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EFFECT OF WATER SUPPLY IN PROCESSING 151
of sodium sulfide by interaction with the carbonate present in the developing solution. If the water is boiled, the colloidal sulfur usually coagulates, when it may be separated by settling or filtration.
B. Vegetable matter is usually precipitated by the salts present in the developer.
C. Animal matter is usually precipitated on mixing the developer, but frequently biological growths and bacteria thrive in a developer and form a slime or scum on the walls of the tank. Some types of these growths act on the sulfite in the developer, changing it to sodium sulfide which fogs the emulsion very badly. The sulfide is removed by developing some waste film in the solution or by adding a small quantity of lead acetate to the developer in the proportion, 25 grains per gallon (0.4 gram per liter).5 Tanks which show a tendency to accumulate slime should be scrubbed out with hot water at regular intervals and then treated with a dilute sodium hypochlorite solution.3 Suspended mineral, vegetable, or animal matter in general has usually no harmful effect on a developer, providing the mixed developing solution is allowed to stand and only the clear supernatant liquid drawn off for use. Mixing the developer with the aid of warm water tends to hasten the rate of settling of the suspended matter.
3. Extracts from decayed vegetable matter or the bark of trees usually discolor developing solutions but are often precipitated if the developer is prepared with warm water and allowed to stand. The staining effect of such extracts with motion picture film is usually negligible.
4. Water dissolves about 2% of air at 70° F. and when a developing agent like hydroquinone is dissolved without the addition of sulfite the oxygen present in the water combines with the developing agent, forming an oxidation product which is apt to stain the gelatin and fog the emulsion. Air in water occasionally collects on the film in the form of little bubbles or airbells which prevent development giving rise to characteristic markings.6 When developing at high temperatures (above 80° F.) dissolved air often causes blisters.7
Mineral waters containing carbon dioxide rarely give much trouble providing the water is boiled first in order to drive off the gas. If carbon dioxide is present in excessive amounts in a developer, it acts in the same way as dissolved air, producing bubbles and airbells on the film, causing blisters.
Hydrogen sulfide gas will cause bad chemical fog in a developer but may be removed by boiling the water or by precipitation with lead acetate before mixing.4 5
Fixation. Calcium and magnesium sulfites are soluble in acetic acid and therefore are not precipitated in fixing baths. Other dissolved salts such as bicarbonates, chlorides, and sulfates are harmless. Suspended matter such as dirt, iron rust, and certain types of vegetable and animal matter usually will coagulate and settle out on allowing the fixing bath to stand.