Serena Panighello

3D laser ablation-ICP-mass spectrometry mapping for the study of surface layer phenomena – a case study for weathered glass

In this work, a multi-elemental 3D laser ablation-ICP-mass spectrometry mapping procedure for high-resolution depth information retrieval to investigate surface layer phenomena is presented. The procedure is based on laser drilling on a virtual grid on the surface, followed by extraction of depth maps along the z-axis (for each element monitored). Using a burst of 50 laser pulses at 1 Hz on each point of the grid, a penetration rate of ca. 150 nm per pulse (in glass) was obtained and a lateral resolution in the order of the laser beam diameter. By ultrafast ICP-MS monitoring of individual ablation pulses (58 ms for a set of 19 elements) using a laser ablation cell with fast signal washout (less than 0.5 s for whole laser pulse), the corresponding peak areas could be consistently integrated, resulting in spatial elemental data associated with individual pulses. The usual laser drilling limitations such as pulse mixing and signal tailing are avoided with this approach. After manipulation of the spatial elemental datasets and quantification, stacks of 50 2D depth maps (for each element monitored) were produced which could be visualized as volume images or time-lapse movies. As a proof of concept, this approach was successfully used to investigate the degradation mechanisms of a medieval, weathered glass artifact by colocalization analysis of selected cross-sectional 2D elemental images in arbitrary planes of the volume images. It was shown that degradation must have started as a result of dealkalinization leading to depletion of alkalis/earth alkalis in glass surface layers and enrichment of network formers (Si and Al), and subsequent worsening by cracking and formation of corrosion pits and so-called spatiotemporal Liesegang rings indicative of radial leaching.

Zanzibar and Indian Ocean trade in the first millennium CE: the glass bead evidence

Recent archaeological excavations at the seventh- to tenth-century CE sites of Unguja Ukuu and Fukuchani on Zanzibar Island have produced large numbers of glass beads that shed new light on the island’s early interactions with the wider Indian Ocean world. A selected sample of the beads recovered was analyzed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to determine the origins of the glass used to make the beads and potential trade relationships are considered. The data show that two major glass types can be identified: mineral-soda glass, m-Na-Al, produced in Sri Lanka (and possibly South India) and plant ash soda glass. The latter comprises three subtypes: two with low alumina concentrations and different quantities of lime (here designated v-Na-Ca subtypes A and B) and one with high alumina (designated v-Na-Al). The v-Na-Ca subtype A beads are chemically similar to Sasanian type 1 glass as well as Zhizo beads found in southern Africa, while v-Na-Ca subtype B compares reasonably well with glasses from Syria and the Levant. While the mineral-soda beads were made in South Asia, it appears likely that at least some of the plant ash beads were made in South or Southeast Asia from imported raw and/or scrap Middle Eastern glass. In contrast, during this period, all beads imported into southern Africa were made of Middle Eastern glass from east of the Euphrates (v-Na-Ca subtype A) and appear to have arrived on ships from Oman and the Persian Gulf. These data suggest that the two sections of the African coast were engaged in different Indian Ocean trade circuits.

Optimization of 2D LA-ICP-MS Mapping of Glass with Decorative Colored Features: Application to Analysis of a Polychrome Vessel Fragment from the Iron Age

2D elemental mapping of glass surfaces by LA-ICP-MS is an interesting technique to elucidate past technologies, establish provenance or understand deterioration processes of ancient, polychrome glass by visualization of the elemental distribution of the glass surface. However, selection of the appropriate LA-ICP-MS conditions for generation of high-quality elemental maps with the highest spatial resolution, lowest signal-to-noise ratio and shortest analysis time is normally a trial-and-error process. In this chapter a computational-experimental strategy is described to optimize the LA-ICP-MS conditions for 2D elemental mapping of polychrome glass by finding the best balance between fluence, beam diameter, repetition rate, scanning speed, gas flow rate and acquisition time. To aid in the initial selection of the optimal LA-ICP-MS conditions for spatial resolution and analysis time, a digital image of the glass was subjected to virtual 2D mapping, using existing software which simulates the actual LA-ICP-MS mapping process. To verify whether these initial conditions would result in an acceptable signal-to-noise ratio during the actual LA-ICP-MS mapping process, they were used to experimentally determine the detection limits for each element via a simple line scan on a “blank” glass, and consequently predict the noise floor in the maps. This strategy was successfully validated (using a modern murrina) and applied to a polychrome glass from the Iron Age yielding more insight into its elemental composition and the mineral sources involved.

Elemental mapping of polychrome ancient glasses by laser ablation ICP-MS and EPMA-WDS: a new approach to the study of elemental distribution and correlation.

In this work a new approach to the physicochemical characterization of polychrome archaeological glasses through elemental maps is presented. It is the first time that elemental mapping both by Laser Ablation-ICP-MS and EPMAWDS was performed on ancient glass. The glass elemental mappings are here proposed as useful tools for a preliminary study of the overall pattern of a glass surface concerning each analyzed element. The visual inspection of the maps gives the distribution of the elements and their degree of homogeneity; this allows the identification of the correlations between elements, in order to get information about chromophores, opacifiers and their associated ores, as well as about the glass deterioration. The LA-ICP-MS quantitative elemental maps of 54 elements were performed on a glass sample area. The concentrations of the element oxides were visualized in pseudo-colors, both in 2D and 3D. EPMA-WDS elemental maps were performed on areas of surface and break-section of the samples, at the interface between bulk and decorations. LAICP- MS elemental mapping can be functional to the determination of the glass chemical composition patterns and of associations between elements, while WDS elemental mapping is mainly aimed to identify the distribution of crystalline phases or to visualize concentration gradients of elements at the interfaces of different areas, such as bulk and decorations. Both the techniques can be functional to the study of glass superficial weathering.