arXiv subjects

Christine Andraud

Publications and source records attributed to Christine Andraud.

4 recordsLinked to original sources

arXiv2020

Edmond Becquerel's colour photographic process involves a Ag/AgCl-based sensitized layer

Edmond Becquerel invented in 1848 the first colour photographic process. The "photochromatic images" he produced raised several questions, among which the photochromic nature of the sensitized layer. Here, we present the first characterization of the sensitized layer created at the surface of a silver plate or a silver foil according to Becquerel's process, which XIX$^{\text{th}}$ century scientists called "silver photochloride". It is constituted by silver nanoparticles dispersed in a micrometric silver chloride grains matrix. It is thus similar to the widely studied Ag/AgCl composites, which suggests new synthesis routes for these photocatalysts. The chemical composition of the sensitized layer has been identified by complementary spectroscopies (EDX, XPS, HAXPES and EXAFS), while its morphology has been studied by electron microscopies (SEM and STEM). These techniques involve X-ray or electron beams, which can have an impact on the silver chloride-based sensitized layer; a large part of this article hence introduces a study of the beam effects and of the way of reducing them.

physics.app-ph↗

arXiv2020

Spectroscopies and electron microscopies unravel the origin of the first colour photographs

The first colours photographs were created by a process introduced by Edmond Becquerel in 1848. The nature of these photochromatic images colours motivated a debate between scientists during the 19th century, which is still not settled. We present the results of chemical analysis (EDX, HAXPES and EXAFS) and morphology studies (SEM, STEM) aiming at explaining the optical properties of the photochromatic images (UV-visible spectroscopy and low loss EELS). We rule out the two hypotheses (pigment and interferences) that have prevailed since 1848, respectively based on variations in the oxidation degree of the compound forming the sensitized layer and periodically spaced photolytic silver planes. A study of the silver nanoparticles dispersions contained in the coloured layers showed specific localizations and size distributions of the nanoparticles for each colour. These results allow us to formulate a plasmonic hypothesis on the origin of the photochromatic images colours.

physics.app-ph↗

arXiv2020

Replication and study of the colouration of Edmond Becquerel's photochromatic images

Edmond Becquerel's process of making photochromatic images, which are the first colours photographs, is still poorly understood. In particular, the origin of the colours of these images gave rise to a long-lasting debate, starting from the publication of Becquerel in 1848 until the end of the 19th century. This photographic process was replicated in the laboratory, and the colouration of the sensitized plates was studied by means of visible spectroscopy. A comprehensive description of the sensitization routes that were adapted from Becquerel's writing is given. The study of the exposition step allowed us to gain insights in the colouration mechanisms of the sensitized plates, and to compare between the two sensitization routes developed by Becquerel in terms of spectral sensitivity and colour faithfulness. The so-called "electrochemically sensitized" samples were found to be more sensitive and to reach a larger chromatic space than the "immersion-sensitized" ones. Finally, a reproducible way of creating coloured samples is suggested.

physics.app-ph↗

arXiv2013

Copper red glazes: a coating with two families of particles

In order to explain the origin of the deep red color of copper glazes on ceramics, a ceramist has elaborated, by firing under reducing atmosphere, a significant number of tiles. The analysis of the structure and composition of a representative sample by TEM and EELS, followed by an optical characterization and an optical modeling using the radiative transfer approach (four-flux model) have enabled a comprehensive explanation of the origin of the color in these glazes. The presence of two families of copper nanoparticles in the vitreous matrix. The first, purely absorbing, of diameter 10-50 nm, essentially creates color by a substractive process. The second, due to its larger diameter, 100 nm or more, mainly acts on color by scattering of the visible light. Both act competitively in the layer. A color chart of all the hues which can be reached by this technique has eventually been theoretically calculated.

physics.optics↗