EEHOP – Electromagnetic Exploration of Historical Oil Paintings
Oil paintings are among the most iconic treasures of our cultural heritage, but time inevitably leaves its marks. Cracks, changes in colour, and surface deformations are visible signs of ageing, while complex chemical reactions continue to take place beneath the surface. Conservators and scientists have long sought to understand these processes in order to develop better strategies for preservation and restoration.
The EEHOP project (Electromagnetic Exploration of Historical Oil Paintings), a collaboration between the Royal Institute for Cultural Heritage (KIK-IRPA) and UCLouvain, funded by the F.R.S.-FNRS, explores an innovative approach: using electromagnetic characterisation techniques to study the hidden transformations in oil paints.
This project is funded by the Fund for Scientific Research (F.R.S.-FNRS) of the Wallonia-Brussels Federation
Why oil paintings deteriorate
Traditional oil paintings are composed of pigments bound together with drying oils, such as linseed oil. Over time, the pigments and the oils interact in complex ways. One key phenomenon is the formation of metal carboxylates, compounds that result from the reaction between metal ions in pigments and fatty acids in the oil binder. These metal carboxylates can play a dual role: while suspected of stabilizing the paint layer, when they cluster and crystalize within or above the paint film, they can cause visible damages such as the appearance of efflorescences or protrusions, the delamination and even the change in colour and transparency of the paint layers.
The mechanism that dictates how and why these compounds form and accumulate is still to be understood, but previous studies indicate that the way ions from the pigments move around in the oil medium and eventually land in specific areas might be linked to the way water – coming into the paint layers from different sources, such as conservation treatments or humidity in the environment – moves within the paint film and interacts with its components.
Understanding how and why such compounds form, and how water affects their development, remains one of the great challenges in conservation science.
LEFT: Central Panel of the Christ on the Cross with Donors retable by Quentin Matsys (oil on wood panel, 1520, Museum Mayer van den Bergh).
The yellow rectangle shows the area of the panel detailed in the figure’s central image.
CENTER: Detail view of the background affected by an irreversible alteration in which the green landscape has turned brown.
RIGHT: Micro cross-section extracted from the altered area observed using an optical microscope (OM) under polarized (POL) and ultraviolet (UV) light.
The yellow arrows indicate the altered pictorial layer in which the original white pigment has partially dissolved due to the formation of metal carboxylates, in this case a mixture of lead soaps and calcium oxalates, giving the pictorial layer a translucid, brownish appearance.
A new approach: exploring changes in the electromagnetic properties of paint films using microwaves
Launched through a collaboration between KIK-IRPA’s Material Science for Conservation Research (MatCoRe) unit and Prof. Isabelle Huynen from UCLouvain’s ICTEAM institute, and funded by FNRS, the EEHOP project explores whether the electromagnetic properties of oils can reveal new clues about how paintings age.
The project focuses on fundamental electromagnetic properties such as electric permittivity – an intrinsic characteristic of every material that dictates how it stores and dissipates electrical energy. By monitoring how permittivity changes as oils dry and age, our researchers expect to track the hidden chemical transformations inside the paint layers and better understand the interactions between the different compounds in the oil paint films and water. This will also refine our understanding of the processes that underly the degradation phenomena linked to the appearance of metal carboxylates.
To investigate this, EEHOP implements advanced techniques from the field of electrical engineering, including microwave spectroscopy using coplanar waveguide (CPW) sensors. The project aims to adapt this non-invasive analytical method to detect subtle shifts in the dielectric behaviour of paint films, thus offering a clearer picture of the chemical processes happening within historical oil paintings as they age.
LEFT: Series of ATR-FTIR images produced by integration of IR spectral bands revealing the location of different carboxylate species in a Ghent Altarpiece (Panel XVI) sample (C091.123). Copper oxalate (Cu-Ox) and copper soaps (Cu-Soap) are spatially correlated in copper-based verdigris-rich layers (L1-L5), suggesting they are produced from the irreversible chemical alteration of the paint materials. Calcium oxalate (Ca-Ox) is found alongside Cu-Ox and Cu-Soap but is mostly concentrated in the surface varnish layer (L5).
RIGHT: FTIR spectra extracted from points A, B and C (white arrows in the ATR-FTIR imaging data) are shown. In spectrum B, taken from the varnish layer (L5), a minority Cu-Ox phase is mixed with Ca-Ox, suggesting that a carboxylate-mediated cationic migration has occurred from the underlying verdigris layers.
From lab experiments to heritage preservation
To test its ideas, the project recreates oil paints based on historical recipes from the early modern period. These model samples are subjected to accelerated ageing and examined with advanced scientific techniques.
The EEHOP project builds on two main lines of research:
- developing a new microwave-based method to measure how oil paint films respond to electromagnetic signals, revealing how their properties change as they age or face humidity.
- modelling how certain compounds inside the paint – such as metal soaps and oxalates – move and interact, and how this relates to a paint’s electrical behaviour.
By combining these approaches, EEHOP seeks to uncover the hidden processes behind the ageing of oil paintings. Linking chemical reactions to the physical properties of a paint film, the project aims to identify early warning signs of oil paint degradation and better understand the role of humidity and ions in ageing.
Protecting paintings for the future
The EEHOP project combines art, chemistry, and engineering in an innovative way. By bringing microwave technology into the field of cultural heritage, it aims to deepen our understanding of how oil paintings deteriorate and to support the work of conservators in protecting these irreplaceable masterpieces. The project’s findings will not only advance heritage science but also provide paintings conservators, restorers, and collection managers with scientifically informed strategies for preventive care and restoration, helping museums and institutions safeguard humanity’s vast oil painting heritage for future generations.
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