Chemical characterization of the stained glass window from the rose window, Siena Duomo (Italy, 1288-1289)

[EN] The chemical composition of nine medieval coloured stained glasses from Duomo (Catedral) of Siena, Italy, has been characterized. They come from the rose window elaborated under the drawing of Duccio di Buoninsegna masterwork (1288- 89 AD). This note explains the results obtained by EMPA, representative of bulk chemistry of several coloured glasses (deep green, olive green, yellow, purple, pink, deep blue, light blue, red plaqué and also uncoloured), as well as the associated trace elements (obtained by Induced Coupled Plasma Mass Spectrometry) that represent the chemical fingerprint of these glasses. The studied samples are sodium-calcium glass (chemical compositions in the range 13-14 wt% Na2O, 56-64 wt% SiO2, 4 wt% MgO, 9-10 wt% CaO, 2,5-4 wt% K2O); thus of Mediterranean tradition.This fact has been found by our team in previous studies (stained glasses from the church of Monestir de Pedralbes, Barcelona) for XIV century glass made at least 40 years later. As a general rule, the conservation state of these sodium glasses is good, except for the cohesion of grisaille to glass mesostase. This study allowed to identify three glass groups developed from different raw material formulations: a first group is constituted by deep green, olive green, light blue and yellow glass; a second one comprises by uncoloured, deep purple, deep blue and pink glass; and finally, the plaqué red glass that shows deeply different composition. Pink and yellow glass was produced following the traditional recipe compiled by Theophilus, a monk who lived at the beginning of XII century. This implies a separate process of raw material purification and a careful control of the redox kiln conditions; thus, these glasses can be considered as traditional or technologically not evolved. The deep blue, light blue and deep purple were obtained after the artisan dosed addition of a cobalt salt. Olive green and deep green glasses were produced with addition of copper (together with manganese and iron), previously prepared as a pigment that has as excipient a potassium glass. The use of potassium glass instead of the sodium glass locally produced strongly suggests that the pigment could be bought directly from Middle or Northern Europe markets and/or could be a sub-product of copper ore melting. On the other hand, the production of sodium red ruby plaqué glass, technologically more evolved and comparable to the coeval potassium glass coming from the Central Europe glass factories, would be outstanding in the XIV century; but we interpret (taking into account its chemical composition) that was produced later and introduced during the restoration conducted at the end of XVII century. We can also note the use of well-dosed lead additions in order to increase the lightness and transparency of coloured glass. Manganese has been a very important element in the Siena workshop glassmaker colour palette achievements. On the other hand, the trace-element chemical fingerprint of the glass allows arguing what kind of mineral salts were used as pigments, as well as the way to introduce it in the uncoloured original glass.[ES] Se han caracterizado químicamente una colección de vidrios arquitectónicos coloreados originales del rosetón del Duomo (catedral) de Siena, Italia, realizados bajo diseño del artista Duccio di Buoninsegna en 1288-89. Esta comunicación expone los resultados obtenidos mediante el empleo de microsonda electrónica de Castaing (mayoritarios) y espectrometría de masas con fuente de plasma acoplado inductivamente (ICP-MS, elementos en traza) en vidrios de varios colores (incoloro, verde oscuro, verde oliva, amarillo, violeta, rosa, azul oscuro, azul celeste, rojo plaqué). Se trata de vidrios sódico-cálcicos (valores en peso alrededor del 13-14 % de Na2O, 56-64 % SiO2, 4% MgO, 9-10 % CaO, 2,5-4 K2O) de tradición por tanto mediterránea. Un resultado semejante se encontró precedentemente para vidrios realizados a lo largo del siglo XIV, cuanto menos unos 40 años después (vidriera de la iglesia del Monestir de Pedralbes, Barcelona). En general, el estado de conservación de estos vidrios es bueno, excepto por lo que se refiere a la adherencia de las grisallas al vidrio base. El estudio desarrollado ha permitido identificar tres grupos de vidrios desarrollados a partir de formulaciones de diferentes composiciones: un primer grupo constituido por los vidrios de color verde claro, verde oscuro, azul celeste y amarillo; un segundo grupo constituido por los vidrios incoloro, violeta, azul oscuro, y malva; y finalmente el vidrio rojo plaqué, de composición netamente diferente a todos los demás. Desde el punto de vista de la obtención de los colores, cabe destacar que los datos químicos permiten deducir que el malva y el amarillo han sido elaborados siguiendo la receta tradicional del monje Theophilus de inicios del siglo XII, utilizando un proceso de purificación especial de las materias primas, y controlando artesanalmente las condiciones redox del horno; en este sentido, estos vidrios se pueden calificar como tradicionales o tecnológicamente poco evolucionados. Los colores azul oscuro, violeta y azul celeste se han obtenido mediante la adición de una misma sal de cobalto en diferentes dosis, y los colores verde oliva y verde oscuro mediante la adición de cobre (junto con manganeso y hierro), previamente preparados en un colorante que tiene como excipiente un vidrio potásico. Este último hecho (el empleo de vidrio potásico no usual en los centros productores del área mediterránea) permite suponer que el color puede haber sido comprado directamente a un fabricante centroeuropeo o que se tratara de un subproducto de fundición de minerales de cobre. Por su parte, la fabricación del vidrio plaqué, tecnológicamente más complejo y comparable al vidrio rojo plaqué potásico centroeuropeo contemporáneo indicaría, si fuera original, la incorporación de este proceso tecnológico al taller local. En opinión de los autores se trata de un vidrio de producción local mucho más tardia, incorporado en la restauración documentada a finales del siglo XVII. Destaca también, como en Pedralbes, el empleo del plomo para aumentar la luminosidad y transparencia del vidrio, en dosis variables atendiendo al color del vidrio. Por otro lado hay que señalar que en Siena el manganeso es un elemento muy importante con el que el fabricante del vidrio jugó intensamente para obtener la paleta de colores. El análisis de los metales presentes como elementos traza, y de las tierras raras, permite establecer sólidas hipótesis sobre el tipo de sales minerales empleadas como colorantes y como fueron incorporados al vidrio incoloro original.Este trabajo se ha podido realizar en el marco de diferentes proyectos financiados (proyecto 3338 de la Fundació Bosch i Gimpera, UB; acción integrada hispano-italiana HI2006-0190: La producción y el comercio del vidrio plano en Europa y en el Mediterráneo Occidental, entre los siglos XI y XV: un estudio arqueométrico) . La caracterización geoquímica de los vidrios mediante MSE e ICP-MS se ha llevado a cabo en los Serveis Científico-Tècnics de la Universitat de Barcelona (SCT-UB);Peer reviewe

Prograde metamorphic evolution and development of chloritoid bearing eclogitic assemblages in subcontinental metagabbro (Sesia-Lanzo zone, Italy)

In the coronitic metagabbroic rocks of the Corio and Monastero metagabbro bodies in the continental Sesia–Lanzo zone of the western Italian Alps, a variety of mineral reactions that testify to prograde conditions from greenschist to eclogite-facies can be recognised. A microstructural and microchemical study of a series of samples characterized by coronitic textures and pseudomorphic replacement of the original igneous minerals has allowed the prograde reactions undergone by the rocks to be established. In completely eclogitized coronitic samples, paragonite, blue amphibole, garnet, epidote, fine grained jadeite and chloritoid occur in plagioclase microdomains (former igneous plagioclase). The mafic mineral microdomains consist of glaucophane and garnet. Complexly-zoned amphiboles constrain changing metamorphic conditions: cores of pre-Alpine brown hornblende and/or tremolite are preserved inside rims of a sodic–calcic amphibole that are in turn surrounded by a sodic amphibole. The main highpressure mineral assemblage, as seen in mylonites, involves glaucophane, chloritoid, epidote, garnet±phengite, ±paragonite. Some layers within the gabbro contain garnet, omphacite, ±glaucophane, and acid dykes crosscutting the gabbro body contain jadeite, quartz, garnet, epidote and paragonite. The presence of chloritoid-bearing high-pressure assemblages reflects hydration of the gabbros during their pre-Alpine exhumation prior to subduction, as well as the composition of the microdomains operating during subduction. The pressure and temperature conditions of gabbro transformation during subduction are inferred to be 450–550 °C at up to 2 GPa on the basis of the chloritoid-bearing assemblages. The factors controlling the reaction pathway to form chloritoid-bearing high-pressure assemblages in mafic rocks are inferred from these observations