made?’ And it has to do with good technology.” Rembrandt she describes as “a great technician of art, who constructed well.” Mainly because of his limited palette, she explains, he knew the capabilities of the few pigments he was using. “And he was smart enough to measure one against the other: if he was using one that wasn’t as great, he would temper or adjust it with a stronger, more stable pigment to give it a helping hand and maintain itself better. He really knew how to handle paint.”

Heidi Sobol’s days of looking at a painting as most people do, simply enjoying the expression or the brushstrokes, are behind her: she has spent too many years sitting close to paintings, considering the saturation levels of the varnish, or the mystery of what is underneath. When asked about her time teaching a course in art conservation at Queen’s in 2016, she recalls that Rembrandt’s Portrait of a Man with Arms Akimbo had just arrived at the Agnes. “I think that painting is spectacular,” she says with feeling. Then she laughs. “And yes, I did look right at his nose!”

[photo of Heidi Sobol with two Rembrandt paintings at the Royal Ontario Museum]
Heidi Sobol at the Royal Ontario Musem.Photo: Tina Weltz

Paintings seen in this story are from the exhibition "In the Age of Rembrandt: Dutch Paintings from the Museum of Fine Arts, Boston"

Rembrandt van Rijn,
Portrait of Aeltje Uylenburgh, 1632.
Oil on panel. 73.7 x 55.8 cm.
Promised gift of Rose-Marie and Eijk van Otterloo,
in support of the Center for Netherlandish Art.

Rembrandt van Rijn,
Reverend Johannes Elison, 1634.
Oil on canvas. 174.0 x 124.5 cm.
William K. Richardson Fund. 56.510.

Rembrandt van Rijn,
Maria Bockenolle (Wife of Johannes Elison), 1634.
Oil on canvas. 174.9 x 124.1 cm.
William K. Richardson Fund. 56.511.

[cover image of the Queen's Alumni Review issue 3, 2019, showing art conservator Heidi Sobol with a painting by Rembrandt]