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 Cox'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.)

[pg 87]

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 quantity 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 space at [pg 88] 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, 18×22 inches.

On a glass 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 quantity of gelatine necessary to coat the paper is regulated by means of a glass 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 glass 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 potassium bichromate. The degree of concentration of the bath, which varies from 2 to 5 per cent. of water, is important. The [pg 89] 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 potassium 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 quantity 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 [pg 90] 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 glass rod fixed at the end of the tray, and placed, prepared side down, on a slightly waxed glass 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 glass.

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 clichÉ; but by [pg 91] holding it over a basin of boiling water, the steam in a few moments rendering it sufficiently pliable to lay it flat between glass 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 glass plate is rubbed with talc, or, which the writer prefers, flowed with a solution of28

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 quantity 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.

[pg 92]

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 glass plate, 6×2 inches, make four or five negatives, 1½×1¼ 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 glass plate of the same size as the first and border like a passe-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, [pg 93] 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 clichÉ 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 [pg 94] 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 glass, 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.

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 glass 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 glass plates permit one to follow the progress of [pg 95] 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 striÆ. 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 glass, 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.

[pg 96]

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 albumen 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 glass, 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 pass 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 shellac 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 shellac.

When the picture must be transferred on small spaces 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.

[pg 97]

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 Brasseur.

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 [pg 98] 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 potassium 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.

[pg 99]

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 [pg 100] in a tepid solution of potassium 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 scraper; 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. [pg 101] 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, 4:100	6 parts

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 glass, 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 scraped 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

[pg 102]

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 glass 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 fine powder	9 parts
	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 substituted 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 quantity of the color matter, India ink, which is ground ex­ceedingly fine.

[pg 103]

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

Soluble glass	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 glass 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

Gelatine	35 parts
Acetic acid, No. 8	250 parts
Alcohol, 95 deg	50 parts
Water	700 parts
Chrome alum, 4:100	60 parts

Dissolve the gelatine in the acid at a moderate heat, add afterwards the alcohol and water, and lastly mix the chrome alum by small quantities at a time.

These substrata are employed to avoid the peeling off of the image. To prevent the entire desiccation of the gelatine, which is the cause of the defect above alluded to, it is advis­able to add glycerine to the washing water after the image is cleared. Some operators recommend a coating of flexible collodion, that is, prepared with castor oil, for the purpose in question. We do not think that necessary when the trans­parencies are not exposed to sunshine. If anything should be applied we would prefer the encaustic.

Carbon transparencies are invaluable for reproducing nega­tives in the original size by the same (carbon) process, or for enlarging by the collodion or gelatine process. For these pur­poses they should be made on the special red tissue manufac­tured by the Autotype Company, of London, Eng. They can, however, be made on the ordinary tissues.

Whatever be the tissue employed, the transparencies for the reproduction of negatives are seldom opaque enough, and should be intensified. This is done by treating them with a [pg 104] very dilute solution of sodium permanganate, which colors them olive green.

Transparencies for lantern slides, etc., are best colored with the couleurs À l'albumine of L. Encausse, sold by J. Reygondaud, Paris (France). They are transparent.33

Toning and Intensifying.—The carbon proofs can be toned and at the same time intensified by reagents acting with chromic oxide.

The dyes or coloring matters precipitated are not opaque, and, as a consequence, not objectionable for transparencies. The following processes are the most employed:

Prepare three solutions as follows:

A.	Ferric sulphate	5 parts
	Water	100 parts
B.	Sodium carbonate	2 parts
	Water	100 parts
C.	Gallic acid	5 parts
	Water	100 parts

Dissolve the gallic acid in warm water. Filter each solu­tion. They keep well.

To tone, the plate is immersed for, say, ten minutes in A, then, after rinsing slightly, it is placed in B for the same period, rinsed again and flowed with C until the desired color is obtained. The tone is a splendid purple black color. If a solution of pyrogallol be substituted to that of gallic acid, the tone is green, and to a green bordering to black when a solu­tion of catechu is used, the catechu exerting at the same time a tanning action on the gelatine. After toning, the plate should be thoroughly washed.

A similar process consists to wet the plate under the tap, then to flow over a mixture by about equal volumes of

A.	Ferrous sulphate	5 parts
	Acetic acid, No. 8	5 parts
	Water	100 parts, filter
B.	Gallic acid	5 parts
	Water	100 parts

[pg 105]

When toned, the plate is well washed, then flowed once with the alum solution and again washed. The tone by this process easily turns to an inky blue not very agreeable. The action should be stopped a little before the desired color is obtained.

It sometimes happens that the image in drying intensifies more then necessary. It can be cleared with a solution of oxalic or citric acid.

A brown sepia is obtained by toning first with potassium permanganate, 1 per cent. of water, then, after washing, with a solution of pyrogallol. If gallic acid be used instead of pyrogallol, the tone is black. By this process a great intensity is obtained. A dilute solution of ammonium sulphide can be employed as a clearing agent.

Pyrogallol and silver nitrate give a warm black tone.

Potassium bichromate followed by silver nitrate form a brick-red precipitate of some opacity.34

Chloride of nickel and potassium ferrocyanate produce a fine brown.

Lime water and alizarine dissolved in alcohol dye violet.

Alizarine and the caustic alkalies produce a variety of tints, from violet to purple, according to the concentration of the solutions.

Lead acetate and alizarine in ammoniacal solution dye purple.

Potassium ferrocyanide and uranium nitrate produce a warm sepia tone. With chloride of nickel the tone is brown.

Ammoniacal solution of coralline diluted with water gives carmine red.

Potassium bichromate and extract of indigo produce a fine greenish tone suitable for landscapes.

Extract of indigo colors blue35

Some of these reactions can be applied to the printing processes with the bichromates, etc. The paper should be coated with galatine. See the Appendix.

Other colorations can be obtained with dyes in utilizing (as shown by Persoz) chromous chromic oxide as a mordant: alizarine, Brazil and yellow wood (morus tinctoria), [pg 106] Fustet (rhus cotinus), etc. The extent of this work does not admit of describing the numerous processes which can be employed; they will suggest themselves to the chemist.

The alkalies employed with the dyes should be employed in diluted solutions, as being liable to produce reticulation. By applying the coloring matters and the mordants thickened with a little starch, the image can be colored with different colors. Lantern slides can be thus colored with great ease.