FEATURE (Reading time 15 MIN)

The science behind a museum visit

When visiting a museum, you don’t immediately think about all the work being done behind the scenes. A world of decay, fading colors, bended canvases and even cracked paint. Tackling these problems involves a lot of science. Because even though some paintings have been around for centuries, research into the ageing process of art works is still in its infancy.

Detail of the world-famous Mona Lisa, painted by Leonardo da Vinci around 1506. The cracks are clearly visible. Photo: Leonardo da Vici, Public domain, via Wikimedia Commons

Perhaps the best-known example of damage to a painting is Leonardo da Vinci's world-famous Mona Lisa. We know this lady with cracks in her skin, an effect known as ‘craquelure.’ Even though this painting technique is now sometimes imitated on purpose, it obviously never was the artist’s intention. Unfortunately, 70% of oil paintings in museums worldwide are damaged in this way.

Climate threat

Our cultural heritage is by now facing a new threat. Climate change increases the frequency of heavy rainfalls, extreme heat and long droughts. These extreme weather conditions are a threat to our monuments, statues and sculptures outside. But indoor objects, such as paintings in museums, are exposed to these conditions as well. The indoor environment of museums needs to remain as constant as possible, and humidity has a devastating effect on these delicate objects. As a result, climate control systems that regulate the air quality have to work harder and harder to keep the conditions constant. And this in a time when sustainability features more prominently on the agenda, which means that the use of energy needs to be scaled down.

It’s therefore important to gain insight into the effects of climate change on our art objects, so that we can prevent or reduce the potential risk that our cultural heritage will suffer damage, or that it will be irretrievably lost.

Several research groups at Eindhoven University of Technology are working to preserve our heritage, often in collaboration with major museums in the Netherlands. They use everything at their disposal to combat the metal soaps, moisture, salt and changes in temperature that cause damage to our paintings. Click on the topic you would like to learn more about. Or continue scrolling down to read about all the topics successively.

Metal soaps, lumps that grow from inside

Cracks in historical oil paintings can be caused by a chemical process in the paint. The pigments in the paint contain metals such as lead and zinc, which can react to fatty acids that are degraded from the oil binder. This leads to the formation of so-called metal soaps. Professor Akke Suiker and assistant professor Emanuela Bosco of the department of the Built Environment investigate the chemo-mechanical processes that occur in oil paintings. Bosco received in 2017 a prestigious Veni grant from NWO for this type of research.

Suiker: “The metal soaps occupy more space than the paint itself, which leads to compressive stresses in the metal soap and tensile stresses around it. Metal soaps manifest as small, growing lumps in the paint. They eventually appear on the painting’s surface, causing the paint to crack in certain locations.” The exact composition of the paint – all the great masters made their own paint – determines the seriousness of this effect. “When we understand that process, we’ll hopefully be able to slow down or even prevent the formation of metal soaps in the future,” Suiker says. Suiker and Bosco, in collaboration with PhD candidate Gijs Eumelen and researchers from the University of Amsterdam, developed a model that can predict when formation of metal soaps will occur, how they will develop, and whether they will cause the paint to crack in certain places.

Red rooftops are discoloring

The paintings in the Mauritshuis in The Hague are also affected by the formation of metal soaps. The red tiles in the painting ‘View of Delft’ by Johannes Vermeer are showing more and more white spots and small cracks. To predict how the metal soaps in this painting will develop, Suiker and his colleagues also wanted to know more about the small-scale mechanical properties of the paint. Based on experiments that were carried out by the Getty Conservation Institute in Los Angeles, the researchers developed a model to calculate the mechanical properties of minuscule pieces of paint. During this process, known as nanoindentation, the small flakes of paint are cast in a resin and mechanically pressed by a tiny needle.

Bosco and Suiker now use the results from that analysis, and the models they developed, in a new Horizon 2020 project of the ERC, called CollectionCare (filmpje). In an alliance with 18 partners, including museums and universities, they want to help museums with the conservation of art objects. They use sensors and big data for this, as well as models that can determine and predict the deterioration of a museum piece.

Moisture, the silent killer

Paintings don’t just face threats from within. An artwork’s environment has a significant influence as well. Moisture in particular causes serious damage. Suiker: “When a painting releases moisture, it shrinks. This results in stresses that can lead to cracks and ruptures. The paint can peel away from the substrate because of this, it almost jumps off so to speak. This process is called spallation, as a consequence of the delamination.” Bosco and Suiker modeled this process in collaboration with professor Norman Fleck of Cambridge University, who received an honorary doctorate from TU Eindhoven in 2014. For this, they determined the material properties that a layer of paint in a historical painting needs to have in order to prevent delamination from occurring. They also investigated what humidity level museums should set to prevent delamination of historical paint layers as much as possible.

Layer of paint stops moisture

But research into humidity goes even further than that. Because humidity can – unfortunately – cause a painting to bend in its entirety, or even tear completely. Sixteenth century wood panel paintings are particularly sensitive to this. They served as religious objects in churches at the time and were painted on wood, because canvas didn’t exist in those days. The vulnerability lies in the layer of paint on the wood.

Bernardus Swaerdecroon, 1646. Damage is visible in the vertical direction, where the three wooden panels are attached to each other. (Rijksmuseum inv. no. SK-A-828).

Leo Pel of the department of Applied Physics wanted to predict up to what level moisture can vary without causing a threat to a panel painting. That is why former doctoral candidate Thomas Arends developed a mathematical model during his doctoral research at the departments of Applied Physics and Mechanical Engineering, that predicts how the panels bend exactly at certain fluctuations in the level of moisture. 

One of the things Arends used for his experiments was a 134-year-old oak door that was on display in the original Rijksmuseum. He used an MRI scanner to look at how the moisture spread throughout the wood. “That allowed us to show that the moisture spreads unevenly throughout the wood,” Arends says. “The layer of paint closes the wood off from moisture on one side, so to speak. As a result, the moisture enters on the back of the wood, causing the painting to bend.”

Thin wood bends further

Former doctoral candidate Thomas Arends was commissioned by the Rijksdienst voor Cultureel Erfgoed (Cultural Heritage Agency of the Netherlands) to investigate the influence of panel thickness on the total bending of wood, and on the timescale during which the fluctuation occurs.

For this purpose, Arends attached an oak plank, which had been coated on one side to imitate the layers of paint, in a small climate chamber with adjustable humidity. The plank was clamped on the bottom side, so that only the top side of the wood could bend. A camera then recorded the extent to which bending occurred, as the humidity level of 50 percent was raised to 90 percent in just a few seconds.

The results from the experiments were then used as the basis for a mathematical model. The model predicts the bending for each fluctuation in humidity, thickness of the wood and thickness of the layers of paint. Arends: “The model shows that the speed of the fluctuations is the most important factor in the damage that occurs. Thin panels are most vulnerable.”

Ageing, old wood breaks fast

Seventeenth century wooden cabinet doors are very sensitive to moisture as well. They are made of several wooden planks that were glued together, and often nailed or glued together with a crossbar. Wood expands in the direction perpendicular to the wood grain, and the crossbar limits that process. This gluing or nailing of the crossbar makes the tensile stresses increase even further. For her doctoral project at TU/e’s Built Environment department, former postdoc Rianne Luimes wanted to investigate how that tearing works mechanically in the cabinet doors of the Rijksmusuem collection. Her research was part of the NWO Climate4Wood project and was carried out in collaboration with the Rijksmuseum. 

Luimes discovered that older wood is less strong than young wood. “A wooden cabinet door ages over a period of several centuries and will therefore be less resistant to fluctuations in humidity,” Luimes says.

Old wood

To reach the conclusion that old wood is less strong, Rianne Luimes subjected pieces of wood from the 14th, 17th and 21st century to a so-called rupture test. During this test, Luimes and her supervisors increased the stress on the wood until it broke. Luimes then validated the results with computer simulations.

Her computer model can now predict whether there is a risk that a certain allowed humidity fluctuation might cause damage to a cabinet door, based on the way in which the planks are glued together and on how old the wood is.

Salt, stone dissolves in front of your eyes

Besides moisture and metal soaps, salt too has a devastating effect on art objects. In fact: salt presents the greatest danger to porous material. Think of statues, pots and vases in museums. But also think of mural paintings in churches and castles. Even entire buildings run the risk of suffering irreparable damage.

Vacationers in southern Europe will often come across churches or statues with a unique honeycomb structure. That’s a typical example of salt weathering. Limestone in particular is very sensitive to salt.

Researcher Leo Pel of the department of Applied Physics looks at the crystallization pressure induced by salt. He investigates the mechanical damage that occurs as a result, and tries to find solutions to this problem.


Salt seeps into a material as a solution and finds its way to the pores of the art object. Pel: “As soon as the temperature increases or the humidity level decreases, the salt crystalizes into a solid substance. When there are significant fluctuations in the humidity level, the salt will dissolve over and over and crystalize again. The salt crystals grow, and as a result the pressure on the material increases.” This can happen on the surface and will be visible there as white powder, for example on a mural painting. It can also happen within the material, in which case it will wait like a silent killer until the pressure increases to such an extent that the material breaks. This is why the latter form is the most dangerous one.

The effects of salt on art objects made of a single material have been extensively researched in recent years. Tiles and frescos however are made of several layers of different materials. How salt behaves between these layers is not known. That is why Pel will soon start a new study into layered objects, in collaboration with the Rijksmuseum and researchers at the universities from Amsterdam, Bologna (Italy) and Pau (France).

Several layers

Researchers within the new project CRYSTINART want to know why the layered tiles in the Rijksmuseum are so sensitive to salt damage. For this purpose, Pel wants to visualize the transport of water and salts using an actual medical MRI scanner in his lab. This unique data will be used for a model that can predict damage. The project hopes to develop new treatment methods to prevent further decay of objects in museums.

Climate system, airbox with stable temperature

Understanding how moisture, metal soaps and salt can damage paintings and art objects is extremely important. But a short-term solution is to make sure that these evildoers can’t even get near a painting. That is why climate systems try to keep the air around paintings in museums as constant as possible. These systems limit the temperature and humidity fluctuations in the entire museum.

Pew heating protects valuable frescos

Researcher Henk Schellen of the department of the Built Environment has been studying these kinds of systems his entire career. “The problems usually arise when a historic building needs to be heated. A church, for example, will suddenly get radiant heating that will release many liters of water, resulting in condensation on the walls and damage to frescos. The installation of floor heating on the other hand, leads to enormous air movements, and this will cause the historical mural paintings on the dome to turn black in the course of time because of the soot from the candles.”

When this happens, people turn to Schellen for the best solution. In many cases, the answer is an air treatment installation, which regulates both the air temperature and the relative humidity, instead of the existing hot-air heating. Pew heating is also a good alternative for heating the entire church.

Anne Frank’s diary saved

Back in 2003 already, the Anne Frank House asked Henk Schellen for advice on the climate in Anne’s original room. The problem was that pieces of wallpaper in the room needed to be protected against the moisture production of the many visitors. Schellen recently advised the Anne Frank House once again, together with PhD candidate Karin Kompatscher this time. They were asked to help protect the world-famous manuscripts of Anne Frank’s diary. Even though these valuable documents were preserved in a display case, before the renovation they still had to cope with temperature fluctuations between 19 degrees Celsius at night and 24 Celsius during the day. Schellen and Kompatscher had to ensure that the temperature constantly remained at 17 degrees Celsius. Because a stable temperature for the manuscripts reduces the risk of chemical deterioration of the paper.

Schellen and Kompatscher used a so-called box-in-box construction for their calculations, where the manuscripts lie safely within the inner compartment of the display box, while the surrounding temperature is strictly regulated with a climate system. Apart from a constant temperature, the solution resulted in dampening the vibrations caused by the visitators walking by on the old wooden floor. In addition, it made it possible to keep the relative humidity very stable in a passive way using silica gel.

Schellen has by now built a database with measurement data covering many years from all kinds of churches and museums in Europe. This allows him and the researchers from his group to continuously develop improved models for the buildings. “Thanks to this, our recommendations are becoming more and more accurate,” Schellen says.

In the Climate for Culture project, he investigated the influence of climate change on climate systems, which need to work harder and harder to keep the conditions constant, while energy use should actually be reduced in order to make buildings more sustainable. In addition, Schellen and former doctoral candidate Zara Huijbregts linked long-term weather predictions to simulation models for historic buildings.

“This allows me to predict the indoor climate of museums and historical buildings more accurately, and for example make recommendations for the climate system of a museum in 2050 at this point already,” Schellens says.

More information:

Hilde van Genugten - de Laat
(Science Information Officer)