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The conservation of acrylic open-air murals

The expansion of acrylic binders during the 60s, used for easel painting, soon led to experimentation with a new technique of mural painting to be applied onto modern cement surfaces, with highly encouraging results...

The paint forms an impermeable membrane which, however, permits transpiration of the water vapour, reducing the risk that the sealing layers become detached when the pressure exceeds the adhesive strength of the paint film.
The cement supports proved effective for the adhesion of these materials, which reacted better on the alkaline substrates of the oil paint and the alkyd enamels formerly used by artists such as Siqueiros, because they did not alter on contact with the cement, once applied. In fact, the acrylic emulsion contains a percentage of ammonium salts which maintains the Ph between 8 and 10 to permit maximum dispersion in water of the acrylic polymer.
Rohm & Haas, the largest producer of acrylic dispersions (Primal or Rhopex in the United States) have promoted them for forty years as the ideal materials for the consolidation of cementitious brick and mortar materials, into which they penetrate in strong dissolutions, adhering to their porous internal structures.
Particular problems are posed when these paint applications of acrylic colours in water are carried out externally, because the surfaces are subject to violent UV radiation, with unexpected variations of temperature between day and night, as well as sudden changes in relative humidity, all of which produce alterations which can occur simultaneously and aggravate the natural transformations of the material in the course of its ageing. They are conditions which are difficult to simulate in the artificial ageing tests, ensuring that any precise reconstruction of the process of alteration will be problematic.
Porous materials exposed in the open air are subject to continued cycles of environmental interaction, involving the solubilization of the saline components which migrate, crystallising and becoming deposited beneath the surface, thereby creating pressure and deformation which disrupt the paint film.
A component of the acrylic emulsion is soluble in water and may then be separated from the aggregate, becoming deposited on the surface and progressively being washed away. This is why the surface frequently shows signs of whitish deposits of surface-active agents and dispersing agents which impede proper appreciation of the pictorial image, and which must be removed carefully using dry-cleaning methods (wishab rubber, broomstick, fibreglass brushes) or by applying distilled water on the surface.
The superficial dust layer is associated with the characteristically low glass transition temperature of these emulsions, which maintains a slight stickiness of the material at room temperature, attracting pollutant dust and fine dust.
Solar radiation has an altering effect on the paint film because while acrylics endure well against U.V.A. rays, they do alter in the presence of U.V.B. rays, causing division in the polymer chain and then reticulation which causes the paint film to become more rigid and fragile.
The use of Tinuvin from Ciba Geigy may enhance the level of surface protection against UV rays, protecting the integrity of the resin and reducing the level of UV rays absorbed in the surface, acting also to reduce discoloration of the fading pigments.
The fixed temperatures cause a progressive hardening of the acrylic film, which unexpectedly splits when subject to very low temperatures below zero.
The problems raised are difficult to resolve because stains and discolorations are irreversible and cannot be masked on painted backgrounds which are flat, coloured and glossy, therefore it is necessary as far as possible to take avoidance measures by ensuring hydrorepellancy, protection from light radiation by means of screens and UV radiation stabilizers, by applying biocides to curtail the growth of microorganisms often found on acrylic films outdoors.
The colorimetric measurements taken over time may reveal progressive transformations, repeating the measurements at the same points indicated by a graphic reference grid. Filling the gaps using a paint integration had the effect of reconstructing the image but it also permitted the re-compressing of a paint film which had been made fragile and riddled with breaks, consolidating the phenomena of detachment deep down.
A number of cases will be presented to demonstrate the approach taken to the conservation of external acrylic mural paintings, such as the painting of Guttuso at the Sacro Monte di Varese and the paintings of Salvo, Testa, Carena, Bertolini, Hsiao Chin and Molin at MACAM, the Museum of open-air art at Maglione Canavese.
In the case of the painting by Guttuso, the alteration of the paint film required the re-adhesion of the film which had become detached by a process of injection and reforming, by applying manual compression and the use of heat to restore planarity, followed by the application of a surface veneer using a polyvinyl butyral resin nebulized on the surface, subsequent to the paint integration.
The paintings of Maglione were cleaned and consolidated, retrieving some of the colour tone which appeared obscured below blankets of dusty white, and after they were touched up they were protected by applying a transpiring siloxane protective hydrorepellant to avoid the continuous cycle of becoming wet over time.

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