Читать книгу A Practical Manual of the Collodion Process - S. D. Humphrey - Страница 7

Table of Contents

SOLUBLE COTTON—MANIPULATION—PLAIN COLLODION—BROMO-IODIZED COLLODION FOR POSITIVES—DITTO FOR NEGATIVES—SOLUTION OF BROMIDE AND IODIDE OF POTASSIUM AND SILVER—DOUBLE IODIDE OF POTASSIUM AND SILVER—DEVELOPING SOLUTION—FIXING THE SOLUTION—BRIGHTENING AND FINISHING THE IMAGE—PHOTOGRAPHIC CHEMICALS.

The chemistry of Photography requires the attention, in a greater or less degree, of every practitioner. It is of the utmost importance, that those who wish to meet with success in the various processes given, should not only be provided with a good selection of chemicals, but also understand the nature of the agent employed. To give a perfectly complete and full list of every agent used would require more time and space than can be given in this work. I shall confine myself to some of the most important, and to such articles as are of the greatest interest to the practitioner.

Soluble Cotton.

I have, in my practice and trade, adopted the term soluble cotton as the one most appropriate, making a desirable distinction from the article sold as gun cotton, they being of a somewhat different nature—gun cotton being the most explosive and least soluble, while the other preparation is more soluble and less explosive.

There are two methods employed in the preparation of soluble cotton; one being by the use of nitric and sulphuric acids, and the other with sulphuric acid and nitrate of potash. The last of these I would recommend as being the most convenient for those who require only a small quantity of cotton. Persons experimenting in the preparation of this article should exercise much care and judgment. A good cotton is not the result of hap-hazard operation. The operator should be acquainted, as nearly as possible, with the quality of the chemicals employed, and the proper mode of manipulation.

Articles necessary.—One quart Wedgewood mortar and pestle, or evaporating dish; one glass rod; one pane of glass, large enough to cover the mortar or dish; one ordinary-sized pail two-thirds full of pure rain or distilled water, and at least ten times that quantity of water at hand; twelve ounces (by weight, avoirdupois) of nitrate of potash (Dupont's refined, pulverized); twelve ounces (by measure) of commercial sulphuric acid; and three hundred and forty grains of clean, pure cotton wool.

Remarks.—It is advisable that the mortar or dish be deep and narrow, as the mixture can be better formed in a vessel of this shape. If not convenient to procure a mortar, a common earthen bowl will answer; glass is objectionable, as the heat generated in the combination of the acid and nitre is liable to crack it. A new pail should not be used, especially if it is painted, as the acids attack the paint, and injure the cotton. I prefer one that has been used for some time, and has been frequently cleaned. A common earthen wash-bowl, or any large glass dish, will answer in place of the pail. Metal pails or vessels should not be used.

Nitrate of Potash (saltpetre) should be dry and finely-powdered. I use none other than Dupont's refined; this is very nearly, if not absolutely, chemically pure.

The commercial Sulphuric Acid (oil of vitriol) of America is of great uniformity of strength, as sold by druggists generally. I use a test-bulb graduated to the proper density, and have been very successful in my experiments.

In some twenty different samples of acid, used in different cities in the United States, I found only one that produced a poor cotton, and this might have been influenced by the moisture of the atmosphere, it being a very rainy day when I used it.

During my recent and somewhat extensive practice, I have thought that the fine long fibres of cotton wool do not make so desirable a soluble cotton as that which is heavy or common. Four or five very careful experiments upon this point, have had the effect to produce in me a strong belief that my ideas are entitled to some consideration. I should not select the finest cotton for making soluble cotton, but now invariably take that which is thick or coarse.

The result of my experience is (other things being equal), that cotton prepared in fine dry weather has a greater degree of solubility than when prepared in a moist atmosphere: hence I would recommend the experimenter to choose fine, clear weather for preparing it.

Manipulation.

Having at hand every article requisite, proceed as follows:—Put the nitrate of potash into the mortar or dish; be sure it is dry and well powdered, and then add the acid; stir them well with the pestle and glass rod, so that the lumps will be all (or nearly so) out, and a pasty solution formed. This operation should not occupy more than two minutes' time. Then put in the cotton, about one-quarter of the whole bulk at a time: it should be well picked apart, so that it may come immediately in contact with the acids, and should be kneaded, with the pestle and glass rod, into the solution, and as soon as wetted, another quarter should be added and wetted as soon as possible; so continue until all is in: then knead with the pestle and mortar for at least four minutes, or until every fibre of the cotton is saturated with the liquid; then the mortar should be covered over with the pane of glass, and allowed to stand for fifteen or twenty minutes; then the entire contents of the mortar should be thrown into the pail two-thirds full of water, and stirred with the glass rod as rapidly as possible: if this rapid stirring is omitted, the cotton will be injured by the action of the acids in combining with the water. The water should be poured off, and another change put into the pail.

After about three changes, the hands may be used in the farther washing. The hands should be perfectly clean, and free from all chemicals. The changes of water and washing should be continued until every trace of acid has disappeared, which can be seen by testing with blue litmus test-paper. After it is thought that the cotton has become free, the water may be squeezed out of a little lump about the size of a pea, and then placed between the fold of the test-paper, and if it reddens the paper, there is acid present, and the washing should be continued until there is no change in the paper. When this is done, the cotton can be put into the folds of a dry towel or cloth (which has been thoroughly rinsed, so that no soap be present), and wrung out as dry as possible, and then it may be picked apart and put aside, exposed to a moderate temperature (say 100° Fahr.) to dry, when it is ready for use.

I employ the method (for convenience, nothing more) of displacing the water by the use of alcohol. [Cutting's patent—see patents.] I wring out the water as before, then place the cotton in strong alcohol, stir and press it, and then pour it off; wring it out again, then put it in a change of alcohol, let it soak for about five minutes, then wring it out as dry as possible, pick it apart, and it will dry immediately, and place it in a close stoppered bottle; or, if wanted for use at once, put it into the dissolving solution immediately.

I will here remark that, since the first edition, I have had occasion to use large quantities of soluble cotton, and have found that if it be kept in an atmosphere of alcohol and ether, its solubility is somewhat improved: that is, in the case of its not being used immediately after its preparation. This is easily kept, by dropping a few drops of ether or alcohol into the bottle containing it, and then sealing close until wanted for use. In the event of the water being displaced by alcohol, it is not necessary to thoroughly dry it, but put in a perfectly close bottle to keep.

Remarks.—There are a few precautions necessary to be observed in the preparation of soluble cotton. I should select a fine clear day, if time is no object; nevertheless I have made a good article in a moderately dense atmosphere. Sulphuric acid has a powerful affinity for hydrogen, consequently, in damp weather, it is more or less reduced by the moisture in the air.

It is advisable to prepare the nitro-sulphuric acid mixture on a roof, or between two doors or windows, where there is a good current of air, in order to prevent the inhalation of white vapors which arise, and are very poisonous to the lungs. As a preventive, in case of inhaling these vapors, I apply the fumes of aqua-ammonia. It is best for every one to have six or eight ounces of this always at hand; it neutralizes all acid that maybe spattered on the clothes, prevents its destructive powers, and restores the color.

Yellow vapors sometimes appear when putting the cotton in contact with the solution: this arises from its not being wet; and when they do appear, the cotton where they are should be quickly put under the liquid and kneaded rapidly, which will prevent a continuance of these vapors. I have had them appear, and used the cotton, and could not observe that any bad effect had been produced.

The temperature is worthy the attention of the operator: if it be low, as in winter, and the cotton be left in the nitro-sulphuric mixture for fifteen or twenty minutes, the whole becomes a thick, stiff mass, bedded together, and has not had proper action, giving a bad article. A good temperature is about 140° Fahr. for the last of the time the cotton is in the mixture. This is not always convenient; so the operator will be governed by circumstances, taking his chance of having a good article. In some cases I have heated a thick iron plate, at a moderate temperature, placing the mortar upon it, and thus aided in regulating the temperature. This is the most convenient method I have employed.

It has been thought advisable to publish in full the account of Edw. Ash Hadow's experiments and investigations upon the subject of soluble cotton. The following is an account of them as it appeared in Humphrey's Journal, vol. VI. p. 12:—

"Having, in my earlier experiments on the collodion process of photography, experienced some difficulty in always producing a collodion of uniform quality with regard to sensitiveness, tenacity and fluidity, although making use of the same materials for its preparation, and this I find being the complaint of many others, it has been my study lately to determine the variations in quality to which the ingredients are liable, and the effects of these variations on the sensitive film, and likewise to ascertain whether the excellent qualities of some samples of collodion depend on the materials in ordinary use, or on some substances accidentally or intentionally added. Researches in the preparation of collodion may appear superfluous, now that it is supplied of the best quality by so many makers; but as some persons of an independent turn of mind still prefer manufacturing their own, I venture to bring forward the subject with the hope of benefiting them. In this beautiful process so much depends for success on the quality of the collodion, that when in possession of a good specimen, it becomes one of the easiest and most simple, and ought to be the most certain of all the processes yet devised; for here no material of uncertain composition is introduced, such as paper, and thus we have nothing to fear from plaster of Paris, alumina, or specks of iron or copper, which continually endanger or modify the calotype process; each ingredient can and ought to be obtained in a state of perfect purity, and with this precaution the degree of success depends upon the skill of the operator himself.

"Of all the substances used in this process, the gun-cotton is usually the only one actually prepared by the operator himself; in this he cannot fail to have observed the great variations in the solubility, and, when dissolved, the transparency and tenacity of the films, to which it is liable; the various processes also that are given appear at first sight unaccountably different, some directing ten minutes, others a few seconds immersion. In consequence of this I have specially examined into the cause of all these variations, with a view to obtain certainty, and also have endeavored to discover how far they affect the sensitiveness of the prepared surface. If we take a mixture of the strongest nitric and sulphuric acids and immerse as much cotton as can be wetted, after some minutes squeeze out the acid as far as possible, then immerse a second portion of cotton, and again express the acids for a third portion of cotton, and so on until the liquid is exhausted, we shall find, on comparing the cottons thus treated, after washing and drying, that there is a gradual alteration in their properties, the first being highly and perfectly explosive, and each succeeding portion less so, until the portion last immersed will be found hardly explosive, leaving distinct traces of charcoal or soot when burned. This may not appear surprising at first sight, as it may be imagined that the latter portions are only a mixture of gun-cotton and common cotton; this is, however, not the case, for if each quantity be immersed sufficiently long, it will not contain a fibre of common cotton, and may yet become charred on burning like unaltered cotton. The most remarkable difference, however, is discovered on treating them with ether containing a little alcohol, when, contrary to what might have been anticipated, the first or strongest gun-cotton remains untouched, while the latter portions dissolve with the utmost ease, without leaving a trace behind, which alone is sufficient proof that no unaltered cotton remains. This difference in properties is owing to the gradual weakening of the acid mixture, in consequence of the nitric acid being removed by the cotton, with which it becomes intimately combined, at the same time that the latter gives out a proportionate quantity of water. In consequence of these experiments, a great many mixtures of these acids were prepared of various strengths, each being accurately known, both to determine whether there were more than one kind of soluble gun-cotton, and, if there were, to ascertain exactly the mixture required to produce the most suitable to photographic purposes. By this means, and by, what I believe has not been pointed out, varying the temperature, at least five varieties were obtained;—first, gun-cotton, properly so called, as before stated, quite insoluble in any mixture of alcohol and sulphuric ether. Secondly, an explosive cotton, likewise insoluble, but differing chemically from the first, obtained by a mixture of certain strength when used cold. If warm, however, either from the heat produced spontaneously on mixing the two acids; or by raising the temperature artificially to about 130°, the cotton then immersed becomes perfectly soluble, producing a third variety; if, however, it be thoroughly dried, it becomes in a great measure insoluble. The fourth is obtained by the use of weaker acids used cold, and the fifth when the mixture has been warmed to 130° previous to the immersion of the cotton; in either of the two last cases the product is perfectly soluble, but there is a remarkable difference between their properties, for on dissolving 6 grains of each in 1 ounce of ether, the cotton treated with warm acids gives a perfectly fluid solution (which is likewise the case with the third variety produced by acids something stronger), while that obtained by the use of cold acids makes a mixture as thick as castor-oil.

"Having obtained these more strongly marked varieties, as well as intermediate kinds, with all gradations of solubility, it was necessary, before I could select any particular formula for preparing the cotton, to compare their photographic properties, with especial reference to sensitiveness, opacity of the reduced silver in negatives, and its color in positives. A certain weight of each being dissolved in a portion of the same mixture of alcohol and ether previously iodized, the comparison was made, by taking the same objects with each collodion in succession, and likewise by pouring two samples on the same plate of glass, and thus exposing them in the camera together, side by side; this last proved to be much the most satisfactory plan, and was repeated many times for each sample, taking care to reverse the order in which they were poured on, that there might be no mistake arising from the difference of time elapsing between the pouring on of the collodion and its immersion in the sensitive bath. By these experiments I had confidentially hoped to have solved the question as to the cause of difference in sensitiveness and other photographic properties of collodion; but in this I was disappointed, for, after repeated experiments, I believe I may safely affirm that they are precisely similar as regards their photographic properties. The same I believe may be said of Swedish paper collodion, judging from a few comparative experiments I have made, and indeed it is difficult to discover what is the superiority of this material over clean cotton-wool; the ease of manipulation which some allege is a matter of taste, but I should decidedly prefer the open texture of cotton to that of a substance like filtering paper, composed of a mass of compacted fibres, the innermost of which are only reached when the acids have undergone a certain degree of weakening by the water abstracted from the outer fibres; and when we consider that from cotton alone we have the means of preparing all varieties of collodion, from the most powerfully contracting and transparent to the weakest and most opaque, and each if required with equal and perfect certainty, there appears to be choice enough without resorting to another material, differing only in being more rare and more difficult to procure. But, although the photographic properties of these varieties of collodion-wool are so similar, other circumstances, such as fluidity, tenacity, and transparency, render its preparation of some importance, and indicate that the acid mixture should always be used warm; and it is chiefly in consequence of this very circumstance, that greater success attends the use of nitrate of potash and sulphuric acid than that of mixed acids; for the former when mixed, produce the required temperature, and must be used while warm, since on cooling the mixture becomes solid, whereas acids when mixed do not usually produce so high a temperature, and being fluid can be used at any subsequent period; another obstacle to their use is the great uncertainty of the strength of the nitric acid found in the shops, requiring a variation in the amount of sulphuric acid to be added, which would have to be determined by calculation or many troublesome trials. When a proper mixture is obtained, the time of immersion is of no importance, provided it be not too short, and the temperature be maintained at about 120° or 130°; ten minutes is generally sufficient; (though ten hours would not render the cotton less soluble, as is sometimes asserted.)

"In using the mixed acids, the limits are the nitric acid being too strong, in which case the product is insoluble, or too weak, when the cotton becomes immediately matted or even dissolved, if the mixture is warm. I have availed myself of these facts in order to produce collodion-wool by the use of acids, without the trouble of calculating the proper mixture according to their strength. Five parts by measure of sulphuric acid, and four of nitric acid of specific gravity not lower than 1·4, are mixed in an earthenware or thin glass vessel capable of standing heat; small portions of water are added gradually (by half drachms at a time, supposing two ounces to have been mixed,) testing after each addition by immersion of a small portion of cotton; the addition of water is continued until a fresh piece of cotton is found to contract and dissolve on immersing; when this takes place, add half the quantity of sulphuric acid previously used, and (if the temperature does not exceed 130°, in which case it must be allowed to cool to that point,) immerse as much cotton, well pulled out, as can be easily and perfectly soaked; it is to be left in for ten minutes, taking care that the mixture does not become cold, and then transferred to cold water, and thoroughly washed; this is a matter of much importance, and should be performed at first by changing the water many times, until it ceases to taste acid, and then treating it with boiling rain-water until the color of blue litmus remains unchanged; the freedom from all trace of acid is insured by adding a little ammonia before the last washing. Cotton thus prepared should dissolve perfectly and instantaneously in ether containing a little alcohol, without leaving a fibre behind, and the film it produces be of the greatest strength and transparency, being what M. Gaudin terms 'rich in gun-cotton.'

"The mixture of nitrate of potash and sulphuric acid is defective chiefly from the want of fluidity, in consequence of which the cotton is less perfectly acted on; this may be remedied by increasing the amount of sulphuric acid, at the same time adding a little water; a mixture of 5 parts of dried nitre, with 10 of sulphuric acid, by weight, together with 1 of water, produces a much better collodion wool than the ordinary mixture of 1 of nitre with 1½ of sulphuric acid. The nitre is dried before weighing, in order that its amount, as well as that of the water contained in the mixture, may be definite in quantity; it is then finely powdered, mixed with the water, and the sulphuric acid added; the cotton is immersed while the mixture is hot, and afterwards washed with greater care even than is required when pure acids are used, on account of the difficulty of getting rid of all the bisulphate of potash that adheres to the fibres, which both acts as an acid and likewise causes the collodion to appear opalescent when held up to the light; whereas the solution should be perfectly transparent."

Plain Collodion.

PLAIN COLLODION.

To dissolve the soluble cotton (pyroxyline), and form plain collodion, proceed as follows:

Take of

Sulphuric ether (concentrated),	10	ounces
Alcohol, from 90 to 95 per cent.,	6	"

Soluble cotton enough to give the solution a consistency such as will allow it to flow evenly over the surface of the glass, and impart to it quite a thick and transparent coating. If the coating is opaque, the cotton has not been properly prepared, the acid mixture has been too weak.

Remarks.—It is desirable for every operator to use chemicals of uniform strength, and the better method to adopt is to employ those purchased from some one respectable manufactory, and not take those furnished by irresponsible and unconscientious parties. At least one-half of the failures experienced by beginners is from want of good chemicals. It is not economy to purchase a cheap article.

Alcohol is an article that can be procured in almost any small village in the United States, and is in general fit for collodion purposes. I have used 88 per cent, in the above proportions, also the intermediate varieties to 98 per cent., and have been quite successful; but feel convinced that the ordinary 98, as marked (which usually stands by actual test 95 to 97 per cent.), is preferable, except in cases where water is employed in dissolving the iodizing salts, when I would use fully 98 per cent.

Before concluding the subject on plain collodion, I will introduce the account given by Mr. E. A. Hadow of his interesting and valuable experiments, as published in Humphrey's Journal, Vol. VI, page 18.

"Having obtained good collodion-wool, the next point of inquiry was with regard to the solvent; to ascertain whether the addition of alcohol beyond what is necessary to cause the solution of the gun-cotton in ether, were beneficial or otherwise. For this purpose ether and alcohol were prepared perfectly pure, and mixtures were made of 1 of alcohol to 7 of ether, 2 to 6, 3 to 5, 4 to 4 and 5 to 3. In one ounce of each were dissolved 6 grains of gun-cotton and 4 grains of iodide of ammonium (iodide of potassium could not be employed, since it requires a certain amount both of water and alcohol to keep it in solution); they were then compared, using a 35-grain solution of nitrate of silver, both by pouring on separate glasses, and likewise by covering two halves of a plate with two samples, as in examining the gun cottons, thus placing them under the same circumstances during the same time; in this way the effect of adding alcohol was very clearly perceived, since the difference between the collodions was much greater than could have been anticipated.

"The first mixture containing only ¹/₈th of alcohol was quite unfit for photographic purposes, from its being almost impossible, even with the most rapid immersion, to obtain a film of uniform sensitiveness and opacity throughout, the surface generally exhibiting nearly transparent bands, having an iridescent appearance by reflected light.

"The second mixture with ¹/₄th of the alcohol is liable to great uncertainty, for if there be any delay in pouring off the collodion the same appearances are seen as in the first, and like it the surface is very insensitive to light, while if the plate be rapidly plunged in the bath, the collodion film becomes much more opaque than before, and is then very sensitive.

"The third proportion of 3 of alcohol to 5 of ether is decidedly the best, giving without the least difficulty a beautifully uniform and highly sensitive film, at the same time perfectly tough and easily removable from the glass if required. A further addition of alcohol, as in the two last collodions, was not attended with any corresponding advantage or increase of sensitiveness; on the contrary, the large proportion of alcohol rendered them less fluid, though with a smaller quantity of gun cotton they would produce very good collodions, capable of giving fine films: the cause of the weakness of the film, observed on adding much of the ordinary alcohol, is the large amount of water it usually contains.

"This surprising improvement, caused by the addition of a certain quantity of alcohol, is referable to causes partly chemical, partly mechanical, for, on examining the films, it will be found in the first, and occasionally in the second collodion, that the iodide of silver is formed on the surface, and can be removed entirely by friction without destroying the transparent collodion film below, while in those collodions that contain more than one-fourth of alcohol, the iodide of silver is wholly in the substance, and in this state possesses the utmost sensitiveness.

"This difference of condition is owing to the very sparing solubility of ether in water, which in the first case prevents the entrance of the nitrate of silver into the film, consequently the iodide and silver solutions meet on the surface; but on addition of alcohol, its solubility enables the two to interchange places, and thus the iodide of silver is precipitated throughout the substance in a state of the utmost division. This difference is clearly seen under the microscope, the precipitate being clotted in the one case, while in the other the particles are hardly discoverable from their fineness.

"The presence of a little water considerably modifies these results, since it in some degree supplies the place of alcohol, and is so far useful; but in other respects it is injurious, for, accumulating in quantity, if the collodion is often used, it makes the film weak and gelatinous, and what is worse, full of minute cracks on drying, which is never the case when pure ether and alcohol are used. Since the ether of the shops almost always contains alcohol, and frequently water, it is important to ascertain their amount before employing it for the preparation of collodion; the quantity of alcohol may be easily ascertained by agitating the ether in a graduated measure glass (a minim glass does very well) with half its bulk of a saturated solution of chloride of calcium; this should be poured in first, its height noted, and the ether then poured on its surface, the thumb then placed on the top, and the two agitated together; when separated, the increase of bulk acquired by the chloride of calcium indicates the quantity of alcohol present, and for this allowance should be made, in the addition of alcohol afterwards to the collodion.

"Water is readily detected, either in ether or alcohol by allowing a drop to fall into spirits of turpentine, with which they ought to mix without turbidity; this is immediately produced if they contain water: for detecting water in alcohol, benzole is a more delicate re-agent than spirits of turpentine (Chemist, xxix, 203). It is also necessary that ether should be free from a remarkable property it acquires by long keeping, of decomposing iodides and setting free iodine, which thus gives the collodion a brown color; the same property may be developed in any ether, as Schonbein discovered by introducing a red hot wire into the vapor in the upper portion of a bottle containing a little ether and water; if it be then shaken up and a solution of iodide poured in, the whole rapidly becomes brown; this reaction is very remarkable and difficult to explain for even a mixture of the ether and nitric acid fails to produce a color immediately. Ether thus affected can only be deprived of this property by rectification with caustic potash."

Bromo-Iodized Collodion for Positives.—No. 1.