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paper, bichromate, water, light, chromic, albumen and gum

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ARTIQUE'S PROCE55.—A process for reproducing plans, drawings, etc. It is practi cally a modification of the carbon process, and is thus described :* " The paper can be prepared with any one of the following solutions : ist. Dissolve 2 parts of ammonium bichromatic and 5 parts of best gum arabic in 15 parts of water and neutralize with a few drops of concentrated aqueous ammonia ; then add ioo parts in volume of whites of eggs 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 that the paper be black when coated, but sufficiently transparent, however, to enable one to see the shadow of objects placed at the back. The coating should not be thick. This is important, in order to allow the light to act through the whole thickness of the preparation when the paper is exposed under the cliché, for, if the film be too opaque or too thick (by addition of too much gum arabic), it would only be 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 renders insoluble this organic sub stance—or any other, such as caseine, gelatine, gum arabic, etc , therefore, whenever the film is not acted on right through the film, the subjacent part, being still soluble, is necessarily washed off and with it the superficially impressed part, that is, the image.

2d. Take io parts of lampblack 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 i part of liquid glucose in zoo parts of water, adding afterward 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 albumen, which we prefer to use, for two months in good condition ; but the sensitive paper does not keep for more than three or four days, 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 3f per cent. solution of potassium bichromate and water applied

to the back with a Buckle brugh.* The bichromate solution should be allowed to soak through 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 method 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 ex posure 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 cer tain 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 insolubility 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 afterward in hot water, is insolubilized with formation of chromium trioxide, + SO,Fe=SO,K, FC,O, (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.

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