Part III.; it must be diluted with three or four parts of water, and

filtered.

Certain facts to which attention has been lately drawn by Dr. Norris and Mr. Barnes in working with dry Collodion, may be advantageously borne in mind when using Oxymel; the preservative solution of which is employed in so dilute a state that the process resembles to a great extent a dry Collodion process. The observations above referred to relate to the quality of the Collodion best adapted for the purpose, and will be found at page 298, to which the reader is referred.

The manipulation of the Oxymel process is very simple. Two flat gutta-percha dishes are provided, the one containing common water and the other diluted and filtered Oxymel. The Collodion plate, on its removal from the Bath, is placed in the first dish, which is gently tilted up and down, to wash away the free Nitrate of Silver. In a few seconds, when the liquid is rendered milky, it is poured away, and fresh water being introduced, the process is repeated _until the oily lines disappear, and the surface of the film becomes smooth and glassy_. The plate is then, after a slight draining, removed to the second tray, and the Oxymel waved backwards and forwards for about half a minute, after which the glass is lifted out and placed vertically on blotting-paper, which must be renewed when it becomes wet and saturated.

The plates may be used any time within a fortnight from the date of their preparation, and it is not necessary to develope immediately after the exposure. The sensitiveness will be considerably less than that of fresh Collodion: from two to five minutes may be allowed with a Stereoscopic view lens having a quarter-inch diaphragm.

Before developing, the film should be gently washed for a few seconds with common water. Solution of Pyrogallic Acid, of the ordinary strength, but previously mixed with a portion of the Nitrate Bath solution, one or two drops to each drachm, may then be poured on in the ordinary way. Use less Nitrate of Silver and more Acetic Acid in hot weather. When discoloration of the developer occurs, mix a fresh portion and proceed as before.

PRECAUTIONS TO BE OBSERVED IN KEEPING PROCESSES.

The plates must be roughened at the edges, and also upon the surface, to make the film adhere.

It is advisable to use a tolerably thick Collodion, giving a yellow film; the pale opalescent films being more easily affected by markings on the glass, and not retaining so much of syrup or Nitrate of Silver upon the surface.

The room in which the plates are prepared must be carefully guarded from scattered pencils of white light; the films are exposed to injury from this cause during the whole of the time occupied in applying the preservative syrup; and hence anything short of absolute chemical darkness will be likely to cause fogging; especially so when free Nitrate of Silver is left upon the film.

The water used for washing away the free Nitrate of Silver before applying the preserving liquid, need not be distilled. Common hard water containing Carbonates and Chlorides, and producing _milkiness_ with Nitrate of Silver, will often suffice. The water of the New River and of the River Thames, with which many parts of London are supplied, may certainly be used; but in the case of a very _hard_ water, containing much Sulphate of Lime, it might perhaps be advisable to substitute clean rain-water, free from brown organic discoloration.

The preservative Oxymel must be carefully filtered, and kept _covered_, in order to protect it from dust. It will also be necessary occasionally, before using it, to run it through a piece of white cambric, to stop back suspended particles, which, if allowed to remain, would be a source of spots. If it becomes mouldy, or discoloured by Silver, or ferments and evolves gas, throw it away.

After the syrup is applied and the plates are drained, stow them in a grooved box perfectly protected from light; or place them in slides, which must be kept scrupulously clean, since any trace of impurity would be likely to produce a stain when the plate was left a long time in the slide. If the preserved plates are kept in a cupboard or box, see that no volatile matter, such as Ammonia, coal-gas, etc., can find entrance.

In changing the plates after the exposure in the Camera, use a large bag made of _several thicknesses_ of black calico, with a square of yellow calico let in at the top; an elastic band securing it round the waist.

THE COLLODIO-ALBUMEN PROCESS.

This process, the theory of which has been briefly explained at page 181, is more sensitive than the one last described, and has the additional advantage of giving _dry_ plates, which do not attract dust, and are less liable to injury. The details of manipulation are complex, but this inconvenience is not so much felt when preparing a large number of plates.

_Cleaning the Glasses._--Success will greatly depend upon the mode in which this part of the process is performed. The layer of Albumen which is applied to the Collodion film tends to swell and to raise the latter in blisters; the most effectual mode of obviating which will be to clean the glass so that the film adheres with unusual tenacity.

The Liquor Potassæ of the Druggists, diluted with three or four parts of water, and rubbed on the glass by a roll of flannel (page 214), is very effectual. A mixture of Tripoli-water and Nitric Acid may however, if desired, be substituted:--

Tripoli 1 drachm. Nitric Acid 30 minims. Water 1 ounce.

Lay the glass flat on a cloth, and rub the surface carefully with a tuft of cotton-wool dipped in the Tripoli; then, before the cream dries, wipe it off with a second tuft, and polish with a third. Lastly, breathe upon the glass, and having ascertained that it is chemically clean, apply the Collodion.

_Coating with Collodion._--Choose a rather thin Collodion which adheres tightly to the glass. A preparation which has been kept a long time after iodizing will usually answer the purpose very well, and, as a rule, a non-contractile, structureless Collodion is better than one which is glutinous and wavy. The degree of sensibility of the Collodion is not thought to have much influence upon the result.

_Coating the Plate._--Apply the Collodion in the usual manner, and allow it full time to set perfectly, before dipping in the Bath, in order to favour its adherence to the glass. With Collodion prepared from anhydrous spirits, about half a minute may be given in cool weather.

_The Nitrate Bath._--Take of

Fused Nitrate of Silver 40 grains. Glacial Acetic Acid 30 minims. Alcohol 20 minims. Water 1 fluid ounce.

Saturate with Iodide of Silver as described at page 204, and filter. An immersion of one minute will be sufficient; after which, give the plate an up-and-down movement, and wash it in plain water, in the manner advised for the Oxymel preservative process, at page 292. Then stand it on blotting-paper, to drain for a minute or two, wipe the back of the glass, and pour on the Albumen.

This Bath may become discoloured after a time; continue to use it until it is of a dark sherry-colour, and then treat it with "Kaolin," in the manner and with the precautions advised at pages 91 and 245.

_The Iodized Albumen._--Procure eggs, fresh laid, or not more than two or three days old. Separate the whites in the same way as for Albuminized paper (p. 241), and mix by the following Formula:--

Albumen 9 fluid ounces. Water 3 fluid ounces. Liquor Ammoniæ 2 fluid drachms. Iodide of Potassium 48 grains. Bromide of Potassium 12 grains.

The Iodide and Bromide should be free from Carbonate of Potash, which is said to cause pin-holes in the Negatives. To ensure the absence of this salt, dissolve the total quantity of both Iodide and Bromide in the three ounces of water advised in the formula; then, previously to adding the Ammonia and Albumen, introduce _an excessively minute particle of Iodine_, enough barely to colour the liquid. The Iodine decomposes the Carbonate of Potash, but it must not be used in excess, since free Iodine possesses the property of coagulating Albumen. Iodide of Cadmium also coagulates Albumen, so that the Iodides of Potassium and Ammonium are the best.

Having mixed the ingredients in the order above given, introduce them into a bottle, and shake it violently until they have thoroughly amalgamated. Then transfer to a tall narrow jar; allow to settle for twenty-four hours, and draw off the upper clear portion for use. Particulars of this part of the process have already been given under the head of Albuminized Paper, to which the reader is referred (p. 241).

The ammoniacal solution of Albumen may be kept for some time in a stoppered bottle without much decomposition. If mucous threads form in it, filter through fine linen.

_Mode of applying the Albumen._--Cover the moist film with the Albumen in the same way as advised for Collodion (p. 216), pouring on at once a sufficient quantity to cause it to spread in an even and undivided sheet; otherwise a veined appearance may be produced, which will show in the development. Return the excess of Albumen into the bottle, and pour it once again upon the plate: the film will remain clear and transparent, if the whole of the Nitrate of Silver has been properly washed away from the Collodion. Lastly, stand the plate nearly vertically on blotting-paper to dry. This will occupy five or six hours; but the process may be hastened by artificial heat.

After the Albumen solution has been used to coat a number of plates successively, it becomes diluted with water; the result of which is, that unequal intensity of image is produced at the upper and lower edge of the film.

The iodized Albumen plates are at this stage of the process nearly or quite insensitive to light, and may be preserved unchanged for many weeks.

_Sensitizing the Albumen film._--When the plate has become thoroughly dry, it is again introduced into the Bath of Aceto-Nitrate of Silver, and allowed to remain for one minute: then washed with water in the same manner as before, but with even greater care, in order to obviate clouding in the development. If blisters should form on drying, it will be found useful to hasten the process by holding the plates to the fire--or a hot iron may be placed in the centre of a covered box and the glasses reared up round the sides. They will thus dry quickly, and there will not be time for the Albumen to swell much by imbibition.

_Exposure in the Camera._--This may be performed at any period within a few weeks from the date of preparation of the plates. For a landscape view with a small Stereoscopic single lens, allow about three minutes in the winter, or one minute and a half in the summer.

_Development of the image._--This can be deferred as long as fourteen days after the exposure, with successful results. Pour water over the plate until the film is thoroughly wetted; then cover it with a solution of Pyrogallic Acid containing one grain of the acid to the ounce of water, and twenty minims of Glacial Acetic Acid. Two drops of a neutral solution of Nitrate of Silver made with forty grains of Nitrate to the ounce of water must be previously added to each fluid drachm of the Pyrogallic. The development, in the case of a landscape view taken with sunlight, commences almost immediately, and may be completed in about ten minutes, but the time occupied in developing will vary greatly with the length of exposure, the quantity of Nitrate of Silver, and the nature of the subject copied--a badly lighted interior, for instance, often taking an hour or longer to appear in all its details. If the developer should discolour before the proper intensity has been obtained, pour it off and mix a fresh quantity.

_Fixing the image._--Hyposulphite of Soda (one ounce to four of water) will be found preferable to the Cyanide of Potassium, as the latter has a solvent effect upon the Albumen. An unusually long time will be required, as the fixing agent must penetrate the Albumen, to reach the Collodion beneath.

Careful washing in water for five or ten minutes removes the excess of Hyposulphite, and the plate may then be varnished in the usual way.

THE DRY COLLODION PROCESS.

The earlier attempts to employ sensitive Collodion plates in a desiccated condition were unsuccessful. The film of Pyroxyline shrinks on drying, and becomes almost impervious to moisture: hence, the developing solution not penetrating properly, density cannot easily be obtained. We are indebted to Dr. Hill Norris, of Birmingham, for establishing the theory of the subject upon a more correct basis. He has pointed out the importance of distinguishing two different conditions of the Collodion surface,[53] viz. the _contractile_, common in newly-mixed Collodion,--and the _short_ or _powdery_, in Collodion which has been iodized with the alkaline Iodides, and kept until much Iodine has been set free. The latter is the most suitable condition for the dry process; and the practical mode of distinguishing between them is by sensitizing a plate and passing the finger across it; if it can be easily pushed away in a firm and connected skin, it will be unfit for the purpose required. In order still further to preserve the film in a condition permeable by the developer, it is recommended to coat it whilst moist with a solution of Gelatine.

[53] See these states of the film more fully described at page 83.

The dry Collodion process, although less sensitive, is more simple than that on Collodio-Albumen, and possesses many of its advantages; but it is less universally applicable, since it depends entirely for success upon the peculiar state of the Collodion, resembling in this respect the Oxymel process already described.

_Mode of preparing the plates._--The glasses are coated with the Collodion in the usual way. Blistering during development being liable to happen in this process as in the last, every care must be taken to make the films adhere with the greatest possible tenacity, both by cleaning the glasses with extra care (see p. 294), and also by allowing the Collodion to set firmly before dipping in the Bath. The plate may be held from twenty to thirty seconds previous to immersion, or even longer, provided the film, when lifted out of the Bath, appears of uniform thickness throughout (see page 218).

The sensitizing having been completed, wash the plates with plain water, exactly in the same way as for Oxymel (p. 292). If Nitrate of Silver be left, clouding will take place in the process of development. After washing, drain for a few seconds, and immerse in the solution of Gelatine.

To prepare this Bath, take of

Nelson's patent Gelatine 128 grains. Distilled water 14 ounces. Alcohol 2-1/2 ounces.

Put the Gelatine in the cold water, and allow it a quarter of an hour to soften and swell; it will then readily dissolve on applying a gentle heat. This may be done in a glazed saucepan or a pipkin of earthenware, taking care not to scorch the bottom part by too strong a heat. Next clarify the solution by adding to it, whilst barely warm, a tea-spoonful of white of egg (previously beaten up with a silver fork), and afterwards heating nearly to the boiling point. The Alcohol must now be added, to facilitate the coagulation of the Albumen. When this takes place and the liquid becomes clear, filter through a clean piece of cambric folded three or four times. If a hot water filtering apparatus can be obtained, the solution may be made to pass through _paper_; but as it tends to gelatinize on cooling, the ordinary mode of filtration commonly fails. The quantity of Alcohol in the above formula is greater than is usually recommended, allowance having been made for a partial evaporation of the spirit.

The filtered liquid may be poured into a flat porcelain dish, or a vertical trough, but in either case it will be necessary to stand the vessel in warm water, in order to prevent gelatinization.

The Collodion plate, thoroughly washed, is to be immersed in this solution and moved up and down for two or three minutes. It is then removed, drained on blotting-paper, and dried. The use of artificial heat in drying will be found a great advantage; it prevents the gelatine from settling unequally upon the plate. Those who possess an apparatus made purposely for drying plates by hot air, will experience no difficulty, but an ordinary deal trunk may be made to answer, with a little management. Cover the bottom of the box with blotting-paper, and having heated one or two "flat irons," place them in the centre: then range the glasses side by side, with the coated surface looking inwards; in a quarter of an hour, or from that to twenty minutes, the desiccation will be complete. If the Collodion plates are prepared in a room containing a fire, they may be reared up side by side at a distance of two or three feet, and in that way may be safely dried without fear of injury, provided white light be excluded.

When dry they can be stowed away in a box; all the precautions given at page 293 being observed. The sensibility remains good for many days, possibly for weeks or months in cold weather.

_Exposure in the Camera._--Allow from four to eight times the exposure of the most sensitive moist Collodion. On a clear summer's day, a sun-lit view may require one minute or a minute and a half, with a short focus Stereoscopic lens, having a diaphragm of a quarter of an inch diameter. The average time however with the same lens would be about twice as much, viz. three minutes.

_Development of the Image._--Make a saturated solution of Gallic Acid in water by the directions given at page 261. Then dissolve forty grains of pure Nitrate of Silver in one ounce of distilled water. Pour into a flat porcelain dish a sufficient quantity of the Gallic Acid solution to flood the plate readily. Then measure it, and to each fluid ounce add _ten minims_ of the solution of Silver, or five minims in hot weather. It is important that no discoloration should occur on mixing these liquids together, to obviate which, observe the following precautions:--Clean the porcelain vessel very carefully with Nitric Acid or Cyanide before use. Employ a pure solution of Nitrate of Silver; and mix it with the Gallic Acid, in preference to adding the Gallic Acid to the Silver solution (read the remarks at p. 179).

The picture may be expected to appear in five or ten minutes, and in one hour, or from that to four hours (p. 298), the development will be complete. It will not be necessary to keep the plates in motion, but simply to lay them side by side in the solution of the Gallic Acid. If in spite of all precautions the developer begins to blacken before the intensity has reached the proper point, it must be poured off and a fresh mixture prepared. This however will not often happen.

Lastly, when a full amount of opacity has been obtained, wash the plate with water, and fix it in a solution of Hyposulphite of Soda, or dilute solution of Cyanide of Potassium.

_Failures in the process._--Stains in the development may arise from using dirty dishes, or glasses which have been left in Gallo-Nitrate of Silver and improperly cleaned. It must be borne in mind that these impurities are not visible to the eye, although they produce the effect of discolouring the developer. A thorough cleansing with strong Nitric Acid or Potash will prove a remedy.

Blisters, unless of large size, may often be disregarded, as they disappear on drying. General cloudiness may depend upon the film having been imperfectly washed. Irregular reduction at certain parts may be due to the Gelatine setting before the plate has become dry, or to stains produced by the finger applied to the upper edge of the plate.[54]

[54] Since the above was written, Mr. Maxwell Lyte has communicated to the 'Photographic Journal' (vol. iii.) a dry process in which a _modified_ Gelatine is used. The change is produced by boiling a solution of gelatine with dilute Sulphuric Acid, which is afterwards neutralized and removed by means of chalk. The result is to destroy the gelatinizing property of the animal substance; the solution retains its fluidity on cooling, and the necessity of employing artificial heat in drying the plates is avoided.