Sept., 1918
T H E JOURNAL OF INDL‘STRIAL A N D ENGINEERING CHEMISTRY
THE RETICULATXON OF GELATINE By S. E. SHHPPABD A N D P. A. ELLEDTT
Received Jury 23, 1918
The reticulation of the surface of negatives is often a source of trouble to photographic workers. This trouble is most likely t o occur in hot weather and is generally produced after fixation, either during or just subsequent t o washing. The wet gelatine layer becomes more or less finely wrinkled or corrugated, the network of puckers forming a pattern, generally extending over the whole of the negative, but sometimes only over part of it. As will be seen from t h e illustrations (Figs. I , 11, 111) the “grain” of the network may vary considerably from very coarse dimensions down t o very fine and even microscopic dimensions. This reticula-
727
facts on the normal swelling and shrinkage of photographic gelatine film which takes place in this treatment and use. There are two aspects t o this: in one we have only t o consider change of mass or bulk; in the other, change of shape. As t o the first, a n y piece of gelatine placed in water within a temperature range of roughly o‘ t o zoo C. swells, a t first rapidly, then more slowly, and finally reaches a limit. Fig. I V shows the curve of this swelling plotted against the time. The limit attained not only depends upon the temperature, hut also upon the character of the gelatine, and, t o a very marked extent, upon the presence of foreign substances, especially electrolytes, in the water. Acid and alkali in particular have a very great influence upon the swelling, as will be seen from
FSG.I
Fie. I1
tion persists with only slight modification after drying. At the same time, as will be seen from the figures, if it occurs on a developed plate, the silver deposit undergoes a redistribution along with the reticulation of the gelatine, accumulating in the raised portions and diminishing or vanishing in the valleys or troughs between. This reticulation has been utilized in some photomechanical processes; thus it is by the reticulation of gelatine t h a t the “grain” of a collotype is produced. It has been employed in the production of irregularly grained “half tone” screens, in which the reticulation pattern takes the place of the cross line rulings of the regular screens. An understanding of the conditions affecting and determining reticulation will not only be of practical use but will ten& t o throw light upon the physicochemical nature of gelatine, and perhaps help toward the scientific spccification of g e l a t i n s for photographic
the curve (Fig. V), which shows results actually obtained with a sample of gelatine. In this curve the ordinates give the amount of water absorbed by I g. air-dry gelatine on final swelling (about 48 hrs.) while the abscissae give concentrations of acid and alkali in normality. As is evident, the swelling or absorption of water is extremely sensitive t o both acid and alkali or, in t,erms of the ionic theory, t o hydrogen and hydroxyl ions. I n fact, the
use. S W E L L I N G O F GELATINE I N WATER A N D ITS SXRIXKAGE
O K DRYING
pro. 111
The immediate cause and mechanism of rcticulation will be best understood if we first consider a few
actual sensibility is of the same order as t h a t of the dyes used as analytical indicators. The minimum ab-
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
728
TIME
FIG.I V
Sorption probably lies in reality a t a point represented b y the dissociation of pure water, according t o t h e equilibrium,
H+
+ OH-
HzO,
b u t this is masked usually b y residual acidity or alkalinity of t h e gelatine. T h e precise determination of this point for a given gelatine will probably prove of some importance in t h e specification of photographic gelatines. It is of great interest since in passing from a n alkaline t o an acid state t h e swelling goes through a very pronounced minimum. 48
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The influence of neutral salts in solution upon swelling is too complex t o be discussed fully. It will be sufficient t o note t h a t some, such as iodides, a c t in the direction of increasing t h e swelling (hydration) ; others, as sulfates, in the direction of diminishing it. So
Vol.
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No. 9
long as only salts of t h e alkali metals a n d ammonia are considered, i t is the acid p a r t or anion which is of chief importance, and these salts have been arranged in a series, indicating their effect upon swelling. The effect of a given salt depends much upon its concentration a n d above all on t h e acidity or alkalinity of t h e solution. Now, turning t o t h e influence of shape upon swelling (and conversely) we find t h a t a dominant factor here is t h a t of the condition in which t h e gelatine first swelled or was cast and dried. Gelatine, in t h e abstract, as a homogeneous material alike in all directions, should, theoretically, tend t o swell or shrink uniformly without change of shape, only altering its mass or bulk. If gelatine could be dried very slowly so t h a t t h e loss of moisture proceeded a t the same rate in all parts of t h e mass then i t would shrink without change of shape, b u t such a condition cannot be real. ized in practice a n d gelatine dries more rapidly on " the surface t h a n in t h e interior, t h u s producing stresses and didortion. I n t h e case particularly important t o us, t h e gelatine is coated on glass or film support and firmly attached t o i t , SO that.one side is eliminated as regards drying, etc. The gelatine cannot spread off t h e plate, so t h a t its swelling and shrinkage are limited t o one direction, v i z . , t h a t perpendicular t o the plane of t h e support (Fig. V I ) , This state of affairs is determined in a d vance by t h e first drying down of t h e jelly (or FIG.V I emulsion) on t h e support; it is not peculiar t o t h e photographic film, since ordinary sheet or leaf gelatine which had been dried on nets shows t h e same tendency t o have its principal expansion-perpendicular t o t h e face of t h e sheet.
Fizziza
P R O D U C T I O N O F RETICULATION
A gelatine film, under normal conditions, can be repeatedly swollen and dried without losing its capacity t o swell and shrink normally t o t h e plate. It is evident t h a t a certain strain must be imposed upon t h e gelatine in drying, which is removed by swelling. If we consider a n ideal unit cube of t h e swelling gelatine, supposed free from all constraint, i t would tend t o expand uniformly in all directions. This ideal uniform expansion corresponds to a uniform swelling pressure, i. e . , a pressure t h e same in all directions. We can consider this resolved into forces perpendicular t o t h e surface, and forces parallel t o t h e surface. Actually, t h e gelatine layer in sheets or on plates does not swell uniformly. T h e forces parallel t o t h e surfaces which would, of course, tend t o remove t h e film from t h e glass or support, must be compensated. This compensation is in a measure external or initially external, being due t o t h e adhesion of t h e gelatine t o a rigid support, b u t i t is chiefly internal, arising from a uniform strain or tension impressed by t h e mode of drying. Now suppose the gelatine layer be subjected t o
T H E JOCR:VAL OF I N D G S T R I A L A N D ENGILVEERIAVGC H E M I S T R Y
Sept., 1 g 1 8
'i 2 9
drastic internal action, excessive swelling and excessic-e susccptible of control, although it must he emphasized dehydration, either successively, or, in a measure, t h a t in any case the balance between hardening and simultaneously, then gelatine jelly will he strained softening agents must he delicately adjusted, and beyond its elastic limit, showing either a total or a t h a t the measure of control is limited. Further, partial reaction. the occurrence or production of reticulation is in a Total reaction would imply the detachment of the very large degree dependent upon the nature of t h e layer from its support, a result which is seen in frill- gelatine. The so-called "hard" gelatines tend readily ing and floating off, as a result of excessive lateral t o reticulation, while the "soft" ones only give transient expansion. signs of it. If, however, the adhesion t o the support is mainAn experiment on this point gave thc. following retained, b u t the newly disengaged tangential or lateral sults: Coating Reticalation io Potassium forces are not entirely compensated, then the strain G*L*f.N* Per cent Mercuric Iodide distribution in the gelatine layer ceases t o he uniform A-Hard ............... 8 stiong. permanent B-Herd ............... 8 strong and we get a local puckering or folding, similar in C.Soft ................ 8 very faint transient D-Soft ................ 8 very faint: transient character t o t h a t produced in the earth's surface by E-Soit ................ S very faint. traneient tangential forces acting on restricted areas of semiThe following results were obtained with combinaliquid igneous rocks. Thus the immediate mechanism of reticulation is the tions of softening and hardening agents 3 and 4. CHBOMIC ACID AND HOT WATER---Chromic acid is, production of restricted tangential dilation, which is of course, a well-known hardening agent for biological partially arrested. ' tissues. Working with 8 per cent hard gelatine, This, however, leaves unsettled the inner physical machine-coated on glass, a I O per cent solution of chemistry of the process, t h a t is, the origin of a n exchromic acid a t 2 0 - t o z z * C., followed by washing cess swelling pressure (the super-pressure) and of a with water a t j 6 " C., was fonndto afford the best conpartial or localized arrest Of this. This can he discussed best in dealing with specific cases of the pro- ditions. duction of reticulation. EXPERIMENTAL PRODUCTION OF RETICULATIOS
4 typical case, which has t h e advantage of following ordinary photographic procedure, is as follows: A Seed 23 plate is "flashed," developed in a standard pyrosoda developer for 4 min. at Zoo F., then rinsed, and fixed in a standard hypo-bisulfite fixing bath a t 80" F. Reticulation was then found t o depend upon the tcmperat.ure of the wash n'ater as follows: Temoerslture 70' F. 80' F. 90' F. 100" F.
Reticulntion
Instead of water, stronger and more definite results were obtained by an after-treatment ivith the following solution: 50 CE. 95 per cent Ethyl Alcohol 40 cc 5 per cent Formaldehyde 110 CE. water
In this case the following factors may hayc played a part: 1-Prehardcned gelatine in the emulsion. 2-Tanning agents produced in development. 3-Excess swelling pressure in hot developer, etc., and particularly in washing. T h a t reticulation can be produced by the combined action of both a swelling or softening agent and a hardening or anti-swelling agent t o restrain this is shown by the production of reticulation by the following combination: NO.
IURDENINC ACEIT Tannic AcM
1 ..........
2.......... 3.. 4..
........ ........
Quinone Chromic Acid Mercuric Iodide
SOSTENWE ACBNT Acetic Aeid Acetic Add Hot water Potassium Iodide
All of these combinations produce reticulation, but I and 2 have only a theoretical interest, as they are difficult t o control. The other th.0 pairs are more
Fro. VI1 P O T A S S I U X U E R C U R I C IonIuc-~~.The solution of mercuric iodide in potassium iodide known as Urucke's reagent was found t o he a convcniciit strength for use. According t o thc formula for this, 1 2 0 g. of mercuric iodide are t o be dissolved in a solution of j o g. potassium iodide in joo cc. water, and the whole diluted t o lono cc. \Ire found, however, t h a t under these circumstances only about 7 2.6 g. of mercuric iodide could be dissolved a t room temperature; nor was this result much affected by heating t o j o ' C. It should bc noticed t h a t these quantities are near t o thoee necessary for the double salt, zICIHg12 or KzHgI,. In this combination, the softening agent is the iodide, or more specifically the iodion, I-, while thc hardening
T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
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or coagulating agent is the mercuric salt, or, again, t h e mercuric ion Hg++. Attempts made t o increase t h e proportion cvf mercnry were without success. A saturated solution of potassium iodide a t 18’ t o 20’ C. was made, containing 128 g. potassium iodide t o IOO cc. of water, or 56.2 g. in I O O g. solution, which agrees fairly with t h e value 59 per cent a t 2 0 ’ C. given as the solubility in Landolt - Bornstein. This solution was s a t FIG. VI11 urated a t 20’ C. with mercuric iodide, t a k ing up 64 g. I n this the ratio of 2KI to HgIz is 0.69, whereas the actual double salt would call for 0.7 3 . This solution was used as a saturated stock solution and Brucke’s reagent is equivalent t o I O parts s;tock saturated plus go parts water. Working with the I O per cent saturated (Brucke’s reagent) and hard gelatine, 8 per cent machine-coated, the following results were obtained:
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Sodium sulfate used in the reticulation process makes the “grain” finer, while after-treatment with formaldehyde increases or conserves the depth of t h e wrinkles. An important conclusion from these experiments is t h a t apparently reticulation may start in more t h a n one way. Thus with the Brucke reagent, and with chromic acid followed by hot water, reticulation proper was generally preceded by t h e appearance of small pock-like markings of about 0 . 2 t o 0 . 3 mm. diameter. These would sometimes align themselves in “streaks,” and in any case seemed t h e foci of t h e subsequent reticulation. These markings are shown in Fig. VII. On the other hand, in t h e reticulation produced by the use of hot water after development and fixation, these initial markings did not appear.
; Duration of Treatment Min.
EFFECT O N THE SILVER IMAGE
It is noteworthy t h a t when t h e reticulating film contains developed silver particles-as
in negatives
EPFECT
............ Small pock marks about 1 mm. apart produced in 40 sec. followed by reticulation which was much lessened in drying ........,... As persistent before, but reticulation somewhat more on drying 5 .............. As before, but with continued treatment reticulation became fainter and vanished on drying IO.. ............ As before but the whole surface finally softened an’d could not be dried, softening and 2814
31/2
10.
’
.............
running After treatment, this was chilled 15 min. on ice, then immersed for 2 min. in 3 per cent formaldehyde. This conserved the reticulation
The formaldehyde after-treatment seems generally necessary with this agent t o “fix” the reticulation. Using soft gelatine, 6 per cent solution machinecoated, and a wide range of concentration of the potassium mercuric iodide solutions, only slight a n d transitory reticulations were observed in the higher concentrations, giving way, however, t o a general softening and liquefaction. Attempts t o overcome this b y preliminary hardening with formaldehyde were not successful. Prehardening with chrome alum showed better results. In t h e case of t h e mercury-potassium iodide combination, while i t is not possible t o increase
FIG.I X
the mercuric iodide ratio above a certain limit, other permanent or temporary hardeners may be added. I n particular it was found t h a t Brucke’s reagent with the addition of 6 per cent of saturated N a z S 0 4 solution gave very fine, uniform reticulation.
FIG. x
after fixation-there is a n apparent migration of t h e silver particles, the ridges being denser, the valleys much less dense or even quite clear. The question arises, whether reticulation is simply a puckering of a sheet grown larger by lateral dilation, larger t h a n the support boundaries, b u t retained on this b y local adhesion, particularly a t the edges, as is indicated in Fig. V I I I , or is a mosaic-like alteration of hardening and softening effects, the ridges being more swollen, t h e valleys more tanned, as suggested by Fig. I X . It is evident t h a t in t h e first case the excess in t h e ridges is simply due t o the total thinning (by the lateral dilation) plus local thickening due t o folding of t h e increasing sheet. I n the other case, t h e greater density in the ridges would be due t o a n actual migration of silver due t o tension, similar t o t h a t occurring on the drying of moisture spots, when t h e tension in drying softens the gelatine and forces t h e particles into t h e periphery of t h e spot. This effect is shown in Fig. X, a drawing made by Mr. M. B. Hodgson
T H E JOCR,VrlL OF I N D U S T R I A L A‘VD EAIGIdVEERING C I I E M I S T R Y
Sept., 1918
73I
m e . XI
from microscopical observations. In the latter case we should haVe in reticulation a great and increasing number of microscopic replicas of such “moisture spots,” tending to run into each other and form one system, like cracks in a drying mass. The theory of alternate softening and hardening, or of differential swelling, couples up readily with the fact already noticed t h a t in many cases reticulation starts from a number of isolated points. When softening and tanning agents arc present together in a gelatine gel, a certain amount of selective adsorption and differential diffusion will occur. A molecule or ion having a tanning action will tend t o be adsorbed or fixed in situ, and its own specific diffusion will be hindered. Molecules or ions having a softening action may modify the action on tanning agents, but their own diffusion will be facilitated by their hydrating and softening action on the gelatine. I t is easy t o see t h a t we should have then a condition of rhythmic coagulation of the gelatine very similar t o t h a t shown in the well-known Liesegang rings. In this latter case, when two salts which react t o form a precipitate are allowed t o diffuse together through a gelatine gel, the precipitate, such as silver chromate or silver halide, is not deposited uniformly, but rhythmically, in alternate rings or layers. Actually it is observed that reticulation generally starts in one or more regions and fills up by the spread of these; in some cases from isolated foci. It seems then t h a t reticulation in its earliest stage involves something like the nucleation of a crystallizing solution. In such a solution, crystallization may start either a t nuclei already present in the solution or by the formation of new ones, but in the latter case there is required a higher degree of supersaturation for crystallization t o start. A t what points in such solution or melt the first nuclei appear is a matter of pure chance and it is apparently much the same mith the start of reticulation, THE
COXNECTION “
BETWEEN
GRAININESS”
RETICULATION
AND
THE
OF PIECATIVCS
In one important case where it is very probable that incipient reticulation is a t work, foreign nuclei
are available. This i s in the case of t h e ordinary development of a photographic emulsion. I t is known that, apart from differences in emulsion, different developing agents and treatments affect the “graininess” of the developed image. By this is not t o be understood thc clementary plate grain, b u t such clumping in second order aggregates as is liable t o be objectionable in projection. This granulation depends upon development, and in the same way, resolving power depends upon development and the developer.’ It is hardly t o he doubted t h a t
Fro. XI1
we have in this case a selective adsorption and differential diffusion of devclopers, producing what amounts t o incipient reticulation, nuclei being formed by the developed silver particles, with their tendency t o adsorb the colloidal reaction products of development, which have tanning or coagulating properties. Consideration of the great change in the swelling equilibrium shown on p a s i n g from an alltdinc t o an acid condition (Fig. V) shows also t h a t the opera1 K. Hure, ”Photographic R e ~ o l v i nPower,” ~ J . Am. OPlicoi Sor., l(1918). 119.
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tions subsequent t o development are very liable t o develop still further any sub-microscopic reticulation, and again t o coarsen the “grain” of t h e image. It is hoped t o follow this u p experimentally when time permits, instruments having been devised for measuring both granularity and minute swellings.
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I n Fig. X I are given photomicrographs of some of the reticulated preparations described, taken with vertical illumination a t IOO diameters. ’
RESEARCH LABORATORY EASTMAN RODAK COMPANY ROCHESTER, N. Y.
LABORATORY AND PLANT METHODS OF ANALYSIS USED IN THE COAL-TAR INDUSTRY. I-CRUDE TARS By J. M. W ~ r s s Received July 20, 1918
INTRODUCTION
.
I n April 1911, S. R. Church’ published a paper which described in some detail the analytical methods as used by T h e Barrett Company a t t h a t time. This was supplemented by a later paper in 1 9 1 3 , ~giving certain revisions and additions which had been made up t o t h a t time. These methods, with others, have been in use in t h e laboratories of The Barrett Company a n d many other companies for a n extended period a n d have been given the test of continued use. Some of t h e methods have been t h e subject of exhaustive investigations t o determine t h e variants which limit the accuracy of t h e tests. For instance, J. M. Weiss presented a paper3 dealing with t h e “free carbon” tests on tars a n d pitches and the factors influencing the results, together with some theoretical consideration of t h e material known as “free carbon.” There have also been numerous publications on t h e testing of t a r products, emanating from various sources, such as t h e Office of Public Roads, t h e U. S. Department of Agriculture, t h e American Society for Testing Materials, t h e American Gas Institute, etc., and also from many individuals, b u t we do not intend t o present here a bibliography of the literature on coal-tar product methods of analysis, b u t merely t o indicate under a few of the tests published the main sources from which we have drawn. About a year and a half ago, we realized t h a t t h e directions of our testing methods were more or less incomplete in detail a n d t h a t in many cases important points were left unemphasized. Accordingly, a chemists’ committee was formed, consisting of S. R. Church, F. J. Gerty, J. B. Hill, K. B. Howell, H. E. Lloyd, J. G. Miller, M. R. Walczak, a n d t h e writer, whose purpose it was t o revise and standardize t h e existing tests. A description of each test was prepared by one or another of the committee and sent for comment t o each member of t h e committee, a majority of whom were actual laboratory workers of long experience. The comments were compiled and, where necessary, experimental work was instituted t o settle points which were in dispute. The methods were not p u t into final form until t h e committee was substantially unanimous regarding all t h e details of t h e methods. We are presenting in this paper a selected list of methods which we believe are of very general interest 1 2
8
THIS JOURNAL, 3 (1910, 227. Ibid., 5 (1913), 195. I b i d , 6 (1914),279.
t o all engaged in the manufacture or use of coal-tar products. This paper will be followed by three others, one dealing with the methods of test for refined tars a n d pitches, another with methods of test for creosote oils and carbolic oils, and the last, benzols and light oils. Many of t h e tests are widely used throughout t h e country a n d have been adopted b y a majority of t h e producers and consumers of t a r products. A number of these tests are the standard methods of such associations as t h e American Railway Engineering Association, American Society for Testing Materials,’ etc. We are $resenting them in t h e belief t h a t i t will be helpful t o those engaged in t h e testing of t a r products t o have these standard methods collected together in compact form convenient for reference. The illus- * trations are mainly assembly drawings, b u t the special apparatus can now be obtained through almost a n y apparatus house. We have furnished detailed plans and specifications of all our special laboratory apparatus to every chemical glassware manufacturer and lab- . oratory supply house of whom we were cognizant. For each test on which we have carried out sufficient exhaustive research, t o enable us t o do so, we have made a statement as t o its accuracy. SAMPLING
Before considering t h e test’s in detail a few words on sampling would not be o u t of place. The sole practical purpose of laboratory testing or analysis is t o obtain information as t o t h e composition, quality, and properties of a given material. Usually, this material is in large bulk. T o make t h e desired test on t h e entire bulk of material would obviously be impracticable as well as ne5essitating frequently t h e destruction of t h e material. I t therefore becomes necessary t o select from the bulk of a material, a portion or sample of same, which shall be representative of t h e entire bulk and which can be tested and its properties determined. Obviously, t h e interest and commercial values attach t o t h e properties a n d qualities of t h e bulk of material a n d only t o t h e properties and qualities of t h e sample as far as the, sample is representative of t h e bulk. It is therefore necessary t o t a k e every possible precaution t o see t h a t t h e sample is in every case representative. The laboratory methods, apparatus, and technique may b e perfect a n d t h e results recorded may be accurate as t o t h e quality of t h e sample tested. If, however, t h a t sample is not representative of t h e bulk, t h e value of t h e test is commercially nil. The entire theory of sampling is therefore based on t h e principle
