Photographic Reproduction Processes Part 8

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 pota.s.sium 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 pota.s.sium chloride or the same quant.i.ty of pota.s.sium 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.

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 cliches, 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 2 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 quant.i.ty of thick India ink, and, this done, beat the whole to a thick froth. In ten or twelve hours the alb.u.men will be deposited and ready for use.

The quant.i.ty of India ink added to the alb.u.men 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 cliche, 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 alb.u.men 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 2 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 alb.u.men, 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 quant.i.ty, beforehand, and for use to sensitize it with a solution of pota.s.sium bichromate at 3 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 cliche 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 gla.s.s plate or a smooth board, gently rubbed with a brush or a soft rag, the parts of the alb.u.men 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 alb.u.men 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 alb.u.menizing his own paper, he can use the alb.u.men paper found in the market for the printing-out silver process generally employed by photographers.

The paper is sensitized from the back with the pota.s.sium 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 alb.u.menized 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 alb.u.men 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 alb.u.menized or gelatinized paper is sensitized from the verso on a solution of pota.s.sium bichromate, dried in the dark and exposed under a positive cliche. 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 pota.s.sa, 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 pota.s.sa at 10 per 100, and the proof finally washed with care.