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= sp.
C
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INDUSTRIAL A S D ESGINEERING CHEMISTRY
heat of cold liquid, P. C. U./lb./" C.
sp. heat of hot liquid, P. C. U./lb./" C.
ACKNOWLEDGJIENT The writer wishes to acknowledge the valuable criticisms of W. H. McAdams and to thank him for permission to use
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the material in example B which was taken from his unpublished notes. RECEIVED January 31, 1933. Contribution 119 from
t h e Evperirnental
Station, E. I d u Pont de Kernours 8: Company.
Improved Process for Physical Development of Plates, Films, and Lantern Slides ALLANF. ODELL,The du P o n t Tiscoloid Company, Inc., Arlington, N. J
Attention is directed to the unusual and litfle to finish one negative a t a time. HE introduction of the Finally, the precipitating silver physical Of known merits of the process of physical developmakes the operation unspeaknegatives, which is an ment. An improz!ed and simplijied process has ably dirty. adaptation of the method used been worked out which makes Physical detelopComplete descriptions of the t o d e v e l o p t h e collodion menl as simple and successful in practice as methods currently in use may emulsion plates of the middle of ordinary 7nethods of photographic dettelopment. be found in the l i t e r a t u r e alt h e l a s t c e n t u r y , has b e e n has been applied to practically ecery ready cited, and in a few availascribed by various authors to The able t e x t b o o k s (1-4, 18). In ( I h ) , K e u h a u s s ("), make and type of plate andfilm on the market. Reneral, these are all a n d Kogelmann ( 5 ) , b u t t h e l a t e r de\yelopments i n t h e &"Jeral lal)oraforY and technical aPPlicafions Qf similar and, aside from those of Sterry ( 1 6 ) and of Neuhauss Ihe method are pointed out where the process method are to be found largely should hai!e an outstanding adivmfage. (13) which consist of a bath of in t h e p u b l i c a t i o n s of t h e hypo, ammonium sulfocyanate, LumiBres and Seyewetz (9) and of Luppo-Cramer (10-12). Wall s o d i u m sulfite, s i l v e r nitrate, (17) claims that the method applied t o dry plates was first and a developer-reducer, and from one proposed by the Lumihres and Sevewetz (8) of a neutral solution of silver suggested bv Abner in 1878. nitrate and sodium sulfite,'all are made up of baths containADVdNTAGES O F PHYSICAL DEVELOPhlEXT ing citric acid in high concentration, metol, and silver nitrate, Physical development, in the sense the term is used today, sometimes with the addition of sodium citrate, sodium sulfite, is to be distinguished from ordinary methods which are gener- gum arabic, etc. Most authors have noted that the image from physical ally employed in all photographic work, sonietimes called "chemical development" in order to introduce a difference in development is composed of an extremely fine grain, and that terminology, by the fact that the developing bath contains a the gradation is beautifully exact. The extreme fineness of silver salt in solution from which colloidal silver is obtained the grain size, coupled with the fact that this grain is formed by reduction with a suitable agent, also in the solution, and independently of the grain of the original emulsion, constitutes the production of the negative image results from the deposi- the chief advantage of the method. One may thus use coarsetion of the nascent silver upon the silver nuclei forming or grain superspeed emulsions to obtain fine-grain images. This originating in the latent image. Both methods are, of course, factor becomes of great importance where enlargements of chemical, and the developing baths resemble each other by unusual size are to be made from small negatives. the further fact that they both contain the same developers Aside from the many beautiful and useful aspects which as reducing agents. Physical development occurs only when physically developed negatives have from an artistic standthe bath contains one of the usual photographic developers, point ( I J ) , such negatives and prints, or enlargements made which must function first in some manner by preparing the from them, should have a definite appeal to the scientist and laboratory worker in nearly any field where photography is latent image to receive the silver deposit (6,7,12,15). Physical development has been little used outside the labo- applicable. ratory, where it has proved a valuable adjunct in research Wherever reproduction as close as possible to the original upon emulsions and upon the theory of image formation, and and with an absence of unnatural contrasts is desired, for information upon the method, while widely distributed intricate detail, and for clean and extraordinarily sharp line through the literature, is rarely found outside of very re- work particularly for measurements, this type is better stricted fields. It has so many advantages, however, that it adapted than the ordinarily used methods of photographic should long ago have been put upon a practical operating development. It should be of value in mierophotography and basis and introduced as a part of laboratory technic wherever cross-section work especially, in biological work (particularly photographic requirements are necessary. photographs from life), and in x-ray work where grain is a The reasons why this has not been done are readily found in problem and undue contrasts are not desirable; i t should the difficulties that surround the present methods. Long ex- prove especially valuable in spectrum photography where posures, up to ten to twenty times normal, are required. The both sharpness of lines and correct comparative densities are development takes from 6 to 48 hours with ordinary imDortant. - negative emulsions. The bath must be freshly renewed every hour SIMPLIFIED PROCESS FOR PHYSICAL DEVELOPMENT or two. The solutions, usually strongly acid, attack the emulsion, and the most careful handlingis necessary t o avoid A study undertaken to improve existing methods of physical marring the surface. It requires all one's personal attention development showed, after repeated experiments with the
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INDUSTRIAL AND ENGINEERING CHEMISTRY
methods of Sterry, of the Lumieres and Seyewetz, alld of Luppo-Cramer, that the chief difficultylay in employment of a large excess of silver which precipitated in the bath, exhausting it rapidly and performing no useful purpose. -4 practical objection was to be found in the high acid concentration also, which made the gelatin film so tender that in most cases it was destroyed. The ideal developing bath would be one in which the silver is precipitated only SO fast as it may be used in the formation of the image; in addition, it should have little or no effect physically upon the gelatin layer. In attempting to adapt the method used by SterrST and by Neuhauss to some such end, it was found that the reduction of silver by developing agents could be regulated by the concentration of sodium thiosulfate in the solution. This property was found to be common to a wide range of salts in fact, but thiosulfate was adopted because it was also a good solvent for silver bromide, which, if left entirely in the emulsion during the process of physical development, will cause fog. This is contrary to opinion, in so far as ordinary development is concerned a t least, as solution of the silver bromide during development is given as a cause of fog. The retarding of the precipitation by the thiosulfate is also a function of the pH of the solution. Organic acids accentuate the retardation and ammonia decreases the effect. That is, in the presence of tartaric or citric acid, the thiosulfate concentration must be decreased, while in the presence of ammonia it must be increased to maintain a given amount of precipitation in a given time. Good developing baths containing thiosulfate may be made up in either acid or alkaline solution, but from practical considerations, with the end in view of preserving the gelatin layer to the best advantage, a neutral bath was finally adopted as best fitting the requirements. A bath containing citric acid and an appreciable amount of chrome alum was found to be equal to the neutral bath, but not superior to it, and possesses the disadvantage of being more complicated to make up. By thus varying the concentration of the thiosulfate alone, a developing bath was finally made in which the precipitation of the silver would continue for several days without the bath becoming exhausted, and the emulsion layer of the negative would not be softened or frilled after being in it for several hours. As is well known, physical development may be carried out either before or after fixation. It has thus been cited often as a laboratory curiosity, and the production of an image by development of a glass-clear fixed negative seems almost miraculous when performed before an audience and the proceedings projected upon the screen. In using the above bath it was found that, if the negative were put into the bath immediately after exposure, it developed and fixed a t the same time, and much stronger images were thus obtained in 2 hours than could be obtained from development subsequent t o fixation in one day. At the same time, the fog, which seems inherent in the development before fixation method, was entirely absent. The solution used was one containing sodium thiosulfate, sodium sulfite, silver nitrate, and amidol for the reducing agent. With five times normal exposure, negatives of normal density can be developed in 1.5 to 2 hours. At the end of this time they are practically cleared, also, but they should be, nevertheless, immersed for a few minutes in acid hardening fix to thoroughly harden the gelatin before cleaning, as well as t o assure fixing. At this point in the development of the process, an experiment which Luppo-Cramer had found to have an accelerating effecton the citric acid-silver nitrate baths which he used ( I I ) , was tried on the neutral thiosulfate bath. This consists in
Vol. 25, No. 8
bathing the plate, before placing it in the developer, in a dilute solution of a mixture of potassium iodide and sodium sulfite. The results were gratifying and far surpassed the accelerating effect the same treatment had in the acid baths used by Luppo-Cramer. It was found that five times exposure in this instance was far too much. Normal exposures gave excellent negatives when treated in this fore bath and subsequently in the neutral thiosulfate, and in all cases twice normal exposure was sufficient to give negatives of normal printing density. The time of development was a t the same time reduced from 2 hours to between 35 minutes and one hour. The plates, however, had to be fixed after development, owing to the shortening of the time in the developing bath and the formation of silver iodide in the emulsion. Practical details of the method follow: The fore bath consists of: potassium iodide, 5 grams; anhydrous sodium sulfite, 12.5 grams; distilled water to make 500 cc. The bath is used in this dilution. Concentrated silver stock solution: crystalline sodium thiosulfate, 80 grams; anhydrous sodium sulfite, 30 grams; silver nitrate, 8 grams; and distilled water to make 500 cc. Dissolve the thiosulfate and sulfite in 200 cc. water, and dissolve the silver nitrate in the remaining 300 cc. Add the silver solution to the thiosulfate solution in a slow stream, stirring vigorously so that the precipitate which forms when the t\To liquids meet is dissolved immediately. Do not reverse the operations, for in that case silver thiosulfate is precipitated and decomposes at once into sulfide and is lost. The solution should be filtered if at all cloudy. After standing a few days, a small amount of brown precipitate forms on the bottom of the bottle, which is negligible and need not be filtered off. The solution is perfectly stable and may be kept indefinitely in white glass bottles. For use, 30 cc. of the above stock silver solution is diluted with 30 cc. water, and t o this is added 60 cc. water in which 0.025 gram of amidol has been dissolved. The solution thus made should be used within 5 to 10 minutes after it is prepared, as directed below. This amount, 120 cc., is sufficient for one 4 X 5 inch plate or its equivalent, using snugly fitting trays in all cases t o utilize all the solution possible. DIRECTIOMFOR EXPOSURE AXD DEVELOPNEXT The plate or film is exposed twice as long as the exposure would be normally made. This should be calculated by means of an exposure meter or by reference to any one of the exposure tables available in photographic annuals. Where this means cannot be applied, as in copying by artificial light, microphotography, etc., the cut-and-try method must be used. It will be found after some experimenting that the exposure time can be cut down but, to begin with, i t is better to overexpose than to underexpose. The plate or film is then transferred to the fore bath contained in a tray, rocking to free the surface from bubbles, and is allowed to remain in it exactly 1.5 minutes. It is then rinsed for a few seconds in a pan of water and placed in the diluted developing solution containing amidol. The development must be made in a tray, and the plate or film must lie flat; otherwise, variations in density occur because of local solution of the silver bromide of the emulsion in the thiosulfate, thus changing the strength of the solution with respect to silver content a t these points. The surface of the emulsion is wiped off with the ball of the finger or with a soft brush while under the developer to insure absence of bubbles and evenness of penetration. The tray is then covered and allowed to stand 35 minutes. The above operations should be carried out a t a temperature of 65" to 70" F. At the expiration of the time given, the density of the image is ascertained by examination of the back of the negative. Nothing can be seen from the front on account of a thin deposit of silver over the entire surface. If the image is not sufficiently deep, the surface of the negative is wiped with a piece of absorbent cotton, and the negative is then placed in the developing bath
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INDUSTRIAL AND ENGINEERING CHEMISTRY
for an additional 25 minutes. If the exposure has been correct, the development will be complete a t the expiration of this period. Little is gained by extending the total development time over an hour, as the process then becomes one of intensification, which is slow in a neutral bath. The plate or film is removed from the silver bath, rinsed lightly, and transferred to the usual acid hardening and fixing bath, where it should remain for several minutes before exposure to white light. Owing to the conversion of the bromide of the emulsion to iodide by the fore bath, the plate receives only slight fixation while in the developer. It also requires 15 to 20 minutes to clear up in the acid fixing bath on account of the low solubility of the silver iodide in this. After fixation, the plate should be cleaned of loose silver deposit with a piece of absorbent cotton and washed in running water for an hour or two, drying as usual afterwards. Any number of plates or film may be developed a t one time, as the process offers no more difficulty than ordinary tank development, and the results should be uniformly successful. The author has used the above method in the preparation of several hundred negatives during the past year. Every make and type of plate, am,and lantern slide found in this market has been used with equal success. It has been tried out on bromide paper with very interesting results. For the development of lantern slides the fore bath need not be used, double exposure should be given, and development should be carried out in the diluted silver bath as in the case of plates and films. The development of slides is complete in 5 to 10 minutes. Very beautiful effects can be obtained on the slides, but they belong to the artistic rather than scientific field. The procedure for bromide paper is the same as for slides, except that the silver bath should be diluted a second time with one to two volumes of water additional. The process is not of particular interest for the development of papers aside from the fact that almost any amount of overexposure can be corrected in the development by inspection of its progress. The developing bath from the plates is richer in silver a t the end of the development than at the beginning, owing t o solution of silver from the emulsion, and may be used a second time or even more if the slight acidity, arising during the course of the silver reduction, is neutralized with a few drops of dilute ammonia, and about one-third the original amount of amidol is added also. The procedure is not, however, particularly advisable. The method of physical development is also applicable to intensification and has been often recommended in the past. It is a little slower than other methods, but has the advantage of being almost rigidly “proportional.” A more vigorous precipitation of silver gives better results here. The same silver bath is used as that given for plates and films, hut, instead of amidol, metol, glycine, or p-amidol phenol is used, a t the same time adding 5 to 7 cc. of a 10 per cent solution of ammonia of 0.92 specific gravity. The negative to be intensified must have been very thoroughly washed; any trace of the double salt of silver and thiosulfate remaining in the film will cause brown stains. The negative is put into the bath either wet or
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dry and is inspected from time to time. The process is complete in 15 to 45 minutes. Needless to say, no metal should come in contact with any of the above silver solution. An excellent and durable silver plate will be given the metal should it occur. In conclusion, the finished negatives from physical development do not appear a t all like the ordinary negative. They are thinner and more delicate in appearance, while possessing equally as good printing qualities. They may be pink, blue, violet, and almost blue-black by transmitted light; by reflected light they appear yellow or gray. The image is built up by surface deposition of the silver, and is not within the emulsion; therefore by reflected light, especially if held against a dark background, the image will be seen as a positive. Dichroic fog, which usually appears on negatives made by physical development, is entirely absent if the foregoing process has been carried out correctly. The contrast obtained is very close to that appearing in the object photographed, and the only possible leeway in the matter of obtaining more contrast is to be found in the selection of the plate, slow emulsions (below 180 H and D) yielding slightly more contrast than rapid emulsions. LITERATURE CITED (1) Angerer, “Wissenschaftliche Photographie,” p. 20, Akad. Verlags., Leipsig, 1931. (2) Cassels, “Cyclopaedia of Photography,” p. 411, Cassell & Co., London, 1911. (3) Clerc, “Photography. Theory and Practice,” p. 255, Isaac Pitman & Sons, New York, 1931. (4) David, “Photographisches Proktikum: Lehrbuch,” 7th and 8th ed., p. 454, W. Knapp, Halle (Saale), 1931. (5) Kogelmann, “Die Isolierung der Substans der latenten photographischen Bilder,” Gras., 1899. (6) Liesegang, Kolloid-Z., 26,47 (1920). (7) LumiBre, LumiBre, and Seyewets, Abstracts Roy. Phot. Soc., 4, 185 (1924). (8) LumiBre, LumiBre, and Seyewetz, Compt. rend., 153,102 (1911); J . SOC.Chem. I n d . , 30,982 (1911). (9) LumiBre, LumiBre, and Seyewetz, J . SOC.Chem. Znd., 23, 1044 (1904), 43B, 579, 732 (1924), 44B,477 (1925); Brit. J . Phot., 51, 866 (1904); Compt. rend., 153,102 (1911), 193,906 (1932); Abstracts R o y . Phot. Soc., 4, 185 (1924), 9,62 (1929); KolloidZ., 35,380 (1924), 37,313 (1925). (10) Luppo-Cramer, Brzt. J . Phot., 50, 305 (1903); J. Soc. Chem. I n d . , 24, 104 (1905); 2. Chem. I n d . Kolloide, 9, 290 (1911); Kolloid-Z., 14, 186 (1914), 36, 326 (1925); Phot. Ind., 21, 504 (1923), 22, 780 (1924); Abstracts R o y . Phot. Soc., 6, 43 (1926). (11) Luppo-Cramer, Kolloid-Z., 25, 175 (1919); 29, 314 (1921). (12) Ibid., 28, 25 (1921). (13) Seuhauss, A m a t e u r Photographer, 16,754 (1898); Phot. R u n d schau, 36, 257 (1899). (14) “Photo-Miniature S o . 207,” Tennant & Ward, New York, 1933 (contains complete resume of subject and detailed manipula(15) (16) (17) (18)
tions). Seyewetz, J . Soc. Chem. Ind., 44B,477 (1925). Sterry, Brit. J . Phot., 45, 260 (1898). Wall, Am. Phot., 16,256 (1922). Wall, Dictionary of Photography, 10th ed., pp. 235-6, Am. Phot. Pub. Co., Boston, 1920.
RECEIVED January 2 5 , 1933. Presented before Section C a t t h e meeting of the American Association for t h e Advancement of Science, Btlantic City, N. J.. December 30, 1932.
HALIBUTOIL AS VITAMINSOURCE.The Vitamin Institute Norway has exported only small quantities of halibut livers, at Oslo has made some preliminary feeding experiments and principally for experimental purposes. Most of the halibut color tests with halibut liver oil, results of which are claimed to fishing is far from the coast, a greater part being in Greenland indicate a content of vitamin A, 50 to 100 times greater than and Iceland waters, so that livers must be stored on ice or salted that of the average cod liver oil, and of vitamin D, 6 to 10 times .to keep them in fresh condition until they can be landed on as great. Other Norwegian tests have given similar results. shore. The demand for halibut livers has made itself apparent The tests have not been extensivp enough to determine the so recently that any comparative methods for storing the livers to maintain quality and potency of vitamin content have not average potency and possible variations caused by season, locality where fish is caught, and methods of preparation. been thoroughly tested.
