Photographic Reproduction Processes

The Platinotype

This process, discovered by William Willis,16 yields very fine impressions which wholly consists of platinum and are, therefore, chemically permanent. It has been described theoretically and practically by Pizzighelli and Kübl in a paper for which the Vienna Photographic Society has awarded the Voightlander prize.17 The following is an abridgment of this important process, as described by the authors:

The paper, calendered or not,18 is sized with gelatine or arrowroot. The color of the proof with the latter size is brownish black, and bluish black with the former.

To prepare the gelatine solution 10 parts of gelatine are soaked in 800 parts of water and then dissolved at a temperature of 60 deg. C. (140 deg. Fahr.), when 200 parts of alcohol and 3 parts of alum are added and the solution filtered.

To prepare the arrowroot solution 10 parts of the substance are powdered in a mortar with a little water and mixed to 800 parts of boiling water, added gradually in stirring. After boiling for a few minutes 200 parts of alcohol are added and the mixture filtered.

These solutions are employed warm. The paper is immersed for two or three minutes and hung up to dry in a heated room, then immersed a second time and dried by hanging it up in the opposite direction, in order to obtain an even coating.

The potassic platinic chloride is an article of commerce. It should be soluble without residue in 6 parts of water and without acid reaction. In this proportion it constitutes the normal stock solution employed in the various formulas.

The standard ferric oxalate solution is also found in commerce. Treated by potassium ferricyanate it should not be colored blue, nor become turpid when diluted with one-tenth part of water and boiled. The former reaction indicates that it contains no ferrous salt, and the latter no basic oxalate.

The authors give the following instructions for preparing the ferric oxalate solution, to which they attach much importance:

Five hundred parts of ferric chloride are dissolved in 5,000 parts of water and heated to boiling, when a solution of soda is added until the liquid becomes alkaline.19 About 250 parts of caustic soda are generally employed for this purpose. The precipitate—ferric oxide—is now washed in warm water until the last washing water is quite neutral to test paper, then drained and mixed with 200 parts of pure crystallized oxalic acid. The mixture is then allowed to stand in the dark for several days at a temperature not exceeding 30 deg. C. (86 deg. Fahr.) At first the solution from green turns to a yellow green, and finally becomes almost brown. At this moment the excess of ferric oxide is filtered out and the liquor submitted to a quantitative analysis, the result of which leads to ascertain the quantity of ferric oxalate in 100 parts of the solution and the excess of oxalic acid. The solution should then be diluted with distilled water, such as it contains 20 parts of ferric oxalate per 100 parts of water, and oxalic acid must be added in the proportion of from 6 to 8 per 100 of the ferric oxalate, taking into account the quantity of acid the solution already contains. The solution should be kept in the dark. It is altered by light.20

IRON CHLORATE SOLUTION
Ferric oxalate solution	100 parts
Potassium chlorate	0.4 parts

This solution is employed to obtain more contrasts.

PREPARATION OF THE SENSITIZING SOLUTION
Platinum solution	12 parts
Ferric oxalate solution	11 parts
Distilled water	2 parts

This solution gives very soft tones with intense black. To obtain more brilliancy we use the following proportions:

Platinum solution	12 parts
Ferric oxalate solution	9 parts
Chlorate of iron solution	3 parts
Distilled water	2 parts

To obtain results comparable to those which the silver printing out process gives, the following mixture is employed:

Platinum solution	12 parts
Ferric oxalate solution	8 parts
Chlorate of iron solution	4 parts
Distilled water	8 parts

For very weak negatives, reproductions of drawings, etc., we use—

Platinum solution	12 parts
Chlorate of iron	11 parts
Distilled water	2 parts21

To obtain proofs not completely black, as, for example, reproductions of lead drawings, the solution may be diluted with half or the whole volume of distilled water. But if the solution be applied on little absorbent surfaces or on paper strongly sized it is not advisable to dilute it.

Preparation, of the Paper.—The paper should be kept slightly moist in order that it does not too completely absorb the sensitizing solution. Therefore, when the atmosphere is very dry, it is well to keep the paper in a damp place, in the cellar for example. Before sensitizing, which should be done by a very diffused light, a quantity of the solution proportionate to the surface to be sensitized (about 15 c.c., for a whole sheet of Rives’ or Saxe paper) must be measured, and spread with a large brush22 on the paper fixed with drawing pins on a board covered with a sheet of blotting paper. When well impregnated, the paper is hung up to dry in the dark room, and as soon as the apparent dampness of the surface has disappeared, it should be dried immediately at a temperature of 30—40 deg. C. (86—101 deg. Fahr). If the paper be dried too rapidly the sensitive compound remains on its surface, and in developing the image does not come out well. If, on the other hand, the drying is too slow, the solution penetrates too much in the paper and the image is wanting of vigor and does not appear very sharp. One cannot depart from this rule that the desiccation from the moment the solution has been applied until the paper is dry should last no more than from twelve to fifteen minutes.

The sensitized paper is hygroscopic and must be preserved in a calcium box. It is a conditio sine qua non that the paper must be quite dry before, during, and after printing, to obtain good results. Dampness is the greatest enemy in this process.

For printing a pad of India rubber should be placed over the platinum paper to prevent it from attracting the atmospheric moisture, and in damp weather it is even advisable to cover it with several sheets of blotting paper previously heated before the fire.

The platinum paper is at least three times more sensitive than the silver paper used in the printing-out process, under the reductive action of light the yellow color of the prepared paper turns brown and then becomes of a lighter color, nearly orange, so that the darker parts of the image often appears more luminous than the dark half tints. No rule can be given to regulate the insolation, but after a few trials it is easy to judge when it is right by observing the progress of the reduction and the color of the image. The orange color indicates the complete reduction of the ferric oxalate. When the details in the lights are faintly visible, the exposure is generally right.

The developer consists of an almost saturated solution of potassium oxalate acidified by oxalic acid, and for use heated to 80—85 deg. 0. (176—184 deg. Fahr.),23 in an agate glazed iron tray placed upon a water bath at the above temperature. By simply drawing the proof over it, the image is at once developed.24

When the proof is thought to be over-exposed, the oxalate solution can be employed at a lower temperature. If, on the contrary, it is under-exposed, the solution may be heated even to the boiling point.

The developer can be used over and over again. It should always have an acid reaction.

According to Mr. Borlinetto a sepia tone is obtained by using the following cold developer:

Saturated solution of potassium oxalate	120 parts
Saturated solution of copper chloride	13 parts
Oxalic acid	1.5 part

After developing the proofs are immediately immersed for fixing in a solution of hydrochloric acid, 1 to 80 of water, renewed so long as the paper is tinged yellow (about three times), leaving the proofs ten minutes in each solution. Lastly, they are washed to remove the acid.

The platinotype has been still improved by Captain Pizzighelli, who devised the following methods of operating by which the impressions are obtained by the continuous action of light, that is, without development, thus rendering the platinotype just as simple as the ordinary printing-out silver process.

In these new processes to the sensitizing solution is added the alkaline oxalate, which effects the reduction of the platinous salt during the exposure to light. Consequently the prepared paper is insolated until the image appears as it should be, or—which is exceedingly useful in cloudy weather—until it is entirely visible but still deficient in delicate half tones, for in the dark the action proceeds and the image developing itself will be found finished in a period which may extend to a few hours. But it can be, however, developed in a few seconds by immersion in a cold or slightly warm solution of sodium carbonate, 1:25 of water. The image is fixed as directed in the foregoing process.

The paper, prepared exactly as in the former process and kept in the calcium box until wanted for use, should not be employed quite dry, but allowed to absorb a little moisture by hanging it in the dark room. Hence, the India rubber and other protecting pads can be dispensed with. They are even objectionable, for dampness is absolutely necessary to promote the chemical changes by which the image is developed.

A. AMMONIO-FERRIC OXALATE SOLUTION
Ferric oxalate solution	100 parts
Neutral ammonium oxalate	18 to 20 parts

B. SODIO-FERRIC OXALATE SOLUTION
Ferric oxalate solution	100 parts
Neutral sodium oxalate	15 to 18 parts

To prepare these two solutions the ammonium or sodium oxalate is dissolved by small quantities at a time, and when the emerald color due to the formation of the double oxalate commences to darken, the saturation being then complete, no more of either salt should be added. The solution is now well shaken with 3 parts of glycerine, allowed to settle and filtered.

Any one of the double oxalates can be used. The ammonium tends to produce softer pictures and bluish tones. To obtain more contrasts a little potassium chlorate may be added.

C. IRON CHLORATE SOLUTION
Solution B	100 parts
Potassium chlorate	0.4 part

D. MERCURIC SOLUTION.
Mercuric chloride solution at 5:100	20 parts
Sodium oxalate solution at 3:100	40 parts
Glycerine	2 parts

SENSITIZING SOLUTIONS.
FOR BLACK TONES.
Platinite solution, 1:6	5 parts
Solution B	6 parts
Solution C	2 parts

FOR SEPIA TONES.
Platinite solution, 1:6	5 parts
Solution C	4 parts
Solution D	4 parts

Intermediate tones are obtained by diminishing the dose of C and replacing it by an equal volume of B. For this process the paper should be sized with

Arrowroot	2 parts
Sodium oxalate at 3:100	100 parts

To dispense with this preliminary sizing Captain Pizzighelli adds gum arabic to the platinite solution, whereby the sizing and sensitizing are done in one operation.

The gum arabic solutions are prepared as follows:

E.	Gum arabic in powder	40 parts
	Sodium ferric oxalate solution, B	40 parts
	Sodium oxalate solution at 3:100	100 parts
	Glycerine	3 parts

Place the glycerine and the gum arabic in a mortar, then, stirring with the pestle, dissolve by adding, little by little, the mixture, heated to 40—45 deg. C. (104—113 deg. Fahr.), of the solution of sodium ferric oxalate and sodium oxalate. Let stand for about two hours and grind again to dissolve entirely the gum arabic. Filter through muslin.

F. Mercuric chloride solution, 5:100	20 parts
Sodium oxalate solution, 3:100	40 parts
Gum arabic in powder	24 parts
Glycerine	2 parts

Dissolve as said above.

SENSITIZING SOLUTIONS.
FOR BLACK TONES.
Platinite solution, 1:6	5 parts
Solution E	6 parts
Solution C	2 parts

FOR SEPIA TONES.
Platinite solution, 1:6	5 parts
Solution C	4 parts
Solution F	4 parts

Mix just before use. The solutions do not keep. The paper prepared by either one of these two processes can be exposed as in the old process, and the image developed bythe hot oxalate solution.

The preparation of wood, canvas, etc., for the platinotype printing need not to be described; it suggests itself.

Causes of Failures

The images are veiled.

This defect may result from various causes, viz.:

1st.	The stock ferric oxalate solution is impaired by a partial reduction of the ferric salt into ferrous oxalate. The solution should be preserved in an orange colored vial, and kept in the closet of the dark room. It should be tested from time to time for the ferrous salt with a solution of potassium ferricyanate. If it does not contain any ferrous oxalate it can be used by adding to it a little of the iron chlorate solution.
2d.	The paper has been exposed to light during the sensi­tizing or the subsequent operations. One should bear in mind that the platinum paper is twice more sensitive than silvered paper.
3d.	The sensitized paper has been dried at a temperature above 40 deg. C. (104. deg. Fahr.)
4th.	Over-exposure.

The proofs are not sharp.

1st.	The sensitive paper has absorbed moisture.
2d.	It is too old. The paper cannot be kept good for over six weeks, unless special care be taken.

According to Mr. Bory, the sensitive paper altered by keep­ing is restored to its original good quality by simply brushing it over with a solution of 0.05 parts of potassium chloride or the same quantity of potassium chlorate in 100 parts of dis­tilled water, or a mixture of these two solutions, or one of iron chlorate.

By treating the insolated paper with these solutions, the image is destroyed, and the paper can be used again. One operates as for sensitizing, taking care to desiccate the paper, as it has been directed.

The proofs are brilliant during the development, but become dull in drying.

The paper not well sized. It has been dried too slowly.

Remember that it should be quite desiccated within fifteen minutes.

The paper is more or less yellow.

1st.	The paper tinted with ultramarine.
2d.	The sensitizing solution or the developer are not suffi­ciently acid.
3d.	The washing (fixing) in the solution of hydrochloric acid was not sufficient to eliminate the iron salts from the paper.

The proofs harsh, devoid of half tones.

1st.	The sensitizing solution contains too much iron chlorate.
2d.	Exposure too short.

The paper is stained.

The brush not kept clean while sensitizing.

Black spots.

They are generally due to metallic dust in the paste of the paper, or from particles of undissolved salt in the platinite solution.

NB: No good results can be expected unless the paper be kept absolutely dry before, during and after exposure, when using the former (original) process.

Impaired sensitiveness of the paper, want of vigor, tinged whites, muddiness, indicate dampness.

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Artigues’ Process

The Artigues process, so called, is, without any doubt, the best to be employed for the reproduction of plans and drawings in lines. It is simple, expeditious, and yields black impressions on a very pure white ground which are absolutely permanent. And this is of the utmost importance when the copies are to be used for military purpose, or kept in archives, such as those of the Patent Office, for example. Should it not require the use of negative clichés, it would certainly super­sede any of the processes previously described; moreover, as it will be seen, it can be employed for many other purposes than that of obtaining duplicates from original drawings. The objection is not even very great indeed, for the design can be, without great trouble, transformed into a negative by the aniline method described in the beginning of this work.

The Artigues process is an adaptation for the purposes in question of the carbon process invented by Poitevin. We shall describe it in extenso.

The paper can be prepared with any one of the following solutions:

1st.

Dissolve 2½ parts of ammonium bichromate and 5 parts of best gum arabic in 15 parts of water and neutralize with a few drops of concentrated aqueous ammonia; then add 100 parts in volume of whites of egg and a certain quantity of thick India ink, and, this done, beat the whole to a thick froth. In ten or twelve hours the albumen will be deposited and ready for use.

The quantity of India ink added to the albumen should be such as the paper be black when coated, but, however, sufficiently transparent for one to see the shadow of objects placed on the back of it, and the coating should not be thick. This is impor­tant in order to allow the light acting through the whole thick­ness of the preparation when the paper is insolated under the cliché, for, if the film be too opaque or too thick (by addition of too much gum arabic), it would be only impressed on its surface, and the image dissolved during the development. The cause of this failure must be explained. Under the action of light the bichromate employed to sensitize the albumen is reduced into chromic oxide which render insoluble this organic substance—or any other, such as caseine, gelatine, gum arabic, etc.; therefore whenever the film is not acted on in its whole thickness, the subjacent part being still soluble, is necessary washed off and with it the superficial impressed part, that is, the image.

2d.

Take 10 parts of lamp black and work it up in a mortar to the consistency of a thin paste by gradually pouring a little of a solution of from 6 to 8 parts of gum arabic and 1 part of liquid glucose in 100 parts of water, adding afterwards the remainder, into which 2½ parts of ammonium bichromate have been dissolved, and filter through flannel. With this, coat the paper by brushing so as to form a thin and uniform film, and pin it up to dry in the dark.

These solutions keep well for a certain period. We have kept the albumen, which we prefer to use, for two months in good condition; but the sensitive paper does not for more than three or four days in taking the usual care. It is more practical—and this is recommended—to leave out the bichrom­ate from the preparations, and to coat the paper, in quantity, beforehand, and for use to sensitize it with a solution of potassium bichromate at 3½ per cent. of water applied on the verso with a Buckle brush.25

The bichromate solution should be allowed to imbue the paper for about one minute, and having brushed it once more, the paper is pinned up to dry in the dark room. It can also be sensitized from the back by floating, if this manner is found more convenient.

When dry the paper is impressed under a negative cliché of good intensity until the design, well defined in all its details, is visible on the back of the paper, which requires an insolation of about two minutes in clear sunshine, and from eight to ten times longer in the shade. In cloudy weather the exposure to light is necessarily very long.

As explained before, the luminous action, by reducing the chromic salt in presence of certain organic substances, causes the latter to become insoluble; consequently if, on its removal from the printing frame, the proof be soaked in cold water, for, say, ten minutes, and, placing it on a glass plate or a smooth board, gently rubbed with a brush or a soft rag, the parts of the albumen or gum arabic preparation not acted on will dissolve, leaving behind the black image standing out on the white ground of the paper. This done, and when the un­reduced bichromate is washed out in two changes of water, the operation is at an end.

As to the theory of this and similar processes, the insolubilization of the bichromate organic substance acted on by light was formerly attributed to the oxidation of the substance by the oxygen evolved during the reduction of the chromic salt into chromic oxide; but from the fact that oxidation generally tends to destroy organic matters, or to increase their solubility, it is more probable that it results from the formation of a peculiar compound of the substance with chromic oxide (J. W. Swan); moreover, gelatine imbued with an alkaline bichro­mate, then immersed first in a solution of ferrous sulphate and afterwards in hot water, is insolubilized with forma­tion of chromium trioxide, Cr2O7K2+SO4Fe = SO4K2+C2O4Fe+C2O3 (Monckhoven). A similar but inverse action occurs, as shown by Poitevin, when gelatine rendered insoluble by ferric chloride becomes soluble by the transformation, under the influence of light, of the ferric salt into one at the minimum.

The writer has improved the above process by simplifying the modus operandi as follows:

Instead of compounding the preparation with gum arabic and the coloring matter, the albumen is simply clarified by beating the whites of eggs to a froth, etc., and the paper is coated by floating for one minute, then hung up to dry in a place free from dust.

If the reader has any objection for albumenizing his own paper, he can use the albumen paper found in the market for the printing-out silver process generally employed by photographers.

The paper is sensitized from the back with the potassium bichromate bath by floating or by brushing. When dry, it is exposed as usual, but for a shorter period than when the preparation contains the India ink or other coloring matters which impede the action of light.

The progress of the impression is followed by viewing, from time to time, the albumenized side of the paper. When the design is visible, well defined and brownish, the proof, being removed from the printing frame, is rubbed with very finely powdered, or, better, levigated graphite, and, this done, immersed in cold water for from fifteen to twenty minutes, when by gently rubbing it under a jet of water with a soft rag, or with a sponge imbued with water, the albumen is washed off from the parts not acted on, leaving the design on a perfectly white ground.

If instead of graphite, or any dry color insoluble in water, lithographic ink, much thinned with turpentine oil, be applied on the print in a light coating which permits one to see the design under it, and if, then, the print be soaked in water and afterwards developed as just directed, an image in greasy ink is obtained. And, furthermore, by replacing the printing by transfer ink, one readily obtains a transfer ready for the stone or a zinc plate to be etched in the ordinary manner.

As usual there are two causes of failures in these processes, viz., under and over-exposures. In the former case the image is partly washed off; in the latter the ground cannot be cleared. The reasons are obvious.

Mr. de Saint Florent gives the following processes:26 A sheet of albumenized or gelatinized paper is sensitized from the verso on a solution of potassium bichromate, dried in the dark and exposed under a positive cliché. After insolation, the proof is washed in water, to which are added few drops of ammonia, then inked all over with an ink consisting of 100 parts of liquid India ink, 7 parts of sulphuric acid and 3 parts of caustic potassa, and dried in a horizontal position. When quite dry, the proof is placed in water, and after an immersion of about ten minutes, rubbed with a soft brush: the image little by little appears, and if the time of exposure be right, it is soon entirely cleared, and, then, if not enough vigorous, it may be inked again. The gloss of the image is removed by means of a solution of caustic potassa at 10 per 100, and the proof finally washed with care.

If in lieu of albumen 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 clichés 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 potassium bichromate, and, when dry, exposed from the verso to the action of light. This done the image is cleared with a somewhat hard brush.

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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 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.)

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 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.

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 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 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 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.

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, 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 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 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.

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.

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 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.

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 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. 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

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.

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 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

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), 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.

Preparation of Red, Yellow, or Blue Tissues

Red Tissue.—Dissolve 10 grams of carmine in 1 liter of aqueous ammonia and evaporate. When the smell of the alkali has almost disappeared, add 1 liter of rain water. Of this take 65 cubic centimeters, add 35 c.c.m. of rain water, and in the solution let soak for an hour 15 grams of very soluble gelatine, add 1 gram of sugar, and dissolve in a water bath. Filter, and take of the mixture a sufficient quantity (25 c.c.m. for a surface 18×24 centimeters) to cover a sheet of paper which has been previously applied upon a glass plate in the following manner: In a tray full of hot water, immerse the plate and the paper; remove the whole in such a manner as the paper remains in contact with the plate; rub out the excess of water with a squeegee, and flow the gelatine over the paper still damp. Let cool on a leveled stand, and when the gelatine is solidified to a consistent jelly, remove the paper from the plate and place it to dry in an oven heated at not over 24 or 25 deg. C.

It is desirable that in drying the paper does not curl up. To that end, apply over it, before it being removed from the plate a wooden frame to which the gelatine, still sticky, will sufficiently adhere to hold the tissue when it stretches in drying.

Yellow Tissue.—Pulverize to an impalpable powder 25 grams of light chrome yellow in tablets (water color), and gradually add in stirring 1 liter of rain water. Take 100 c.c.m. of this and into it let soak for an hour 15 grams of the same gelatine used for the red tissue, add 1 gram of sugar, then proceed as above.

Blue Tissue.—In a liquid consisting of 85 c.c.m. of rain water and from 12 to 15 c.c.m. of blue ink, such as sold by stationers, let soak for an hour 15 grams of the same gelatine and 1 gram of sugar, and proceed.

Preparation of Transfer Paper.—Two kinds of transfer paper are employed—the enamel and plain transfer paper.

To enamel the paper: Dissolve 100 parts of barium nitrate in 500 parts of water, and, on the other hand, 200 parts of sodium sulphate in the same quantity of water. Mix, wash well the precipitate—barium sulphate—by decantation, and when well drained, mix to the following solution:

Gelatine, Coignet’s	300 parts
Glycerine	80 parts
Ultramarine blue	1 part
Crimson lake	0.1 part
Water	2,500 parts

Let soak the gelatine for, say, one hour, dissolve by heat, then add by small quantities, stirring violently, 4 parts of chrome alum dissolved in 250 parts of hot water. Filter through flannel and coat the paper as directed to prepare the tissue. The mixture should be employed immediately after adding the chrome alum.

The plain transfer paper is prepared in the same manner, leaving out the barium sulphate and the coloring matters.

Preparation of the Silver Paper.—Immerse the paper for two minutes in a solution of—

Sodium chloride (common salt, dry)	2 parts
Lemon juice	1 part
Water	100 parts (filter)

When dry and wanted for use, sensitize the salted paper by floating for one minute on—

Silver nitrate	8 parts
Nitric acid	0.1 part
Water	100 parts

On its removal from the silver bath, sponge the paper between sheets of blotting paper and hang it up to dry.

ENCAUSTIC FOR SINGLE TRANSFER PROOFS.
White wax	25 parts
Mastic	3 parts
Turpentine	100 parts

Dissolve by heat, first the mastic, then the wax, and keep for use in a large mouthed vial.

MATT VARNISH.
Sandarac	6 parts
Mastic	6 parts
Lavender oil.	0.5 parts
Ether	100 parts

When dissolved, add 30 parts of benzine. The opacity of the film varies with the quantity of benzine added; by excess the varnish dries transparent.

WATER COLORS WHICH RESIST THE ACTION OF LIGHT.
Red.	Indian red.	Light red.
Orange.	Mars yellow.
Blue.	Cobalt blue.	French blue.	Smalt.	New blue.
Brown.	Raw umber.	Burnt sienna.
Green	Terre verte.
Yellow.	Cadmium yellow.	Yellow ochre.	Roman ochre.

---

Appendix

Although we intended to only describe the printing processes without the use of silver salts, we thought it would be well to complete this work by giving the most practical and interesting processes ever published to obtain permanent photo­graphs; as they may give rise in the hand of experimenters to useful applications.

From time to time processes are published under “queer” names, which are based on the well known actions of reagents on the ferric salts reduced by light. They are derived from those described in the following pages.

We call specially the attention of the reader to the process of Poitevin, by which one can experiment with every ferric salts, citrate, lactate, oxalate, tartrate, benzoate, etc., by simply exciting with the corresponding acid. Observe that to obtain good results the paper should be strongly sized; it is a sine qua non, although not recommended by Poitevin.

C.J. Burnett’s Process(1857)

“A capital process for many purposes,” says Mr. Burnett, “is to float or steep the paper in a mixed solution of bichro­mate of potash and sulphate of copper, as for Hunt’s chromo­type process.36 I have mixed gelatine, or occasionally grape sugar, or both with the solution;37 but instead of developing it with nitrate of silver, as in chromotype, wash out the salt unaltered by light, and develop by floating on a solution of ferrocyanate of potassium. The purple red color of the copper salt which now forms the picture may be modified or changed in many ways,38 viz., by soaking the picture, after the ferrocyanate of potassium has been washed out of the lights, in a solution of sulphate of iron. Solutions of gallic acid, tannic acid with alkalies of carbonate, may also be employed to modify or change the color. This process has the advantage that one may regulate the exact tone (black or useful neutral tint) to the greatest nicety by the time we allow the print to remain in the iron toning bath.”

Godefroy’s Process (1858)

Float the paper upon the following solution for three minutes and hang it up to dry:

Uranium nitrate	30 to 60 parts
Silver nitrate	8 parts
Water	100 parts

39

The sensitiveness increases in proportion to the quantity of uranium nitrate. With the above formula the paper can be exposed in the camera, or, for printing, under a negative cliché.

In printing an exposure of five seconds in diffused light gives an image perfectly visible, and a grayish black tone; ten seconds gives a vigorous image almost of a black color; in from fifteen to twenty seconds the image is very strong, with the color of an engraving. In sunshine the action is neces­sarily much more rapid.

The impression is developed by immersion in

Ferrous sulphate	8 parts
Tartaric acid	4 parts
Sulphuric acid	1 part
Water	100 parts

The image is rapidly developed. It is fixed by washing in water.

De la Blanchere’s Process (1858)

Uranium nitrate	25 parts
Distilled water	100 parts

Filter the solution and keep it in the dark.

The paper should be sized with a gelatine solution at 5 per 100 of water, and, when dry, kept in the dark.40 It is sensi­tized by floating five minutes.

The exposure under a negative varies from fifteen to twenty minutes in the shade, and from one to three minutes, at the most, in sunshine. As a rule, it is advisable to somewhat under­expose in order that the development be regular, progressive, under control.

The image is developed by floating, or immersion in

Silver nitrate	2 parts
Distilled water	100 parts
Nitrate acid, C.P.	a trace

When the image is intense enough it is washed in several changes of water, then toned in a solution of gold at 1 per 1,000 of water acidified with traces of hydrochloride acid.41

The following bath develops slowly, and gives very rich purple tones without toning:

Nitrate of silver	3 parts
Nitrate of uranium	1 part
Nitrate of cadmium	1 part
Alcohol	10 parts
Water	100 parts
Nitric acid	traces

The developing solutions should be as little acid as possible, but not neutral, for then the proofs would be veiled and grayish.

The image can also be developed in a solution of gold, or in a very weak solution of mercuric chloride at 1 per 10,000. The proof must be extremely well printed and left for from two to five minutes in the mercuric solution. If the time of exposure is right, the image will change but little in the solu­tion, and will take, when treated with silver nitrate, the most splendid tones.

The proofs should be carefully washed when finished. If they were developed with silver, they must be immersed in diluted aqueous ammonia, which will perfectly clear the whites. If developed with chloride of gold, the water should be heated to 60 to 80 deg. C. (140 to 176 deg. Fahr.)

Houdoy’s Process (1858.)

The paper is floated upon a lukewarm solution of gelatine at 5:100, and when dry, on a bath of uranium at 10 or 15 per 100 of water. After exposure to the sun the image is developed with a solution of silver nitrate acidified with acetic acid. The exposure varies, according to the nature of the negative, from one to ten minutes; it must be long enough for the image being developed in from thirty to forty seconds. It is then removed from the silver bath and placed in the following:

Ferrous sulphate	3 to 8 parts
Acetic acid	2 parts
Water	100 parts

In this bath the image takes a great vigor and appears entirely on the surface of the paper. When the proof has been too long exposed it should be washed slightly before placing it in the iron bath. Developed, the image is, gener­ally, of a sepia tone, which can be turned to black by a solu­tion of chloride of gold, 1:1,000, washing afterwards as usual.

Nièpce de St. Victor’s Process (1859)

Red Prints

Float the paper for fifteen or twenty seconds on a 20 per cent. solution of nitrate of uranium and dry before the fire in the dark room. This paper can be prepared many days before use. Expose in sunshine from eight to ten minutes, according to the intensity of the light and the quality of the negative, then wash in moderately warm water (50 to 60 deg. C.) for a few seconds. This done, immerse in a solution of red prussiate of potash at 2 per cent. of water; in a few moments the proof will become of a fine blood-red color, like “sanguine.” Wash, etc.

Green Prints

Make a red print as above described, immerse it for a few minutes in a solution of nitrate of cobalt and dry it without washing. Fix then in a solution of sulphate of iron at 20 per cent. of water and 4 of sulphuric acid. Wash and dry before the fire.

Violet Prints

Prepare the paper in the uranium bath, expose, wash and develop in a solution of chloride of gold, 1:200, until the proof has assumed a fine violet color. Wash in several changes of water.

Blue Prints

Sensitize the paper with a red prussiate of potash solution at 20 per 100. Let dry, expose until the proof is slightly blue; immerse it for five or ten seconds in a saturated solu­tion of bichloride of mercury, wash only once and immerse in a solution of oxalic acid—saturated when cold—heated to about 55 deg. C. Wash in three or four waters and let dry spontaneously.

Black Prints

Float the paper on a mixture by equal volumes of a solution of iron perchloride and another of uranium nitrate, each at 10 per 100 of water. Expose and develop on a saturated solution of gallic acid.

Dr. T.L. Phipson’s Process (1861)

Take a solution of perchloride of iron and, having precipi­tated the peroxide with ammonia, collect the precipitate on a filter and wash it with boiling water. Add the precipitate in excess to a warm solution of oxalic acid. A beautiful emerald green solution is obtained, which must be a little con­centrated by evaporation and then set aside in a dark room for use. The paper is floated for ten (?) minutes upon the green solution of ferric oxalate, to which has been added a little oxalate of ammonia and hung up to dry in the dark.

Expose under a negative for from ten to twenty minutes, according to the weather, and wash well the paper with rain water. Spring water will not do on account of the lime it may contain, which will form oxalate of lime in the paper (insoluble). When all the non-decomposed oxalate is washed from the proof, a feeble image of oxalate of protoxide of iron, scarcely visible, is left on the paper. To develop it and to obtain the vigor, the tone and color of silver prints proceed as follows:

Plunge the proof for a little while in a (weak) solution of permanganate of potassium to which a few drops of ammonia have been added; in the bath the image becomes brown and distinctly visible. It is then withdrawn and immersed in a solution of pyrogallic acid for half an hour, after which it is washed and dried.

The image thus obtained can hardly be distinguished from silver prints; the tone is soft, brilliant and permanent.

This process is quite original and interesting. The theory is as follows: Under the action of light the ferric oxalate is reduced in the ferrous salt, insoluble, which, after the print has been cleared from the ferric oxalate, is oxidized and reduced into ferric oxide by the alkalized permanganate, the latter then forming colored compounds with reagents.

It has been lately published in England under the name of “kallitype,” a new process—or old, ad libitum—which con­sists in developing the image in ferrous oxalate by a peculiar silver compound whose formula is given below. The paper is prepared by brushing with a strong solution of neutral ferric oxalate dried rapidly—which is a sine qua non when using deliquescent salts; and after exposure the image is developed, etc.

Silver nitrate	50 grains
Sodium citrate	800 grains
Potassium bichromate	1 to 2 grains
Water	10 ounces

“Dissolve the silver nitrate in 1 ounce of water, the citrate and bichromate in the remainder and mix. The precipitate—silver citrate and chromate—is then dissolved by adding 1 dram of ammonia.880, and after 35 drops of strong nitric acid has been added the solution is ready for use.”

This process reminds us that of Robert Hunt (1842), and that of more recent date (1863), of Borlinetto, who developed the image in black with a silver nitrate alcoholic solution, 1:500, and after washing the picture in a solution of citric acid, 1:10, fixed it by aqueous ammonia. But, although that is not absolutely necessary, we would advise one working this, or similar processes in which a silver salt is employed for developing, to fix the image, after treatment with citric acid to clear the proofs from iron salts, in a solution of ammonium sulphocyanate—which has not the injurious effect of sodium thiosulphate (hyposulphite)—in order to prevent the paper to be tinged by the reduction of the silver nitrate which is mechanically retained in its fiber.

The solution of ammonium sulphocyanate should be com­pounded with auric chloride to tone the picture at the same time it is fixed; thus:

Ammonium sulphocyanate	35 parts
Gold terchloride	0.15 part
Water	350 parts

The solution can be used over again.

In the processes devised by Dr. Phipson, Monckhoven and other authorities, the double ammonio-ferric oxalate is rightly recommended instead of the simple oxalate. Not only is the preparation more sensitive to the luminous action, but better half tones are obtained. As usual, it is advantageous to size the paper with starch.

The ammonio-ferric oxalate is prepared by precipitating ferric chloride or sulphate by aqueous ammonia, then washing the precipitate collected on a filter until the washing water be neutral or does not evolve the smell of ammonia. The precipitate is then placed in an evaporating dish, and by small quantity is added a hot solution of ammonium oxalate until it is nearly (not entirely) dissolved, when the solution is set aside for a few hours, then filtered and evaporated to crystallization. For use, the crystals of ammonio-ferric oxalate are dissolved in the proportion of 1 for 5 of distilled water. The solution as well as the crystals should be kept in the dark.

If one object to the trouble of crystallizing, the solution can be prepared by dissolving the ferric oxide in a hot solution of 30 parts of ammonium oxalate and 25 parts of oxalic acid in 180 parts of water observing that the oxide must be in excess.42

The following sensitizing solution gives also excellent re­sults:

Ammonio-ferric oxalate	10 parts
Ammonio-ferric lactate	4 parts
Water	100 parts

After exposure, which varies from five to ten minutes, according to the intensity of the light and the printing quality of the negatives, the picture appears negative from formation of ferrous oxalate. It may be developed in a great many ways: by a solution of silver nitrate at 2 or 3 per cent. of water acidified slightly by an organic acid—citric acid, for example—or a diluted solution of ammonio-nitrate of silver, which most likely constitutes the best developer; the image is black and consists of metallic silver and ferric oxide, with formation of silver oxalate, which dissolve in the ammonia. If the print be treated by a weak solution of aqueous ammonia, the image turns green, then brown, and if, before the latter coloration is obtained, gallic acid or pyrogallol be added, the image becomes bluish-black or brown-black. In the same cir­cumstances tannin (gallo-tannin) produces a blue-black image; catechu-tannin43 and quino-tannin give green, etc. Employed as a developer, potassium ferricyanate develops an image in prussian blue, and auric chloride one in the characteristic vio­let metallic gold. To fix the images obtained by the latter re­actions, it suffices to wash them in a few changes of water, and, if developed with silver, they can be toned by any of the alkaline solutions of auric chloride used in the printing out silver process, etc.

The photographs obtained by all these processes are perma­nent.

Dr. J.B. Obernetter’s Process (1863)

Copper chloride	100 parts
Ferric chloride, sol. sp. gr. 1.5	13 parts
Hydrochloric acid, conc. C. P.	12 parts
Water	1,000 parts

Float the paper on this solution for about two minutes and hang it up to dry. The keeping quality of the prepared paper is remarkable; it has been kept for two years without apparent change; its sensitiveness is at least one-third greater than that of silver albumen paper. Unless developed within an hour or two, the vigor of the proof is much impaired; after twenty-four hours a print can be taken over on the same.

When exposed, only a faint image is visible. It should be fixed in the following solution:

Potassium sulphocyanate	12 parts
Sulphuric acid, conc.	1 part
Sensitizing solution	10 to 12 parts
Water	1,000 parts

A print is floated on this solution, face downward, for three or four minutes, taking care to agitate the liquid as little as possible; the print is afterwards immersed and another one floated in its place, thus proceeding until all the prints are immersed or the solution can hold no more. A fresh solution is then added to strengthen it: the older the solution the more rapidly and better it works. In this developer copper cyanide is precipitated on the parts acted on by light, and this exactly in the proportion to the luminous action. The time of immer­sion depends on the method selected to finish the proofs; it its from five minutes to half an hour. If the proof is immersed for, say, twenty-four hours, the image comes out in a relief which may bring the shadows to two lines in depth. When well developed and thoroughly washed, the proof can be dried and the subsequent operations made at any convenient time.

Various processes may be employed to give to these proofs the tone required; thus: the prints well washed are placed in a solution of ferricyanate of potassium at 6 to 12 per 100 of water, where they take a red color increasing in intensity. If left over night the color becomes a splendid velvet deep red with perfect clear whites. To obtain the color of silver photographs one hour’s immersion is sufficient. After this operation the proofs are washed until the water is no more tinged yellow.

By immersion in

Ferrous sulphate	100 parts
Iron sesquichloride	40 parts
Hydrochloric acid	80 parts
Water	200 to 300 parts

the proofs undergo the following gradation of colors: red, red­dish violet, blue-violet, black and greenish black. As soon as the desired color is obtained, the proofs are washed in acidified water and dried.

The most beautiful purple violet is obtained by leaving the proofs in the iron solution until green-black, and then washing for a moment in a dilute solution of sub-acetate of lead.

A brown-black may be produced by treatment, after washing, with an ammoniacal solution of hypermanganate of potash.

A weak solution of nitrate of silver also yields very fine pictures, but the exposure should be very short, and the proofs must be fixed in water containing a small quantity of oxalate of ammonia.

In order to impart to the proofs the gloss of silver photo­graphs, they should be albumenized in the ordinary manner, and the albumen insolubilized by well known means.

The chemical actions in this process I explain in the follow­ing manner: On the paper there are Fe2Cl3 and CuCl, the latter in excess. By the action of light, and according to the transparency of the negative, Fe2Cl3 is reduced to FeCl, while CuCl suffers no alteration.

If the paper be immediately placed in an absolutely dry room after exposure, the picture remains unchanged. In a moist atmosphere FeCl attracts moisture and, with a part of CuCl, is so decomposed that Fe2Cl3 is formed together with Cu2Cl.

After this action has commenced, if the proof be not immediately immersed in a solution of sulphocyanate of potas­sium, Cu2Cl passes over to a higher combination of chlorine, and the paper is again fit to be impressed anew by the action of light.

As long as FeCl or even Cu2Cl is present, if the print is immersed in the sulphocyanate solution, sulphocyanate of copper is immediately formed on the reduced parts, while on the others the sulphocyanide of copper, formed and dissolved by the sulphocyanide of potassium in excess, becomes decom­posed with water in soluble sulphocyanide of copper and deposited as such on the parts already covered with the salt.

Frequently the prints appear yellow from formation of the double sulphocyanide of copper, but the color disappears by washing in water. Red coloration is due to decomposition into ferrocyanide of copper.

L. Liesegang’s Process (1865)

Pour ammonia into a nitrate of uranium solution, wash the precipitate of uranate of ammonia in distilled water, then dis­solve in citric acid.

Mix this solution of citrate of uranium and a little of a solution of chloride of gold with a paste prepared by dissolv­ing tapioca in hot water. The quantity of chloride of gold must be small and the heat not too great, otherwise the gold would be reduced.

Spread the mixture with a sponge on the paper, which takes a brilliant yellow color, and expose when quite dry; the proofs have the delicacy and vigor of albumen prints.

The proofs come from the frame with a bluish-black color; they should not be toned, but merely fixed by washing until the yellow color of the paper has disappeared.

The color of the picture can be changed to a purple by a solution of chloride of tin.

Guarbassi’s Process (1867)

The paper is floated in the dark for four or five minutes on a saturated solution of bichromate of potash. When dry, it is printed a little longer than for silver prints and afterwards floated, face upwards, on a water bath until all the unaltered bichromate is dissolved. It is then immersed in the following solution, which improve by use and tones the pictures to a reddish color:

Saturated solution nitrate of mercury, as free from acid as possible	4 parts
Saturated solution bichromate of potash	1 part
Distilled water	28 parts

This solution should be prepared, filtered and allowed to stand for some time before use. The print is left in the bath until it has assumed an intense red color, the whites remain­ing perfectly pure. It is then washed and put in another bath to obtain a brownish tint. This bath is thus composed:

Conc. aqueous ammonia	2 parts
Distilled water	100 parts

The print must be immersed at once, and when, in a short time, it has assumed the proper color, it should be washed immediately.

The picture is toned in a very diluted solution of chloride of gold, 1:7,000, in which the color passes from a light brown to a deep black or a violet black tone, when it is washed in two changes of water.

A. Poitevin’s Process (1870)

“I use a paper prepared with iron sesquioxide rendered sensitive to light by tartaric or, better, citric acid in concen­trated solution. This paper, after desiccation and exposure to light, possesses the property of reducing the solution of silver nitrate and that of chloride of gold, and of turning blue with a solution of potassium ferncyanate in the parts where light has reduced the iron sesquichloride into the oxide at the minimum.”

“To coat the paper with an equal layer of iron sesqnioxide, I brush it with a tuft of fine linen dipped in a solution of iron perchloride at 10 or 12 per cent. of water, and dry the sheets in the dark. I immerse afterwards these sheets, one after the other, in a tray containing aqueous ammonia, in such a man­ner as to well wet each sheet successively. A sufficient num­ber of sheets being immersed, I pour off the ammonia in a vial, and, in the tray, I wash them several times, and remove them one by one to hang them up to dry, even in full light, the iron sesquioxide not being sensitive to light.”

“The paper can be prepared in quantities beforehand. To use it I apply upon each sheet a solution of citric acid at 30 or 35 per cent. of water44—which may be done by daylight—and let them dry in the dark.”

“Exposed under a negative of the ordinary intensity, the paper is impressed in sunshine in a few minutes; in the shade it requires about the same time as chloride of silver paper.”

“After exposure the image is not visible, and without being obliged to shelter it from light, I immerse the print in a solution containing about 1 per cent. of silver nitrate. This solution can be used over and over again, by adding to it a little of the silver salt. It does not become turpid by use; it simply turns slightly green from formation of iron nitrate. The image appears soon and rapidly becomes vigorous; in half an hour it will be completely developed. When the exposure is sufficient the color is deep sepia, but not so intense if the quantity of citric acid is feeble. No fixing is necessary; it suffices to wash in several changes of waters.”

“The image can be toned with great facility by a weak solution of gold or of platinum chloride, or, better, by a mix­ture of these two salts. If the impressed paper be treated by a very diluted solution of potassium ferrocyanate, one obtains very pretty blue proofs.”

“A weak solution of gold chloride develops a violet image. A solution of platinum chloride has no effect.”

“All the various phases of this printing method can be followed in full (diffused) light; there is only the desiccation of the paper when sensitized with citric acid, which requires to be done in the dark.”