The dimensional stability of the support is of fundamental importance for the conservation of the color layers and the underlying ground preparation. This stability is tied to the deformations and the dimensional variations that can occur on all supports that are organic in nature each time there are environmental variations of temperature and humidity. Maintaining relative humidity at a constant level would avoid the negative influences on adhesion mechanisms between the various elements in the pictorial work: color, ground preparation, support. Nevertheless, each of these materials responds in a different manner to the hygrothermetric variations, each with its own shrinkage and swelling, flexibility and resistance to traction and compression curves. In general, wood, canvas or metal supports can become deformed, swollen, and contracted without damage – to the ground preparation and pictorial film.
Control and prevention
Due to the fact that it is not possible to obtain and maintain a balanced situation among the various components of the painting, nor is it possible to suppress the hygroscopic behavior of the organic materials that make up the work, the best maintenance conditions must be oriented toward the control of the climatic stability and toward the research of a homogeneous distribution of the forces of the structural whole of the work (with primary intervention on the support) in a such a way as to prevent damage.
Paintings on wood panel: dimensional variations
In terms of paintings on wood panel, one must keep in mind that the wood tends to find an equilibrium with its environment by modifying the humidity it contains with the changes in the surrounding humidity.
When the environmental humidity increases the wood tends to absorb humidity and expands. When the amount of environmental humidity decreases, the wood tends to contract. These dimensional variations are generally accompanied by changes in the shape of the wood due to its anisotropic nature (the dimensional variations are not equal in the different directions of the wood: see anisotropy).
Paintings on wood panel: deformations
The deformations can return to the initial state when the factors causing them cease (elastic deformation); or, the deformations can be permanent. Paintings on wood panel are subject to various types of deformations (warping or “imbarcatura”, twisting or “svergolatura”, curvature or “falcatura”, arching or “arcuatura”) depending on the cut of the board (radial, transversal, tangential sections), the type of wood and the presence of the surface layers of ground preparation and colour that act as a barrier in the exchange of humidity with the environment. This is accentuated when the area on the back of the panel is not coated or protected (see humidity gradient) A panel with both sides uncoated usually remains fairly plane; but when the back is uncoated it tends toward a permanent bowing with the convexity toward the painted side.
Paintings on wood panel: support structure
Paintings on wood panel are rarely made from a single board. Therefore, the influence of various elements (wood panel, frame, batten, systems of nails, et.) must be taken into consideration when analysing the mechanisms that regulate the deformations. With the changes in the hygrothermic conditions each one of these elements tends to work against the movements of the other. Strong tensions are created that can cause deformation, separation at the joins of the boards and separation of the support elements that can cause damage and detachment of the pictorial film.
Paintings on wood panel: binding elements
The application of the batten (more or less rigid) to the wood panel, conditions the free movements of this latter. A batten which is glued to the panel restricts every type of dimensional variation or deformation of the panel as a whole. A batten which is nailed on limits or inhibits bowing of the panel, but does permit a certain play because the nails have a slight tendency to move. Movable battens help keep the panel plane and they guaranty that the wood can swell and contract. When there are drastic changes in the humidity, the movable battens help avoid splits, detachments and breakage of the support. Having said this, movable battens are not always and in every case the best system. In those situations when the pictorial film is very precarious (given that the object can be conserved in environmental conditions with limited variations in the humidity) rigid battens are preferred. In this manner, the rigid support pieces reduce to a minimum the deformations that could damage the pictorial layers.
Canvas paintings: the effects of time
The deformations that the canvas undergoes over time – in addition to chemical and structural deterioration – are highly influenced by the properties of the canvas itself, the type of fibre (hemp, linen, cotton), the weave, its weight, as well as the way it was treated and prepared (tensioning, enervating, the application of adhesives). Slack and bagging are principally due to slow, gradual deformations resulting from the weight of the work itself (the combination of the weight of the color, the ground preparation and the canvas). This phenomenon is known as “creep”, that is, deformation as a result of a constant load. Creep is influenced by the relative humidity, by the tensions brought on by the strainer or stretcher and by the behavioral differences in the weft and the warp. It has also been seen that the deterioration of the fibers in the fabric is more accentuated in the upper area of the painting, and in particular along the edge and the angles because of the greater tensions caused by the weight of the work itself.
Paintings on canvas: the strainer and stretcher
When analyzing the deformations of a painting on canvas, the tensions that are created by the stretcher or the strainer must also be considered. The stretcher was originally conceived as a fixed structure functioning primarily as a support for the canvas. Over time it has transformed into a tensioning device known as the strainer. The tensioning function also brought about the use of canvases that were lighter and more flexible, rather than the previously rigid and self-supporting canvases. The type of strainer and the amount of tension imparted determine the distribution of the deformations of the canvas along the vertical and horizontal axis. When the canvas relaxes, the tensions created by “keying” out the four corners of the strainer are a source of new asymmetries that often create cracks in the direction opposite the right angles of the strainer. Slack in the canvas – in addition to compromising the adhesion of the paint film because of the mechanical stress produced – can also cause the internal borders of the strainer/stretcher to mark and deform the painted surface.
Canvas paintings: humidity
With the changes in humidity in the air, the canvas – like wood – is subject to dimensional variations. With the increase in the relative humidity the canvas lengthens, while with the evaporation due to the lowering of the environmental humidity the canvas tends to contract and shrink. Unlike wood, the elasticity of the canvas tends to decrease over time, the fibers become more rigid and fragile and the deformations become permanent. In order to improve the elastic behavior of the canvas under environmental solicitations, the lining canvases undergo repeated tensioning. Then, an adhesive is applied that in addition to other purposes, has the purpose of fixing the imposed deformation. The canvas (intended here as that with the most commonly found tabby or plain weave) is also subject to dimensional variations in two directions. The transversal thread of the weft, in fact undergoes greater fluctuations with respect to the warp threads when the humidity in the air changes.
Manfredi Faldi – Claudio Paolini
Estratto da: Artis (Art and Restoration Techniques Interactive Studio), Direzione scientifica: Manfredi Faldi, Claudio Paolini. Cd Rom realizzato da un gruppo di istituti di restauro europei, coordinati dall’Istituto per l’Arte e il Restauro Palazzo Spinelli, con il determinante contributo della Commissione Europea nell’ambito del programma d’azione INFO2000.
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