Phase field theory of polycrystalline solidification in three dimensions

A phase field theory of polycrystalline solidification is presented that is able to describe the nucleation and growth of anisotropic particles with different crystallographic orientation in three dimensions. As opposed with the two-dimensional case, where a single orientation field suffices, in three dimensions, minimum three fields are needed. The free energy of grain boundaries is assumed to be proportional to the angular difference between the adjacent crystals expressed here in terms of the differences of the four symmetric Euler parameters. The equations of motion for these fields are obtained from variational principles. Illustrative calculations are performed for polycrystalline solidification with dendritic, needle and spherulitic growth morphologies.Comment: 7 pages, 4 figures, submitted to Europhysics Letters on 14th February, 200

Production technology of Nabataean painted pottery compared with that of Roman terra sigillata

The Nabataeans, who founded the city of Petra (southern Jordan) in the late first millennium BCE, are noted for the production of a distinctive very fine pottery with painted decoration and a wall thickness sometimes as little as 1.5 mm; this pottery appears largely locally made and not widely circulated. Using a combination of OM, SEM with attached EDS, surface XRF, and XRD, it is shown that the Nabataean fine pottery bodies were produced using semi-calcareous clays which were fired to temperatures of about 950 °C. In contrast, published data indicate that contemporary and in many ways apparently functionally equivalent Roman terra sigillata, which was traded throughout the Roman Empire, was produced using fully-calcareous clays which were fired to temperatures in the range 1000–1100 °C. Furthermore, the high gloss slip applied to Roman terra sigillata is fully vitrified whereas the red-painted decoration applied to the Nabataean pottery is unvitrified. The more robust Roman terra sigillata is therefore better suited as tableware for serving and consuming food than would be the case for Nabataean fine pottery, and would be a more successful export material

The use of micro-XRD for the study of glaze color decorations

The compounds responsible for the colours and decorations in glass and glazed ceramics include: colouring agents (transition metal ions), pigments (micro-and nano-precipitates of compounds that either do not dissolve or recrystallize in the glassy matrix) and opacifiers (microcrystalline compounds with high light scattering capability). Their composition, structure and range of stability are highly dependent not only on the composition but also on the procedures followed to obtain them. Chemical composition of the colorants and crystallites may be obtained by means of SEM-EDX and WDX. Synchrotron Radiation micro-X-ray Diffraction has a small beam size adequate (10 to 50 microns footprint size) to obtain the structural information of crystalline compounds and high brilliance, optimal for determining the crystallites even when present in low amounts. In addition, in glass decorations the crystallites often appear forming thin layers (from 10 to 100 micrometers thick) and they show a depth dependent composition and crystal structure. Their nature and distribution across the glass/glazes decorations gives direct information on the technology of production and stability and may be related to the color and appearance. A selection of glass and glaze coloring agents and decorations are studied by means of SR-micro- XRD and SEM-EDX including: manganese brown, antimony yellow, red copper lusters and cobalt blue. The selection includes Medieval (Islamic, and Hispano Moresque) and renaissance tin glazed ceramics from the 10th to the 17th century AD

Nucleation and Bulk Crystallization in Binary Phase Field Theory

We present a phase field theory for binary crystal nucleation. In the one-component limit, quantitative agreement is achieved with computer simulations (Lennard-Jones system) and experiments (ice-water system) using model parameters evaluated from the free energy and thickness of the interface. The critical undercoolings predicted for Cu-Ni alloys accord with the measurements, and indicate homogeneous nucleation. The Kolmogorov exponents deduced for dendritic solidification and for "soft-impingement" of particles via diffusion fields are consistent with experiment.Comment: 4 pages, 4 figures, accepted to PR

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Early Islamic lustre from Egypt, Syria and Iran (10th to 13th century AD)

This paper presents a study of a representative selection of lustre ceramics dating from the last quarter of the 10th century AD to the second half of the 13th century AD from Egypt, Syria and Iran. The study concentrates on the structure and chemistry of the lustre itself over the historical period considered and has found a number of significant similarities between the production centres studied. Previous work on the reproduction of lustre under laboratory-controlled conditions allows the archaeological data to be related to the historical technological aspects of lustre production. The results obtained, although restricted to the limited number of samples studied, have demonstrated the occurrence of significant differences and similarities between lustre productions during this period. The possible reasons for these changes are discussed

Temperature resolved reproduction of medieval lustre

Luster is a golden metallic-like decoration produced on glazed ceramics since early Islamic times (Iraq, 9th AD). Luster is obtained by the reaction of a luster paint and the glaze surface over which it is applied. A temperature-resolved XRD experiment was designed to study the high temperature reactions in the luster paint while the luster layer is formed. The luster paint composition has been made based on the original luster paints found during the excavation of the 13th AD workshop site at Paterna (Valencia). The sulfo-reducing atmosphere created during the decomposition of cinnabar promotes the reduction of Cu2+ containing compounds to Cu+ and the presence of Hg vapours delays the precipitation of metal silver. Moreover, evidence of the formation of a melt in which the copper and silver-containing compounds dissolve has also been obtained. The thickness of the luster paint applied results in the formation of luster layers of different hues and colours. The use of a mixture of copper and silver paint results in the formation of dark-brown luster layer similar to the ones produced in early Islamic times in Iraq and showed also the characteristic blue iridescence