'The Martyrdom of Saint Hippolytus' is one of Bouts' last paintings. He died before he could finish it. Based on stylistic analysis, there is a hypothesis that the left panel, featuring the patrons' portraits, may have been painted by Hugo van der Goes. This hypothesis is reinforced by research conducted using infrared reflectography, abbreviated as IRR. Yet, what is IRR exactly, and what insights can it provide?

We are continuously surrounded by electric and magnetic waves, collectively referred to as electromagnetic radiation. We are quite familiar with some of it: we perceive the waves ranging from 380 to 750 nanometres in length as visible light. Wavelengths ranging from 750 to 300,000 nanometres are classified as infrared. Although we cannot see them directly, we can convert them into the visible spectrum using special cameras.

Infrared reflectography or IRR employs a specific spectrum of infrared waves, typically ranging from 950 to approximately 1700 nanometres. These waves penetrate the paint layers. To an IRR camera, the paint layer is as transparent as the varnish layer is to the human eye.

Below the paint layer lies the underdrawing. This initial outline of the composition is typically executed with charcoal, black chalk, or ink. These materials contain carbon, which absorbs infrared rays, whilst the white preparation layer reflects the rays. Due to these characteristics, images made with IRR can make the underdrawing visible.

The underdrawing gives us invaluable insights. Frequently, the underdrawing is created by the master artist himself. As part of the design process, an artist makes compositional changes. These alterations offer us a glimpse into their quest for the ideal composition. The underdrawing thus provides insight into the painter's creative process even before he starts painting.

In the case of 'The Martyrdom of Saint Hippolytus’, this underdrawing has been hidden under the paint layer for 500 years. Thanks to IRR, we can once again see the underdrawing. This furthers our understanding of the creative process. You can explore some of the key findings of the IRR study using the screen on the table.