CONSERVATION OF THE ARCHITECTURAL DECORATION OF MISSION SAN XAVIER DEL BAC, TUCSON, ARIZONA

Abstract

The Mission of San Xavier del Bac, located seven miles south of Thcson, Arizona, on the San Xavier Tohono O'odham (Papago) Indian Reservation, is considered to be the finest example of Spanish colonial architecture in the United States. The interior of the church is almost entirely decorated in a late Mexican baroque style, comprising wall paintings as well as polychromed and gilded sculptural and architectural elements. Dating from the late eighteenth century, this·decoration is of a very high standard and is surprisingly intact. The mission ~hurch is a living monument, served daily by Franciscan fathers whose order constructed the present church c .1778-97, with the help of the indigenous Tohono O'odham Indians. The east transept chapel of 'La Madre Dolorosa' (Sorrowing Mother) contains polychromed architectural relief work and wall paintings. The latter portray La Nuestra Senora del Rosario (Our Lady of the Rosary), La Virgen Nitla (The Young Virgin Mary), and mourning figures of the Sorrowing Mother and St John the Evangelist on either side ofthe central Crucifixion. The niches and oval medallions contain relief and free-standing sculptures of Franciscan saints. The wall paintings are executed on a very fine buff-coloured and water-sensitive preparation layer applied over a lime-sand plaster. The pigments have been applied a secco with an aqueous tempera binder which is fairly sensitive to moisture. The relief decoration is composed of painted plaster moldings and cornices, th~ finest of which is the highly ornate frame of the central crucifix which consists of a stylized floral band and scalloped channels coloured with transparent glazes applied over metallic leaf. Examination of cross:-sectional samples taken from this area reveals the following stratigraphy: lime-sand stucco overlaid with two gesso preparation layers, a coarse layer covered by a finer one, both containing calcium sulphate anhydrite. Madder lake and copper resinate glazes have been applied over silver leaf on a red coloured bole. This has been overlaid with a 61i-thick varnish layer. In addition to natural earths, the pigments employed in the wall decoration include: Prussian blue, vermilion and orpiment. Although there is some minor evidence of rising damp and hygroscopic salts,. except for the areas where it is clear that there have been infiltrations of rainwater in the past (the vault), moisture does not seem to have been a major agent. of deterioration. Extremely low relative humidity, .high temperature fluctuations and high natural' and ultraviolet light levels appear to be the primary causes of the localized detachment of the painted and relief decoration. The original polychromy is largely obscured by thick layers of dust, candle soot, and bird and bat excrement as well as by the hardened nests of mud dauber wasps (Sceliphron caementarium) . During some previous effort at restoration, possibly early in the twentieth century, a fixatiye was liberally applied to the surfaces. The intention seems to have been to revitalize the original colours. The fixative, which appears to have:darkened and turned orangebrown, encouraged contraction of the paint and preparatory layers at low levels of relative humidity and resulted in the separation of these layers from their support. Analysis of the fixative using Fourier transform infrared analysis shows that it is a n.atural tree resin, possibly from the plesquite tree (Prosopis juliflora). Very unsightly filling oflacunae,'oone in 1968, is evident in the scene of the Virgen Nina. The incompatible filling material, which