The photosensitive agent of this process fine-tuned by Niépce and Daguerre in 1832 was the residue of lavender oil distillation. • Niépce and Daguerre obtained this residue evaporating lavender oil by heating it until they got a dry product. After evaporation, only a dark brown tar desiccated by heat is left, becoming hard and brittle. Nowadays the lavender oil and the evaporation process is usually replaced with the use of
violin rosin that's melted without any further treatment. • Niépce and Daguerre would then dissolve a small amount of this tar in alcohol, then pour the solution on a well polished silver plate. Modern photographers interested in the process found out that the solution has to be about 1%. Any dirt or grease on the plate will show on the final image. • After the alcohol evaporation, a uniform white deposit remained on the plate. The thus prepared plate was exposed to light in the
camera obscura for about 7 to 8 hours. • After exposure, the plate was put upside down above a tray holding oil of white petroleum (something like
kerosene or
turpentine). The fumes of this kerosene were sufficient to develop the image without any further treatment. The process gives directly positive images, since the white deposit remains on the plate, at places that were touched by light, while the kerosene fumes render transparent the zones that were not illuminated. However, with the effect of reflections on the metal appearing at places where the white deposit has become transparent, images can be seen as positive or negative. ==References==