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MATERIALS FOR PROCESSING APPARATUS 145
but slate is not recommended as it often splits along planes of cleavage as a result of crystallization. Some varieties of soapstone are not resistant to fixing baths, and tend to disintegrate where the sodium thiosulfate crystallizes out.
A satisfactory cement for joining large pieces of soapstone, as in constructing a tank, can be prepared from 1 part whiting, 2 parts litharge, thoroughly mixed and made into a putty with boiled linseed oil. A mixture of litharge and glycerine is recommended for cementing small fittings into the tanks.
Practical Suggestions
Materials suitable for constructing various types of photographic apparatus are as follows.
Small Apparatus
Allegheny metal is one of the most satisfactory materials known. Nickel, Monel metal, Mond, and Corronil metal are suitable for use in developing solutions.
Small Tanks
Since these containers are generally used for a variety of purposes, they should be resistant to most photographic solutions. Suitable materials are glass, enameled steel, hard rubber, teak wood or spruce impregnated with paraffin wax, wood or metal coated with "Oxygenated Asphalt," and well-glazed porcelain or stone ware. Allegheny metal, Monel, Mond, or Corronil Metals and Nickel with pressed seams or joints soldered on the outside are satisfactory for washing or developing and for fixing purposes when the tanks are to be used intermittently.
Deep Tanks
Alberene stone, well-glazed stoneware and wood (cypress) are suitable for developing and fixing baths. Lead-lined wooden tanks are fairly satisfactory for developing solutions provided the joints are lead burned and not soldered. Plain wooden tanks are satisfactory but they tend to accumulate slime. Tanks of paraffined wood can be used if the wood is properly joined together with strips of untreated wood as explained above. Tanks of portland cement have been found satisfactory for developers of low alkali content. Metal or wooden tanks coated with "Oxygenated Asphalt" are excellent providing the base material is not exposed.
Tubes, Sprockets and Idlers for Motion Picture Developing Machines
Hard rubber, chemical lead, Allegheny metal and Pyrex glass have been found satisfactory for developing tubes. Lead gathers a deposit of silver from the fixing bath, and in time this tends to obstruct the tube, but this deposit can be removed by scraping. Brass or copper tubing should not be used since both materials affect developers and are corroded by fixing baths. Idlers and sprockets should preferably be made of hard rubber or Allegheny metal according to the purpose for which they are intended. Metal tubing should not be soldered with solders containing tin.