Amateurs and professionals, who would have to purchase their distilled water from outside sources, but who are supplied with ordinary drinking water, should use freshly boiled water for the preparation of the baths. Boiling con verts the bicarbonate of lime, which forms the larger part of the dissolved salts, into the insoluble carbonate of lime, coagulates most of the organic matter, and expels the dissolved gases, which can re-dissolve only after the water has been cooled.
A few substances form traces of precipitate in presence of the sulphate of lime which exists in boiled water, but if the baths are made up in relatively small quantities it is quite easy to filter them before use. The filtration of large quantities of solutions, such as are required in the cinematograph industry, introduces many serious complications, which can be avoided either by the use of distilled water or of water purified by chemical means.
As a general rule, therefore, distilled water is not necessary for the preparation of photo graphic baths, 1 in spite of the instructions to the contrary which are given in various formulae.
276. Preparation of Photographic Baths. For the weighing-out of small quantities, the balance pans are covered with small sheets of paper, which are renewed for each fresh substance. To avoid the formation of dust, each sheet is wetted under the tap before being thrown away. For large quantities the substance is gradually introduced into a tared receptacle. 2 A bottle large enough to contain the total volume of the bath is half or two-thirds filled with freshly boiled warm water, into which the substances are introduced in the prescribed order with constant stirring, taking care that each substance is fully dissolved before the next is added. After the complete solution of all the con stituents, the bottle is filled with boiled water and, if necessary, the solution filtered or decanted off.
For the preparation of baths in large quanti ties, the work is carried out in a large vessel or tank, in which the final volume is adjusted either to a mark on the inside of one of the walls or to a notch on the handle of a wooden stirrer placed in contact with the bottom. Having introduced a certain quantity of water, the substances, contained in a linen net fixed on a wooden frame, are successively dissolved by sprinkling them with water. When all the
constituents are dissolved, the level is brought up to the mark by the addition of the requisite amount of water.
277. Filtration. Filtration through paper is usually much slower and more costly than through either fabric or absorbent pads. The process is best carried out in a wide funnel (cone 6o1 with a cylindrical stem, which is plugged with a piece of cotton wool or a lightly compressed piece of sponge, or, alternatively, the cone of the funnel may be fitted with a flannel or chamois leather bag. The three last mentioned materials may be used repeatedly, if washed in running water and set to dry immediately after each filtration.
With large bulks of liquid, filtration is usually done through felt pads, each pad being kept for a special solution. In such cases, however, it is often preferable to stand the solution in a reservoir with a tap fitted at a little distance from the bottom. After the solution has cleared by sedimentation the top layer is drawn off, and it is then only necessary to filter the small quantity at the bottom, which con tains the sediment and any surface scum. The decantation may also be carried out by a syphon.
278. Stock Solutions. Stock solutions may be used with advantage for all substances which are as stable in solution as in the dry state, and which are constantly being used in small quantities at a time. Weighing is then replaced by a measurement of volume, calculated from the concentration of the solution.
It has often been proposed to use saturated solutions for stock solutions, but their concen tration, although well defined if maintained at a constant temperature, is liable to considerable variations. The salt, which crystallizes out on cooling, is often deposited in a difficultly soluble mass, so that when the temperature rises again the solution is no longer saturated. All concentrations which exceed the satur ation value at 400 F., the lowest temper ature usually reached in any commercial establishment, even during the interruption of work at the week-end, should therefore he avoided in the preparation of stock solutions.