Calcium Tartrate Tetrahydrate Preconsolidation of Salt-Contaminated Limestone at Mission San José Y San Miguel De Aguayo

Abstract

The highly carved limestone of the mission church of San José y San Miguel de Aguayo in San Antonio, Texas is considered to be the zenith of artistic expression of eighteenth century Spanish baroque architecture in the continental United States. Salt contamination, leading to flaking and delamination of the basal stone of the celebrated sacristy window, has caused accelerated disfigurement and instability of the carved surface. Physical and chemical analyses were performed on original and newly quarried stone to assess the performance of various consolidation treatments in the presence and absence of salts. Petrographic and chemical analyses identified the stone as a dense, impure micritic limestone and the salts as extrinsic gypsum and nitrates. Subsequent testing examined the effects of a tartrate-based preconsolidant followed by an ethyl silicate consolidant on saltimpregnated and non–salt-impregnated stone. Results from the analysis and testing, together with observations during field treatment, suggest that the application of a tartrate-based preconsolidant (ProSoCo Hydroxylating Conversion Treatment/HCT), followed by ethyl silicate (ProSoCo Conservare OH 100 Stone Strengthener/OH100) consolidation, was successful in stabilizing the fragile stone surface, even in the presence of salts, and did not negatively reduce the water absorption and water vapor permeability of the stone. After consolidation, the stone was grouted with a moderately hydraulic lime grout (St. Astier 3.5), fractured and delaminated stones were micropinned with alumina ceramic pins and Paraloid B-72 adhesive, and losses were filled with a hydraulic lime mortar. In addition, the entire surface was cleaned with a commercial, low-pressure/low-volume, microabrasive system (Sensi-Clean) utilizing precipitated chalk.