American cinematographer (Feb-Dec 1929)

Four American C i n e m a t o g r a p h e r September, 1929 tor, (d) a hardening agent. (a) Although many solvents of silver halides are available, the most satisfactory for fixing purposes are sodium and ammonium thiosulfate. A solution of ammonium thiosulfate of a given concentration fixes more rapidly than a corresponding solution of sodium thiosulfate'" but ammonium thiosulfate is more expensive and commercial grades are apt to contain free sulfur. For most purposes sodium thiosulfate fixes sufficiently rapidly and is entirely satisfactory. (b) Practically any acid will function as an anti-staining agent because it has merely to neutralize the alkali in the developer carried over by the films and thereby retard oxidation of the developing agent. Organic acids such as citric, tartaric, oxalic, lactic, malic, maleic, and acetic, are more suitable than the inorganic acids because they are less dissociated and therefore have a correspondingly less tendency to precipitate sulfur from the hypo. Practically all of the sole organic acids yield with aluminium salts complex aluminium ions which are not effective hardening agents.8 Acetic acid has been found to be the most generally efficient acid for use in fixing baths but much is still to be desired from the standpoint of a perfect acid for this purpose. Of the acid salts, sodium bisulfite and potassium metabisulfite are the most generally used but they are not suitable for compounding a hardening fixture bath containing alum because the reserve acidity of these salts is not sufficient to prevent the precipitation of aluminium sulfite on the addition of alkali. (c) It is imperative when compounding an acid fixing bath to have a quantity of free acid present in order to prevent discoloration of the bath by developer oxidation products and also to prevent the precipitation of the hardening agents by the alkali in the developer. This free acid tends to cause the precipitation of sulfur from the hypo especially at temperatures above 70° F. It is therefore necessary to add some substance which will prevent the precipitation of sulfur without impairing the anti-staining properties of the acid. Two types of substances act in this manner. The first of these consists of the alkaline sulfites of which sodium sulfite is the most common. Since hypo decomposes in the presence of acid to form sodium sulfite and sulfur, it is considered that the addition of sulfite to an acid fixing bath retards the decomposition of the hypo because it tends to reverse the action by virtue of its mass action. The second type of preservative consists of the alkali salts of organic acids which are commonly referred to as "buffer salts" and of which sodium acetate is a typical example. The effect of this type of substance is to buffer or reduce the hydrogen ion concentration of the acid employed below the limit at which sulfur is precipitated from hypo by acids. (d) The hardening of gelatin may either be temporary or permanent. Temporary hardening agents raise the melting point and prevent the swelling of gelatin only while the gelatin is in contact with the hardening solution. A concentrated solution of sodium sulfate is a typical temporary hardener.' The hardening produced by such substances is reversible, that is, the gelatin will subsequently absorb water and swell. Such hardeners will not be discussed because their application is limited to tropical development when the temperature of the various photographic solutions is above 75° F. Permanent hardening is characterized by a reduced absorption of water (swelling) by the gelatin during subsequent washing. Various materials may be used for permanently hardening gelatin such as formalin, quinone, tannin, organic developer oxidation products, and certain inorganic compounds. Formalin, quinone, and developer oxidation products harden gelatin only in alkaline or neutral solutions and their application is therefore limited to use in developers or to the hardening of completely washed film. Of the inorganic compounds, the salts of iron, chromium and aluminium exert the most powerful hardening action on gelatin. Salts of aluminium are perhaps the most satisfactory hardener because they are colorless, readily soluble in water, and do not form colored compounds with the common developing agents either in acid or alkaline solutions, while they give satisfactory hardening provided the wash water is not above 75° to 80° F. Aluminium chloride tends to hydrolyze when dissolved in water, forming a white precipiate of aluminium hydroxide, but the double salts af aluminium sulfate with sodium or potassium sulfate called alums form a clear solution and are therefore to be preferred. Sodium and potassium alum are equally efficient but with ammonium alum an evolution of ammonia takes place after the fixing bath becomes alkaline and this tends to produce dichroic fog.2 III. Standard Tests for Determining the Properties of Fixing Baths To date very few methods of representing the properties of a fixing bath in numerical terms have been published. The following tests, however, in no way give absolute measurements but they have proved satisfactory as a means of comparing the various formulas examined. 1. Time of Fixation The time to clear Eastman negative motion picture panchromatic film when viewed against a black background was taken as a measure of the time of fixation. In order to maintain uniformity the film was immersed directly in the fixing bath and not previously developed in order to eliminate any error caused by swelling, which might influence the rate of diffusion of the hypo into the gelatin film and thereby affect the rate of fixation. 2. Acidity One hundred cubic centimeter portions of the fixing bath were titrated with MQ25* developer using phenolphthalein as an indicator. The results are expressed as the number of cubic centimeters of developer required to neutralize 100 c.c. of fixing bath. 3. Sulfurization Life The time required for the average acid fixing bath to precipiate sulfur at ordinary temperatures (65° to 70° F.) is relatively long, so that in order to increase the rate of precipitation of sulfur and reduce the time required for the experiments, samples of the fixing bath were kept in glass-stoppered bottles at 110° to 115° F. and the time noted for the first visible turbidity. It was considered that a -(Formula of MQ^ Developer) ►JVote Eton ...... Hydroquinone Sodium sulfite Sodium carbonate Potassium bromide Wafer to 1.2 5 grams 3.75 grams 7 5.00 grams 2 5.00 grams 1 . 5 grams 1.0 liter TIME. TO CLEAR MOTION PICTURE. PANCHROMATIC NEGATIVE FILM. TIME TO CLEAR MOTION PICTURE POSITIVE FILM Curves showing the time to clear Eastman motion picture positive and negative film in hypo solutions of various concentrations at different temperatures. bath which was not stable under these conditions for one-half day was highly undesirable, whereas if the bath remained clear for three days at 1 1 0° to 1 1 5° F. it was found to remain clear for about one month at normal temperatures and therefore would be quite satisfactory for use at ordinary temperatures. 4. Hardening The relative hardening produced by a given fixing bath was determined as the temperature at which the emulsion left the support after treatment in the following standard manner: Strips of Eastman motion picture positive film Y& inch by 5 inches were exposed so as to produce three densities at one end of the strip, ranging from approximately 0.5 to 2.5 after development. These strips were developed in MQ25 diluted 1 : 1 for 2 minutes, rinsed 30 seconds, fixed 5 minutes in the fixing bath to be tested, and washed 15 minutes in running water. The temperature of the solutions up to this point was maintained between the limits of 65° to 70° F. The film was then placed on a wooden frame which was immersed in a beaker containing 750 c.c. of water at 70° F. The temperature of the water in the beaker was raised from 8° to 10° F. per minute and the temperature at which the gelatin emulsion left the support was taken as the melting point or a relative measore of the hardening produced by the fixing bath. The apparatus as used is illustrated in Fig. 1 . The temperature at which the gelatin emulsion reticulated was not recorded after the first few experiments because it was found that this value is related to the melting point by a definite ratio. The degree of hardening produced depends on a large number of factors all of which must be maintained constant if consistent results are to be obtained. Some of these factors may be tabulated as follows: 1. Nature of emulsion used. 2. Alkalinity of the developer and time of development. 3. Composition of rinse or stop bath. 4. Time in rinse or stop bath. 5. Concentration and relative composition of fixing bath. (Continued on Page 19) -bOO 400 300 600 CONCENTRATION GRAMS No.j SjOj ■ 5Ht0 /LITER SOLUTION