Photographic Reproduction Processes Part 10

From time to time during the exposure the print in the photometer is examined, and when a certain picture is printed to a certain shade, or when the one next by commences to appear or is faintly printed, etc., the exposure of the tissue is sufficient. This, as the reader has already inferred, is a matter of experience, the guide being the knowledge of the intensity negative tested as above explained.

_Development.-_The carbon prints are developed either on a sheet of paper upon which it should remain (single or simple transfer), or on a provisory support to be afterwards transferred on paper or any other material (double transfer).

_Simple Transfer.-_This process is quite simple: The impressed tissue and a sheet of paper coated with alumed (insoluble) gelatine are immersed face to face in cold water, and when the tissue is softened both are removed, one superposed on the other, and the whole, being placed on a gla.s.s plate and covered with a thin oil cloth, is firmly pressed into contact with the squeegee. The rationale of applying under water the tissue on the gelatinized paper is to avoid the interposition of air bubbles.

To operate by simple transfer the tissue should be impressed under a reversed negative. The reason is obvious.

_Double Transfer.-_By this method the carbon prints are generally developed on porcelain or opal plates, which more easily than gla.s.s plates permit one to follow the progress of the development and to retouch the imperfections before transferring the picture on paper.

In order that the image does not adhere on the provisory support a little of the following mixture is spread over the plate, which is then pretty strongly heated, and, when it has cooled down, polished lightly with a piece of white flannel to obtain a very thin and even layer free from striae. If the plate has not been used before for the purpose in question, it should be waxed a second time in the same manner:

Yellow wax 4 parts Rosin 1 part Turpentine or benzine 250 parts

The plates can be developed on the plates so waxed, but for "full gloss,"

that is, for enameled pictures, a film of collodion is applied on the plates, which then, instead of being waxed, should to be simply flowed with a solution of India rubber 1 to 100 of benzole:

Ether 250 parts Alcohol 250 parts Castor oil 1 part Pyroxyline 5 to 6 parts

When the plate is coated and the collodion film set, it is immersed in water until greasiness has disappeared and wanted for use. Then the tissue, previously soaked in water, is applied upon it (taking care to avoid air bubbles) and squeezed, lightly at first, with some force afterwards, to insure a perfect contact.

Zinc plates are also employed as provisory supports instead of gla.s.s, opal or porcelain plates. The modus operandi is exactly the same.(30) The plates should be well planed, free from scratches, etc., and well polished to obtain glossy pictures without one having recourse to a film of collodion. For matt pictures, i.e., without gloss whatever, the plate should be finely granulated, and when waxing a very light pressure should be exerted to remove the excess of wax, else it might be quite impossible to strip off the picture in transferring on paper.

For double transfer on biscuits, objects in alabaster, porcelain, wood, any even or curved rigid materials, flexible supports are employed to develop the pictures. These supports are prepared by fastening alb.u.men paper on a board and evenly brushing over the following hot compound, filtered through flannel, which, when dry, is polished with a cloth: Stearine 15 parts Rosin 3 parts Alcohol 100 parts

The flexible supports should be waxed, then collodionized for full gloss, as the gla.s.s, porcelain and metallic plates.

Another method which the writer recommends is the following, due to Mr.

Swan: Immerse a sheet of paper in a solution of India rubber, 4:100 of benzole, and let dry, which requires a few minutes. This is the flexible support. Then after exposure, brush over the India rubber solution on the carbon tissue, apply upon it the support when the benzole is evaporated, and pa.s.s the whole under a rolling press to secure adhesion, then develop.

To transfer, soak the proof in tepid water, apply it on the material prepared, as it will be explained further on, and when dry, imbue the support from the back with benzole, to soften the India rubber, and strip.

To dispense with a rolling press, the proof may be developed on lacquered vegetable paper prepared by immersion in a solution of 10 parts of red sh.e.l.lac in 100 parts of alcohol. After developing the proof is coated with alumed gelatine, and when dry transferred as usual. To strip off it suffices to imbue the paper with alcohol in order to dissolve the sh.e.l.lac.

When the picture must be transferred on small s.p.a.ces or on small objects the most simple method-the most effective, perhaps-is the following, devised some years ago by the writer and now employed for the ornaments of "articles de Paris:" Prepare the provisory support as usual, but with a thicker film of collodion; then, after developing and coloring, if necessary, the picture is coated with gelatine, to which may be added some zinc white or other colored substance to form a ground. This dry, strip off, immerse the pellicle in water to soften the gelatine and transfer on the material collodion side up.

The proofs should be developed within three or four hours after insolation, for the luminons action continues pretty actively in the dark, and this for a long time; thus: a proof rightly exposed in the morning behaves as one over-exposed if developed in the evening, and after a certain period either can not be developed or refuses to adhere on the support. However, the proofs can be kept for three weeks, may be more, before development, if the soluble bichromate be washed off, the tissue sponged and dried rapidly in the warm season. This capital improvement is due to Mr. Charles Bra.s.seur.

It has been said that before being applied on the support the proof should be immersed in water to soften the tissue. The time which it should be allowed to absorb water has an importance which must not be neglected. If it do not remain long enough to be soaked through, small invisible air bubbles are formed on its surface, and interposing themselves between the image and the support, form minute, brilliant, silver-like spots on the finished picture; and, if the temperature of the water is above 20 deg. C.

(68 deg. Fahr.), the image will be more or less reticulated. The temperature depends a good deal of the softness of the gelatine; 15 deg.

C. (59 deg. Fahr.) is safe, except, however, when the thermometer is in the thirtieths (90th Fahr.), when the water should be cooled down a few degrees lower, but not at the melting ice temperature, for then the proof would not adhere well. As a rule, the tissue should remain in the cold water until it becomes flat and shows a tendency to curl up. It is at this very moment that it should be squeezed on the support.

The proofs should not be developed immediately after transferring. The adherence is greater and the pictures finer and devoid of defects when the development is made half an hour, and even an hour, after. If developed too soon the picture will be partly, and even entirely, washed off.

Hence, a number of transfers can be prepared beforehand, placing them, face to face one upon another, in order that the tissue does not dry, which is quite essential.

To develop, the plate, with the tissue adhering to it, is placed in water heated to 30 deg. C. (80 deg. Fahr.), where it is left rocking the tray occasionally until the paper rises up by itself at the corners, when taking hold of it by one corner, it is stripped off, leaving behind the image buried in soluble gelatine. Should the paper offer any resistance whatever, the gelatine should be allowed to become more soluble by increasing the temperature of the water, or by a longer immersion. There is, in fact, no objection to this. The plate-and that is a good method-can be placed in an upright position in a tin box, made ad hoc, and left therein in warm water until the paper detaches itself and the image is partly developed _and the bichromate washed off._ This done, the plate is held in an inclined position on a tray filled with water at 35 deg. C.

(95 deg. Fahr.), which is dashed with a wooden spoon on the image to clear it from the non-acted-on gelatine. Presently one can judge whether the exposure is right. If it is too short, the half tints in the shadows are washed off, unless the negative be too intense, when a similar effect also occurs in the whites. If it is too long, either the image is with difficulty cleared or remains undeveloped. In the latter case, it is recommended by some operators to increase the temperature of the developing water to near the boiling point, and, for local clearing, to pour it on. This we find objectionable, for the half tints are easily washed off. A better process, when the picture can not be cleared by water at 50 deg. (122 deg. Fahr.), or thereabout, is to use a solution of common salt at 5 or 6 per cent. of slightly warm water.(31) It is even preferable to finish the development in a tepid solution of pota.s.sium sulpho-cyanide, 12:100. The dissolving action is long, but not only, as said above, the half tints are best preserved, but blistering and local washing-off are avoided.

After development the plate is rinsed under the tap, then flowed two or three times with a solution of chrome alum at 1 per cent. of water, then washed, and finally allowed to dry spontaneously.

It is objectionable to use a strong solution of alum, and in it to immerse the plate for any length of time; the gelatine is considerably hardened-which is not necessary-and more liable to crack by time in being thoroughly desiccated. We discard the common alum which we found liable to produce a slight reticulation.

Two defects are complained of by the beginners, viz., the want of adherence of the deep blacks, and, especially, the isolated and fine lines when the picture is a reproduction of an engraving, a drawing, etc., and the liability in half tone pictures of the delicate details being washed out. The first defects are avoided by pouring a solution of boric acid on the transitory support before applying the tissue and developing at a low temperature with salted water. The second from an imperfect knowledge of the properties of gelatine acted on by light in presence of a salt of chromic acid. One should bear in mind that the degree of solubility of gelatine so acted on, as also its degree of impermeability-which is important in certain processes of photogravure-is proportionate to the degree of insolation; thus, when not impressed, bichromated gelatine dissolves in water heated to about from 25 to 30 deg. C. (77 to 80 deg.

Fahr.), and when acted on between 30 and 100 deg. C. (86 to 112 deg.

Fahr.), according as to the degree of insolation, that is, of reduction of the chromic salt, the latter temperature being that of insolubility of the parts the most acted on. The very delicate half tints do not, generally, stand a temperature higher than 35 deg. C. (95 deg. Fahr.), and, therefore, as the degree of insolubility of the various parts cannot be ascertained, a priori, it is advisable during the development to increase gradually the temperature of the water from this degree, and not to exceed 45 deg. C. (113 deg. Fahr.), in order to obtain the most perfect result from a negative of good intensity. Indeed, by placing the supports on a rack and immersing the whole in water heated to 30 to 35 deg. C. (86 to 95 deg. Fahr.), the image will clear up by itself to perfection in a certain period. This method is excellent for proofs in lines. Those from the grained negatives employed in photogravure are still more perfectly developed in a tepid solution of pota.s.sium sulphocyanate, since the impressions wholly consist of insoluble parts (the lines) and gelatine not acted on.

_Retouching.-_The retouches are easily made. They should be done before transferring when working by the double transfer process.

The transparent spots, and any parts which should be altered, are retouched with the material of the tissue dissolved in warm water; the whites are cleared with a sc.r.a.per; and any parts which are not intense enough, or which should be blended by the addition of half tints, are worked on the proof-to which a tooth has been given by rubbing with cuttle-fish powder-by means of a stump and an appropriate color, a mixture of lamp-black and carmine, for example, in very fine powder.

The proofs can also be colored by chemical means (see further on), or with water colors employed with a solution of chrome alum, 1 to 200 of water, or gilt, silvered or bronzed with metallic powders applied with the gilder's size thinned with turpentine on the proof previously coated with a thin layer of alumed gelatine.

_Second Transfer.-_To transfer, a sheet of enameled or simple transfer paper is immersed in tepid water until the gelatine is softened and feels slippery to the fingers. The support is then placed under water at ordinary temperatures-not under 16 deg. C. (60 deg. F.)-for two three minutes, then rubbed with a camel brush to remove the air bubbles, which might be formed on the surface of the image, when, without draining, the gelatinized paper is laid upon it, covered with the thin oil cloth, and pressed into contact with the squeegee, commencing in the center to the sweep off the water, then repeating the operation for the other half, as explained to apply the tissue on the provisory support. When the whole is quite dry, which requires three or four hours, the edges are cut with a penknife and the whole stripped off. It may happen that the proof is covered with minute, silver-like brilliant spots, which are nothing else than very small air bubbles interposited between the carbon proof and the transfer paper. They are caused by the gelatine paper not having been sufficiently softened or not laid on the proof with proper care. The defect may also arise from the transfer paper coated with not sufficiently thick gelatine.

To transfer on any rigid material, the proofs on flexible supports are coated by floating on the following gelatine solution, then allowed to dry, and, when wanted for use, immersed in tepid water to soften the gelatine and secure adherence:

Gelatine 50 parts Water 400 parts Solution of chrome alum, 6 parts 4:100

_Development on Absorbing Materials.-_The development of carbon prints on absorbent material-such as canvas and palettes to be painted in oil, etc.-cannot be made in the ordinary manner on account of the impossibility to eliminate entirely the chromic salt which tinges the material yellow.

To turn the difficulty, it suffices to wash off in several changes of cold water all the unaltered bichromate from the prints on their removal from the printing frame, and to proceed as usual, or the prints can be allowed to dry and transferred at some future time.

Canvas should be prepared by brushing with a solution of aqueous ammonia in alcohol, 5:20, to remove greasiness until the thread is apparent, and, when dry, rubbed with sand to grain it-or to give a tooth, as it is termed-then rubbed dry with a solution of soluble gla.s.s, 1 to 10 of beer.(32)

Palettes should be rendered impervious, or nearly so, by flowing upon them a solution of alumed gelatine, which is allowed to penetrate into the pores of the wood and the excess sc.r.a.ped off when solidified, when the surface may be whitened, if necessary, as for printing on wood box, q.v.

Opals, porcelain, or ivory should be prepared with the following substratum:

Gelatine 50 parts Water 400 parts Chrome alum, 4:100 6 parts

Very fine carbon proofs having the appearance of pictures on opal plates are made by transferring in the following manner, devised by the author:

Develop on the ground surface of a gla.s.s or porcelain plate, well waxed, to obtain a matt picture, or in the ordinary manner for "full gloss," and when the image is retouched or colored, apply a thin coating of gelatine, let dry and coat with the following opaque collodion:

A. Ether, conc. 100 parts Alcohol, 95 deg 90 parts Pyroxyline 7 parts B. White zinc in very 9 parts fine powder Castor oil 3 parts Alcohol 10 parts

Grind in a mortar, adding ultramarine blue and carmine, or a little of any suitable coloring matters, and mix to A. When the collodion is dry, which requires a few hours, strip the whole or back with strong white or colored paper before stripping. A solution of gelatine with glycerine, white zinc, etc., may be subst.i.tuted for collodion when the pictures are employed as ornaments on wood, etc. Carbon prints on celluloid are now made for similar purposes.

OPAL GELATINE SOLUTION Gelatine 150 parts Glycerine 15 parts Zinc, white 40 parts Water 600 parts

To which some coloring matters may be added according to taste. Grind the white with the glycerine and a little water, mix to the gelatine dissolved in the remainder of water, and filter through canvas. Apply the mixture moderately hot, 30 deg. C. (86 deg. Fahr.)

_Transparencies.-_The transparencies are printed on a special tissue sold under the name of "diapositive." It differs from the ordinary tissue in this, that the mixture contains a greater quant.i.ty of the color matter, India ink, which is ground exceedingly fine.

The proofs for transparencies should be printed deeper than those to be seen by reflection, and developed on thin gla.s.s plates, free from any defects, and coated with either one of the following substrata:

Soluble gla.s.s 5 parts White of eggs 15 parts Water 20 parts

The whole is beaten up to a thick froth and allowed to subside, when the clear liquid is decanted, filtered through flannel and the gla.s.s plates coated. The substratum should be allowed to dry for a few hours, and rinsed under the tap before use.

The other substratum consists of