Non-invasive Raman identification of crystalline and glassy phases in a 1781 Sèvres Royal Factory soft paste porcelain plate

Abstract

Both European and Asian historical records report that Jesuits were at the origin of enamelling technology transfers from France (and Italy) to Asia during the 17 th century. A mobile Raman setup equipped with a high magnification (x200) microscope objective with long working distance is used to identify the use of European (arsenic-rich) cobalt in 17 th and 18 th century porcelains: twenty soft-paste porcelains from Rouen (L. Poterat' Factory), Saint-Cloud, Paris (Pavie', Hébert', Chicanneau', and Bellevaux' factories), Lille, Mennecy, and Vincennes factories, of which almost all were produced before 1750, belonging to the Cité de la Céramique Collection (formerly Musée national de Céramique, Sèvres), have been analysed in order to get a more representative view of the type of cobalt ore used. A large proportion of blue decors exhibit signs of lead arsenate, even for productions after ~1750, which proves the wilful interest given to arsenic phases in magnifying blue hues. Different lead arsenate signatures are observed, one assigned to the product of reaction of As-rich cobalt ore with lead-based glaze and another characteristic of added arsenic. At least two groups of glaze are identified: a lead-rich one and a mixed lead-alkali glaze. Porcelains made during the second half of the 18 th century exhibit the very strong As-O modes characteristic of the voluntary addition of a lead-arsenate forming compound. Comparisons are made with Qing Chinese productions exhibiting similar Raman signatures and expected to have benefited from transfers of technologies.

Non‐invasive on‐site Raman study of polychrome and white enamelled glass artefacts in imitation of porcelain assigned to Bernard Perrot and his followers

“…Precipitation of colloidal gold with arsenic salt appears to be a characteristic of Perrot's methods whereas the Kunckel's method using precipitation with tin salts was more common in the 18th century production of ruby glass and Cassius purple glaze The narrow ~820 cm −1 peak is an indicator of the lead arsenate apatite phase Na 1 − x − 2y K x Ca y Pb 4 (AsO 4 ) 3 that is also formed by cooling on the reaction of cobalt ores containing arsenic (CoAs 2 , CoAs 3 , CoAsS, etc. ) from European mines with lead‐based glass (see further) . The broadness and position of the peak vary consistently with the parent arsenate phases, as identified by scanning electron microscopy–energy‐dispersive X‐ray spectroscopy and microdiffraction in some ancient glazed pottery and distinguishing the different phases from their sole Raman signature requires further studies.…”

“…There can only be two thermally stable blue chromophores present in glassy silicates: (a) Co 2+ ions dissolved in the silicate network or hosted in a pigment as a crystalline phase stable in the (molten) glass, namely phase‐hosting cobalt ions (cobalt silicate, cobalt aluminate, and spinels) and (b) a phase‐hosting S 3 − chromophore such as lazurite (a feldspar‐like mineral present in lapis lazuli rocks) or ultramarine (a synthetic zeolite) . Co 2+ ions dissolved in the glassy silicate matrix do not show a specific Raman signature, in contrast to the pigments that have very characteristic Raman signatures . In particular, the use of European arsenic‐rich cobalt ores leads to the precipitation of a lead arsenate apatite phase that is characterised with a strong ~820‐cm −1 band, in the case of lead‐containing glasses/glazes coloured with this type of cobalt source .…”

“…Co 2+ ions dissolved in the glassy silicate matrix do not show a specific Raman signature, in contrast to the pigments that have very characteristic Raman signatures . In particular, the use of European arsenic‐rich cobalt ores leads to the precipitation of a lead arsenate apatite phase that is characterised with a strong ~820‐cm −1 band, in the case of lead‐containing glasses/glazes coloured with this type of cobalt source . The reaction between the lead‐based glass and the As‐rich cobalt ore generates phases with varying compositions as Na 1 − x − 2y K x Ca y Pb 4 (AsO 4 ) 3 apatite according to the glass composition and the firing temperature.…”

“…However due to the higher number of electrons involved in the As─O bond, its intensity will be stronger than that of the Co─O one. Consequently, the variations in the resulting solid solutions as a function of composition and firing conditions induce slight shifts in the peak wavenumbers of the ~820 cm −1 band as well as changes in the bandwidth and intensity . It is also worth mentioning that discrimination of these possible different phases from their sole Raman signature requires further multi‐technique studies.…”

Breaking Preconceptions: Thin Section Petrography For Ceramic Glaze Microstructures

“…These procedures have both the advantage to preserve the samples from the sectioning and to cut on time and costs. Sometimes, the samples are prepared in the form of crosscut pieces of several millimetres for the Raman study [85][86][87][88][89][90][91][92][93]. The use of thin sections for the Raman investigations within the frame of Cultural Heritage studies is quite rare, despite what happens in other research areas such as mineralogy and petrology [94][95][96][97].…”