CHAPTER II.. Formulæ for Solutions Required in the Collodion Process.

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.

Purified Ether, sp. gr. ·720 5 fluid drachms. Purified Alcohol, sp. gr. ·825 3 " " Pyroxyline 3 to 5 grains. Pure Iodide of Cadmium or Ammonium 4 grains.

Formula No. 2.

Rectified Ether, sp. gr. ·750 6 fluid drachms. Spirits of Wine, sp. gr. ·836 2 " " Pyroxyline 2 to 4 grains. Iodide of Potassium or Ammonium 3 to 4 "

If the operator wishes to prepare a stock of the plain Collodion, and to iodize as required, the last formula will stand thus:--

Rectified Ether, ·750 3 fluid ounces. Alcohol of ·836 2 fluid drachms. Pyroxyline 8 to 14 grains.

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:--

Alcohol, sp. gr. ·836 1 fluid ounce. Iodide of Potassium 16 grains.

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 of Potassium may 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 at page 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. Anhydrous Alcohol 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 30 grains. Nitric Acid 1/20 minim, or Acetic Acid (glacial) 1/6 minim. Alcohol 15 minims. Distilled water 1 fluid ounce.

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 with Nitrite of 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, and Nitric 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 appears reddened, 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 (see p. 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.

Sulphate of Iron, recrystallized 12 to 20 grains. Acetic Acid (glacial) 20 minims. Alcohol 10 minims. Water 1 fluid ounce.

FORMULA No. 2.

Pyrogallic Acid 2 grains. Nitric Acid 1 drop. Water 1 fluid ounce.

FORMULA No. 3.

Solution of Protonitrate of Iron 1 fluid ounce. Alcohol 20 minims.

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. 1 is the most simple, since the solution can be used as 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 of persalt. When it is too weak, add more of the Protosulphate. The muddy deposit which settles 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 a small portion of 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, use three grains of 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 2 to 12 grains. Common Water 1 fluid ounce.

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 of Carbonate of Potash in 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.

Bichloride of Mercury 30 grains. Distilled Water 1 fluid ounce.

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.