SECTION II.. _positive Printing by Development._

Positive Printing by Development.

Negative printing processes will be found useful during the dull winter months, and at other times when the light is feeble, or when it is required to produce a large number of impressions from a Negative in a short space of time. The plan of development also enables the operator to obtain Positives of greater stability than those yielded by the direct action of light.

Three processes may be described, the first of which gives Positives of an agreeable colour, but the second, on Iodide of Silver, the greatest permanency under unfavourable conditions.

NEGATIVE PRINTING PROCESSES UPON CHLORIDE OF SILVER.

Positives may be obtained by exposing paper prepared with Chloride of Silver to the action of light until a faint image is perceptible, and subsequently developing by Gallic Acid; but in this process it is difficult to obtain sufficient contrast of light and shade; the impression, if sufficiently exposed and not too much developed, being feeble, with a want of intensity in the dark parts. By associating with the Chloride an organic salt of Silver, such as the Citrate, this difficulty may be overcome, and the shadows be brought out with great depth and distinctness.

The papers are salted with a mixed Chloride and Citrate as in the formula for the Ammonio-Nitrate Process. They are then rendered sensitive upon a Bath of Nitrate of Silver containing either Citric or Acetic Acid, which are used in Negative processes to preserve the clearness of the white parts under the influence of the developer.

The formula at p. 246 may be modified with advantage: use double the quantity of Gelatine, and half the amount of Citrate and Chloride.

The Bath of Aceto-Nitrate is prepared as follows:--

Nitrate of Silver 30 grains. Glacial Acetic Acid 30 minims. Water 1 fluid ounce.

Float the papers (Papier Saxe or Papier Rive) upon the Bath for three minutes, and suspend them to dry in a room from which actinic rays are perfectly excluded.

The exposure to light,--which is conducted in the ordinary printing frame, the Negative and sensitive paper being laid in contact in the usual way,--will seldom be longer than three or four minutes, even upon a dull day. It may be regulated by the colour assumed by the projecting margin of the paper; but it is quite possible to tell by the appearance of the image when it has received a sufficient amount of exposure:--the whole of the picture should be seen, excepting the lightest shades, and it will be found that very few details can be brought out in the development which were altogether invisible before the Gallic Acid was applied.

The developing solution is prepared as follows:--

Gallic Acid 2 grains. Water 1 fluid ounce.

In very cold weather it may be necessary to employ a saturated solution of Gallic Acid, containing about four grains to the ounce; whereas in warm weather the image will develope too quickly, and Acetic Acid must be added (see the remarks at the end of the process, p. 266).

To facilitate the solution of the Gallic Acid, stand the bottle in a warm place near the fire. A lump of Camphor floated in the liquid, or a drop of Oil of Cloves added, will to a great extent prevent it from becoming mouldy by keeping; but if once mould has formed, the bottle must be well cleansed with Nitric Acid, or the decomposition of the fresh Gallic Acid will be hastened.

Pour the solution of Gallic Acid into a flat dish, and immerse the Prints two or three at a time, moving them about, and using a glass rod to remove air-bubbles. The development is rapid, and will be completed in three or four minutes. If the Print developes slowly, becomes very dark in colour by continuing the action of the Gallic Acid, but shows no half-tones, it has not been exposed sufficiently long to the light. An over-exposed proof, on the other hand, developes with unusual rapidity, and it is necessary to remove it speedily from the Bath in order to preserve the clearness of the white parts; when taken out to the light, it appears pale and red, with no depth of shadow.

The extent to which the development should be carried depends upon the kind of Print desired. By pushing the action of the Gallic Acid, a dark picture not much altered by the fixing Bath will be produced. But a better result as regards colour and gradation of tone will be obtained by removing the Print from the developing solution whilst in the light red stage, and toning it subsequently by means of Gold; in which case it will correspond both in appearance and properties to a Positive obtained by the direct action of light (see the remarks at page 167).

When it is intended to follow the latter plan, the action of the developer must be stopped at a point when the proof appears lighter than it is to remain; since the Sel d'or Bath adds a little to the intensity, and the image becomes somewhat more vigorous on drying.

Wash the Prints in cold water in order to extract all the Gallic Acid. Then tone with Sel d'or in the manner described in the next Section, and fix in the usual way. The whites will with care be kept pure; or with only a faint yellow tinge, which is not objectionable.

Upon comparing the developed Prints with others obtained by the direct action of light upon the same sensitive paper, it is evident that the advantage is slightly on the side of the latter; but the difference is so small that it would be overlooked in printing large subjects, for which the Negative Process is more especially adapted. The colour of both kinds of Positives is the same, or perhaps a shade darker in the developed proofs, which are usually of a violet-purple tone, but sometimes of a dark chocolate-brown.

A developing process with Serum of Milk.--The use of "whey" as a vehicle for Chloride of Silver has something the same effect as that produced by adding a Citrate. This may be traced to the presence of the Milk Sugar and of a portion of uncoagulated Caseine left in the Serum.

The only difficulty in the process is to coagulate the milk in such a way as to separate the greater part but not the whole of the Caseine. Milk which has become sour, or to which an acid has been added, is not considered so good for the purpose as that which has been treated with rennet; and even when rennet is used it must be of the best quality or its action will be imperfect. The serum must filter clear through, blotting-paper; but it should not run very rapidly, or in all probability the whole of the Caseine has been separated, and the fluid contains little besides sugar. The whey which is left after cheese-making, commonly answers the purpose, if clarified by beating it up with the white of an egg and subsequently boiling and filtering. Globules of oil must be separated as far as possible, or they will produce a greasiness of the paper.

See the Vocabulary, Part III,, Art. "Milk," for further particulars.

Salt the prepared Serum with Chloride of Sodium or Ammonium; in quantity about eight or ten grains to each fluid ounce, and render sensitive upon the same Bath as that recommended for the Citrate Process.

A NEGATIVE PRINTING PROCESS UPON IODIDE OF SILVER.

Iodide of Silver is more sensitive to the reception of the invisible image than the other compounds of that metal; and hence it is usefully employed in printing enlarged Positives from small Negatives, by means of the Camera. The great stability of the proofs upon Iodide of Silver will also be a recommendation of this process when unusual permanency is required.

Take of

Iodide of Potassium 160 grains. Water 20 fluid ounces.

The best paper to use will be either Turner's Calotype, or Whatman's or Hollingworth's Negative; the foreign papers do not succeed with the above formula (p. 258).

Float the paper on the iodizing Bath until it ceases to curl up and lies flat upon the liquid: then pin up to dry in the usual way.

Render sensitive upon a Bath of Aceto-Nitrate of Silver containing 30 grains of Nitrate of Silver with 30 minims of Glacial Acetic Acid to each ounce of water.

When the sheet is quite dry, place it in contact with the Negative in a pressure frame, and expose to a feeble light. About 30 seconds will be an average time upon a dull winter's day, on which it would be impossible to print at all in the ordinary way. On removing the Negative nothing whatever is seen upon the paper, the image being strictly invisible in this process unless the exposure has been carried too far.

Develope by immersion in a saturated solution of Gallic Acid, prepared in the manner described at page 261. The image appears slowly, and the process may last from 15 minutes to half an hour. If the exposure has been correctly timed, the Gallic Acid appears at length almost to cease acting; but when the proof has been over-exposed, the development goes on uninterruptedly, and the image becomes too dark, partaking more of the character of a Negative than a Positive. The usual rule, that under-exposed proofs develope slowly but show no half-tones, and that the over-exposed develope with unusual rapidity, is also observed in the process with Iodide of Silver.

After the picture is fully brought out, wash in cold, and subsequently in warm water, to remove the Gallic Acid, which, if allowed to remain, would discolour the Hyposulphite Bath. Then fix the Print in a solution of Hyposulphite of Soda, one part to two of water, continuing the action until the yellow colour of the Iodide disappears. The fixing Bath ought not to produce much change in the tint. If the Positive loses its dark colour on immersion in the Hyposulphite, and becomes pale and red, it has been insufficiently developed. The theory of this part of the process should be understood:--It is particularly the second stage of the development of a Photograph (see p. 144) on which the fixing Bath produces no effect; and therefore a considerable change of colour in the Hyposulphite indicates that too little Silver has been deposited, and the remedy will be to push the development, adding a little Aceto-Nitrate to the Gallic Acid if the strength of the Bath be found insufficient to yield dark tones.

The colour of Positives developed upon Iodide of Silver is not agreeable, and they become blue and inky when toned with gold. By fixing the proof in Hyposulphite of Soda which has been long used and has acquired sulphuretting properties, the tint is much improved; but the permanency of the Print under unfavourable conditions is lessened by adopting that mode of toning.

A NEGATIVE PRINTING PROCESS UPON BROMIDE OF SILVER.

By substituting the Bromide for the Iodide of Silver in the above process, the proportions and details of manipulation being in other respects the same, a more agreeable colour is obtained.

Paper prepared with Bromide of Silver is less sensitive than the Iodide, but an exposure of one minute (in the printing frame) will usually be sufficient even on a dull day. The image is nearly latent, but sometimes a very faint outline of the darkest shadows can be seen. The proportion of Bromide used is likely to influence this point; the sensitiveness being diminished, but the image showing more of the details before development, when the quantity of the Silver Salt is reduced to a minimum.

Either English or French papers may be used, but in the latter case the Bromide should be dissolved in Serum of Milk (p. 262), or it will be difficult to obtain a good surface picture. The proportion of Bromide may be five grains to the ounce of Serum.

These proofs, even when simply fixed in plain Hyposulphite of Soda, are superior in colour to the Positives printed by the last formula upon Iodide of Silver; and the permanency is very great if the development be sufficiently pushed. The use of the Serum of Milk gives an advantage in resisting the oxidizing influences to which Positives are liable to be exposed (p. 150).

GENERAL REMARKS ON NEGATIVE PRINTING.

Printing by development should not be attempted until the manipulation of the ordinary process by direct exposure to light has been acquired.

Perfect cleanliness is essential. The salting or iodizing solution and the Aceto-Nitrate Bath must be filtered clear, as the effect of small suspended particles in producing spots is more seen when the image is brought out by a developer.

It will be necessary to be far more careful in excluding white light than in the ordinary process; and when Iodide of Silver is used, all the precautions required in the case of Collodion Negatives must be taken.

Observe particularly that the dishes are kept clean, or the Gallo-Nitrate of Silver will be rapidly discoloured (read the remarks at page 179).

Stereoscopic Negatives and small portraits are not successfully printed by development; since it is difficult to obtain the most elaborate definition, and there is a slight tendency to yellowness in the white parts. Positives may be developed upon Albumen paper, but the Gallic Acid is apt to discolour the lights.

In printing by development upon Chloride of Silver, the theory of the subject must be particularly studied. When the weather is cold and the light bad, the development of the image proceeds slowly, the Gallic Acid Bath remains clear, and good half-tones are obtained; but under opposite conditions, the developer may become turbid and the shadows be lost by excessive deposit of Silver. This over-development will be remedied by printing the Negative in a more feeble light (near to the open window of a room), and by adding Acetic Acid to the developer, about 5 or 10 minims to the ounce, so as to bring out the image more slowly. The intensity of action is thus lessened, and if the picture be not under-exposed, the half-tones will be good.

Observe also when preparing papers with Citrate, that if too much Carbonate of Soda be added in neutralizing the Citric Acid, Carbonate of Silver will be deposited in the paper, the effect of which is to remove by degrees the acidity of the Nitrate Bath, and to produce over-development and excessive sensibility to light.

The colour of the proofs when taken from the Gallic Acid should be light red; the gradation of tone not being usually so perfect when the development is carried into the second or black stage.

It is not recommended to prepare too large a stock of the salted papers, as they will probably be liable to mouldiness and decomposition unless kept perfectly dry.