Cobalt absorption bands for the differentiation of historical Na and Ca/K rich glass

Abstract

The main purpose of this publication is to define sensing parameters by means of optical spectroscopy to study and characterize ancient glass. Because during different times different flux agents were used, a chemical individuation of these elements provides a first dating classification. Normally, X-ray-based chemical methods are applied to this kind of investigation. On the basis of the Ligand-field theory, the absorption band positions of a transition metal ion undergo a change when the ion is placed in different chemical environments. A total of 25 cobalt coloured Roman, post-medieval and industrial glasses were analyzed by means of SEM-energy dispersive X-ray spectroscopy to group them according to their chemical composition. Optical spectroscopy was applied to build a method to study glass with this low-cost and nondestructive technique. It is proved that the cobalt absorption band situated around 535.5 nm for sodium-rich glasses (Roman and industrial material) is shifted towards 526.5 nm for calcium-potassium-rich glasses (post-medieval window glass). This feature was employed as a marker in the case study of the Renaissance window of the Church Wardens from the Church of Our Lady in Bruges, Belgium to distinguish the post-medieval material from the 19th and 20th century restoration material.

Characterisation of Roman and Byzantine glasses from the surroundings of Thugga (Tunisia): Raw materials and colours

“…the position of absorption bands substantiates that it has a soda -based vitreous matrix [29]. Further, sample n. 49 exhibits a different spectrum in the NIR region: the peak linked with the Fe 3+ absorption at 320 nm is more intense and it is better defined than in the case of samples of site 550.…”

“…Achievable information from reflectance spectra are linked to properties of light reflection, which results in a chemical and chromatic characterisation of the gems. This kind of analysis, not so much used on amorphous transparent materials, provides indeed a huge amount of information about both chromophore and glassy matrix constituent [29,30,31]. Moreover in some cases the position of absorption bands in FORS spectra is affected by the vitreous matrix, as it will be highlighted in the following [32].…”

“…On the other hand, it is difficult to reveal and quantify cobalt by XRF when iron is present in not negligible amounts since Co Kemission is hidden by Fe K line. The presence of cobalt oxide can instead be easily seen in FORS spectra due to three characteristic absorption bands (540-590-640 nm) which can be shifted towards lower wavelengths (first band at 526 nm) in presence of potash glass matrix [29]. Moreover, the cobalt used to produce ancient glasses could be an impurity in iron and nickel rich minerals so that iron and nickel presence must also be considered together with cobalt.…”

Authenticity screening of stained glass windows using optical spectroscopy

“…Apparent is the common spectral position of the first cobalt absorption band of all blue fragments close to 534.2 nm (±1.6). Ceglia et al correlates the spectral shift of this band to the composition of the historical glass26. Glasses with a calco-potassic composition have a band situated around 526.5 nm (±1.5 nm) shifting towards longer wavelengths (535 nm ±2 nm) in case of a soda based composition.…”

Composition and microstructure of maiolica from the museum of ceramics in Ascoli Piceno (Italy): evidences by electron microscopy and microanalysis

“…Blue decorations were obtained by employing cobalt compounds or a blue cobalt-rich frit, as highlighted by FORS spectra, in which the three absorption bands of Co(II) ions in a distorted tetrahedral coordination geometry are evident in the range 500-700 nm [14][15][16]. This preliminary inspection, besides highlighting the presence of Co(II), also point to a similar composition of the glazes, as no significant differences emerged among the position of Co(II) absorption bands recorded on different samples [16], as shown in Fig. 4.…”