The Experts below are selected from a list of 25008 Experts worldwide ranked by ideXlab platform

Maria João Melo - One of the best experts on this subject based on the ideXlab platform.

  • Barium, zinc and strontium yellows in late 19th–early 20th century oil paintings
    Heritage Science, 2017
    Co-Authors: Vanessa Otero, Marta F. Campos, Joana V. Pinto, Márcia Vilarigues, Leslie Carlyle, Maria João Melo
    Abstract:

    This work focuses on the study of the 19th century yellow chromate pigments based on barium (BaCrO_4), zinc (4ZnCrO_4·K_2O·3H_2O) and strontium (SrCrO_4). These pigments, which are reported to shift in hue and darken, have been found in 19th century artworks. A better understanding of their historic manufacture will contribute to the visual/chemical interpretation of change in these colours. Research was carried out on the Winsor & Newton (W&N) 19th century Archive Database providing a unique insight into their manufacturing processes. One hundred and three production records were found, 69% for barium, 25% for zinc and 6% for strontium chromates, mainly under the names Lemon, Citron and Strontian Yellow, respectively. Analysis of the records shows that each pigment is characterised by only one synthetic pathway. The low number of records found for the production of strontium chromate suggests W&N was not selling this pigment formulation on a large scale. Furthermore, contrary to what the authors have discovered for W&N chrome yellow pigments, extenders were not added to these pigment formulations, most probably due to their lower tinting strength (TS). The latter was calculated in comparison to pure chrome yellow (PbCrO_4, 100% TS) resulting in 92% for barium, 65% for zinc potassium and 78% for strontium chromate pigments. This indicates that W&N was probably using extenders primarily to adjust pigment properties and not necessarily as a means to reduce their costs. Pigment reconstructions following the main methods of synthesis were characterised by complementary analytical techniques: Fibre optic reflectance spectroscopy, X-ray diffraction, micro-Raman and micro-Fourier transform infrared spectroscopies. These pigments can be clearly distinguished on the basis of their infrared CrO_4 ^2− asymmetric stretching fingerprint profile (between 1000 and 700 cm^−1) and of their Raman CrO_4 ^2− stretching bands (850–950 cm^−1). This enabled their identification in historic paint samples: a tube of late 19th century W&N Lemon Yellow oil paint and micro-samples from paintings by three Portuguese painters, António Silva Porto (1850–1893), João Marques de Oliveira (1853–1927) and Amadeo de Souza-Cardoso (1887–1918). The good correlation found between the reconstructions and historic samples validates their use as reference materials for future photochemical studies.

P J Groot - One of the best experts on this subject based on the ideXlab platform.

  • fast in Database cross matching of high cadence high density source lists with an up to date sky model
    Astronomy and Computing, 2018
    Co-Authors: B Scheers, S Bloemen, Hannes Muhleisen, P Schellart, A Van Elteren, Martin L Kersten, P J Groot
    Abstract:

    Abstract Coming high-cadence wide-field optical telescopes will image hundreds of thousands of sources per minute. Besides inspecting the near real-time data streams for transient and variability events, the accumulated data Archive is a wealthy laboratory for making complementary scientific discoveries. The goal of this work is to optimise column-oriented Database techniques to enable the construction of a full-source and light-curve Database for large-scale surveys, that is accessible by the astronomical community. We adopted LOFAR’s Transients Pipeline as the baseline and modified it to enable the processing of optical images that have much higher source densities. The pipeline adds new source lists to the Archive Database, while cross-matching them with the known cataloguedsources in order to build a full light-curve Archive. We investigated several techniques of indexing and partitioning the largest tables, allowing for faster positional source look-ups in the cross matching algorithms. We monitored all query run times in long-term pipeline runs where we processed a subset of IPHAS data that have image source density peaks over 170,000 per field of view (500,000  deg−2). Our analysis demonstrates that horizontal table partitions of declination widths of one-degree control the query run times. Usage of an index strategy where the partitions are densely sorted according to source declination yields another improvement. Most queries run in sublinear time and a few ( 20 % ) run in linear time, because of dependencies on input source-list and result-set size. We observed that for this logical Database partitioning schema the limiting cadence the pipeline achieved with processing IPHAS data is 25  s.

P Schellart - One of the best experts on this subject based on the ideXlab platform.

  • fast in Database cross matching of high cadence high density source lists with an up to date sky model
    Astronomy and Computing, 2018
    Co-Authors: B Scheers, S Bloemen, Hannes Muhleisen, P Schellart, A Van Elteren, Martin L Kersten, P J Groot
    Abstract:

    Abstract Coming high-cadence wide-field optical telescopes will image hundreds of thousands of sources per minute. Besides inspecting the near real-time data streams for transient and variability events, the accumulated data Archive is a wealthy laboratory for making complementary scientific discoveries. The goal of this work is to optimise column-oriented Database techniques to enable the construction of a full-source and light-curve Database for large-scale surveys, that is accessible by the astronomical community. We adopted LOFAR’s Transients Pipeline as the baseline and modified it to enable the processing of optical images that have much higher source densities. The pipeline adds new source lists to the Archive Database, while cross-matching them with the known cataloguedsources in order to build a full light-curve Archive. We investigated several techniques of indexing and partitioning the largest tables, allowing for faster positional source look-ups in the cross matching algorithms. We monitored all query run times in long-term pipeline runs where we processed a subset of IPHAS data that have image source density peaks over 170,000 per field of view (500,000  deg−2). Our analysis demonstrates that horizontal table partitions of declination widths of one-degree control the query run times. Usage of an index strategy where the partitions are densely sorted according to source declination yields another improvement. Most queries run in sublinear time and a few ( 20 % ) run in linear time, because of dependencies on input source-list and result-set size. We observed that for this logical Database partitioning schema the limiting cadence the pipeline achieved with processing IPHAS data is 25  s.

Joséphine Simonnot - One of the best experts on this subject based on the ideXlab platform.

Vanessa Otero - One of the best experts on this subject based on the ideXlab platform.

  • Barium, zinc and strontium yellows in late 19th–early 20th century oil paintings
    Heritage Science, 2017
    Co-Authors: Vanessa Otero, Marta F. Campos, Joana V. Pinto, Márcia Vilarigues, Leslie Carlyle, Maria João Melo
    Abstract:

    This work focuses on the study of the 19th century yellow chromate pigments based on barium (BaCrO_4), zinc (4ZnCrO_4·K_2O·3H_2O) and strontium (SrCrO_4). These pigments, which are reported to shift in hue and darken, have been found in 19th century artworks. A better understanding of their historic manufacture will contribute to the visual/chemical interpretation of change in these colours. Research was carried out on the Winsor & Newton (W&N) 19th century Archive Database providing a unique insight into their manufacturing processes. One hundred and three production records were found, 69% for barium, 25% for zinc and 6% for strontium chromates, mainly under the names Lemon, Citron and Strontian Yellow, respectively. Analysis of the records shows that each pigment is characterised by only one synthetic pathway. The low number of records found for the production of strontium chromate suggests W&N was not selling this pigment formulation on a large scale. Furthermore, contrary to what the authors have discovered for W&N chrome yellow pigments, extenders were not added to these pigment formulations, most probably due to their lower tinting strength (TS). The latter was calculated in comparison to pure chrome yellow (PbCrO_4, 100% TS) resulting in 92% for barium, 65% for zinc potassium and 78% for strontium chromate pigments. This indicates that W&N was probably using extenders primarily to adjust pigment properties and not necessarily as a means to reduce their costs. Pigment reconstructions following the main methods of synthesis were characterised by complementary analytical techniques: Fibre optic reflectance spectroscopy, X-ray diffraction, micro-Raman and micro-Fourier transform infrared spectroscopies. These pigments can be clearly distinguished on the basis of their infrared CrO_4 ^2− asymmetric stretching fingerprint profile (between 1000 and 700 cm^−1) and of their Raman CrO_4 ^2− stretching bands (850–950 cm^−1). This enabled their identification in historic paint samples: a tube of late 19th century W&N Lemon Yellow oil paint and micro-samples from paintings by three Portuguese painters, António Silva Porto (1850–1893), João Marques de Oliveira (1853–1927) and Amadeo de Souza-Cardoso (1887–1918). The good correlation found between the reconstructions and historic samples validates their use as reference materials for future photochemical studies.