The Use of Science in Art Conservation

By Louisa Ren

Vincent van Gogh was known for his vibrant paintings. Photo by Fan Yang on Unsplash.

Even in the earliest days of human existence, humans have always created art to express creativity. It is considered by some to be a hallmark of what makes humans human and often tells us about how past cultures developed, hence why art conservators want to preserve art for future generations [1].

Art comes in many forms, such as paintings, sculptures, architecture, literature, music, crafting, and more. What counts as art can be subjective, and what kind of art should be preserved can be even more so, but that is not the main topic of discussion for this article, nor does this article aim to discuss art history and ethics in great depth. There is a lot of science involved in art conservation too, and this article is a simple attempt to discuss a few of the many concepts involved in conserving paintings.

Although the terms ‘art conservation’ and ‘art restoration’ are sometimes used interchangeably, they are actually not the same thing. Art conservation is more about examination and preventative care from further damage, whereas art restoration is more about trying to restore an artwork from damages to mimic its original state. Many art preservation projects will involve a combination of both [2]. This article will begin with some colour and paint chemistry, followed by a few lab techniques used by art conservation scientists to assess damage and paint losses, and will end off briefly discussing what is done for the artwork afterwards.

Photo by Tabitha Turner on Unsplash

But First, What Even is Paint?

Generally, the key components of paint are a binder and pigment(s), and sometimes a solvent and additives [3]. Binders in paint are a liquid component that holds the pigments together and allows for the paint to be spread. It also forms a film on the surface when it dries. In the past, eggs, glue, and vegetable oils were used as common paint binders. Nowadays, paint binders tend to be natural or synthetic resins like vinyls, but this can depend on what kind of paint it is.

Pigments are inorganic or organic compounds (often made into powder form that gets mixed with binders and solvents) that can be used to colour other things when combined with a binder [4]. A pigment gets its colour from electrons in d-orbitals of the material’s atoms transitioning between energy states. Transition metals like cobalt or cadmium tend to be used for inorganic pigments because their ions have partially filled electron d-orbitals, making them available for repulsive interactions with ligands. The repulsion between the electrons in the d-orbitals of the transition metal ion and the electrons in the ligand causes the d-orbital electrons to be excited into a higher energy state. However, the electrons will be split into high energy and low energy groups because not all electrons will be raised by the same energy amount. The energy difference between the high and low energy groups during electron excitation is what causes absorption of a specific wavelength of light from the visible spectrum, and the colour seen in the pigment is complementary to the colour wavelength that was absorbed [5]. A similar process happens with organic pigments but usually with conjugated double bonds instead [6]. Some colours need more energy to be absorbed than others [5].

Making paints is probably one of the oldest forms of applied chemistry [7]. Natural clay earth pigments known as ochre, which were made from mixtures of clay, sand, and ferric oxide (also known as iron(III) oxide, occurring naturally as the mineral hematite), are the oldest pigments found so far that were used by humans. An ochre pigment processing workshop dating back around 100,000 years was found in Blombos Cave in South Africa [8]. The earliest known drawing was also found there in 2011, etched on a rock as criss-crossed line patterns with red ochre and dates back 73,000 years [9].

Over many millennia of recorded history later, civilisations across the world developed more ways to make pigments and dyes. Some of these used rare materials, like the bright lapis lazuli blue that could only be extracted from Afghanistan mines and was worth more than gold in medieval times, or the Tyrian purple extracted from the mucus of sea snails (Bolinus brandaris, or Murex brandaris) that only members of the Byzantine imperial court were allowed to use [10]. Some pigments were even later found to be harmful, such as the brilliant emerald shades of Scheele’s green and later Paris green that were popular in the 19th century, both of which contained arsenic (copper arsenic and copper(II) acetate triarsenite, respectively) [11]. In more modern times, where we have a better understanding of paint chemistry, these pigments can be more easily replicated through industrial processes with synthetic polymers, and many are generally less harmful than they used to be [7].

How do Pigments in Artwork Degrade Over Time?

Pigments degrade and fade over time due to environmental factors such as light or moisture, and past conservation or restoration efforts can contribute to further deterioration too [12]. Organic pigments in particular are more prone to the chemical bonds breaking down from light (especially when it is ultraviolet light), causing gradual discolouration. A notable example of this would be a lake-based pigment known as eosin Y that was popular with 19th century artists like Vincent van Gogh. The strong light absorption from the many double bonds alternating with single bonds (conjugated π system) in the chemical structure gives the pigment its vibrant red colour, which also quickly fades after prolonged light exposure [13]. Many of van Gogh’s canvas paintings were displayed in houses with insufficient light control, so the faded colours posed a challenge for art conservators to identify the eosin pigments on the paintings that used them. Fortunately, these fading pigments leave behind traces of the bromine atoms from the eosin Y chemical structure on the canvas, which can be identified using spectroscopy [14].

Humidity can also gradually affect the state of paint on artworks. As humidity increases, moisture from the atmosphere can accumulate on the painting’s surface and oxidise pigments in the paint, contributing to its discolouration. For example, some of the yellow cadmium paint used in Edvard Munch’s painting The Scream (ca. 1910) have turned off-white and begun flaking. Studies showed the discolouration is likely due to moisture interacting with chloride compounds in the paint, causing the cadmium sulfide (CdS) in the yellow oil-based paint to gradually oxidise into white cadmium sulfate (CdSO4) [15]. Excessive humidity can also encourage mould or mildew growth on the canvas. Low humidity isn’t good either because it can cause the water content binding the artwork together on a molecular level to dry up, leaving the (often fragile) artwork to become brittle and break apart. Some materials will also expand or contract when humidity changes, which can cause them to be weakened [16]. For these reasons, artworks need to be kept in environments with controlled humidity, but this isn’t always easy to control when the artwork is displayed in a museum for thousands of people to see everyday.

The oldest human-made drawing found so far, etched with red ochre onto silcrete stone.

Where Art and Science Meet

Conserving or restoring a painting is multidisciplinary and not a ‘one size fits all’ process, but knowing the chemical composition of the paints used by the original artist and the history of the painting (e.g., how it was created, whether there were any past restoration attempts) is usually a good start. Art conservation wasn’t always done with a scientific approach; some conservation and restoration attempts in the past have resulted in more damage to the painting in the long term, whether due to mishandling or using chemicals that turned out to be too harsh for the paint. The risks of adding more damage to the artwork is partly why art conservation projects are sometimes controversial, especially on famous irreplaceable artworks [17]. With a more scientific approach used nowadays in the field — where conservators prefer more precise minimal intervention methods, the conservation process often starts with assessing the painting using microscopic chemical analysis methods such as Raman spectroscopy, FT-IR (Fourier Transform Infrared Microspectroscopy), X-rays, or microfadeometry, just to name a few common ones

Raman spectroscopy is widely used in science, and in this case is particularly useful for identifying individual pigments and their products from degradation. It is a non-destructive chemical analysis technique that analyses interactions between light (often as a laser), and the chemical structure bonds to find out information about the chemical structure and other properties [18]. Knowing this information about an artwork can also provide insight on its original state and help prove its authenticity.

FT-IR is another microscopic chemical analysis technique useful for identifying inorganic and organic pigments, as well as any varnishes or other protective chemicals the artist may have used. It works by analysing the infrared spectrums of samples (i.e. how much light is absorbed by the sample) that could be as small as 1 nanogram. This is useful because conservation scientists are able to take as little as possible from the artwork. FT-IR is a quicker but more complicated form of UV-vis spectroscopy as it uses more than one light frequency on the sample at the same time, allowing for more accuracy [19].

X-radiography is also often used to reveal details about the artwork that the human eye can’t easily see (such as holes or tears in the canvas), and can show hidden layers of underdrawing or underpainting, as well as previous changes to the painting [20]. This information is useful when identifying forgeries or determining a timeline for the artwork’s creation. X-rays use the amount of electromagnetic radiation absorbed by the artwork to produce the radiograph, but some pigments show up better than others. Paints made with heavier elements, like lead white paint, tend to absorb a lot of radiation, so they are less likely to show up on the radiograph compared to carbon black paints, which don’t absorb much radiation. This is due to carbon black paint being made of atoms with less electrons and protons in the nucleus. The amount of radiation used in x-rays for art conservation is significantly less than for medical x-rays, so damage to the artwork is not generally a concern for conservators [21].

Microfadeometry is a relatively newer non-contact and mostly non-destructive technique that is useful for finding out how light exposure affects the colour of pigments. It uses a microfading tester instrument that focuses a tiny amount of UV-filtered light from its powerful xenon arc lamp onto an area of between 0.3 mm to 0.4 mm on the artwork. This can mimic years of light exposure on the pigments, providing useful information to art conservators about optimal display conditions for the artwork [22].

While non-invasive methods are preferred, sometimes they will analyse tiny microscopic samples of material from the artwork as well, as long as the samples are carefully documented and from areas of pre-existing damage on the artwork. Chromatography techniques are often used to identify varnishes and binders in paint mixtures by separating the components in samples [23]. Using these techniques, conservation scientists can find out more about the properties of the materials used, which can give insight on how the artist created it. They can also determine the deterioration factors or risks that the artwork may be susceptible to and advise art conservators on how to proceed.

People working on art preservation projects have to be careful when handling artworks so they don’t accidentally cause more damage to them. Some attempts to clean valuable artworks have even been controversial, despite the intention to restore it to how the artist had intended [17]. A painting will naturally deteriorate and accumulate impurities over time, especially if it has not been well cared for. Completed paintings will usually have a layer of transparent varnish over the paint that can help protect it from dust and dirt, but older varnishes made from natural materials are more susceptible to deteriorating. As the varnish ages, it can lose transparency and affect the colour of the paints underneath as well. Based on the information gathered from examining the artwork, the old varnish might be removed and replaced by a newer and more durable varnish. The new varnish is chosen based on what is known about the painting’s chemical composition and the environment where it will be displayed [24]. Outright repainting over the artwork is considered unethical, but damaged areas on the canvas or sections that suffered paint loss could be carefully restored by a well trained art conservator in a process known as inpainting. However, this process must be documented to comply with art ethics guidelines that require all changes made in the art restoration process to be easily identifiable and reversible [25].

When the field of art conservation combines art with science, we can see the results on display in museums and galleries even centuries after the artist has passed. Despite the relatively imprecise beginnings of the field, art conservation still grows more multidisciplinary as newer technologies have been developing in recent years, including methods like removing accumulated impurities on artwork with lasers, repairing and reforming damaged areas with nanoparticles, or using bacterial enzymes to clean dirt [17]. Painting as an artform has come a long way since the earliest ochre cave paintings, and hopefully, future generations will still be able to see artworks from centuries ago with their own eyes too.

It can be difficult to control conditions for preserving paintings, while also allowing the public to enjoy art. Photo by Andrew Neel on Unsplash.

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