Fresco

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Fresco is an ancient technique of painting on masonry walls. The color in Diego Rivera's murals at the Detroit Institute of Arts (Color Plate 6) was applied to damp plaster (hence the term fresco, or "fresh" in Italian). The material used to paint frescos does not conform to our usual definition of a paint, which specifies the presence of a binder. The pigment is ground in water, which has no binding strength. Water is a vehicle, or diluent, which evaporates during the drying process. The water is used to dilute the pigment, allowing it to be spread easily over the plaster and to assist in the drying/binding process.

The binder is the result of a chemical process, resulting in a mechanical bonding of the particles of pigment. The color mixture of pigment and water is brushed onto a freshly plastered surface. The plaster is a mixture of slaked lime (calcium oxide in solution with water, forming calcium hydroxide) and an aggregate (sand, marble dust, or a volcanic ash called pozzolana). As the plaster dries, the particles of pigment are pulled into the surface of the plaster and locked in place by particles of calcium hydroxide, which convert to calcium carbonate as the plaster dries (Figure 2.5). The plaster acts as ground, support, and binding agent. Fresco is the one painting medium in which all of the component parts merge to form a single unit. A cross section from Rivera's mural (Figure 2.6) reveals a simple structure with pigment particles integrated within the surface of the lime mixture. (See Appendix J for an analysis of pigment-plaster integration.) Underlying the apparent simplicity of this medium, however, is a process complex in its chemical interactions and demanding in its execution.

Fresco is a marvelous medium for painting large-scale works that are meant to be assimilated into an architectural space. It is compatible with masonry and is durable and quickly executed. The difficulties for the painter are that extensive preparatory work must be done (in the form of sketches, full-size drawings called cartoons and small-scale color sketches), large areas must be painted quickly and without alteration, and the work of assistants and tradesmen must be orchestrated in the often cumbersome environment of scaffolding and restrictive architectural spaces.

The wall surface is built up of layers of different formulations of lime, water, and aggregate as shown in Figure 2.5. Each successive layer contains a greater percentage of lime, gradually increasing the binding power of the plaster. Layering ensures even, slow drying, which reduces the likelihood of cracking. Because most frescos are large in scale, the intonacco, or surface layer, is applied in sections that can be painted in one day before the plaster dries. These sections are called giornate, or a day's work. In addition to making the painting process more manageable for the

Fig. 2.6. A cross section from the fresco Detroit Industry by Diego Rivera showing the accumulation of pigment particles (thin dark band) at the surface of a layer of plaster.

Although this band appears as a discrete layer, the pigment particles are actually interlaced with crystallites of calcium carbonate in the plaster. See analysis of this cross section in Appendix J.

Fig. 2.6. A cross section from the fresco Detroit Industry by Diego Rivera showing the accumulation of pigment particles (thin dark band) at the surface of a layer of plaster.

Although this band appears as a discrete layer, the pigment particles are actually interlaced with crystallites of calcium carbonate in the plaster. See analysis of this cross section in Appendix J.

Seams Between Giornate

Fig. 2.7. The line drawing superimposed over a photo of Vaccination from Detroit Industry by Diego Rivera indicates the seams between giornate, or sections of the fresco completed in one day. The seams usually follow the contours of forms in the image.

painter, the seams between giornate act as expansion joints, alleviating some of the stresses on the surface that could lead to cracking. Figure 2.7 shows the arrangement of giornate for the section Vaccination from the mural cycle by Diego Rivera at the Detroit Institute of Arts. The seams are aligned with the edges of forms in the image. Complex color mixtures would need to be matched from day to day (nearly impossible to do given the inconstancy of pigment and plaster mixtures and the inevitable fluctuations of atmospheric conditions) should a seam pass through a form or area of the image. The schemes for the giornate are very carefully plotted when the cartoons are drawn. The image in the cartoon (Figure 2.8) is transferred directly to the damp plaster by means of pouncing (charcoal dust or pigment forced through a series of small punctures along the lines of the drawing), or by scribing along the lines in the cartoon with a pointed wooden or metal tool that incises the line into the soft plaster.

Painting of the image then proceeds in the prescribed sequence. Each giornate must be completed in 4 to 8 hours, after which time the plaster has dried too much to permit the adhesion of the pigment particles within its surface. The painting cannot be revived after the plaster dries to make alterations or corrections without having to chip away the in-tonacco and start over again. It is possible to paint over the fresco to make alterations, but this procedure requires the use of a binder in the paint (egg, casein, or animal glue have been used), which then produces a film on the plaster. In addition to making corrections, this technique, called secco, is sometimes used to apply pigments that are not compatible with lime (pigments containing copper, for example).

The finished state of a fresco has a somewhat dry, nonglossy ap

Fig. 2.7. The line drawing superimposed over a photo of Vaccination from Detroit Industry by Diego Rivera indicates the seams between giornate, or sections of the fresco completed in one day. The seams usually follow the contours of forms in the image.

pearance. The surface may become slightly more reflective with age or from having been burnished with a fine abrasive, a practice sometimes employed on Roman frescos. In most cases, there is no sense of the surface being coated with a layer of paint. The image is simply a part of the wall. Fresco has the potential to produce brilliant color as well as a feeling of monumentality through its affinities with architecture (Figure 2.9).

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Pencil Drawing Beginners Guide

Pencil Drawing Beginners Guide

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