Photographic Reproduction Processes Part 9

If in lieu of alb.u.men paper, one employs paper prepared with a thin coating of gelatine, and dissolves the not acted on gelatine in warm water, a very fine positive image is obtained by means of acidified inks which will fix themselves on the bare paper.

Positive impressions from positive cliches can also be obtained in operating in the following manner: On its removal from the printing frame the proof is washed, sponged between sheets of blotting paper, then covered with not acidified India ink mixed with pota.s.sium bichromate, and, when dry, exposed from the verso to the action of light. This done the image is cleared with a somewhat hard brush.

THE CARBON PROCESS.

The carbon tissue is seldom prepared by photographers. However, for the sake of completeness, we shall give the formula of the mixtures most generally employed, and describe the manner of coating the paper on a small scale.

_Preparation of the Tissue.-_The gelatine generally recommended to compound the mixture is the Nelson's autotype gelatine. Coignet's gold label gelatine, mixed with a more soluble product, such as c.o.x's gelatine, for example, gives also excellent results.

Gelatine 110 parts Sugar 25 parts Soap, dry 12 parts Water 350 parts

The coloring substances consist of:

FOR ENGRAVING BLACK.

Lamp-black 20 parts Crimson lake 2 parts Indigo 1 part

FOR WARM BLACK.

Lamp-black 3 parts Crimson lake 3 parts Burnt amber 2 parts Indigo 1 part

FOR SEPIA Lamp-black 2 parts Sepia of Cologne 18 parts

FOR PHOTOGRAPHIC RED BROWN.

India ink 3 parts Crimson lake 4 parts Van Dyck brown 4 parts

For blue, Turnbull's blue is employed; for yellow, light chrome yellow; for red, carmine dissolved in aqueous ammonia, evaporating, then adding water, etc. (See further on.)

To prepare the mixture, dissolve the sugar and soap in the cold water, add the gelatine, let it soak for an hour, then dissolve it in a water bath and mix by small quant.i.ty the colors finely ground together and wetted to the consistency of a paste. After filtering through flannel the mixture is ready for use.

For coating, the method devised by Mr. Alf. Harman has been found excellent in the hands of the writer, not only for the purpose in question, but also for coating paper with gelatinous or viscous (gum arabic) preparations.

"Take two tin dishes, such as used for the development of the carbon prints; arrange one on your bench tilted to an angle; the lower angle is intended to receive the warm water for keeping the gelatine mixture to a proper temperature. Into this angle of the tray arrange another tray somewhat smaller, and keep it from touching the bottom of the outer one by the insertion of any small article that will suggest itself. Into the inner tray the gelatine mixture is to be poured."

"The actual making of the tissue can now be proceeded with, and is so simple and certain as not to be believed until put to the test. Purchase a roll of paper-hanger's lining paper of good quality, cut it into widths of about one and a half inch less than the width of your inner tray, and in length of, say, thirty inches. For the success of the operation it is necessary that the paper be rolled up the narrow way. Now having just sufficient water at a temperature of 100 deg. Fahr. (38 deg. C.) into the outer tray, pour the gelatine mixture into the inner one, and take one of the lengths of rolled paper, and, holding it by both ends, gently lower it on the surface of the gelatine; then at once slowly raise the end of the paper, which will unroll itself and become beautifully coated in far less time than it takes to describe. Twenty sheets may be coated in a quarter of an hour, and be equal in all respects to that made by the most expensive machine."

In the description of this method of coating, Mr. Harman does not explain how the gelatine should be allowed to set before hanging up the paper to dry, which is, however, obviously important. It is as follows: Place on the tray a smooth board a little larger than the sheet of paper, leaving a small s.p.a.ce at the end furthest from the body, and slowly, without a stop, draw off the paper, prepared side uppermost, on the board upon which it should remain until the gelatine is set. If the paper curls up, wet the back a little with a sponge before coating.

The following coating method, due to Mr. Chardon, is excellent for sheets of paper of the ordinary photographic size, 1822 inches.

On a gla.s.s plate placed on a leveled stand, is laid a sheet of paper previously wetted, which is then flattened into contact with an India rubber squeegee, taking care to remove the air bubbles interposed. The quant.i.ty of gelatine necessary to coat the paper is regulated by means of a gla.s.s rod held by an iron lath, which serves to handle it; at each end of the rod is inserted a piece of an India rubber tube whose thickness regulates that of the gelatine layer. The mixture is poured from a small teapot, at the opening of which has been adapted a bent gla.s.s tube about three-sixteenths of an inch in diameter, between the rod and the lath, so that by a simultaneous motion, one can equalize the gelatine as it is poured on. When the gelatine is set the paper is hung up to dry. In drying, the gelatine contracts, and, necessarily, causes a deformation of the tissue, which curls up at the edges and loses its planimetry. To prevent this, while the gelatine is almost dry, the tissue is placed under pressure until quite desiccated. Dumoulin advises to apply on the film, while still soft and tacky, a wooden frame, which, by adhering to it.

keeps the tissue perfectly plane as it dries.

[Chardon's method of coating]

_Sensitizing.-_The tissue is sensitized in a bath of pota.s.sium bichromate.

The degree of concentration of the bath, which varies from 2 to 5 per cent. of water, is important. The tissue sensitized in a weak bath is less rapidly acted on by light and yields more contrasts than when imbued in a concentrated one. The former should consequently be employed for printing weak negatives, and the latter for those which are intense. A bath compounded with 30 parts of pota.s.sium bichromate, 1,000 parts of water and 2 parts of aqueous ammonia, is used for printing negatives of the ordinary intensity, the tissue being, then practically of the same sensitiveness, a silvered paper insolated to obtain a print not over-exposed. For intense negatives the ammonia should be discarded and replaced by the same quant.i.ty of chromic acid.

The time of immersion has also a certain influence on the results. The less the tissue is allowed to absorb the solution the less sensitive it is, but also the more the tendency of the half tints to be washed off during the development. Generally the tissue should remain immersed until it lies flat and the edges just commence to curl up, unless white and black impressions are desired, but even then it is preferable to operate as said above, using a bath at 2 per cent.

For use the bichromate bath should be cooled down to 15 deg. C. (59 deg.

Fahr.), and much lower in summer, say 10 deg. C. (50 deg. Fahr.), and kept at about this temperature by placing pieces of ice around the tray. At 20 deg. C. (68 deg. Fahr.) the prints are more or less granulated; above this the gelatine is softened and the reticulation greater; at 25 deg. C. (75 deg. Fahr.) it may dissolve.

The addition of alcohol to the bichromate bath-sometimes recommended to harden the film and allow it to stand a higher temperature, and to hasten the desiccation of the tissue-is objectionable, for the spirits tend to reduce the bichromate, which is transformed into the green salt, and, therefore, a partial or complete insolubilization of the gelatine is the result.

Aqueous ammonia added to the sensitizing solution has for its object to permit one to keep the sensitive tissue for a somewhat longer period, but it renders it less sensitive. If enough be added to turn the solution yellow weak prints are obtained.

The bichromate bath should be renewed often. It does not keep owing to the presence of gelatine and other organic matters which it dissolves and which cause the reduction of the chromic salt even in the dark. The tissue prepared in such a bath is not very sensitive and the image develops with difficulty, and even cannot be developed at all.

As said above, the tissue is well sensitized when its edges commence to curl up. It is then removed from the bath by drawing it on a gla.s.s rod fixed at the end of the tray, and placed, prepared side down, on a slightly waxed gla.s.s plate, rubbing it with an India rubber squeegee to remove the superflous liquid, when it is hung up to dry.

While wet the bichromated tissue is insensitive; the sensitizing can therefore be made by daylight, but the drying should of course be done in the dark room, that is in a room lighted by a candle or the sunlight filtered through a deep orange window gla.s.s.

_Caution.-_The soluble bichromates are very poisonous. By absorption they produce skin diseases not without danger and very difficult to cure. Hence when handling the wet tissue the fingers should be protected by India rubber tips, and any yellow, stains on the hands should be rubbed with a dilute solution of aqueous ammonia, and the hands well rinsed in water.

_Drying.-_When the tissue dries rapidly it adheres well on the support upon which it is applied for developing and yields brilliant images which are easily cleared. On the other hand, were it allowed to dry slowly the adherence would not be so complete, the image dull and developing with difficulty. They may even refuse to develop at all from the insolubilization of the gelatine.

In winter and in the cool days of spring and autumn, the gelatine dries quick enough in the air, but when the weather is warm and damp, the gelatine, drying very slowly, may be so softened as to run off, or to produce an entirely objectionable reticulation, or the defects above mentioned. This may be avoided by drying it pinned up in a box, or a closet, over quick-lime.

When dry, the tissue is generally wrinkled, brittle, breaks easily in handling and cannot be laid flat on the cliche; but by holding it over a basin of boiling water, the steam in a few moments rendering it sufficiently pliable to lay it flat between gla.s.s plates, where it should be kept under pressure until wanted for use.

The writer always dries the tissue in the following manner, which he devised about sixteen years ago.(27) And not only the least trace of reticulation is avoided, but the tissue, drying quite flat, lies in perfect contact with the negative, which is quite important to obtain proofs exactly sharp all over.

A clean gla.s.s plate is rubbed with talc, or, which the writer prefers, flowed with a solution of(28) Yellow wax, pure 1 part Benzine, pure 100 parts

then strongly heated, allowed to cool and rubbed clean (apparently) with a piece of flannel. After once more repeating this operation the plate is coated with the following plain collodion:(29)

Ether, conc. 250 parts, in volume Alcohol, 95 deg 250 parts, in volume Pyroxyline 3 parts

When the film is set, the plate is immersed in filtered water until greasiness has disappeared, when on its removal from the bichromate bath the tissue is laid, without draining, upon it and pressed into contact with the squeegee to remove the excess of liquid and, with it, the air bubbles interposited. The tissue is then allowed to dry in the air on the collodionized plate in the cold season, or, when the weather is warm and damp, in a box in the bottom of which is placed a quant.i.ty of quicklime in earthen dishes. When dry, the plates are placed one upon another, wrapped in paper and kept in a dry place. When wanted for use the tissue is stripped off and will be found quite flat with a beautiful surface to print upon.

One should avoid to keep the sensitized tissue in a moist and warm atmosphere, for in less than ten hours it becomes insoluble even in complete darkness. It should neither be kept in the air contaminated with gaseous reductive matters, such as the products of the combustion of coal gas and petroleum, sulphydric or sulphurous emanations from any source, the fumes of turpentine oil, etc., which, by reducing the chromic salt, cause the insolubilization of gelatine, prevent the print to adhere on the support or the clearing of the image, which may even refuse to develop.

The sensitive tissue keeps well for three or four weeks in cool and dry weather, and no more than eight or ten days in summer unless well desiccated and kept in a preservative box. If kept too long the image cannot be developed.

_The Photometer.-_The time of exposure is regulated by means of a photometer. Of all the photometers which have been devised for that purpose we do not know any one more practical than that suggested in 1876 by Mr. J. Loeffler, of Staten Island. It is made as follows: On a strip of a thin gla.s.s plate, 62 inches, make four or five negatives, 11 inch, exposing each one exactly for the same period and developing in the usual manner, but without any intensification whatever. It is even advisable to reduce the intensity if they were opaque. Fix, etc., and apply a good hard varnish. Now cover the back of these negatives with strips of vegetable paper or transparent celluloid, or, better, of thin sheets of mica, in such a manner as there be one thickness on the second negative, two on the third, three on the fourth, etc., leaving the first one uncovered. Then place on the whole a gla.s.s plate of the same size as the first and border like a pa.s.se-partout.

_The Negatives.-_For the carbon process the negatives should be intenser than those intended for printing out on silver paper. However, good proofs may be obtained from any negatives, so to say, by varying the strength of the bichromate solution, as, also, by _using the tissue freshly sensitized for weak negatives,_ in order to obtain vigor, and _for strong negatives, the tissue two or three days after its preparation,_ when it yields better half tones. Printing dodges are also resorted to.

That the most commonly employed consists to varnish the back of the negatives with a matt varnish, or to stretch on the same a sheet of mineral paper upon which the retouches are made by rubbing graphite, chrome yellow, pink or blue colors to strengthen the shadows or the whites, as the case requires. As a rule, it is advantageous to cover the printing frame with tissue paper, whatever be the quality of the negatives.

The negatives should be bordered with deep yellow or orange-red paper to form what is termed a "safe edge" upon which should rest the tissue in order to prevent the margin from being insolubilized by the reductive action of light. If this precaution were neglected it would be impossible to strip off the paper without tearing the proof when the tissue is applied on the support upon which the image is to be developed.

Before exposing it is advisable to ascertain what the printing qualities of the negative are by making on silvered paper a proof of it-_not over-printed-_and another of the photometer, both being exposed at the same time and for the same period. This done, compare the proof from the negative cliche with those of the photometer, and mark the negative with the number of that of the photometer to which it corresponds, stating the shade of the proof next to it; for example: _No. 2_; _No. 3 faint, or commences to appear,_ etc. This No. 2 and the observation will indicate the intensity of the negative and serve as a guide for printing on the tissue, since, as before explained, the silver paper is practically of the same sensitiveness as the tissue prepared for negatives of the ordinary intensity.

_Exposure.-_To print, the tissue is laid over the negative, taking care that it covers the safe edge, and a strip of silvered paper placed in the photometer, then both the printing frame and the photometer are exposed to light side by side.

Unless the negative be weak, when more vigor is obtained by exposing in sunshine, the printing should be done in the shade. It is a well-known fact that the part of the bichromated film corresponding to the half tones in the lights are not sufficiently impressed in comparison to the blacks while impressed in direct sun's light in this as well as in the collotype, photogravure and other processes with the chromic salts, because the luminous action through the bare gla.s.s, or nearly so, which in the negative represent the shadows and half blacks, is more energetic in proportion than through the other parts, from which it results that these parts being most acted on are made deeply insoluble through the thickness of the film, and then require to be cleared by a treatment with water at a higher temperature than the parts representing the half tints in the lights of the picture, which are but superficially and slightly insoluble, can stand.