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MATERIALS FOR PROCESSING APPARATUS 143
Glass
Glass apparatus well annealed, free from ribs, and with the corners rounded off, is quite satisfactory and is one of the most resistant materials available. For the storage of strong alkalis, special resistant glass should be used. Owing to its fragility, however, glass is not suitable for large tanks.
Impregnated Fibrous Materials.
Tanks prepared with fibrous materials impregnated with varnish or lacquer develop cracks with use, thus permitting access of the solutions to the under layers. Such tanks are entirely unsatisfactory for use with solutions containing strong alkalies, or with fixing baths, because these solutions disintegrate the fibrous materials through crystallization as explained later under "Porcelain and Glazed Earthenware."
Containers made from most laminated phenolic condensation products can be used with photographic solutions, with the exception of strong oxidizing solutions. Some samples of these materials have been found to swell and warp out of shape when used with strongly alkaline solutions.
Wood
Wood is fairly satisfactory for developing, fixing, and washing purposes, and is cheaper than any other available material. It has the disadvantage that, unless strongly braced, tanks have a tendency to warp out of shape. In many localities fungus growths accumulate on the outside of the washing tanks which must be removed frequently, while the inside of wash tanks often become coated with a layer of slime which necessitates frequent cleaning. Wooden containers also become permanently discolored if they are used for dye solutions. The most satisfactory varieties of wood for the construction of tanks are cypress, spruce, redwood, maple, and teak.
Paraffined Wood
Although certain woods such as cypress and teak are frequently used for the construction of containers for photographic solutions, paraffin impregnated wood is much more satisfactory. It also possesses the additional advantage that it does not tend to accumulate slimy layers as rapidly as unwaxed wood. The chief disadvantage of paraffined wood is that it is too heavy for the construction of large equipment which is to be handled manually. Methods of impregnating wood with paraffin have been investigated by Eberlin and Burgess,* who found that the best results were obtained with cypress and spruce by soaking in water for twelve hours, and then immersing in molten paraffin wax for two hours at around 257°F. (125°C.).
The soaking serves to swell the wood and in the hot paraffin bath the water in the pores is replaced by paraffin. The wood should be wiped thoroughly with a cloth on removing from the paraffin bath so as to remove the excess wax. Water-tight joints with paraffined
* "Impregnating Wood with Paraffin," L. \V. Eberlin and A. M. Burgess, Ind. Eng. Chem 19, (1927) 87. Revised 1928.