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144 CINEMATOGRAPHIC ANNUAL^
wood are best made by grooving the pieces of wood to be joined together, as for a T-joint, and inserting tightly a small piece of unparaffined wood in the groove. When placed in water the untreated strip swells and completely caulks the seam.
Porcelain and Glazed Earthenware
Porcelain, glazed biscuit ware, and tile material are usually unsatisfactory because the glaze invariably cracks, causing minute fissures into which the solution penetrates and crystallizes. The crystals then grow and cause the biscuit ware to disintegrate, incidentally causing the glaze to chip. Tanks of high grade, dark brown earthenware, glazed on both sides are especially recommended for storing ordinary developing and hypo solutions, but should not be used with strong alkalies.
Rubber, Rubber Composition, Nitrocellulose and Asphaltum
Materials
Pure hard rubber will withstand practically all photographic solutions at normal temperatures. Some so-called hard rubber tanks are made from a mixture of asphalt or rubber composition with an excess of mineral filler. Such tanks are somewhat brittle, warp under heat, and when used as containers for solutions disintegrate in the same manner as porous earthenware. Smooth surfaces reduce the tendency to etching since less strain is exerted on the walls during the crystallization process.
Rubber sheeting and rubberized cloth are often used for coating the inside of wooden trays and troughs, and are very satisfactory. Cheap rubber sheeting or tubing often contains an excess of free sulfur which reacts with photographic developers and causes chemical fog.* Pure gum rubber materials are quite satisfactory.
A tarry material called "Oxygenated Asphalt" used for sealing storage batteries and supplied by the Standard Oil Company, has been found to be a satisfactory protective coating for use with all kinds of photographic solutions. This material is applied, while hot, as a thick coating over the metal or wood and if a smooth surface is desired the coating can be smoothed out by the use of a blowtorch. Nitrocellulose lacquer (E. K. Lacquer No. 5119) is useful for coating wooden articles such as racks for handling motion picture film, although several coatings are usually necessary, either by brushing, spraying, or dipping. Small apparatus constructed of nitrocellulose sheeting is satisfactory for use with almost every type of aqueous solution.** Wooden tanks lined with this material have also proved satisfactory.
Slate and Alberene Stone
These materials are very suitable for constructing large tanks for containing developing solutions. For fixing solutions, Alberene Stone (a gray, finely crystalline variety of soapstone) is quite satisfactory,
♦"Chemical Fog" by J. I. Crnbtree. Amet. Ann. Phot. 33. (1919) 20.
** "Plastic Cellulose in Scientific Research,'1 K. Hickman and D. E. Hyndman, J. Frank Inst. 207 (1929) 231.