Metal Soaps and Visual Changes in a Painting by René Magritte – The Menaced Assassin, 1927

The Menaced Assassin by René Magritte is a major surrealist painting
from the collection of the Museum of Modern Art (MoMA). A recent conservation
treatment coupled with scientific analysis has characterised some puzzling and
visually disturbing surface phenomena once attributed to mold growth.

A detailed examination of the painting under the microscope has evidenced that
the formation of radiating dark micro cracks is responsible for the overall speckled
appearance of the painting. These web-like cracks often surround a small white
crystalline particle and are generally accompanied by aggregates of very small white
particles sitting on the top of the surface. The examination also revealed the presence of larger flat pustules of a translucent and softer material with no apparent visual impact. An overall whitish haze is visible as well over the areas painted in red and brown earth tones.

Samples of both white and translucent material were analysed by Fourier transform infrared spectroscopy (FTIR) and confirmed the presence of metal soaps. Past exhibition conditions of the painting and composition of the ground layer may have played a role in the formation of the soaps, in particular the coarse calcium carbonate rich priming layer and presence of kaolin as identified by FTIR and scanning electron microscopy combined with electron dispersive spectroscopy (SEM-EDX).