T. Frederick Hardwich

PREPARATION OF COLLODION.

This includes—the soluble Paper;—the Alcohol and Ether;—and the iodizing compounds.

The formulÆ for Negative and Positive Collodion, and for the Nitrate Bath and developing fluids, are given in the second Chapter.

THE SOLUBLE PAPER.

Pyroxyline may be prepared either from cotton wool or from Swedish Filtering-paper. Most operators prefer the latter, from its giving a product of constant solubility, and yielding a fluid solution.[30] The Cotton Wool however is better adapted for use with the Sulphuric Acid and Nitre, since the Paper, from its closeness of texture, requires a longer immersion in the mixture.

[30] Swedish filtering-paper may be procured at the operative chemists', at about five shillings the quire. Each half-sheet has the water-mark "J. H. Munktell."

Preparation of a Nitro-Sulphuric Acid of the proper strength.—There are two modes of preparing the Nitro-Sulphuric Acid: first, by mixing the acids; second, by the Oil of Vitriol and Nitre Process. The former is the best in cases where large quantities of the material are operated on, but the amateur is recommended to begin by trying the Nitre Process (p. 190) as the most simple.

PREPARATION OF NITRO-SULPHURIC ACID BY THE MIXED ACIDS.

The operator may proceed in either of two ways; first, by taking the strength of each sample of acid, and mixing according to fixed rule; second, by a more ready plan, which may be used when the exact strength of the acids is not known. Each of these will be described in succession.

a. Directions for mixing according to fixed rule.—This process is given from Mr. Hadow's original paper in the 'Quarterly Journal of the Chemical Society.' It is certain in its results if the strength of both acids be accurately determined.

A very perfect process for taking the strength of Nitric Acid is by means of powdered Marble or Carbonate of Lime, as described in various works on practical Chemistry. Sulphuric Acid may be estimated by precipitating with Nitrate of Baryta, and weighing the insoluble Sulphate with the proper precautions.

The specific gravity is not a criterion of strength to be perfectly relied on, but if it be adopted as a test, the following points must be attended to.

1st. That the temperature of the acid be at or near 60° Fahrenheit; the density of Sulphuric Acid especially is, from its small specific heat, greatly influenced a change of temperature.

2nd. The sample of Nitric Acid must be free from Peroxide of Nitrogen, or only slightly coloured by it. This substance, when present, increases the specific gravity of the acid without adding to its available properties. A yellow sample of Nitric Acid will therefore be somewhat weaker than is indicated by the specific gravity.

3rd. The Oil of Vitriol should yield no solid residue on evaporation. Sulphate of Lead and Bisulphate of Potash are often found in the commercial acid, and add much to its density. Oil of Vitriol containing Sulphate of Lead becomes milky on dilution.

The formula for a definite Nitro-Sulphuric Acid, of the proper strength for making the soluble Pyroxyline, may be stated thus:—

HO NO₅, 2 (HO SO₃) + 3½ HO

or

	Atoms.	Atomic weight.
Nitric Acid	1	54
Sulphuric Acid	2	80
Water	6½	58
		192

Having found the percentage of real acid which is present,[31] the following calculation will give the relative weights of the ingredients required to produce the formula:—

Let	{	a	=	percentage of real	Nitric Acid,
		b	=	""	Sulphuric Acid,
	then	5400 a	=	quantity of	Nitric Acid,
		8000 b	=	"	Sulphuric Acid,
192 - 5400 a - 8000 b			=	"	Water.

[31] Tables are given in the Appendix for calculation by specific gravity; but direct analysis of the acids is the most certain.

Observe that the numbers in the calculation correspond to the atomic weights recently given; and that the amount of water is derived from the total atomic weight, viz. 192, minus the sum of the weights of both acids.

Hence if the samples of acid employed are too weak for the purpose, the formula for the water gives a negative quantity.

The weight of mixed acids produced by the formula is 192 grains, which would measure somewhere about two fluid drachms. Ten times this quantity forms a convenient bulk of liquid, in which about 50 or 60 grains of Paper may be immersed.

In weighing corrosive liquids, such as Sulphuric and Nitric Acid, a small glass may be counterbalanced in the scale-pan, and the acid poured in carefully. If too much is added, the excess can be removed by a glass rod, or by "the pipette" commonly employed for such a purpose.

The following example of a calculation similar to the above may be given:—

100	parts of the	Oil of Vitriol	=	76·65	real acid.
"	"	Nitric Acid	=	65·4	real acid.
therefore 8000 76·65 = 104·3 grains of Oil of Vitriol. 5400 65.4 = 82·5 " Nitric Acid 192 - 104·3 - 82·5 = 5·2 " Water.

Multiplying these weights ten times, we have

Oil of Vitriol		1043	grains.
Nitric Acid		825	"
Water		52	"
		——
Total weight of the Nitro-Sulphuric Acid	}	1920	grains.

Having prepared the acid mixture of a definite strength by the above formula, the paper must be immersed according to directions given at page 191.

b. Process for mixing Nitro-Sulphuric Acid, the strength of the two acids not having been previously determined.—Take a strong sample of Nitric Acid (the yellow Nitrous acid, so called, succeeds well), and mix it with Oil of Vitriol as follows:—

Sulphuric Acid	10	fluid drachms,
Nitric Acid	10	"

Now immerse a thermometer and note the temperature;[32] it should be from 130° Fahr. to 150°. If it sinks below 120°, place the mixture in a capsule, and float upon boiling water for a few minutes.

[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 becomes matted, 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 actually dissolved in 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 be insoluble, dry it by a gentle heat and apply a flame: a brisk explosion indicates that the Nitro-Sulphuric Acid employed is too 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.

There is a third condition of Pyroxyline, different from either of the above, which may be puzzling:—the fibres of the 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 of Chloride 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 retained mechanically 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 weight plus 54 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, and commercial Nitre, 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 into a 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.

The time of immersion required varies 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 quarter of 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 the Bisulphate of Potash formed 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 a transparent film 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 case it 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 on partially, 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 that a drachm and a half of water in 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, inasmuch as 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 it will 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 been shaken frequently, the action is complete, and the lower stratum of fluid may be drawn off and rejected. Pure Carbonate 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 is Carbonate 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 Potash is also a common impurity; it may be detected by Chloride of Barium.

A third impurity of Iodide of Potassium is Chloride of 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 Potash is 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 (vide Part 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 contaminated with 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.