RELIEF, PHOTOGRAPHS IN In the many processes of making photographs in relief, the well-known property of chromated gelatine losing its water-absorptive power after exposure to light is taken advantage of. Negatives having over-strong contrasts give the most satisfactory results ; in producing such a negative, the background should be black and the sitter's face and hair powdered, and lighted in such a way that the head stands out in the greatest possible contrast to the background.
The negative is printed upon a plate prepared as follows : Mix together 2 oz. of gelatine, I oz. of gum arabic, io oz. of water and 48 mins. of glacial acetic acid, and melt by the application of heat. Pour a quantity on a bevelled sheet of glass to the thickness of two or three millimetres, and let it set and dry in a horizontal position. The plate is sensitised by immersing it for a few minutes in a 3 per cent. solution of ammonium bichromate, with an excess of ammonia, and dried in the dark. The plate requires a longer exposure than if bichromate alone is used, from a quarter to half an hour being required in full sunlight, the printing frame being so placed that the sun's rays fall upon it as perpendicularly as possible. After printing, in which a certain
amount of guesswork is necessary, the prepared plate is taken from the frame and placed in a 2 per cent, solution of alum to which a 2 per cent. solution of citric acid has been added. After soaking for several hours considerable relief is obtained, which is suitable for cast ing in plaster-of-paris, or, after dusting with graphite, can be electrotyped. In the latter case, varnish the edges of the relief plate before or after printing, so as to prevent the film from stripping when it is in the acid sulphate of copper bath. (See also " Acrograph," " Cameo," " Bas-reliefs," " Electro-phototypy," " Galvano graphy, Photographic," " Gelatine Reliefs," Photo - lithophane," " Photopolygraphy," " Photo-sculpture," " Plaster Casts from Photo Reliefs," " Woodburytype," etc.)