Wall painting has a very ancient tradition in the Andean area . The oldest examples are those of the Cerro Ventarrón religious sanctuary, the only ones known from Pre-Ceramic period, dated back to over 4000 years. This pictorial tradition was developed through the pre-Hispanic centuries. It is worth noticing that the first paintings appear on the north coast of Peru (Cerro Ventarrón, Kotosh and Cerro Sechín), then on the central coast (Garagay). The wall decoration patterns were at the beginning very simple and schematic, and then become more complex. During the period of the Early Horizon (900 − 200 BC) the polychromy on monuments was developed, with in particular the paintings and painted reliefs of the temples of Puncurí and Cerro Blanco, in the Nepeña Valley in the north of Peru.
During the Early Intermediate Period (200 BC − 700 AD), the mural art flourished. Two major styles coexist: the "Lima" style on the central coast, and the "Mochica" style in the north. Both styles continued during the Middle Horizon (700–1000 AD). The first style is characterized by the repeated use of a pattern of intertwined snakes. In the "Mochica" style, real scenes were painted, with incisions used as preparative drawings. Several archaeological sites illustrate this style and Mochica iconography including the Huaca de la Luna (Moche valley), the El Brujo complex (Chicama valley), La Mina (Jetepeque valley), Pañamarca (Nepaña valley) and the Huaca Pintada (Lambayeque valley) on the northern coast. For later periods, little evidence has been preserved but no major variations seem to have been introduced. The Mochica style was divided into five phases. Most of the known Mochica murals belong to phase IV (Early Intermediate Period) or to phase V (Middle Horizon). The paintings of the Middle Horizon are characterized by the splitting of the scenes into several square surfaces which follow each other. To the south of Peru, on the highlands, in the large urban center of Tiahuanaco, several large stone temples were built between 700 A.D. and 1200 A.D. There were also adobe walls, some of which were painted . Few examples remain from the Late Intermediate Period (900–1440 AD). Also, few murals are dated from the Late Horizon during which the Inca Empire prevailed. Vandalisme, the rainy climate in the Sierra and the iconoclasm of the colonial era all contributed to the massive destruction of these paintings, which were probably much more numerous. However, in the south coast, among the archaeological sites belonging to this period, La Centinela in the Chincha valley, Incahuasi and Guarco in the Cañete valley and Tambo Colorado in the Pisco valley, have preserved inca-style paintings . Several other sites were found on the central or northern coast, like La Fortaleza, but very few in the interior of the country. However, some historical writings like the chronicles of Garcilaso de la Vega testify to Inca murals but unfortunately they have disappeared.
In summary, the oldest examples of mural painting were discovered on the north coast, where presumably the pictorial technique appeared in the Pre-Ceramic period. Mural painting was then developed during the Early Horizon through the Chavín style on the north and central coast. On the other hand, in the south, except for the few fragments of mural painting recently discovered in Cahuachi, and some others on a Early Horizon site that is now destroyed , no other vestige bears witness to murals on the south coast. Consequently, according to these archaeological data, it seems that the technique of mural painting was adopted and mastered very early by the cultures of the north, while the Paracas and Nasca cultures in the south do not seem to have had a predilection for this type of polychrome architectural decoration.
The Nasca, whose culture appeared on the south coast of Peru in the 2nd century BC and disappeared around the 7th century AD, have left many vestiges that testify to the importance of the development of polychromy in cultural expression. In particular, objects of worship such as ceramics and textiles were produced by them on a large scale. Indeed, the Nasca seem to have been masters in the art of rich polychromy on ceramic [4, 5] and on textile [6, 7]. Ceremonial buildings also seem to have been a favorite support for developing polychromy, precisely at Cahuachi, the major ceremonial center of the Nasca during the beginning of the Early Intermediate Period (200 years BC to 350 years AD). Archaeological excavations carried out in 2006 by Giuseppe Orefici revealed the existence of fragments of paintings on the walls of the great pyramid of Cahuachi. These are, up to now, the first examples of mural painting discovered on the site. They are one of the few murals discovered on the south coast.
The Cahuachi site is located in the center of the Rio Grande basin, 42 km from the Pacific coast (Fig. 1). It was erected on the south bank of the Rio Nasca, in the middle of the valley and of natural mounds. The presence of water, rare in this arid region, made it a sacred place: a huaca. The occupation of the site was very extensive since the site surface is estimated to be 24 km². Excavations have continued since the 1950s, started by W. Strong, then continued by H. Silverman and finally, since 1982 by CISRAP (Centro Italiano Studi e Ricerche Archeologiche Precolombiane) team. Until now, only part of the site has been excavated: the area where the most imposing buildings are concentrated.
The central sector of Cahuachi is made up of monumental ceremonial buildings, with the "Great Pyramid", the "Great Temple", as well as other major structures. The monumental district extends east and west, linearly, with a high density of construction. Oldest buildings were constantly covered by new structures, until the abandonment of Cahuachi (around 400 AD). In general, the walls of the structures of the old platforms were dismantled and their surfaces were covered with embankments in order to reconstruct new platforms on the top.
We can thus identify four construction phases and a fifth phase during which the abandonment of the site was ritualized [9, 10]. The most monumental structures were erected during the phases 2 (200 BC to 0) and 3 (0 to 250 AD): they consisted of several platforms two to three meters high, superimposed on each other. Phase 3 coincides with the peak of the Cahuachi ceremonial center activity. The oldest traces of wall paintings date from the third phase of construction, then other painted decorations were applied during the fourth phase (250–400 AD) [11, 9]. During the phase 4, the interior of the Great Pyramid changed of use and space was divided with narrower passages [10, 12].
Some fragments of wall paintings taken from the Cahuachi site were analyzed in the laboratory in order to characterize their materials and techniques . This was the first analytical study on Cahuachi murals and on fragments directly transferred to the laboratory from an excavation site, which permitted to avoid any conservation treatment or museum collection storage.
2. Archaeological Context And Sampling
The central area of Cahuachi is made up of predominant monuments such as the Great Temple, the Orange Pyramid, the Temple of the Escalonado and the Great Pyramid. They are connected to each other and to several other minor temples by squares, enclosures, passageways, staircases.
The Great Pyramid, 30 m high, is the tallest building. Its stepped architectural form and the abundant vestiges of ritual objects seem to indicate that it was the focal point of religious activity. The first fragments of mural paintings were discovered in this building.
The state of conservation of the paintings is poor, because the coating detaches from the wall and falls. As a result, the remains of wall paintings splitted into small fragments of painted plaster were found at the base of the walls (Fig. 2.a).
They are rare and too fragmentary to allow the original motifs to be reconstructed. In the upper part of the pyramid (phase 3), only fragments of red paint were found, while in a lower area (phase 4), several colors were present: yellow, orange, red, green, white, black and purple. Black or white lines seem to emphasize the outlines of patterns. Some sections of the walls may have been painted in only one color while others may have been decorated with polychrome patterns or scenes.
According to the first observations in situ, the preparation of the support and the pictorial technique seem to be relatively close to the contemporary cultures of the central and northern coast: the surface of the adobe walls is covered with mud-based coatings . This plaster is generally thick because it was used to even out the surface of the walls by filling in the hollows between the adobes (Fig. 2.b).
On the fragments of paint found in the lower part of the Great Pyramid, we note the presence of large incisions, generally underlined by a black line which was employed to separate two colors. The paint was applied after the incision on the clay plaster. Among the Great Pyramid paint samples, we found some with a thin white layer (plaster or whitewash) applied to the base plaster, and on which the pictorial layer was executed. In some cases, several layers of paint overlap one another or are separated by a layer of plaster. This recalls the painting practices used by northern cultures. The samples were chosen among the many fragments detached from the wall. We have chosen them with the aim of getting the most complete paint stratigraphy as well as the different colors and types of coatings.
The samples from our study were taken from four areas of Cahuachi (Fig. 3). All the fragments of mural painting were taken from three distinct areas of the Great Pyramid (Y8): two sectors belonging to the upper platforms of the pyramid (samples 16N, 17R, 18J, 19R) and a sector in the lower part of the pyramid (samples 01V, 02V, 03J, 04R, 05R, 06R, 07R, 08N, 08J, 09N, 10N, 11N, 12N, 13V).
Finally, two plaster fragments (14G and 15G) come from the Escalonado temple.
By observing the thickness of the coating of the fragments taken on site, we can distinguish a thin layer on the surface for the majority of the samples. The stratigraphy generally displays a first, coarser layer, which essentially has the function of equalizing the surface of the wall, and a second one, thinner and smoother, intended to receive the paint.