Emulsion processes in photography

Preface

It has been the endeavour of the Author to collect, in the form of a u handy-book,” those Photographic Emulsion Processes which have been received with most favour by experienced workers, and with which he is practically acquainted himself ; and, at the same time, to give theoretical explanations of some of the phenomena which are met with in these particular processes.

To avoid misconception, the Author wishes it to be understood that much of the original substance of the work, except where special references have been given, has already appeared in the columns of the Photographic News , or in papers read before the Photographic Society of Great Britain.

The Author also wishes to make grateful acknowledg¬ ment to the Editor of the Photographic News for the permission accorded to use several woodcuts which illustrate the work.

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PHOTOGRAPHY WITH EMULSIONS.

Chapter I. Peeliminaey Considerations

The term emulsion is derived from the Latin word 11 emulgere,” to milk out, and the definition of it as found in the dictionary is, u any milk-like mixture prepared by uniting oil and water by means of another substance.” F or our photographic technology this is hardly a correct definition, for by it we mean a sensitive salt of silver in very minute division, held in suspension in some viscous body, such as gelatine, or, more commonly, collodion. With the sensitive salts held in the latter we propose to deal firstly, and then to show the peculiarities of, and the difference in its manipulation of the second.

We shall first deal with some theoretical considerations connected with the preparation of the emulsions, and subsequently give the mode of preparing the plates.

An emulsion in collodion in its simplest form, when required for producing negative pictures, may "be considered to be simply pure silver bromide held in suspension in collodion, and so well prepared that when a plate is coated with it, a homogeneous film results; a film which, in fact, is equal in sensitiveness and in physical qualities to any which can be prepared by the bath process.

The reason why silver bromide is chosen in preference to the iodide or chloride is, that the iodide is very diffi¬ cult to emulsify in collodion which is of such a consistency as shall pour ; and that the chloride, if not slower, is less easily amenable to the alkaline method of development than the bromide. The chloride can be emulsified as is well known, but is better adapted for a printing process, and in that respect is without a rival for deli¬ cacy. It will be seen, however, that both the iodide and chloride, or both together, are used in emulsions in com¬ bination with the bromide ; and that when so used they have certain qualities which are said to improve the deve¬ loped image. W e may as well state, at the outset, that in our opinion there is nothing to compare with pure bromide u solus,” for general work, but this opinion must be taken for what it is worth.

Collodion emulsion processes are divided into two classes : one in which the emulsion is made up and used without any preliminary extraction of the soluble salts necessarily present in the collodion after the double decomposition of the soluble bromide, &c., and the silver nitrate; and the other where these soluble salts are ex¬ tracted. In the first process the plates are washed, and then usually receive a coating of some preservative, whilst in the second they are generally left to dry spontaneously, and are used without any preservative, though it is not uncommon to dissolve resinous matter in the collodion, to act as a temporary varnish. The theoretical value of the preservative we may subsequently allude to briefly, when other matters have been considered.

rfhe plain collodion with which the emulsion is to be made shall be first dealt with, distinguishing the quali¬ ties necessary for the unwashed and for the washed emulsion.

Some emulsion workers have laid it down as an axiom that the pyroxyline for the two processes should differ, while others declare that this is unnecessary. Again, some declare that to gain good density the pyroxyline should contain a percentage of organic matter, presumably to be capable of acting on the silver bromide during deve¬ lopment, or by forming some definite compound with silver. Our own experience is, that for securing density organic matter is unnecessary, though it may improve sensitive¬ ness ; and we have found in some instances that density was absolutely impossible to attain where organic matter was present. We shall touch on the question of density of the image further on.

If a preservative be used as a sensitizer, there can be no doubt that a collodion should be used which is as porous as possible, to enable it to surround the particles of the sensitive salt. This porosity has also another advan¬ tage, which is, that when the preservative is washed off previous to development, the sensitive salt is immediately accessible to the action of the developer. It is such a collodion that is recommended for dry plates prepared with the aid of the bath, more particularly in the collodioalbumen process, though in this process the sensitive salt is more especially contained in the albumen, and it is there¬ fore necessary that a fair quantity of the latter should be on the plate, which is accomplished by this porosity of the collodion film. For any emulsion process we consider a horny collodion objectionable, owing to the difficulty that exists in making the developer penetrate through the film. A [horny collodion has, however, one advan¬ tage in that it acts as a varnish to exclude the air from the sensitive salts enclosed within it. In the following formula? which are given for the preparation of pyroxylines, one will produce an ordinary tough film, and the other a fairly porous film, and consequently a rather powdery pyroxyline.

The solvents of the pyroxyline should be as pure as practicable to secure the maximum of sensitiveness, and this has been shown to be the case by that indefatigable experimentalist, Mr. H. Berkeley, in a communication he made to the British Journal of Photography during the last year. There is, for instance, no doubt that when methylated alcohol is used, there may be a lack of sensitiveness, and even a production of fog. The ordinary methylated ether, however, will be found, as a rule, to be sufficiently pure.

Effect of a Base upon Pyroxyline . — The general -way of forming an emulsion in collodion is to dissolve the soluble bromide, &c., in the collodion, and then to add silver nitrate to it, either in excess, or in defect, and experience has shown that the sensitiveness depends to some extent on the metal which forms a part of the bromide. Mr. Warnerke states that the pyroxyline enters into combination with the metal, in some instances altering it in chemical, and there¬ fore of necessity in physical, qualities. As an instance, he adduces the action of ferrous chloride on gun-cotton. That such action does take place must he admitted, hut up to the present time we have no researches which point out what those changes are. It must also not he forgotten that some of the haloids enter into combination with alcohol ? good examples of this are to he found in calcium chloride and zinc bromide, the latter salt entering into combination with a considerable evolution of heat. It is highly probable that this compound may re-act on the pyroxyline which is in solution, but from analyses which have been made it seems that the metallic base does not enter into absolute combination. In one case, which was brought under the notice of the writer by Mr. Warnerke, the pyroxyline in solution had been in contact with a copper salt, and a blue colour was noticeable after the pyroxyline had been desiccated, but this would seem to arise from the traces of the insoluble form of salt which nearly always accompanies this particular bromide (see page 9). The caustic properties of silver nitrate, too, will probably affect the pyroxyline, and it will be noticed, in some of the processes described, that the contact of the silver salt with the collodion is insisted upon strongly. The writer’s own experi¬ ence is, that not much benefit is derived from such contact, but undoubted authorities on the subject have prescribed it.

The Cause of Fog in Emulsions. — Every student in emulsion work has found, and will find, that the chief obstacle that he has to overcome is the tendency for the plate's prepared with it to fog on development, and it has taken a great deal of experimental work to enable it to he overcome. The reason of the fog-giving emulsion has been obscure ; hut the writer ventures .to think that the very recent researches that have heen made on the sub¬ ject have explained in a great measure, if not entirely, its raison d'etre ; and, in spite of being tedious, or of being told that the reader knows all about it, a summary of what is known of the matter is appended.

Setting aside the collodion from the question, and merely taking into consideration the sensitive salts employed, we may arrive at very definite results. It has been asserted that a neutral combination between two substances can never take place ; for example, if we mix potassium chloride with silver nitrate we shall never be able to get pure silver chloride, how¬ ever much we may wash it — that either the soluble potassium or silver salt will always be in excess, though in the minutest quantities. This certainly is the case theoretically, because do what you will, and wash as long as you like, there still must be some infinitely small part of the soluble salt left behind. Now for ordinary chemical reactions, where the products of these combinations have to be weighed, the residual impurity may be inappreciable, being so infinitesimal that no balance yet constructed can show them. Though a balance may be inoperative, yet, as is well known, light is able to show us impurities in a substance which may not be one millionth part of a grain in weight. By passing the light from the heated vapours of the substance and its impurity through a prism, and noting its spectrum, we may be able to detect the latter.

The spectroscope will not tell us at present, however, whether the silver or potassium is present as nitrate, chloride, bromide, oxide, &c.

The question now arises, is it possible to detect this? Is it possible, for instance, to say whether any infinitesimal quantity of oxide be present? In the majority of cases the question would be an open one ; but where we are dealing with silver salts which are sensitive to light, and which are amenable to deve¬ lopment, we think we can give an affirmative answer. Sometimes, however, the impurities in the bromides can be shown by weighing. For, in a paper read before the Photographic Society of Great Britain on the 8th of February, 1876, Mr. Warnerke stated that on testing the different bromides, he found that considerable variation from the theoretical quantities necessary to combine with silver nitrate was observable. Thus he found that one grain of silver nitrate requires

Of potassium bromide 1*35 grains, theoretically, 1*429 grains, Of ammonium bromide 1*80 ,, ,, 1*78 ,,

Of cadmium bromide 1*005 ,, ,, *885 ,,

The cadmium salt, he states, was not anhydrous, therefore this may account for one discrepancy ; the potassium and zinc point to the presence of oxide, whilst the ammonium seems to indicate the presence of free haloid. Be this as it may, a careful experimenter has found these discrepancies.

Let us see how potassium bromide may be contaminated in its preparation. We find that the mother liquor from the sea water brine is treated with chlorine, and that this takes the place of the bromine in combination with the magnesium ; the yellow liquid is agitated with ether, which takes up the bromine, and this ethereal solu¬ tion is treated with potash in solution. The bromine forms the bromate and bromide of the alkali, and when the alkali is nearly saturated, it is decanted off and further treated. Now, from what we have said before, it is more than probable that the bromide is contaminated with the alkali, however well it may be separated : the traces of alkali may even be so small as to be undetected by litmus paper.

Again, the bromides of the alkaline metals are pre¬ pared by acting on the alkalies with an excess of bromine, a similar reaction to that above taking place. The bromate is decomposed by ignition, and this heating alone tends to decompose the bromide, in which case we should have the oxide of the alkali left behind. In good preparations it would be excessively small, but still sufficient, for the purpose we shall indicate presently.

The bromides of the metals may be similarly contami¬ nated. Take zinc as an example ; the metal is easily oxidized, and the zinc oxide is soluble in zinc bromide, as it is in the chloride. In all these cases, then, it is possible we may have traces of oxide with the bromide. Again, there are some metals which form two bromides, as that of copper ; and experience shows that it is very hard to get the compound fully saturated with bromine without having part of it in the less saturated state.

If such bromides, contaminated with the oxide, or con¬ taining the lower combination of bromine, be brought in contact with silver nitrate, we shall have two separate reactions to consider. In the case of the oxide contami¬ nation, when silver nitrate is in excess, we shall have —

Potassium bromide, Potassium oxide, and Silver nitrate

zKBr + K20 + (*+2)Ag NOs

Give Silver bromide, Silver oxide, and Potassium nitrate

Or, besides the silver bromide, we shall have silver oxide formed.

In the case where we have a sub-bromide of the soluble metal formed, wTe have, taking copper as a sample —

Cupric bromide, Cuprous bromide, and Silver nitrate

* Cu Br2 + Cu Br + (2z+2) Ag H03

Give Silver bromide, Silver sub-bromide, and Cupric nitrate

Or, we have silver sub-bromide formed ; if it be preferred, this compound may be supposed to be made up of silver bromide and metallic silver (Ag2 Br=Ag Br+Ag), though from experiments already published, we have proved it to be the sub-bromide.

Evidence of the most unmistakable character points to the chemical theory of the formation of the photo¬ graphic image — evidence so strong, and so well known, that it would be out of place to record it here. Briefly we may say that the action of light on a silver haloid seems to be to reduce it to a simpler type, which we may call the sub-haloid. Thus

Silver bromide gives Silver sub-bromide and Bromine.

A g2 Br2 = Ag2 Br 4“ Br.

If this be admitted, we can proceed a step further ; but if not, the subsequent argument falls to the ground. If any reader will not admit it, he should read all the various evidence that has been adduced since the time when Scheele first made his experiments with silver chloride, and we doubt if he can remain unconvinced, more particularly in regard to the visible image formed on the bromide and chloride. It is a pertinent question to put, as to whether the visible and the invisible (or developable) image are of the same nature ; which may be answered by another question : Can the line be drawn where the image is invisible ? If so, what is the boundary between the two? If we admit the theory of the forma¬ tion of the visible image, it seems hardly logical to deny a similar formation for the invisible or photographic image. It is quite possible that beings with more acute sight than ourselves might be able to see the image which we cannot, as we know certain insects can hear sounds which do not affect our auditory nerves. Coloured particles are visible when put together en masse , but if only a few coloured particles are present in a mass of colourless particles, it is quite certain that they may re¬ main undetected.

If we may, then, be allowed to beg the question, or rather to assume that silver bromide is reduced- by light to the sub-bromide as indicated, the following explana¬ tions may be proceeded with.

We have seen that we may have oxides and sub¬ bromides contaminating the bromides, and in a similar way we may have oxides and sub-chlorides contami¬ nating the chloride. In a communication to the Philo¬ sophical Magazine , which was reprinted in the photo¬ graphic journals, the writer showed that it was possible to develop an image on a film never exposed to light, but which was in contact with a film (during the operation ot development) on which an invisible image had been im¬ pressed. The explanation there offered seems every way to meet the requirements of the case, which is, that where a nucleus, if it may so be termed, exists, there the silver from the adjacent bromide will be deposited in preference to any other part of the film. Such a nucleus is found in the silver sub-bromide or sub-chloride obtained by ex¬ posing a plate to light. However produced, we may assume it will act in a similar manner.

The case of the oxide is not so clear; but a little experiment will throw light on it. Prepare silver oxide as an emulsion in collodion ; dissolve (say) 6 grains of silver nitrate in an ounce of plain collodion, and add to it 2 grains of potash in alcohol. This will give an emulsion of oxide of silver. Now wash it, and treat it as a washed emulsion, and add a drachm of it to an ounce of a washed emulsion which works perfectly free from fog ; coat a plate, and develop it. It will be found that inevit¬ able fog is produced. In this case the silver oxide (pre¬ sumably partially reduced to the metallic state, since the oxide is an unstable compound) acts as the nucleus on which the silver bromide is reduced to the metallic state by the alkaline developer.

It must be borne in mind that the invisible image must necessarily be composed of very minute particles of the altered silver salt. If, then, such a small number of particles distributed over a film are suffi¬ ciently powerful to form nuclei for the development of the image, the same minute quantity of oxide, or chemi¬ cally produced sub-haloid of silver, might be capable of producing the same results. The above, then, seems to be the explanation of fog in emulsion plates. Now as to the remedies.

On the Elbnination of the Causes of Fog from Emulsions. — It is well known that when we have an excess of soluble haloid, freedom from fog is secured. In some recent experiments we carried out, we found that silver bro¬ mide is formed before any other silver compound, except the iodide, when the sensitive salt is formed from haloid salts, and not from the halogens themselves. Thus, if potassium bromide be contaminated with potash, we shall have both silver bromide and silver oxide formed, if an ex¬ cess of silver nitrate be added ; but if there be a defect of the nitrate there will not be a trace of silver oxide, but only silver bromide. Again, if we take bromide of copper, which is usually contaminated with the sub-bromide as already stated, it will be found that the bromide is all util¬ ized before the sub-bromide is attacked at all ; and if, in addition to the bromide, we have a metallic chloride pre¬ sent which may be contaminated with sub-chloride, the order in which they will combine with the silver nitrate is, — bromide, chloride, sub-bromide, sub-chloride. Thus, if there be only sufficient silver nitrate added to an emulsion to combine with the two first on the list, the other two will be left in the emulsion as harmless compounds. The ration¬ ale of the elimination of fog from the finished emulsion in which there is at first an excess of silver nitrate is thus demonstrated. We have now the theoretical explanation of Major Russell’s statement that a little soluble bromide must be left in the film when silver bromide is formed by the bath in the usual way, and it may be remarked.

parenthetically, that whether the image he developed by the alkaline or acid methods, the same result must hold good. Supposing we have an emulsion which contains bromide, sub-bromide, and oxide of copper, and also a slight excess of silver nitrate. The addition of certain metallic chlorides or of hydrochloric acid will at once con¬ vert the sub-bromide and oxide into the chloride of silver* leaving harmless compounds behind : the metallic chlo¬ rides which are of use are those which readily part with chlorine, and which, therefore, preferably form more than one chloride, such as copper, cobalt, gold, platinum, &c. When other chlorides, such as of the alkalies, are employed, the needful substitution may not take place, because the affinity of the alkali for the chlorine is greater than for the sub-bromide ; and therefore the elimination of the sub¬ bromide is not effected. Thus, if all the silver nitrate in original excess be converted into silver chloride, we have the silver sub-bromide to get rid of. Now, supposing we are using sodium chloride as a corrective, then we should have

A g2 Br + NaCl,

which can form no new saturated silver compound, since an atom of metallic silver, sodium, bromine, or chlorine cannot be left in a free state ; but if we use (say) cobalt chloride, we have —

Ag2 Br -f- CoCl2, which can form —

Ag Br + AgCl + CoCl.

The CoCl, or sub-chloride of cobalt, is harmless, an d can be washed out of the film.

It will, therefore, be seen how it is that addition of these chlorides to a washed emulsion will give freedom from fog.

Secondly, if an excess of silver nitrate be used, it is evident that something else besides a mere chloride will be required, since the sub-salts and oxides may be formed. This we find in the employment of an acid, or of a halogen itself, or both together, added to the collodion, and to be most rapidly effective. Whatever is used is best added to the soluble salts before the silver nitrate is added.

Suppose nitric acid alone be employed, then any oxide or carbonate will immediately be attacked, as also any of the sub-bromides — such as of copper. Again if aqua-regia be employed, we know that chlorine is evolved in an extremely nascent state, and that this would attack either oxide or sub -bromide, fully saturating the unsatisfied atom in the latter. If, now, silver nitrate be added, silver bromide and chloride would result with some compounds (perhaps such as the chlorate), which would be as inert as producers of fog as the silver nitrate itself.

If a halogen be employed without any acid, the same result would occur. Thus, suppose we had as impurities an oxide and a sub-bromide, and that we added a solution of bromine to it, we should get the oxide changed to a bromide and bromate (the latter salt of. which is experi¬ mentally proved to be inert), and sub-bromide changed to a bromide.

If the halogen be added last, when there is an excess of silver, it is probable that until all the latter is converted it will exert no un fogging action ; but if an acid, such as nitric acid, be added, it will exert its proper influence, though slowly ; for it will convert any oxide or compounds of the oxide into nitrate, and from the silver sub-bromide dissolve away the loose atom of silver, converting the sub¬ bromide into bromide and nitrate. Thus : —

Or, at all events, a fresh combination will be made, which is unacted upon by the developer.

There is also a method of eliminating fog from the dry plate when it is coated, without doctoring the emulsion at all. This need not apply only to washed emulsions, but it can be effected during the washing of the plates prepared by the unwashed emulsion. In addition to the elimination by the acids, and by the metals forming two bromides or chlorides, we can further effect it by using a solution of potassium bichromate, permanganate of potash, or per¬ oxide of hydrogen. The reason of this seems to be due to oxidation ; the cause at present is somewhat obscure, but the writer is engaged in experiments as to the cause. It may be, in the first case, that a minute quantity of silver bichromate is formed by the oxide, or that the free silver atom of the sub-bromide is oxidized, and then formed into silver bi-chromate ; with the second it may be that the manganese salt is substituted for the silver salt, and is inert ; and in the last case it may be that the silver salt is per-oxidized, and forms an oxy-bromide, which is un¬ affected by the developer. This seems probable, since ozone has the same effect on the fog.

It may not be uninteresting to note an 'experiment which throws some light upon this point, though it is not conclusive. If a plate be coated with emulsion, and be allowed to thoroughly darken in the daylight, and then drops of these destructives be placed on the film, and allowed to act for a few minutes, it will be found, after washing, that the spot where the bichromate and the hydrogen peroxide have been dropped the colour and appearance of the film remain unaltered. Where the manganese has been, the film is slightly brown. Now it the film be treated with sodium hyposulphite, the parts where all the three have been will become transparent, showing that everything, except the collodion, has been dissolved away, whilst on the rest of the plate there will remain a delicate layer of metallic silver.

Be the theory what it may, the treatment holds good. Perhaps the application of nitric acid is the safest, but to that we shall refer later on.

Density of the Film. — In securing the sensitiveness of a film, and resulting intensity of the developed image, there are two points which must be considered : — 1st, the opacity of the film ; and, 2nd, the method of development.

Density of a film, it will be seen, is an absolute neces¬ sity to secure the maximum of sensitiveness, since if a film be not opaque to the rays of light which are useful for forming the invisible image, there must be a loss equivalent to the rays which pass through. Some of the processes, therefore, which give transparent films pro¬ bably owe their insensitiveness in a great measure to this fact. But it must be borne in mind that there are some rays which may be allowed to pass through, and which will yet cause no loss in this direction. Thus, for photo¬ graphic work generally, we may say that the blue rays are the active ones. If, therefore, by any means, we can stop all these rays from passing through, we may allow a transmission of yellow and red rays without diminish¬ ing the sensitiveness. Again, if we mean to develop plates by the alkaline or ferrous oxalate developer, where the supply of silver is derived from the film itself, we are bound to take into consideration the amount of silver necessary to give opacity to the developed image in the highest lights. Practically, it is found that if, over every square inch of plate, there is four and a-half grains of metallic silver, this is attained ; anything less, therefore, would give a lack of density, anything over would tend to give harsh negatives. This consideration applies only where no subsequent intensification is carried out. F ortunately, the densities necessary to secure opacity to the blue rays, and to give sufficient opacity to the image, are found to be coincident.

From what has been said, it will also be evident, theo¬ retically, that with a given amount of silver bromide we are likely to get most sensitiveness if the film, when ex¬ amined by transmitted light, transmits the red rays ; and this is the case as proved by experiment. A film trans¬ mitting red rays is much more sensitive to the blue rays than one which transmits all colours, though, for obtain¬ ing sensitiveness to the redrays, and these alone, of course the more red light absorbed the better. As at present these are not the rays which photographers look to, we may take it that the red transmitting films are the most useful for ordinary work. Now, mathematical investiga¬ tion teaches us that if we split up silver bromide into particles, so as to transmit red light when held in a film, that they are of a smaller size than if they are divided so as to transmit blue light through the film. Hence, for a given thickness of film, more of the former would be affected by the incident light than of the latter, and the probabilities are that the reduction of the sensitive com¬ pound to the metallic state would take place to a greater extent in the former than in the latter. In the writer’s experience this is the case. It is always more difficult to secure density with a film that transmits blue light than with one which transmits red.