AIC's 41st Annual Meeting

The retreatment of previously consolidated stone has become an increasingly common occurrence and an important area of research in the field of architectural conservation. The San José Convento Column within the San Antonio Missions National Historical Park in San Antonio, Texas is a prime example for consideration. Initially analyzed, tested and treated in 1993 by the Architectural Conservation Laboratory at the University of Pennsylvania, the column today is now in need of additional treatment. Recent analysis has determined that while the 1993 treatment had been largely successful in stabilizing the active flaking and decohesion of the stone surface, microcracks originally present have advanced and original surface detachment has worsened, largely due to clay swelling deterioration. It is likely that this deterioration mechanism existed in 1993, but was less visible due to the extent of detachment and flaking. Over the past 19 years, further research has been performed on modified ethyl silicates that inhibit clay swelling while restoring the grain-to-grain cohesion of argillaceous stone. In order to determine the most appropriate consolidation treatment for the column, several consolidants including a modified ethyl silicate were tested to determine the most appropriate method for in situ application.

Research undertaken for this study has focused upon the continuing deterioration of the column due to swelling of clays inherent within the stone. Local argillaceous limestone samples similar to the column were taken from the perimeter wall of the mission and previously studied during the initial 1993 treatment tests. These samples have been used for the purposes of this research. These samples, consisting of 2-inch square cubes and 2-inch x 2-inch x 0.2-inch thick coupons were treated with four different consolidants including a pre-treatment with an anti-swelling agent. Untreated and treated samples were then subjected to physico-mechanical testing to evaluate the effect of these treatments on stone strength, water absorption, permeability, and wet-dry cycling. Bulk samples taken from the column were analyzed through XRD to identify the swelling clay mineral composition of the stone. New methods of assessing the consolidation effects involving resistance drill testing were used to measure depth of penetration and efficacy of application on a large boulder recovered from the site. Durability of the stone was determined by wet/dry cycling coupled with SEM-EDS analysis to visualize the interaction of the consolidants with clay particles within the stone. The information and data obtained from this study will inform the future retreatment of the column needed to resolve the microcracking visible today. The conclusions drawn from this work will also contribute to the broader issues of retreatment and life cycle of treatments on previously consolidated stone.