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Craquelure in easel paintings: a matter of time

Craquelure in easel paintings: a matter of time

 

Ludovic Pauchard, CNRS – laboratory FAST (Fluides, Automatique et Systèmes Thermiques), University Paris-Saclay, France

 

Time impacts artworks as soon as they leave the artist’s hands: in general, colours and light/dark contrasts may be altered, the original shades may be disrupted, … Furthermore, one of the main alterations that memorize the “life” of a painting lies in cracking. Cracking is a process in which cracks irreversibly break bonds and consequently separate the matter. Hence, cracks are the mark of time on the matter. Even if cracks may fade over time, their underlying presence persists. Cracks exhibit a wide variety of morphologies in most easel paintings.1 Moreover, the complexity of these morphologies lies in the possible superimposition of crack networks deep in the paint layer. Besides, cracks are specific of the matter: the pattern of parallel cracks in the forehead of Mona Lisa is distinguishable from that of the landscape or the sky without any preferential orientation (see illustration). Hence, the presence of cracks in easel paintings reveals matter as will be highlighted throughout this manuscript.

Multispectral imaging showing some visible crack patterns in the painting “Mona Lisa”. Directional and deep cracks appear in the face and left chest. The long thick parallel lines in the face correspond to cracks that extend. The long, thick parallel lines in the face correspond to cracks that extend from the surface layer to the underlying layers.

Early after the paint process, drying or premature cracks may form in young paint layers.

They result from internal tensions, or internal stresses, induced by the drying process.2 The last process governs the solidification of the paint layer. Such cracks present fairly wide lines and are often attributed to defects in the paint process due for example to the use of the wrong amount of drier or oil in the paint. In contrast, networks of thin lines provide the so-called craquelure that develops later in the life of the painting. The presence of cracks is due to the existence of strong mechanical stresses in the material.3 Depending on the properties of the paint layer and the way this last may resist the mechanical stresses, the cracks will be formed as a result of the stress release. 

Drying cracks or craquelure form networks of lines, exhibiting various lengths and widths, more or less connected to each other. Whatever, these lines interact with the underlying image composing the easel painting. Looking more carefully at a pattern of cracks, a multitude of morphologies can be revealed: “herring-bone” patterns caused by the wedges in the corner of the frame, “cobweb” caused by a force applied at one point in the surface, shock, due to possible transportation, the radiation including light and ultraviolet, variation in relative humidity rate and temperature, or paintings flood, “parallel cracking” when a canvas painting has been rolled up, or due to brush-marks memorization in specific paints, as many patterns resulting from the history of the painting. Such examples of morphologies give clues to the life of the painting.

The variation in crack patterns across a painting can have a significant impact on the viewer’s perception of the image.4 In general, craquelure draws a complex interconnected network made of more or less contrasted lines. These lines offer a wide range of tones and depths and generally emphasize the surface of an easel painting. They can be viewed as undesirable since the aspect of the painting is drastically modified. The illusion of the painting can be compromised by such visual features, impacting the perspective of envelopment intended by the painter. However, craquelure can be perceived as familiar traceries that give a sense of authenticity. Would Mona Lisa be Mona Lisa without its craquelure? Due to the presence of craquelure, the paintings look older; the market value of the painting is increased since related to the time elapsed. Cracks may also be desirable for their aesthetic qualities which break the monotony of a flat surface. Besides, as a fingerprint of the work of art, their presence is a powerful source of information. Hence, craquelure has been overlooked for a long time. The analysis of the craquelure of Mona Lisa recorded on photographs taken since 1880 enabled to authenticate Mona Lisa and to remove the doubts expressed after the theft of the painting in 1911. Besides, craquelure plays an undoubted role in Authentication in Art as a forgery detective. A forger inevitably and involuntarily left the mark of his time to attempt to replicate craquelure. Indeed, elements of contemporary composition may be used as well as the method to accelerate the ageing process and the creation of cracks. The in-depth classification of the craquelure in easel paintings over years is a way to discriminate the ones formed during ageing and those created in an accelerated manner for example through temperature variations. The last process implies changes in the craquelure features in terms of length, width, connection or orientation of the cracks. Hence, craquelure cannot lie!

Moreover, the strong link between morphologies of cracks and the matter is a way to obtain precious information on the type of materials as well as the techniques used by the artist, as well as the conditions of conservation. Indeed, the wide variety of crack patterns reflects the complexity of an easel painting in terms of chemical, physical and mechanical properties. These properties are inherent to the composition of the paint layer and evolve over the years. In that way, it is of importance to consider the paint layer itself.

An easel painting is a layered structure composed of inorganic and organic compounds. Its formulation is not a standard feature as it can vary a lot from one product to another. Nevertheless, the main components are solid pigments and a binding medium, including many additives brought by the artists then colour makers over the years. 

In particular, the binding medium affects the mechanical properties of the paint layer. This component has been changed many times over the centuries: from water or egg (tempera paint) to oil leading to softer paintings and finally to acrylic systems, synthetic resines, by now. Depending on the time evolution of each component under their mutual interactions, the physicochemical and mechanical properties of the paint layer evolve in a variety of ways.

At the earliest stages of the painting lifetime, the drying consists of a chemical process where successive oxidative and polymerization mechanisms lead to the formation of a tridimensional network and of a solid film where the pigment particles are entangled and bound. In particular, during the application of paint, brushstrokes result in a preferred direction impacting the drying process. This may generate cracks in the same direction. In contrast, a spreading without preferred direction results in uniform drying without favouring any one direction in the craquelure. Even if craquelure is not formed early after the paint spreading, the direction of application may be memorized so impacting the direction of craquelure formed over time.

Together with the solvent evaporation, the paint layer tends to shrink, but it is geometrically constrained by the presence of the support, panel or canvas, on which it adheres. Too much tension leads to the appearance of cracks or the peeling of the paint. These processes allow the mechanical stresses in the layer to be released. If young paint layers exhibit a significant ductility to withstand stresses, ageing tends to make paints more glassy by chemical changes. Hence if ageing paints are more likely to be stiffer over time, they can become either stronger or weaker, so providing various mechanical responses to environmental effects.3 The increase in the paint stiffness is often accompanied by embrittleness of the layer that becomes more and more sensitive to external effects. This results in a decrease in the ability to withstand stresses leading to craquelure formation. By the way, ageing increase the sensitivity to the ambient conditions, such as humidity rate, temperature variations or deformations of the support, for example. Age crack has sharp edges, thinner than the premature cracks, and, contrarily to the last, usually penetrates deeper in the layered structure. 

The changes of the material properties are combined with the environmental changes as well as its method of conservation that are both specific to the history of the painting. Hence, craquelure is a characteristic pattern of a period or a country. Hence, Bucklow proposed a classification of the crack pattern morphologies based on the geographic location: it is Italian, Flemish, Dutch or French typical styles leading to distinct crack patterns with specific features (straight cracks in Flemish or curved ones in the French style).1 

To perceive the mark of time in a classical crack pattern in a paint layer, careful observation of the features of the pattern is required. Generally, a network of cracks invades the plane of the layer following well-defined rules of Physics.5 Indeed, cracking is a generic process of growth when tensions, or mechanical stress, exist in the material. Mechanical stresses whether internal due to the drying process or due to ageing and environmental effects can give birth to cracks. To release the stress, a defect, e.g. any imperfection of the layer, will be always revealed as a nucleation centre of a crack. The propagation throughout the layer takes place at a speed depending on the mechanical properties of the material. Then the crack path is governed by tensions in the layer. A crack always propagates in a direction perpendicular to the direction of maximum tensions. After propagation, the component of the stress normal to the crack surface is released in its vicinity. The trajectory of a future crack is modified to satisfy the condition imposed by the existing crack. Consequently, the new crack connects to its neighbour always perpendicularly. Progressively craquelure invades the plane of the paint layer through successive generations, so revealing a hierarchical organization. This results in the division of the plane of the layer through adjacent polygonal domains exhibiting a more or less regular size. At the final stage of the process, the temporal order of the fracturing can be approximately reconstructed. Indeed, the longer cracks are the first to appear, the cracks of second-order connect to the first one, and so on …

If craquelure can be viewed as desirable for its aesthetic qualities or as a powerful source of information, existing craquelure can however evolve drastically over time. During the evolution of the craquelure, the number of cracks possibly increases as well as their length, width and depth impacting dramatically the painting. Such damages result in flaking that causes the paint layer to dislodge from the support resulting in a loss of material in the paint. 

Hence, an objective of Conservation and Restoration deals with the stability of an existing crack pattern so that to prevent its possible evolution over time. Due to the careful analysis of the shape of the craquelure over time, it is possible to detect possible unstable areas and to advise on preventive conservation conditions. 

Besides, craquelure is a pathway for moisture from the surrounding to penetrate deep into the paint layer so favouring the layers separated from each other. The so-called blanching that is induced by excessive humidity, or even more damaging flood, is still intensified by the presence of cracks. 

Over time, the process of restoration is a complicated deal mainly due to the presence of craquelure since cracks may affect the curator operations. Craquelure possibly collects dirt particles from the surrounding. Besides, the treatment that consists of cleaning or removing aged and discoloured varnish on the top surface of the painting generally implies the use of solvents. During this operation, the presence of craquelure can promote the solvent imbibition inside the layered structure possibly resulting in further damages such as swelling or formation of new craquelure.

To avoid the craquelure evolution and especially the dramatic separation of the layer from the support, it is sometimes necessary for the cracks to be repaired by techniques suitable for conservators. In this way, the repair is integrated to preserve the visual appearance of artworks. However initial craquelure is associated with irreversible ruptures and the impact of time on the restoration is still not well known.

Craquelure is a characteristic of the material properties and notably reveals the way a paint layer evolves with ageing. Besides, the analysis of the morphology or cracks is a non-destructive way to obtain important details to authenticate paintings. Making craquelure meaningful provides relevant information about the methods and materials used by the artists, as well as the changes in the lifetime of the paintings.

Illustration:

Reproduced from F. Giorgiutti-Dauphine, L. Pauchard Journal of Applied Physics 120 (6), 065107 (2016) with the permission of AIP Publishing.

  1. S. Bucklow, “The description of craquelure patterns,” Stud. Conserv. 42, 129–140 (1997). [] []
  2. P. de Willigen, A Mathematical Study on Craquelure and Other Mechanical Damage in Paintings, WBBM Report Series 42/MOLART (Delft University Press, 1999). []
  3. M. F. Mecklenburg and C. S. Tumosa, “Mechanical behavior of paintings subjected to changes in temperature and relative humidity,” in Art in Transit, edited by M. F. Mecklenburg (National Gallery of Art, Washington, D.C., 1991) pp. 172–214. [] []
  4. S. Bucklow (1994). The effect of craquelure on the perception of the pictorial image. Zeitschrift für Kunsttechnologie und Konservierung, 8(1), 104–11. []
  5. F. Giorgiutti-Dauphine, L. Pauchard Journal of Applied Physics 120 (6), 065107 (2016). []