Abstract
Color-induced changes in stones due to heating can be useful in determining the degree of degradation generated in their components and structure. We evaluate the effect of heating in three types of limestones, useful in building and restoration, corresponding to biomicrite (BC), biosparite (BS), and intramicrite (IM), in the range of temperature from 100°C to 600°C. Our analyses indicate that the strongest changes in reflectance occur in the samples of intramicrite, followed by biomicrite, being biosparite showing the smallest ones. Additionally, IM samples suffer the largest modifications in effective porosity due to the thermal treatment; in contrast, BS and BC samples show smaller temperature-induced alterations associated with smaller changes in reflectance. Moreover, we reveal that most of the thermally induced variations are associated, in this range of temperature, to the dehydroxylation of goethite that transforms into hematite, as well as by the burning and partial release of the organic matter present in the rock. Colorimetry analyses show that the sequence of thermally induced changes can be followed by the evolution of the three coordinates, L∗(lightness), a∗(red-green), and b∗(yellow-blue), of the CIE 1976 L∗a∗b∗space.
Highlights
Limestone is the most abundant type of rock in southeastern Mexico and constitutes the basis of the building legacy in pre-Columbian, colonial, and modern periods
Limestones are mostly composed of Journal of Spectroscopy calcium carbonate (CaCO3), with an appearance, which generally ranges from white, passing through a great variety of colors. ese colors are related to the roughness of the surface of the stone, to the content of organic matter, and to the presence of quartz, feldspar, and clay minerals, among many other factors [4, 6]
We study the influence of heating on the optical properties and color changes in three types of limestones. e range of thermal treatments were from 100°C to 600°C, in which the major damage of the structure can be avoided. e analyses of the optical properties were performed by diffuse reflectance in the visible region of the electromagnetic spectrum. e changes in color are analyzed and described in terms of the International Commission on Illumination CIE 1976 L∗a∗b∗ color space
Summary
Limestone is the most abundant type of rock in southeastern Mexico and constitutes the basis of the building legacy in pre-Columbian, colonial, and modern periods. Ancient Maya used different types of limestone to develop structures, buildings, and sculptures, as can be observed in numerous Maya archeological zones such as Uxmal and Chichen Itza. Limestones are a fundamental part of several historical buildings and monuments, dating from the colonial period. Limestone is used for various purposes in the building industry and in the restoration of historical monuments such as stonework and sculptures [1,2,3]. E natural color of this type of sedimentary rock reflects the fundamental characteristics of its mineral composition and natural pigments [4, 5]. Limestones are mostly composed of Journal of Spectroscopy calcium carbonate (CaCO3), with an appearance, which generally ranges from white, passing through a great variety of colors. ese colors are related to the roughness of the surface of the stone, to the content of organic matter, and to the presence of quartz, feldspar, and clay minerals, among many other factors [4, 6]
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