[32]In the preparation of soluble cotton, and indeed in all Photographic manipulations, a thermometer is almost indispensable. Instruments of sufficient delicacy for common purposes are sold in Hatton Garden and elsewhere, at a low price. The bulb should be uncovered, to admit of being dipped in acids, etc., without injury to the scale.
[32]In the preparation of soluble cotton, and indeed in all Photographic manipulations, a thermometer is almost indispensable. Instruments of sufficient delicacy for common purposes are sold in Hatton Garden and elsewhere, at a low price. The bulb should be uncovered, to admit of being dipped in acids, etc., without injury to the scale.
A preliminary experiment with a small tuft of Cotton Wool (cotton shows it better than paper) will then indicate the actual strength of the Nitro-Sulphuric Acid. Stir the tuft in the mixture for five minutes. Remove with a glass rod, and wash with water for a short time, until no acid taste can be perceived. If the Wool becomesmatted, and gelatinizes slightly on its first immersion in the acid, or if, in the subsequent washing, the fibres appear to adhere and to be disintegrated by the action of the water,the Nitro-Sulphuric Acid is too weak. In that case add to the acid mixture.
Oil of Vitriol, 3 drachms.
If the cotton was actuallydissolvedin the first trial, an addition of half of a fluid ounce of Oil of Vitriol may be required.
Supposing the cotton not to be gelatinized and to wash well, then wring it out very dry, pull out the fibres, and treat it in a test-tube with rectified Ether,[33]to which a few drops of Alcohol have been added. If it beinsoluble, dry it by a gentle heat and apply a flame: a brisk explosion indicates that the Nitro-Sulphuric Acid employed istoo strong. In that case, add to the twenty drachms of mixed acids, one drachm of water, and test again, repeating the process until a soluble product is obtained.
[33]Observe that the Ether be pure; if it contains too much water and Alcohol, it will not dissolve the Pyroxyline, or will yield an opalescent solution.
[33]Observe that the Ether be pure; if it contains too much water and Alcohol, it will not dissolve the Pyroxyline, or will yield an opalescent solution.
There is a third condition of Pyroxyline, different from either of the above, which may be puzzling:—the fibres ofthe Cotton mat together very slightly or not at all on immersion, and the washing proceeds tolerably well; the compound formed is scarcely explosive, and dissolves imperfectly in Ether, leaving little nodules or hard lumps. The ethereal solution yields, on evaporation, a film which is opaque instead of transparent. In this case (presuming the Ether to be good) the acid mixture is slightly too weak, or the temperature is too low, being probably about 90°, instead of 130° to 140° (?).
When the acid mixture has been brought to the proper strength by a few preliminary trials, proceed according to the directions given at the next page.
PREPARATION OF NITRO-SULPHURIC ACID BY OIL OF VITRIOL AND NITRE.
This process is recommended, in preference to the other, to the amateur who is unable to obtain Nitric Acid of convenient strength. The common Oil of Vitriol sold in the shops is often very good for Photographic purposes; but it is best, if possible, to take the specific gravity, when any doubt exists of its genuineness. At a temperature of 58° to 60°, specific gravity 1·833 is the usual strength, and if it falls below this, it should be rejected. (See Part III. for 'Impurities of Commercial Sulphuric Acid.')
The Nitre must be the purest sample which can be obtained. Commercial Nitre often contains a large quantity ofChloride of Potassium, detected on dissolving the Nitre in distilled water, and adding a drop or two of solution of Nitrate of Silver. If a milkiness and subsequent curdy deposit is formed. Chlorides are present. These Chlorides are injurious; after the Oil of Vitriol is added, they destroy a portion of Nitric Acid by converting it into brown fumes of Peroxide of Nitrogen, and so alter the strength of the solution.
Nitrate of Potash is an anhydrous salt,—it contains simply Nitric Acid and Potash, without any water of crystallization; still, in many cases, a little water is retainedmechanically between the interstices of the crystals, and therefore it is better to dry it before use. This may be done by laying it in a state of fine powder upon blotting-paper, close to a fire, or upon a heated metallic plate.
The sample must also be reduced to a fine powder before adding the Oil of Vitriol; otherwise portions of the salt escape decomposition.
These preliminaries having been properly observed, weigh out
Pure Nitre, powdered and dried, 600 grains.
This quantity is equivalent to 1¼ ounce Troy or Apothecaries' weight;—and to 1¼ ounce Avoirdupois weightplus54 grains. Place this in a teacup or any other convenient vessel, and pour upon it.
Stir well with a glass rod for two or three minutes, until all effervescence has ceased, and an even, pasty mixture, free from lumps, is obtained.
During the whole process, abundance of dense fumes of Nitric Acid will be given off, which must be allowed to escape up the flue or into the open air.
A modification of the formula.—The above formula will invariably succeed with a good sample of acid and pure Nitre. When tried however with Oil of Vitriol rather weaker than ordinary, andcommercialNitre, it may fail, the cotton being gelatinized and dissolved. When such is the case, the addition of water must be omitted or the quantity reduced from one drachm and a half to half a drachm.
GENERAL DIRECTIONS FOR IMMERSING, WASHING, AND DRYING THE PYROXYLINE.
The mixture of Sulphuric Acid and Nitre requires to be used immediately after its preparation, as it solidifies intoa stiff mass on cooling; but the mixed acids may be kept for any length of time in a stoppered bottle.
When Cotton is used, the fibres should be well pulled out, and small tufts added one by one to the acid mixture, stirring with a glass rod in order to keep up a constant change of particles. The Paper is cut into squares or strips, which are introduced singly.
In either case the quantity must not be too great, or some portions will be imperfectly acted upon; about 20 grains to each fluid ounce of the mixture will be sufficient.
Thetime of immersion requiredvaries from ten minutes with Cotton, to twenty minutes or even half an hour with the Paper. When an unusually large proportion of Sulphuric Acid is used, as in the case of a weak sample of Nitric Acid, the Cotton should be removed at the expiration of six or seven minutes, as there is a tendency to partial solution of the Pyroxyline in the acid mixture under those circumstances.
It is an advantage in some cases to prepare the material at a high temperature, but unless the proportions of the Acids are strictly according to Mr. Hadow's formula, solution of the Cotton may take place if the thermometer indicates more than 140°.
After the action is complete, the Nitro-Sulphuric Acid is left weaker than before, from addition of various atoms of water necessarily formed during the change. Hence, if the same portion be used more than once, an addition of Sulphuric Acid will be required.
Directions for Washing.—In removing the Pyroxyline from the Nitro-Sulphuric Acid, press out as much of the liquid as possible, and wash it rapidly in a large quantity of cold water, using a glass rod to preserve the fingers from injury. If it were simply thrown into a small quantity of water and allowed to remain, the rise in temperature and weakening of the acid mixture might do mischief.
The washing should be continued for at least a quarterof an hour, or longer in the case of Paper, as it is essential to get rid of every trace of acid. When the Nitre plan has been adopted, a portion of theBisulphate of Potashformed adheres to the fibres, and if not carefully washed out, an opalescent appearance is seen in the Collodion, resulting from the insolubility of this salt in the ethereal mixture.
If no acid taste can be perceived, and a piece of blue litmus-paper remains in contact with the fibres for five minutes without changing in colour, the product is thoroughly washed. It is however a safe plan to place the Pyroxyline in running water and allow it to remain for several hours.
Lastly, wring it out in a cloth, pull out the fibres, and dry slowly, by a moderate heat. After drying, it may be kept for any length of time in a stoppered bottle.
RECAPITULATION OF THE GENERAL CHARACTERS OF PYROXYLINE PREPARED IN NITRO-SULPHURIC ACID OF VARIOUS DEGREES OF CONCENTRATION.
The acid mixture too strong.—The appearance of the cotton is not much altered on its first immersion in the mixture. It washes well, without any disintegration. On drying, it is found to be strong in texture, and produces a peculiar crackling sensation between the fingers, like starch. It explodes on the application of flame, without leaving any ash. It is insoluble in the mixture of Ether and Alcohol, but dissolves if treated with Acetic Ether.
The acid mixture of the proper strength.—No agglutination of the fibres of the cotton on immersion, and the product washes well; soluble in the ethereal mixture, and yields atransparentfilm on evaporation.
The acid mixture too weak.—The fibres of the cotton agglutinate, and the Pyroxyline is washed with difficulty. On drying, the texture is found to be short and rotten. It does not explode on being heated, but either burns quietly with a flame, leaving behind a black ash—in which caseit consists simply of unaltered cotton,—or is only slightly combustible, and not explosive. It dissolves more or less perfectly in glacial Acetic Acid. When treated with the ethereal mixture, it is acted onpartially, leaving behind lumps of unchanged cotton; the solution does not form an even transparent layer on evaporation, but becomes opaque and cloudy as it dries. This opacity however may be seen to a small extent with any sample of Pyroxyline, if the solvents contain too much water.
In using Swedish Paper in place of Cotton, the Pyroxyline formed in too weak a Nitro-Sulphuric Acid is usually insoluble in Ether and Alcohol, and burns slowly like unchanged paper.
By studying these characters, and at the same time bearing in mind thata drachm and a half of waterin the quantities of acid given in the formula (p. 188) will suffice to cause the difference, the operator will overcome all difficulties.
PURIFICATION OF THE SOLVENTS REQUIRED FOR COLLODION.
The purity of the Ether employed is a matter of as much importance in the manufacture of a good Collodion as that of any other ingredient; this point must be attended to in order to secure a good result.
There are four kinds of Ether sold by manufacturing chemists; first, ordinary rectified Sulphuric Ether, containing a certain percentage of Alcohol and of water; specific gravity about ·750. Second, the washed Ether, which is the same agitated with an equal bulk of water, to remove the Alcohol: by this proceeding the specific gravity of the fluid is reduced considerably. Third, Ether both washed and re-rectified from a caustic alkali, so as to contain neither Alcohol nor water; in this case the specific gravity should not be higher than ·720. Fourth, "Methylated" Ether, manufactured at a lower price than the others.
Rectified Ether of 750° is not to be depended on, inasmuchas the specific gravity is often made up by adding water instead of alcohol. Methylated Ether should be used only when economy is an object, as it is prone to acidity and less certain in its properties.
Some of the qualities which render Ether unfit for Photographic purposes, are as follows:—a peculiar and disagreeable smell, either of some essential oil, or of Acetic Ether; an acid reaction to test-paper; a property of turning alcoholic solution of Iodide of Potassium brown with unusual rapidity; an alkaline reaction to test-paper; a high specific gravity, from superabundance of Alcohol and water.
The Ether which has been both washed and redistilled is always the most uniform in composition, and especially so if the second distillation be conducted from Quicklime, Carbonate of Potash, or Caustic Potash. These Alkaline substances retain the impurities, which are often of an acid nature, and leave the Ether in a fit state for use.
The redistillation of Ether is a simple process: in dealing with this fluid however the greatest caution must be exercised, on account of its inflammable nature. Even in pouring Ether from one bottle into another, if a light of any kind be near, the vapour is apt to take fire; and severe injuries have been occasioned from this cause.
Purification of Ether by redistillation from a caustic or carbonated alkali.—Take ordinary rectified Sulphuric Ether, and agitate it with an equal bulk of water to wash out the Alcohol; stand for a few minutes until the contents of the bottle separate into two distinct strata, the lower of which—id est, the watery stratum—is to be drawn off and rejected. Then introduce Caustic Potash, finely powdered, in the proportion of about one ounce to a pint of the washed Ether; shake the bottle again many times, in order that the water—a small portion of which is still present in solution in the Ether—may be thoroughly absorbed. Afterwards set aside for twenty-four hours (not longer, or the Potash may begin to decompose the Ether), when itwill probably be observed that the liquid has become yellow, and that a flocculent deposit has formed in small quantity. Transfer to a retort of moderate capacity, supported in a saucepan of warm water, and properly connected with a condenser. On applying a gentle heat, the Ether distils over quietly, and condenses with very little loss; care must be taken that none of the alkaline liquid contained in the body of the retort finds its way, by projection or otherwise, into the neck, so as to run down and contaminate the distilled fluid.
A more economical plan of purifying Ether is, without previous washing with water, to agitate with Carbonate of Potash or with Quicklime, and redistil at a moderate temperature.
In order to preserve Ether from decomposition, it must be kept in stoppered bottles, nearly full, and in a dark place. The stoppers should be tied over with bladder and luted, or a considerable amount of evaporation will take place, unless the neck of the bottle has been ground with unusual care. After the lapse of some months, probably a certain amount of decomposition, evidenced by the liberation of Iodine on adding Iodide of Potassium, will be found to have taken place. This however is small in amount, and not of a character to injure the fluid.
Rectification of Spirits of Wine from Carbonate of Potash.—The object of this operation is to remove a portion of water from the spirit, and so to increase its strength. Alcohol thus purified may be added to Collodion almost to any extent, without producing glutinosity and rottenness of film.
The salt termed Carbonate of Potash is a deliquescent salt,—that is, it has a great attraction for water; consequently when Spirits of Wine are agitated with Carbonate of Potash, a portion of water is removed, the salt dissolving in it and forming a dense liquid, which refuses to mix with the Alcohol, and sinks to the bottom. At the expiration of two or three days, if the bottle has beenshaken frequently, the action is complete, and the lower stratum of fluid may be drawn off and rejected.PureCarbonate of Potash is an expensive salt, and a commoner variety may be taken. It should be well dried on a heated metal plate, and powdered, before use.
The quantity may be about two ounces to a pint of spirit; or more, if an unusually concentrated Alcohol is required.
After the distillation is complete, a fluid is obtained containing about 90 per cent, of absolute Alcohol, the remaining 10 per cent, being water. The specific gravity at 60° Fahrenheit should be from ·815 to ·825; commercial Spirit of Wine being ·836 to ·840.
PREPARATION OF THE IODIZING COMPOUNDS IN A STATE OF PURITY.
These are the Iodides of Potassium, Ammonium, and Cadmium. The properties of each are more fully described in Part III.
a.The Iodide of Potassium.—Iodide of Potassium, as sold in the shops, is often contaminated with various impurities. The first and most remarkable isCarbonate of Potash. When a sample of Iodide of Potassium contains much Carbonate of Potash, it forms small and imperfect crystals, which are strongly alkaline to test-paper, and become moist on exposure to the air, from the deliquescent nature of the Alkaline Carbonate.Sulphate of Potashis also a common impurity; it may be detected by Chloride of Barium.
A third impurity of Iodide of Potassium isChlorideof Potassium; it is detected as follows:—Precipitate the salt by an equal weight of Nitrate of Silver, and treat the yellow mass with solution of Ammonia; if any Chloride of Silver is present, it dissolves in the Ammonia, and, after filtration, is precipitated in white curds by the addition of an excess of pure Nitric Acid. If the Nitric Acid employed is not pure, but contains traces of free Chlorine,the Iodide of Silver must be well washed with distilled water before treating it with Ammonia, or the excess of free Nitrate of Silver dissolving in the Ammonia would, on neutralizing, produce Chloride of Silver, and so cause an error.
Iodate of Potashis a fourth impurity often found in Iodide of Potassium: to detect it, add a drop of dilute Sulphuric Acid, or a crystal of Citric Acid, to the solution of the Iodide; when, if much Iodate be present, the liquid will become yellow from liberation of free Iodine. The rationale of this reaction is as follows:—The Sulphuric Acid unites with the base of the salt, and liberates Hydriodic Acid (HI),a colourless compound;but if Iodic Acid (IO5) be also present, it decomposes the Hydriodic Acid first formed, oxidizing the Hydrogen into Water (HO), and setting free the Iodine. The immediate production of a yellow colour on adding a weak acid to aqueous solution of Iodide of Potassium, is therefore a proof of the presence of an Iodate. As Iodate of Potash renders Collodion insensitive, this point should be attended to.
Iodide of Potassium may be rendered very pure by recrystallizing from Spirit, or by dissolving in strong Alcohol of sp. gr. ·823, in which Sulphate, Carbonate, and Iodate of Potash are insoluble. The proportion of Iodide of Potassium contained in saturated Alcoholic solutions varies with the strength of the spirit (videPart III., article Iodide of Potassium).
Solution of Chloride of Barium is commonly used to detect impurities in Iodide of Potassium; it forms a white precipitate if Carbonate, Iodate, or Sulphate be present. In the two former cases the precipitate dissolves on the addition of pure dilute Nitric Acid, but in the latter it is insoluble. The commercial Iodide however is rarely so pure as to remain quite clear on the addition of Chloride of Barium.
b.The Iodide of Ammonium.—This salt may be prepared by adding Carbonate of Ammonia to Iodide of Iron,but more easily by the following process:—A strong solution of Hydrosulphate of Ammonia is first made, by passing Sulphuretted Hydrogen gas into Liquor Ammoniæ. To this liquid, Iodine is added until the whole of the Sulphuret of Ammonium has been converted into Iodide. When this point is reached, the solution at once colours brown from solution of free Iodine. On the first addition of the Iodine, an escape of Sulphuretted Hydrogen gas and a dense deposit of Sulphur take place. After the decomposition of the Hydrosulphate of Ammonia is complete, a portion of Hydriodic Acid—formed by the mutual reaction of Sulphuretted Hydrogen and Iodine—attacks any Carbonate of Ammonia which may be present, and causes an effervescence. The effervescence being over, the liquid is still acid to test-paper, from excess of Hydriodic Acid; it is to be cautiously neutralized with Ammonia, and evaporated by the heat of a water-bath to the crystallizing point.
The crystals should be thoroughly dried over a dish of Sulphuric Acid, and then sealed in tubes; by this means it will be preserved colourless.
Iodide of Ammonium is very soluble in Alcohol, but it is not advisable to keep it in solution, from the rapidity with which it decomposes and becomes brown.
The most common impurity of commercial Iodide of Ammonium is Sulphate of Ammonia; it is detected by its sparing solubility in Alcohol. Carbonate of Ammonia is also frequently present to a large extent, in which case an alkaline Collodion and eventually an alkaline Nitrate Bath will be produced.
e.Iodide of Cadmium.—This salt is formed by heating filings of metallic Cadmium with Iodine, or by mixing the two together with addition of water.
Iodide of Cadmium is very soluble both in Alcohol and Water; the solution yielding on evaporation large six-sided tables of a pearly lustre, which are permanent in the air. The commercial Iodide is sometimes contaminatedwith Iodide of Zinc; the crystals being imperfectly formed and slowly liberating Iodine when dissolved in Ether and Alcohol. Pure Iodide of Cadmium remains nearly or quite colourless in Collodion, if the fluid be kept in a cool and dark place.
FORMULÆ FOR SOLUTIONS REQUIRED IN THE COLLODION PROCESS.
Section I.—Solutions for direct Positives.Section II.—Solutions for Negative Photographs.
SECTION I.
Formulæ for Solutions for direct Positives.
The solutions are taken in the following order;—The Collodion.—The Nitrate Bath.—Developing fluids.—Fixing liquids.—Whitening solution.
THE COLLODION.
Formula No. 1.
Formula No. 2.
If the operator wishes to prepare a stock of the plain Collodion, and to iodize as required, the last formula will stand thus:—
Dissolve the Pyroxyline, and let the fluid stand for forty-eight hours to subside, then draw off clear, with a siphon.
To each fluid ounce of this plain Collodion add about two fluid drachms of the following iodizing mixture:—
Of the two formulæ above given, the first is considered the best, but the second may be substituted for it when highly rectified spirits cannot be obtained. Iodide of Ammonium chemically pure is perhaps superior to any other Iodide for preparing a portrait Collodion, but Iodide of Cadmium, with addition of free Iodine, possesses better keeping properties, and gives very good results. A mixture of the two Iodides may also be used advantageously, or Iodide ofPotassiummay be combined with Iodide of Cadmium: this preparation has been much recommended, but the Collodion will be liable to produce a spotted film unless the salts are quite pure.
The exact quantity of Pyroxyline will vary with the temperature at which the preparation was made. The Collodion should flow smoothly on the glass and remain free from crapy lines on setting. When Iodide of Cadmium is used, the tendency to glutinosity will be a little greater than usual, which must be obviated by the directions given atpage 83.
The film, after dipping in the Bath, should appear opalescent and not too yellow and creamy. Pale-blue films yield very good Positives, but with more liability to failure than thicker films (p. 109).
If the Positives are not perfectly clear and transparent in the shadows, dissolve 5 grains of Iodine in an ounce of Spirits of Wine (not methylated), and add a few drops until the Collodion assumes a golden-yellow colour.
In hot weather advantage will be gained by somewhat increasing the quantity of Alcohol in Collodion; the evaporation of the solvents being retarded, and the film rendered less liable to become dry before development.AnhydrousAlcohol of Sp. Gr. ·796, may be mixed with pure Ether of ·715, even to the extent of equal parts; but this is the extreme limit, and with the strongest spirit ordinarily obtainable, the Collodion will often become somewhat glutinous if the proportions (by measure) of 5 parts of Ether to 3 of Alcohol be exceeded.
Collodion prepared by Formula No. 1, and iodized with Iodide of Cadmium, may be kept for weeks or months without much loss of sensitiveness; but when Alkaline Iodides are employed as in the second Formula, Iodine is liberated, and the fluid becomes at last brown and insensitive.
THE NITRATE BATH.
Nitrate of Silver which has been melted, in order to expel Oxides of Nitrogen, is always the most certain in its action: but the heat must not be raised too high or the salt will be contaminated withNitriteof Silver.
In the Vocabulary (see Part III.) directions are given for the preparation and purification of Nitrate of Silver; also for the testing of distilled water, and the best substitutes when it cannot be obtained.
The Bath must be saturated with Iodide of Silver, andNitric Acid neutralized if it be present. Nitrate of Silver however which has undergone fusion is free from Nitric Acid.
Weigh out the total quantity of crystals of Nitrate required for the Bath, and dissolve in about two parts of water. Then take a quarter of a grain of Iodide of Potassium to each 100 grains of Nitrate, dissolve in half a drachm of water, and add to the strong solution; a yellow deposit of Iodide of Silver first forms, but on stirring is completely re-dissolved. When the liquid is clear, test for free Nitric Acid by dropping in a piece of blue litmus-paper. If at the expiration of two minutes the paper appearsreddened, Nitric Acid is present, to neutralize which, add solution of Potash or Carbonate of Soda (not Ammonia) until a distinct turbidity, remaining after agitation, is produced (an excess does no harm). Then dilute down the concentrated solution with the remaining portion of the water, stirring all the time, and filter out the milky deposit. If the liquid does not at first run clear, it will probably do so on passing it again through the same filter.
Lastly, add the Acetic Acid (previously tested for impurities, see Part III.) and the Alcohol to the filtered liquid.
As the bulk of the Bath becomes lessened by use, fill it up with a solution containing 40 grains of Nitrate to the ounce, which will be found sufficient to maintain the strength nearly at the original point.
The common practice of occasionally dropping Ammonia or Potash into the solution, to remove Nitric Acid liberated by free Iodine in the Collodion, is not recommended (seep. 89).
When the Bath becomes old, and yields Positives which are highly intense or stained, and slightly foggy, with a deficiency of half-tone, it will be advisable to precipitate it with a Chloride and prepare a new one.
THE DEVELOPING FLUIDS.
Either of the three following formulæ may be used, according to the taste of the operator:—
FORMULA No. 1.
FORMULA No. 2.
FORMULA No. 3.
In all these formulæ, if distilled water is not at hand, read the directions in the Vocabulary, Part III., Article "Water," for the best substitute.
Remarks upon these Formulæ.—Formula No. 1is the most simple, since the solution can be usedas a Bath, the same portion being employed many times successively. If it acts too rapidly, lessen the proportion of Sulphate of Iron. An addition of Nitric Acid, half a minim to the ounce, makes the image whiter and more metallic; but if too much is used, the development proceeds irregularly, and spangles of Silver are formed.
The Alcohol and Acetic Acid render the development uniform by causing the solution of Protosulphate to combine more readily with the film. The latter also has an effect in whitening the image and increasing its brightness.
Solution of Sulphate of Iron becomes red on keeping, from a gradual formation ofpersalt. When it is too weak, add more of the Protosulphate. The muddy deposit whichsettles to the bottom of the Bath is metallic Silver, reduced from the soluble Nitrate upon the plates.
Some operators add pure Nitrate of Potash to this developing solution, to form asmall portionof Protonitrate of Iron. It is said to improve the colour slightly. The proportions are 10 grains of Nitrate of Potash to about 14 or 15 grains of Protosulphate of Iron.
Formula No. 2.—In this formula, if the colour of the image is not sufficiently white, try the effect of increasing the amount of Nitric Acid slightly. On the other hand, if the development is imperfect in parts, and patches of a green colour are seen, usethree grainsof Pyrogallic Acid to the ounce, with less Nitric Acid. A few drops of Nitrate of Silver solution added to the Pyrogallic, immediately before use, will augment the energy of development when blue and green spots occur.
Formula No. 3, or Protonitrate of Iron, does not require any addition of Acid; but it will be advisable, in some cases, to add to it a few drops of Nitrate of Silver immediately before developing. It gives a bright metallic image, resembling that obtained by adding Nitric Acid to Protosulphate of Iron.
The following process is commonly followed for preparing Protonitrate of Iron:—
Take of Nitrate of Baryta 300 grains;—powder and dissolve by the aid of heat in three ounces of water. Then throw in by degrees, with constant stirring, crystallized Sulphate of Iron,powdered, 320 grains. Continue to stir for about five or ten minutes. Allow to cool, and filter from the white deposit, which is the insoluble Sulphate of Baryta.
In place of Nitrate of Baryta, the Nitrate of Lead may be used (Sulphate of Lead being an insoluble salt), but the quantity required will be different. The atomic weights of Nitrate of Baryta and Nitrate of Lead are as 131 to 166; consequently 300 grains of the former are equivalent to 380 grains of the latter.
THE FIXING SOLUTION.
Cyanide of Potassium is usually preferred to Hyposulphite of Soda for fixing direct Positives; it is less liable to injure the purity of the white colour. The percentage ofCarbonate of Potashin commercial Cyanide of Potassium is so variable that no exact directions can be given for the formula. It is best however to use it rather dilute—of such a strength that the plate is cleared gradually in from half a minute to a minute.
The solution of Cyanide of Potassium decomposes slowly on keeping, but it will usually retain its solvent power for several weeks. In order to escape inconvenience from the pungent odour evolved by this salt, many employ a vertical Bath to hold the solution; but in that case the plates must be carefully washed before fixing, as the Iron salts hasten the decomposition of the Cyanide.
THE WHITENING SOLUTION.
By a gentle application of heat the corrosive sublimate dissolves and forms a solution as nearly as possible saturated at common temperatures. The addition of a portion of Muriatic Acid enables the water to take up a larger quantity of Bichloride; but this concentrated solution, at the same time that it whitens more quickly than the other, is apt to act unequally upon different parts of the image.
Before applying the Bichloride, the image is to be fixed and the plate well washed. Either the Protosulphate of Iron or the Pyrogallic Acid with Acetic (p. 223) may be used for the development; but the whitening process is more rapid and uniform in the latter case.
SECTION II.
Formulæ, etc., for Negative Solutions.[34]
[34]The same Collodion and Nitrate Bath may be used both for Positives and Negatives if required; but there are a few minor points of difference which are included in the following remarks.
[34]The same Collodion and Nitrate Bath may be used both for Positives and Negatives if required; but there are a few minor points of difference which are included in the following remarks.
THE COLLODION.
FORMULA No. 1.
FORMULA No. 2.
When the Collodion and Iodizing mixture are kept separate, the second formula will stand thus:—
To each fluid ounce of this plain Collodion add 2 fluid drachms of the following Iodizing solution:—
When the temperature of the Nitro-Sulphuric Acid used in making the Pyroxyline is high (140° to 155°), it often happens that the Collodion is too fluid with 4 grains of soluble paper to the ounce, and forms a blue transparent film of Iodide on dipping the plate in the Bath. In thatcase, increase the quantity of Pyroxyline from 4 grains to 6, or even to 8 grains to each ounce.
If the Collodion is glutinous, and produces a wavy surface, with less than 4 grains of Pyroxyline to the ounce, it is probable that the Alcohol is too weak, or that the soluble Cotton is badly made.
If flakes of Iodide of Silver are seen loose upon the surface of the film, and falling away into the Bath, the Collodion is over-iodized, and it will be impossible to obtain a good picture.
After the Collodion has been employed to coat a number of plates, the relative proportions of Alcohol and Ether contained in it become changed, from the superior volatility of the latter fluid: when it ceases to flow readily, and gives a more dense film than usual, thin it down by the addition of a little rectified Ether.
In dissolving the Pyroxyline, any fibrous or flocculent matter which resists the action of the Ether, must be allowed to subside, the clear portion being decanted for use. The Iodide of Potassium is to be finely powdered, and digested with the spirit until dissolved; it is better not to apply any heat. Both Iodide of Ammonium and Iodide of Cadmium dissolve almost immediately, if the salts are pure.
The Collodion must be kept in a cool and dark place. When prepared with Iodide of Ammonium or Potassium it becomes at length high coloured and insensitive. The free Iodine may then be removed by a strip of pure zinc or silver-foil.
When sensitiveness is not an object, many prefer working with an old, coloured Collodion, finding that it gives more intensity. It has been shown atpage 97that a peculiar change takes place in Collodion after iodizing, by which the intensity of the image is increased.
Directions for using Glycyrrhizine in Collodion.—The action of this material has been described atpage 114. The Collodion should be iodized with the Iodide of Cadmiumonly, or with a mixture of the Iodides and Bromides of the alkalies. The condition which calls for the employment of Glycyrrhizine is that often found in a newly made and rather glutinous Collodion, viz. sensitiveness of film, with good half-tones, but insufficient intensity in the high lights. Dissolve the Glycyrrhizine in Alcohol (not Methylated) in the proportion of 5 grains to the ounce: this solution may perhaps keep unchanged for three or four months. To each ounce of the Collodion add from one to four drops, and expose in the Camera a few seconds longer than before. The effect of the Glycyrrhizine upon the Collodion may not be fully produced immediately; if so, the fluid must be set aside for twenty-four hours.
Use of Nitro-glucose in Collodion.—Nitro-glucose is a substance analogous to Pyroxyline, but more unstable. When added to Collodion iodized with the alkaline Iodides, it slowly decomposes, liberates Iodine, lessens the sensitiveness to a certain extent, and confers intensity. Like Glycyrrhizine, it may be used to remedy feebleness of the image, and to give opacity to the blacks. Prepare the Nitro-glucose by the directions given in the Vocabulary, Part III. Dissolve twenty grains in an ounce of pure spirit, and agitate with powdered chalk to remove free acid. Add from five to eight drops to each ounce of Collodion. In a few days, more or less, according to temperature, the Collodion will deepen in colour, and will be found on trial to produce a more vigorous picture.
Collodion for hot Climates.—In this case the Iodide of Ammonium should be avoided, as unstable and prone to change colour. Iodide of Cadmium may be substituted, which has been shown to remain quite colourless when dissolved in Alcohol and Ether.
Collodion iodized with the Iodide of Potassium will usually keep for about six weeks or two months; but no certain rule can be given, much depending upon the condition of the Ether and the heat of the weather.
Plain Collodion may retain its properties unimpaired forfive or six months, sometimes much longer; but there is a tendency to a formation of the acid principle (p. 85); and hence, on the addition of an alkaline Iodide to old Collodion, the coloration is commonly very rapid. The structure of the transparent film may also be injured by keeping plain Collodion for too long a time.
Photographers who wish to operate with Collodion in hot climates will find it advantageous to carry with them the prepared Pyroxyline and the spirituous solvents, observing that the bottles are carefullyluted, and that a bubble of air is left in the neck of each, to allow for the necessary expansion, which might otherwise burst the glass or force out the stopper.
THE NITRATE BATH.
This solution may be prepared by the same formula as that given for direct Positives atpage 203, acidifying the solution with Acetic Acid in preference to Nitric Acid.
THE DEVELOPING SOLUTION.
In place of Distilled Water, pure Rain-Water may be used (see Part III., Art. "Water").
The quantity of Acetic Acid required will vary with the strength of the Acid and the temperature of the atmosphere. An excess enables the manipulator to cover the plate more easily before the action begins, but when the picture is taken in a dull light, is apt to give a bluish, inky hue to the image. In cold weather, use less of the Acetic and twice the quantity of Pyrogallic Acid. With Collodion prepared from Spirits nearly anhydrous, and iodized with Iodide of Cadmium, the full quantity of Acetic Acidwill be required, as there is sometimes a little difficulty in making the developer flow up to the edge of the film.
If the image cannot be rendered sufficiently black, two or three minims of the Nitrate Bath solution may be added to each drachm towards the end of the development.
If the solution be kept for some time after its first preparation, it becomes brown and discoloured. In this state it will still develope the image, but is less likely to give a clear and vigorous picture. A solution of Pyrogallic Acid in Acetic Acid will keep for many weeks, and may be diluted down when required for use.
The following is a good formula:—
To one drachm add seven drachms of water.
THE FIXING LIQUID.
For remarks on the Cyanide of Potassium Fixing Bath, see the last Section,page 207.
MANIPULATIONS OF THE COLLODION PROCESS.
Thesemay be classed under five heads:—Cleaning the Plates.—Coating with Iodide of Silver.—Exposure in the Camera.—Developing the image.—Fixing the image.—In addition to this, the present Chapter will include in separate Sections directions for the choice and management of lenses, for copying engravings, manuscripts, etc., and for taking stereoscopic and microscopic photographs.
CLEANING THE GLASS PLATES.
Care should be taken in selecting glass for use in Photography. The ordinary window-glass is inferior, having scratches upon the surface, each of which may cause an irregular action of the developing fluid; and the squares are seldom flat, so that they are apt to be broken in compression during the printing process.
The patent plate answers better than any other description of glass; but if it cannot be procured, the "flatted crown glass" may be substituted.
Before washing the glasses, each square should be roughened on the edges by means of a file or a sheet of emery-paper; or more simply, by drawing the edges of two plates across each other. If this precaution be omitted, the fingers are liable to injury, and the Collodion film may contract and separate from the sides.
In cleaning glasses, it is not sufficient, as a rule, to wash them simply with water; other liquids are required to removegrease, if present. A cream of Tripoli powder and Spirits of Wine, with a little Ammonia added, is commonly employed. A tuft of cotton is dipped in this mixture, and the glasses are well rubbed with it for a few minutes. They are then rinsed in plain water and wiped dry with a cloth.
The cloths used for cleaning glasses should be kept expressly for that purpose; they are best made of a material sold as fine "diaper," and very free from flocculi and loosely-adhering fibres. They are not to be washedin soap and water, but always in pure water or in water containing a little Carbonate of Soda.
After wiping the glass carefully, complete the process by polishing with an old silk handkerchief, avoiding contact with the skin of the hand. Some object tosilk, as tending to render the glass electrical, and so to attract particles of dust, but in practice no inconvenience will be experienced from this source.
Before deciding that the glass is clean, hold it in an angular position andbreatheupon it. The importance of attending to this simple rule will be at once seen by referring to the remarks made atpage 39. In the Honey preservative and Collodio-Albumen processes it is especially needful that the glasses should be thoroughly cleaned, on account of the tendency which the film has to become loosened or to blister during the development and washings. Caustic Potash, sold by the druggists under the name of "Liquor Potassæ," is very efficacious, or in place of it, a warm solution of "washing Soda" (Carbonate of Soda). Liquor Potassæ, being a caustic and alkaline liquid, softens the skin and dissolves it; it must therefore be diluted with about four parts of water and applied to the glass by means of a cylindrical roll of flannel. After wetting both sides thoroughly, allow the glass to stand for a time until several have been treated in the same way; then wash with water and rub dry in a cloth.
The use of an alkaline solution is usually sufficient to clean the glass, but some plates are dotted on the surface with small white specks, not removable by Potash. These specks may consist of hard particles ofCarbonate of Lime, and when such is the case they dissolve readily in a dilute acid,—Oil of Vitriol, with about four parts of water added, or dilute Nitric Acid.
The objection to the use of Nitric Acid is, that if allowed to come in contact with the dress, it produces stains which cannot be removed unlessimmediatelytreated with an alkali. A drop of Ammonia should be applied to the spot before it becomes yellow and faded.
When Positives are to be taken, it is advisable to use additional care in preparing the glass, and especially so with pale transparent films and neutral, Nitrate Bath.
After a glass has been once coated with Collodion, it is not necessary in cleaning it a second time to use anything but pure water; but if the film has been allowed to harden and become dry, possibly dilute Oil of Vitriol or Cyanide of Potassium may be required to remove stains.
When glasses have been repeatedly used in photography they often become at length so dull and stained, that it is better to reject them.
COATING THE PLATE WITH THE COLLODIO-IODIDE OF SILVER.
This part of the process, with that which follows, must be conducted in a room from which chemical rays of light are excluded. It is inferred therefore that the operator has provided himself with an apartment of that kind.
The most simple plan of preparing the room is to nail a treble thickness of yellow calico completely over the window, or a part of it, the remainder being darkened. To this a single thickness of a waterproof material made by coating linen with gutta-percha may be added as a further security against the entrance of white light, the smallest pencil of which admitted into the room would cause fogging.
It is often convenient to illuminate by means of a candle screened by yellow glass. A dark orange yellow, approaching to brown, is more impervious to chemical rays than a lighter canary-yellow. Lamps suitable for the purpose are sold by the manufacturers of apparatus and chemicals.
Before coating the plate with Collodion, see that the fluid is perfectly clear and transparent, and that all particles have settled to the bottom; also that the neck of the bottle is free from hard and dry crusts, which, if allowed to remain, would partially dissolve and produce striæ upon the film. In taking small portraits and stereoscopic subjects, these points are of especial importance, and every picture will be spoiled if they are not attended to.
A useful piece of apparatus for clearing Collodion is that represented in the following woodcut.