Los modelos cristalográficos del Museo de la Geología (Facultad de Ciencias Geológicas, Universidad Complutense de Madrid, España). Identificación mineralógica y textural

Abstract

En este trabajo se realiza el inventario completo de una colección de modelos cristalográficos que se exponen en el Museo de la Facultad de Ciencias Geológicas de la Universidad Complutense de Madrid. Dicha colección consta de 312 piezas; 186 fueron catalogadas en una publicación anterior (López-Acevedo, 2006), mientras que las 126 restantes aparecieron con fecha posterior y no pudieron ser incluidas en este primer inventario. Estos modelos, además, se estudian e identifican mediante la utilización de técnicas no destructivas, lo cual ha permitido diferenciar dos grupos de ejemplares. Un primer grupo, más numeroso, son cerámicos y atribuibles a Jean Baptiste Romé de L´Isle (1736-1790). Pudieron ser modelados a mano, a partir de mezclas de illita - caolinita (16% - 84%) con cuarzo, óxidos de hierro, chamota e incluso cenizas óseas. Las marcas dejadas por el desgrasante indican que fueron facetadas antes de su cocción. Los elementos contaminantes detectados demuestran que la cochura se realizó en un horno de porcelana. El segundo grupo de modelos consta de 14 piezas de yeso. La presencia de abundante Zn en el engobe indica que pueden ser posteriores a las de cerámica y no guardar ninguna relación con ellas.

The beautiful geometrical shapes often shown by minerals must have been the historical reason for human beings to become interested and started their studies on them. To explain these symmetric morphologies, those early scientists made idealised replicas, bigger than the original minerals they were studying, that is, crystal models that provided the necessary measurements and verifications to establish the first crystallographical laws. The earliest collections of models known were made of ceramics by Jean Baptiste Romé de L´Isle (1736-1790) and stand out for their aesthetical value and their great interest from the point of view of History of Crystallography (Amorós, 1978). Subsequently some other scientists made their own collections using different materials, especially wood. There are also few collections made of porcelain, with ornamental or merely commercial aim (Touret, 2004). The models of Romé were sculpted on clay and then fired on a stove. In fact the exact procedure followed to make these pieces and how could Romé reach such precision with this soft material, or how he avoided that they got distorted during the firing remain unknown. This research is focused on the study from this point of view of some pieces belonging to the collection of the Departament of Crystallography of the Complutense University of Madrid (Spain). This collection consists of 312 models. 292 of them are ascribable to Romé de L´Isle, while the 20 remaining show totally different characteristics. During this research, non-destructive identification techniques have been used, such as binocular lenses, hydrostatic balance and environmental scaning electronic microscope. Moreover, usual procedures in art restoration have been adapted to carry out DRX diagrams based on microsamples. They have allowed to identify some components and even make a quantitative estimation of their relative proportions. The obtained results point out that the group of pieces ascribable to Romé could have been made of mixtures of illite-kaolinite in a proportion of 16% - 84%, with quartz, iron oxides, grog and even ashes of bone. They must have been hand-sculpted and faceted before their firing. This is deduced from the desgrasante dragging marks shown in the faces of the models, reminding the slickenside kinematic indicators (Dobla s, 1998). The superficial pollution they show,consisting of Pb, Fe, Ag and Sb, proves they were fired in a porcelain stove. The presence of repeated pieces points out they are the remainder of different collections. Plates I to VII show pieces classified according to the same criteria used by Romé de L´Isle (1783). Romé considered “primitive” forms that he modified truncating its corners and bavelling its edges. Finally, the group of pieces designated as “different” are made of gypsum, predictably made with a mold and painted with a slim layer of non-fired clay. The abundant presence of Zn in the engobe that decorates them indicates they can be dated later than 1837 and they do not have any connection with the ceramic ones.