Documentación y reconstrucción virtual en restauración de obras pictóricas de gran formato: el lienzo mural de la farmacia Zambrano

The aim of this paper is to explain the convenience of photogrammetry and virtual reconstruction applied to the restoration of large format canvas. This study presents the application of these techniques to the restoration and musealization of a late 19th century mural canvas painting attached to the ceiling of the Zambrano Pharmacy (Granada, Spain). The painting is an example of the allegorical motifs that could be found in 19th and 20th century pharmacies. It represents a group of cherubs and allegorical figures of Science or Pharmacy sitting in the clouds; the scene is surrounded by an architectural frame. The mural painting by Francisco Moron & Lujan (Granada, 1846 - Huercal-Overa, Almeria, 1899) shows his signature in the lower right corner of the painting. The painting was in a good overall condition, but presented considerable yellowing and darkening (due to exposure to nearby pollution from the street, dust, etc.) that required its cleaning to reveal its true colors. In 2018, the pharmacy was acquired by the University of Granada, initiating its transfer to the conservation laboratories for its restoration, with two aims: recovering its original appearance, and preparing for its display in the “Ciencia, ciudad y cambio” exhibition (Hospital Real of Granada, February 6th to may 17th 2019); to this day, the painting is located in the crossing of the Hospital Real. Once the intervention started, its large dimensions (7.6 x 3.3 m) made it impossible to obtain a complete high-quality orthogonal image using traditional photography methods. Therefore, it was decided to use photogrammetry for the correct documentation of: a) the initial state of the painting, b) the different restoration phases (initial documentation, mechanical cleaning of the reverse, removal of the protection of the front, cleaning of the front and pictorial reintegration), and c) the final result after the restoration process. Furthermore, this canvas was attached to the ceiling presumably with an animal glue adhesive, and during its intervention it was observed that the imprint of an earlier mural painting was attached to its reverse. Since the reverse of the canvas was hidden by the final mounting system, it was necessary to document the imprint of the previous mural painting adhered to the reverse of the canvas. To recreate the original painting and to allow a correct study and comprehension of this work, a virtual reconstruction based on the photogrammetric documentation of the reverse of the painting was achieved. The photogrammetric processing allowed us to obtain high-quality orthogonal images (10000 x 5000 px), thus demonstrating the suitability of this technique for the documentation of a large format canvas. The images obtained were also useful to study the dimensions of the paint, with a total area of 25 m2. Agisoft PhotoScan Professional was used for the photogrammetric model; the three-dimensional (3D) models and the textures were transferred to a 3D free software (Blender) for the rendering and recreation of the models. On the other hand, the virtual reconstruction was made using Adobe Photoshop to recover the entire painting. The methodology consisted in working with different layers to paint the missing parts of the motifs and the simulated architecture; then, the missing parts were reconstructed based on the preserved paint (24.5% of the total area) and, finally, textures and filters were incorporated to simulate the appearance of a mural painting. This research has proved photogrammetry is suitable for the documentation of a restoration process for large format pictoric works, since this technique allows to obtain high resolution orthophotos from the different intervention phases. Additionally, the virtual reconstruction has proven to be a useful tool for the documentation of the painting, its registration and its visualization; it can also recreate the decorative pattern and original colors.