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.