M. C. López de Azcona

Chromatic parameters as performance indicators for stone cleaning techniques

To determine the degree of soiling of urban buildings subject to environmental pollution and, at a later stage, to assess the effectiveness of cleaning techniques on the types of stone used in the construction of these buildings, it is very useful to define the chromatic parameters (L*,a*, b*,C*,WI, and YI) before and after treatment. A comparative study of these parameters and the original color of the stone is carried out. The best cleaning performance is achieved using a hot water jet at a temperature of 90°C and a pressure of 120 bars. In areas with significant black encrusted gypsum and calcite layers, these cannot be removed completely with water. Thus, chemicals must be applied, the most appropriate being sodium hydroxide-based products. Using these cleaning techniques, it is possible to obtain chromatic parameters that come close to those of the stone in its original state, allowing for an increase in yellow hues resulting from the preservation of the patina inherent to the natural aging process that stone surfaces undergo.

DEGRADATION OF BUILDING MATERIALS OF THE TOLEDO CATHEDRAL (SPAIN)

The Toledo Cathedral was built with different types of stones (mainly granites and carbonates) which display varied weathering features. It is concluded that rock degradation induced by pollution is weak. The alteration has been mainly due to climatic controlled factors, such as humidity, thermal changes and frosting. These processes resulted in scaling of the stone blocks, alteration of the granite texture to sand, and the development of discontinuities within the rocks.