Wall paintings are the oldest known form of painting, dating back to the prehistoric paintings in the Altamira cave in Spain and the Lascaux Grotto in France. In the last decades of the 20th century, the conservation and restoration treatment of two Renaissance masterpieces of wall painting, Michelangelo’s frescoes in the Sistine Chapel in Vatican City and Leonardo da Vinci’s Last Supper (1495–98) in Milan, drew the world’s attention to the environmental and structural vulnerabilities of these treasures.

Commonly, large paintings placed into architectural niches are considered “mural paintings,” even those stretched over stationary or expandable wooden bars in the manner of easel paintings. Strictly speaking, however, “wall paintings” are distinguished from other murals by virtue of being executed directly onto primary wall supports, which are typically plaster, concrete, masonry, or stone. Wall paintings are integral to architecture, in both a material and aesthetic sense. The conservation of wall paintings inevitably concerns not only the paintings themselves but also the larger context of adjacent building materials, building maintenance, use, and preservation. Depending upon their construction and the degree of involvement of the wall support, wall paintings’ conservation and restoration needs may be closely allied to those typical of easel painting or to those of porous stone (see Paintings on canvas, above, and Stone sculpture, below).

From the point of view of conservation, different types of wall paintings have features in common, though the techniques of restoration required for each can differ greatly in detail. In buon (“true”) fresco, pigments mixed only in water are painted directly onto a freshly prepared layer of damp lime plaster. Pigments are permanently bound to the plaster as a result of a chemical change, as the fresh lime becomes calcium carbonate upon drying. In fresco secco (“dry”), the artist applies paints to already dried plaster. The stability of these paintings depends upon the presence of a binding medium—such as egg, oil, gum, or glue—mixed with the pigments to adhere them adequately to the wall surface. This type of painting is found in the wall paintings of ancient Egypt. In marouflage, a more modern variety of wall painting, paintings on canvas are mounted to the wall using an adhesive.

Chief among the hazards to all these types of wall paintings is excessive moisture. Damp may rise through the walls, originating at the level of ground contact and spreading upward. Prevention of rising damp is sometimes achieved by cutting into the wall beneath the mural and inserting a “damp course” of water-impermeable material or a high capillary tube that draws and deflects the harmful accumulation (see Architecture, above). These avenues of intervention are, however, often prohibitively expensive due to the complex engineering they require. If these approaches are not possible, amelioration of the problems may be achieved by reconfiguring drainage at the exterior of the building, and thereby reducing the overall quantity of available moisture. Damp may also come from the outside wall, where direct infiltration of rainwater may penetrate through the substrate to the face of the painting, evaporating at the paint surface. In this instance, localized building repairs or efforts to shield the exterior wall may attenuate the problem. Moisture may also result from condensation on a cold mural surface, a phenomenon common in churches, tombs, or buildings that are heated only intermittently or that are subject to excess ambient moisture generated by the respiration of crowds of visitors. More continuous and uniform heating of the wall may adjust this situation, provided that ambient air is not dried so rapidly that “efflorescence” (the formation of salts) occurs. Lastly, water damage caused by leaking roofs, clogged drainpipes, and faulty plumbing is easily stopped by repairing these systems. Conscientious maintenance is the best preventative treatment.

Damages to wall paintings due to moisture may include blanching, drip staining, and delamination of paint layers due to efflorescence. Crystallized salts may form above, below, or within the painted image, resulting in disintegration or obfuscation of the image and creating a salty “veil.” The conservator must avoid coating the painting with a water-impermeable material, such as wax or resinous products, so that the damp can penetrate freely without meeting a barrier at the inner surface; when evaporative sites are blocked, moisture will move laterally, expanding areas of damage. Problems such as mold growth and mildew are secondary results from overly damp environments.

Another enemy of wall paintings is more insidious and also more pervasive. Due to the worldwide use of fossil fuels and automobile emissions, concentrations of sulfur dioxide in the atmosphere have markedly increased. In the presence of moisture, pollutants forming sulfuric acid can quickly erode the calcium-carbonate component of most cement- and lime-based wall paintings. This “acid-rain” effect converts calcium carbonate to calcium sulfate. The volume of the sulfate crystal is almost twice that of the original carbonate of the mural, which causes internal pressure within the pores of wall fabric that can lead to fracturing. Further, the sulfate has a greater capacity to absorb moisture, thus perpetuating and exacerbating the cyclic wet-dry process of decay. Polluted environments can bring about the blackened, sooty surfaces associated with fossil-fuel particulates to a wall painting and can also discolour certain pigments traditionally found in Renaissance paintings, such as white or red lead, malachite, and azurite.

In the face of such damage from moisture and pollution, the conservator works to halt the causative agents of deterioration and then proceeds to stabilize insecurities such as spalling plaster or flaking paint. Many new conservation treatments were developed in the second half of the 20th century: chemical poultices, gel technology, and ion-exchange resins have allowed advances in cleaning methods, reduction of salt deposits, and consolidation techniques. Natural or synthetic adhesives and inorganic consolidants are now utilized, but they must be chosen for compatibility with the paint medium and used with discretion to avoid film-forming blockages. Hypodermic injection of adhesives followed by light pressure while drying has also become an effective way to mitigate many problems of detached paint or wall support.

Conservators often develop solutions in the face of a specific problem. For example, after the flood of the Arno River in Florence in 1966, Italian conservators developed drastic but necessary and highly expert methods to transfer frescoes from decayed walls. These range from the strappo technique to the stacco a massello. While in practice these methods are not always clearly distinguishable, strappo, the more radical procedure, consists of gluing canvas firmly to the surface of the fresco and then pulling and easing away a thin layer of the plaster containing the pigment particles of the fresco. The bond between the facing and the fresco must be stronger than the internal cohesion of the plaster. Excess plaster is removed from the back, revealing the thinned fresco in reverse. This thinned pictorial layer is then fixed to a rigid support after recoating the reverse with materials optically simulating the original underlying plaster. Unfortunately, much of the original surface character of the wall and density of the pigment layer is sometimes irreversibly altered by this technique, so the method is now seldom used. Less intrusive is the stacco method; a thicker layer of plaster is retained along with the fresco and is smoothed flat on its back surface before the composite rigid layer is mounted to a prepared support. Lastly, in the procedure called stacco a massello, the least intrusive to the fresco but more challenging transfer procedure due to mass and weight, the wall painting is removed with its entire original substrate. This feat requires bracing the wall with counter-forms to avoid damages due to torque, vibration, and other mechanical strains. Selecting the method of transfer depends greatly on the stability of the painting, the type of deterioration encountered, and the limitations of size, weight, and practicality.

Whenever possible, transfer techniques are abandoned in favour of conservation and restoration treatments carried out in situ, with the conservator working from the surface and preserving as much original building fabric, character of surface, and contextual meaning as possible. The art conservation community, including art historians and preservation specialists, generally hold that murals and wall paintings are physically and aesthetically dependent upon their architectural context. The so-called “site-specific” nature of the paintings is valued, and the character of the original site is maintained as nearly as possible; relocation may cause diminishment of meaning or appreciation. The disciplines of wall and mural painting conservation, engineering, and architectural conservation are symbiotic, and each specialty is increasingly called upon to contribute to a holistic preservation plan.