humidity and temperature, which can cause the oil paint to crack. For photographs with substantial damage, the expertise of an oil paintings conservator might be required for treatment. Crayon and pastel hand-coloured photographs have a powdery surface which must be protected for preservation purposes. Historically, crayon and pastel coloured photographs were sold in a frame under a protective layer of glass, which was often successful in reducing the amount of handling and smudging of the photograph surface. Any conservation work on crayon or pastel colour-photographs must retain these original frames and original glass to maintain the authenticity and value of the object. If the photograph is separated from its original enclosure, it can be stored in an archival quality folder until it is framed or cased. Auxiliary materials In the United States, many commercially sold, hand-coloured photographs were packaged and framed for retail sale. Early 20th century hand-coloured photographs were often mounted on mat-board, placed behind a glass frame, and backed by wood panel slats, cardboard, or heavy paperboard. A backing sheet was often glued to the back of the mat-board. Unfortunately, the paper products produced and used during the late-19th and early-20th centuries are highly acidic and will cause yellowing, brittling and degradation of hand-coloured photographs. Metallic inclusions in the paper can also oxidize which may be the cause of foxing in paper materials. Wood panel slats will also off-gas causing further degradation of the photographs. Simple conservation of these fragile materials can be carried out by the adventurous amateur. A hand-coloured photograph should be removed from the frame, retaining any original screws or nails holding the frame together. Wood panels, acidic cardboard slats, and acidic backing paper can be removed from the frame and mat-board and discarded, retaining any identifying information such as stamps or writing on the backing paper. The mat-board on which the photograph is mounted, even though acidic in nature, cannot be removed and replaced due to the intrinsic value of this original mounting. Often the artist's signature and the title of the photograph are inscribed on the mat-board. The best way to limit degradation is to store the photograph in a cool, dry atmosphere with low light. The hand-coloured photograph should be replaced in its original frame, held in place with archival quality acid-free paper paperboard, and closed with the original nails or screws. Related techniques Hand-colouring should be distinguished from tinting, toning, retouching,