Interventions on wood supports
Contrary to popular belief, wood is a very resistant material: in fact its deterioration is not due to intrinsic causes but to its environment – or rather, to environmental imbalances (variations in humidity or the on-slaught of biotic agents such as fungi, bacteria or insects).
For this reason, the restoration of a painting on wood panel must not be made with the conviction that the solution to the problem will be solved by a specific intervention or by the materials employed; it must be evaluated in direct relation to the stability of the environmental conditions where the art work will be placed.
The alterations caused by fungi that can be found on wood supports situated in high-humidity environments can damage the cell walls of the wood, causing a loss of consistency as well as chromatic alterations. Among the most dangerous of the changes caused by fungus attack is “rot” that destroys the lignin (white rot) and the cellulose. Brown rot fungi attack only the cellulose, resulting in the retraction of the lignin and its cracking, giving the wood a characteristic cuboidal cracking along and across the grain (soft rot). The alterations caused by bacteria are not as serious: for bacteria – as well as for mold – the treatment calls for the use of antibiotics, and here the sterilizing effects of gamma rays have been studied. Traditional treatments using toxic gas or other products normally employed in disinfestation treatments are also efficient.
An infestation by xylophagous insects is often manifested by bore dust coming out of the emergence hole. X-ray analysis can clearly point up the areas of decay; they usually appear concentrated right below the preparatory layers and in the areas of the panel joins. The main criteria for a correct disinfestation treatment are: reaching the most internal layers with impregnating substances as well as the non-reaction of these substances with the preparatory and paint layers while still providing protection over time. Currently, the most valid treatment is the use of methyl bromide gas in airtight chambers at ordinary pressure and temperature. However, the methyl bromide may not reach all the areas of the panel due to the presence of wax or varnish. In addition, it can not insure protection over time; it is therefore preferable to use a treatment based on chlorophenols or other insecticides.
This operation is meant to re-establish a sufficient level of cohesion and solidity in the wood fibers that have been decayed by xylophagous insects or by fungus flora and bacteria. The need for consolidation, as well as the consolidant itself, are aspects of an intervention that must be carefully evaluated: the substance used must be compatible with the work, it must penetrate and distribute itself uniformly without creating tensions nor changing the aesthetic appearance of the wood. Additionally, once the consolidant has penetrated into the work, it can not be completely removed: this type of intervention must therefore be considered irreversible. The most efficacious techniques require impregnation using a brush or by immersion of the object or – more rarely – by vacuum suction of acrylic resins in suitable solutions and after the appropriate protection of the pictorial film.
The strong tensions in the wood support created by hygrothermic variations can cause separations of the boards, splits and breaks in the panel. In the past, dove-tail inserts were placed along the splits and the joins: these inserts had to be of very well seasoned wood, of the same type of the panel itself, and with the grain of the inserts in the same direction as the general grain of the panel. More recently rectangular grooves were cut and filled with wood inserts of the same shape. Today, the structural intervention on the wood is done by cutting V-shaped grooves (with the point toward the pictorial layer) running down the entire length of the split or join. The grooves are then filled by gluing in small pieces of seasoned wood that follow the grain of the original. The triangular shape and its dimensions guaranty an exact fit in the groove. This shape also reduces contractions to a minimum, and allows for the smallest portion of the wood insert to be placed very close to the pictorial layer and at the same time maintaining a broad surface for gluing.
Straightening and protection screens
The straightening out of bowed and deformed panels is generally discouraged in that it can cause serious damage to the painted surface. The techniques employed for this type of intervention require the forced straining of the wood – with its tendency to return to its pre-treatment condition – in order to keep the surface flat. The straightening of the panel can be effected by varying the degree of humidity on the back surface of the panel – either by warming or by the imbibition of water – or by making a series of incisions running along the grain into the panel back, flattening the curvature, and inserting wooden elements into the grooves. Both systems require the immediate construction of support battens. In light of the fact that the curvature of wood panel paintings is a result of an imbalance between the back and the front of the panel, it is advisable to apply a waterproof protection to the back in order to maintain a balance with the humidity gradient.
Paintings on wood panel have a tendency to warp and split with changing hygrothermographic conditions. In order to anticipate and to correct these deformities various types of support systems have been developed. Among these the most elaborate is a system of movable battens. It consists of a series of movable crosspiece supports placed at regular intervals in contact with the panel. Other battens placed perpendicularly to them hold these crosspieces in place. The old systems of fixed batten supports (cradling) inhibited the movement of the wood panel and have caused splitting and breaking of the wood. The cradle is usually replaced by a movable batten system. Today, the tendency is toward more complex structures that reduce the risk of the battens becoming blocked. Before there is splitting or breakage in the structure of the wood panel, and in the realm of the natural movements of the panel, the batten system should be the first to give way.
Interventions on wood supports – Cross-pieces and other support structures
The support of a painted wood panel is made up of one or more planks joined together and by a system of “backframes” that can be simply one or more crossbeams. Even the frame can make up part of the entire support structure (engaged frame). Battens were inserted directly into the back of the panel by cutting and thinning the width of the panel where the batten was then inserted and held in place with nails, glue and by varying the shapes of tracks and sliding crosspieces. The main function of these battens was to support the planks that made up the panel. In addition, they helped to keep the structure flat, to prevent and to correct the deformities by distributing the induced and transmitted forces occurring with the variations in humidity. Any modification made to the battens must always be preceded by an evaluation of their functionality to the whole. The best conditions must be found so that the support does not create a negative reaction on the more rigid layers of ground preparation and pictorial film.
Restoration. Interventions on wood supports – Transfer
When the entire support structure is compromised, and when every other solution at saving and salvaging the panel is exhausted, a transfer may be required. This intervention consists in transferring the painted layers from the original support to another one. The methods, tools and materials used in this intervention are still in the experimental stages. This is due to the fact that the transfer always brings with it serious risk for the work, which in any case, it is profoundly altered. The procedure consists in the gradual demolition of the wood support, or, by the separation of the pictorial layers from the support by softening the ground preparation which loses its adhesive strength. The new support must guaranty absolute stability for the pictorial film and it can be made from balsa wood. It can also be constructed of a box of bakelite between two layers of fiberglass. Between this structure and the paint layers, there is a sheet of polystyrene that allows for future restoration interventions.
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, con il determinante contributo della Commissione Europea nell’ambito del programma d’azione INFO2000.
Quest’opera è distribuita con Licenza Creative Commons Attribuzione – Non commerciale – Condividi allo stesso modo 4.0 Internazionale.