Craquelure – Introduction
As soon as the painting is finished, regardless of the technique used, it begins to deteriorate, both due to the action of environmental agents and its own intrinsic instability. Physical, chemical and biological phenomena alter the material of which the work is composed of in various ways, for example by causing mechanical tractions which are in turn responsible for much of the damage found on the support, on the preparation and also on the pictorial film.
From a careful examination of the cracks (or craquelure) it is often possible to trace the causes that generated them and, consequently, to obtain useful information on the structure of the work. The cracks reach the deepest layers or are limited to the surface of the painting, branching out according to variable, straight or concentric networks which, intersecting each other, form the characteristic islands of the most varied sizes and shapes.
The phenomenon can be highlighted with the use of macrophotography, further enhanced with raking light and studied in comparison with transillumination, transmitted infrared and X-ray radiography tests.
By observing a painting illuminated with a beam of raking light, we can highlight how mechanical tractions have over time given rise to cracking (craquelure), to lifting of color and to the loss of the pictorial film itself.
The scholars who have dealt with the first phenomenon, that of cracking, have indicated two categories of craquelure linked to two different periods of the work: drying craquelure (or paint stress craquelure) and ageing craquelure (also called noble).
The craquelure consists of a breakage line, an island, the edge, the opening and the breakage depth. The breakage line corresponds to the cut that is formed through the pictorial layers. Its size and form can vary depending on its cause. The intersecting of various breakage lines creates an island of material of different sizes and forms. The edge is the external profile of the break. It can be clean and sharp, or irregular and rounded.
The differences in these elements and their subsequent study has made it possible to classify the different types of craquelure.
Drying cracks are only found of the surface layers of varnish and color. They are a result of abnormal contractions of the materials as they are applied. Therefore, they form during the first few weeks after the paint has been applied, although the phenomenon can continue over a period of years.
The breakage line in this case is broad and irregular. In rare exceptions, the width of the break can reach 1 cm or more. The depth is limited to the layer of paint that is still soft enough to undergo plastic deformation. It forms as a result of internal forces during the drying process. The edges of the islands have a fluid and lumpy appearance and the wide fissures often leave the underlying layer in view.
Paint stress craquelure
The paint stress craquelure is so called as it is formed in the initial phase of the life of the pictorial film and is originated by internal chemical variations that involve contractions of the material itself, causing large and irregular crushing lines. This type of craquelure is rare to observe, as it is essentially due to an incorrect structuring of the painting or to the use of unsuitable materials, or to not having followed some rules that the painter is usually careful to apply. Causes of its formation can be the application of a less flexible layer of paint than the underlying layer, or the application of the latter on a glossy and non-absorbent surface, both conditions in which one of the primary rules of painting is missing: fat over lean. Drying craquelures are also observable when methods and materials that accelerate the drying of the layers have been used improperly.
Ageing cracks form when the pictorial film and the preparatory layer are already completely dry, and therefore can no longer sustain the forces exerted by the dimensional fluctuations of the support. The breakage line is thin, uniform, relatively straight or only slightly curved and it can go on for a fair distance before it intersects with another break. The islands that are formed are in relationship to the thickness of the layers: the thicker the layers are the larger the interval between the breakage lines. The type and nature of the support obviously influence the form of the fissure because they are at the origin of the various tensions. Where the tensions are the greatest, we will find a high density of craquelures, for example near the corners and the margins on canvas paintings and along the grain of the wood for paintings on panel. The edges have sharp angles indicating that the break occurred after the drying process had finished.
The ageing craquelure shows fine and deep crushing lines and develops in the already dry paint and therefore unable to resist mechanical, accidental or internal traction to the structure. As the humidity and temperature of the environment vary, the supports are in fact subject to dimensional variations: the wood swells and shrinks (favoring phenomena of arching, curving, warping and twisting), the canvas loosens and stretches.
In most cases this craquelure does not limit the reading of the painting, it is practically irrelevant from a conservative point of view and can indeed add a certain “charm” to the work from an aesthetic point of view.
Detachment of the islands
In other cases, however, both drying and ageing craquelure can achieve disfiguring effects and, interacting with other factors, compromise the stability of the work, causing buckling and loss of color.
These particular conditions can cause the complete detachment of the various islands from the support with the consequent loss of matter. Macro photography shows us one of the areas of the painting by now strongly compromised.
Ageing cracks begin at the support level and open up in thin breaks forming a “V” with the vertex toward the bottom. The crack passes through all the layers from the ground preparation to the varnish. Under x-ray radiography, ageing cracks are seen as dark lines that are very different from the image formed by drying cracks. Even under transillumination or transmitted infrared, the depth of the fissure is evident. Obviously, this is visible under infrared conditions only if the support is highly transparent, as is the case with this canvas.
Cracks that are not present on the surface but that can be seen by these means could be a result of overpainting of part of the work, or a result of using an old painting as the support for a newer one. This type of use is not unheard of in the field of forgery.
The analysis of craquelures as a proof of the authenticity of a painting must always be made with care. It is true that the presence of craquelures has always been a factor in the authenticity (or better – the age) of a work. However, it is also true that there are numerous methods used to create it artificially. It is however certainly more difficult to create a false cracking pattern that penetrates even the ground preparation, with concave islands and edges that curve away from the support in the typical “cupping” form, than it is to obtain a simple pattern of drying cracks by making one layer slide over another. Nevertheless, at the same time not everything that shows up in an x-ray should be considered authentic. Some of the systems used in art forgery, such as the rolling up of the painting on canvas with a fragile preparation after accelerating the drying of the paint, create deep cracks, and the appearance is very similar to naturally formed ones.
Consumption of the craquelure
When the edges are formed they break at right angles, but over time they have a tendency to wear down. It would be interesting to observe this detail of cracking under the microscope or with macrophotography. However, the forger has not necessarily overlooked this fact.
As an example, we show a possible imitation of craquelures in this painting that at one time was attributed to Spinello Aretino. It recently appeared for sale with the declared supposition that it could probably be a forgery from the 1800’s.
Manfredi Faldi – Claudio Paolini
Estratto da: Artis (Art and Restoration Techniques Interactive Studio), Direzione scientifica: Manfredi Faldi, Claudio Paolini.
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