Natural or Artificial? Multi-Analytical Study of a Scagliola from Estoi Palace Simulating Imperial Red Porphyry

Abstract

In this paper the characterization of a gypsum plaster sample from the end of the 19th century simulating imperial red porphyry using a multi-analytical approach is presented and discussed. The results of X-ray diffraction (XRD), thermogravimetric and differential thermal analysis (TGA-DTA), physical and mechanical properties are summarized. In order to have further insight into the microstructure, polarized light microscopy (PLM), scanning electron microscopy coupled with energy dispersive X-ray spectrometer (SEM-EDS), and micro Raman spectroscopy analyzes were also made. They helped to clarify the main issues raised by the other complementary analytical techniques and allowed the establishment of interrelations between the different properties, providing important information about the materials, the skills, and the technological development involved in the art of imitating noble stones with gypsum pastes. This study also contributes to our knowledge concerning the preservation of these types of elements that are important in the context of European decorative arts and rarely reported in the literature.

Measuring the Burning Temperatures of Anhydrite Micrograins in a High-Fired Medieval Gypsum Mortar

“…Note that all grains exhibit lower crystallinity, i. e. broader bands, at their borders due to thermal or mechanical damage undergone during preparation of the binder (burning, grinding, etc.). In the same sense, we interpret the fibrous crystals in Figure d, showing a typical habitus of primary as well as of burned anhydrite,–,, as naturally formed minerals with stronger border effects due to the crystal shape. Thus, the imaging approach not only provides access to measurements of single microstructures but also supports the data interpretation.…”

Raman band widths of anhydrite II reveal the burning history of high‐fired medieval gypsum mortars

“…Unreacted remnants of anhydrite II, either found in clusters of firing products or spread within the hydrated gypsum matrix, provide access to the burning history of high‐fired medieval gypsum mortars. Morphological changes of thermal anhydrite grains observable by polarised light and scanning electron microscopy (SEM) are discussed in the literature, but quantitative results can only be gathered by a spectroscopic analysis. Raman microspectroscopy turned out to be extraordinarily suitable, because it combines high structural sensitivity with submicrometric lateral resolution enabling to pinpoint individual anhydrite crystallites in mapping measurements of thin‐sectional samples of medieval mortars …”

Characterisation of stucco marble coatings towards their preservation

“…Resultados semelhantes haviam sido obtidos em estudos de duas amostras de stuccomarmo pertencentes a dois palácios portugueses: uma a imitar mármore do Palácio de Monserrate [24], e outra a imitar pórfiro vermelho imperial do Palácio de Estoi [25].…”

“…Um dos raros trabalhos que procurou caracterizar os materiais realmente usados neste tipo de revestimentos foi levado a cabo no âmbito do projecto europeu ENVIART, no final dos anos 90, do qual resultaram várias publicações [9,11,[22][23]. Em Portugal, com excepção de algumas amostras analisadas no âmbito dos trabalhos de doutoramento da primeira autora (uma de imitação de mármore, do Palácio de Monserrate [24] e outra de pórfiro vermelho imperial, do Palácio de Estoi [25]) e de Vieira [15], desconhece-se a existência de outros estudos, lacuna que se estende a vários países da Europa.…”