Why Cracks in Ancient Works Reveal Many Secrets

That’s the Mona Lisa you see in this picture, made entirely of cracks and fissures. Even without color, the crack pattern differs depending on the pigment and binder. We can even see cracks in the depths of the painting, for example in the forehead, whose parallel cracks are clearly distinguishable from landscapes or skies that have no particular orientation.

Cracks are one of those changes that remember the life of a painting. With wide morphological variation in most of the easel paintings, they are of interest to art historians and restorers alike.

First of all, it’s a way to authenticate paintings. Analysis of the Mona Lisa’s cracks captured in photographs taken as far back as 1880 helped authenticate the painting and dispel any doubts that were expressed after it was stolen in 1911.

Multispectral image reveals cracks in parts of the Mona Lisa – Codex Images International, 2007

The cracks are the “fingerprints” of the artwork. By trying to reproduce it, a counterfeiter inevitably and involuntarily leaves its mark on its time. Thus, chemical compounds discovered at a much slower period than that attributed to a work could be used to artificially develop cracks. A thorough classification of cracks in easel paint over the years is a way of distinguishing between those formed during aging and those created in an accelerated manner, for example by variations in temperature. The cracks don’t lie!

Study crack to better understand work and the creative process

The cracks also show the materials and methods used by the artists. They do not form randomly, but obey the laws of physics and mechanics: cracks propagate while being guided by environmental tension (eg fabrics). Once the tension is released, the crack organization differs in a number of characteristics, such as its density or orientation.

These characteristics are affected in particular by the stiffness and thickness of the coating, and sometimes by the direction of brush strokes, the heterogeneity of the paint, as well as the way the paint demands: drying, aging of the paint, deformations introduced by supports (wood panels, canvas).

Thus the cracks make it possible to obtain information about the entire table. We study them using “multispectral imaging” which records the full spectrum of colors, from ultraviolet to infrared, with extreme spectral precision.

Cracks with a characteristic pattern also form in the dried mud.
Cracks with a characteristic pattern also form in the drying mud – Ludovic Pauchard / The Conversation

The cracks then appear as sudden variations in luminosity. We searched it over all multispectral images to find and differentiate these changes in depth in the image layers (paint layers, which can be heterogeneous with mixtures of pigments of different sizes and different stiffnesses, and solvents in different layers).

This imaging technique maintains the integrity of the work. It is used as an adjunct to the structural analysis of pictorial materials, with the help of which sample analysis allows, among other things, the identification of the pigments used and other imaging techniques, such as fluorescence under UV or X-ray lighting for example.

For example, an image can highlight three specific areas. The highly visible parallel, vertical cracks on the Mona Lisa’s forehead run deep, down to the surface of the poplar panels that support the painting; they have the same direction as the wood grain. Thus, these cracks appear to be strongly related to the strain transmitted by the supports over time.

The cracks obtained by laboratory experiments made it possible to model (1) the effect of deformation of the support on the cracks in the rigid layer model (front of the Mona Lisa) or vice versa (2) the absence of this effect in the layer of the soft model paint (landscape);  (3) no cracks in the low thickness model layer (1µm)
The cracks obtained by laboratory experiments made it possible to model (1) the effect of deformation of the support on the cracks in the rigid layer model (front of the Mona Lisa) or vice versa (2) the absence of this effect in the layer of the soft model paint (landscape); (3) absence of cracks in a model layer with a small thickness (1μm) – F. Giorgiutti-Dauphiné and L. Pauchard, Journal of Applied Physics (reproduced by permission of AIP Publishing)

On the other hand, in the sky or landscape, the cracks form a network that delimits more or less regular polygons, without any preferential orientation, much like the decimetric cracks that form in a dry lake. This crack no longer follows the deformation of the support, the painting has been able to absorb this one mechanical stress, as a result of the drawing material being less brittle than that used on the painting surface.

It’s the absence of cracks that underscores the interest in other areas of the painting. Indeed, the veil around the Mona Lisa’s face was most likely painted using a pictorial technique based on the successive application of very thin layers, that is, lightly charged in pigment. This technique, “sfumato”, makes it possible to play on the effect of depth and shading of the image. However, a coating is generally free from cracks when the thickness is thin enough. This is why there are no visible cracks in these particular areas of paint.

This hypothesis has been supported by laboratory studies. The studies were carried out using model materials using well-calibrated pigments, under controlled compaction (drying) conditions, in controlled sublayers so as to properly separate the physical mechanisms involved. Art paints are complex media because of their geometries (layer superposition) and the materials used (pigments with variable mechanical properties in a mixture of volatile and non-volatile solvents).

Craziness gives a sense of authenticity

The cracks also stick to the paint. They are very interested in conservation and restoration. Variations in crackle patterns in a painting can have a significant impact on the viewer’s perception of the image. In general, crack draws a complex interconnected network of more or less contrasting lines. These lines can be considered unwanted, because the appearance of the table changes radically. The illusion of a painting can be compromised by such visual characteristics, which can diminish the pervasive perspective the painter intended.

But the cracks can also be considered familiar imprints, which lends a sense of authenticity. Would the Mona Lisa be the Mona Lisa without the cracks? The cracks give the painting an older look; their market value, when tied to elapsed time, can be increased. Crackles are also desirable for their aesthetic qualities which break up the monotony of flat surfaces.

However, the crack network must not expand uncontrollably according to the variation of environmental conditions (humidity, temperature) and cause phenomena such as release or loss of void material. Studies on the stability of the fracture network as a function of the image material and its stress conditions are ongoing.

This article was produced by The Conversation and hosted by 20 Minutes.

Posted in Art

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