1996-04

1996-04

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The goal of this research project was to study the deterioration of porous stone in the marine environment of the Pacific Coast, and to develop and test appropriate preservation methods, using the facade of the Royal Presidio Chapel as a case study.

Visual inspection of the Chapel indicated that lateral movements, meteoric and ground water, and soluble salts are the main causes of deterioration of the Chapel’s facade and walls.

A small section of the foundation was excavated, using proper archaeological techniques. The foundation consists of stone rubble with mud mortar.

A total of 27 samples of stone, mortars, renderings, patching and repair materials, salt efflorescence, and paint were taken and analyzed. In an attempt to locate the quarry from which the facade stones were extracted, we visited thirteen quarries and took stone samples which were subsequently characterized in the laboratory.

The facade contains two types of stone of differing resistance to weathering and stability against salt crystallization. No previous preservation studies of these two stone types are known to the project team.

A variety of soluble salts were identified. While the sulfates are concentrated on the surface, there is a remarkable increase of the chloride and nitrate concentration towards the interior of the wall. No explanation for this phenomenon could be found. High concentrations of hygroscopic salts were found in all samples.

Patching and repair mortars consisted of Portland cement based on mortars or proprietary mixtures. Their excessive hardness and reduced permeability for water has led to separation and spalling of many of these repairs.

A hydrological evaluation demonstrated that ground moisture, leading to rising damp, is the result of landscape irrigation or other applied water, not due to a high water table.

The paint was removed from part of the facade to study the distribution of the two types of stone, and of the various patching and repair mortars. A paint remover was identified that can be used for paint removal on the facade.

No chemical consolidation experiments on the deteriorated stone of the facade was attempted. An explanation for this departure from the original proposal is given elsewhere in this report.

The scientific studies were accompanied by historic research. Historic photographs were identified and duplicated, to document changes in architectural features and the state of preservation of the Chapel.

Recommendations for future studies and immediate and future interventions, designed to slow down the deterioration, are given at the end of this report.

This research was made possible through Grant MT-0424-4-NC-14 from the National Center for Preservation Technology and Training (NCPTT).

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