Material on which the photograph is depicted.
Paper: Salt papers are generally bleached more than later silver photographs. However, all silver photographs - regardless of the process - can turn yellow. Fading is therefore an important indicator that it is a silver photograph.
Glass: Glass pictures are only pictures that actually consist of glass. Pictures that are only framed in glass are categorized under the carrier material. Glass images should be viewed in bright light to clearly see the tonal values (avoid reflections). In glazed paper photographs, the image carrier is often only clearly visible when they are framed. Attention: Daguerreotypes are always airtightly glazed, the carrier material is always a silver-plated copper plate.
Iron plate: Metal pictures are magnetic. This can be easily detected using an ordinary magnet.
Leather or wax linen: In pannotypes, the structures of leather or wax linen are clearly visible.
Porcelain or stoneware: In photoceramics, the fired glaze is usually visible.