Scientific American 19, 9.5.1863
The following remarks were written by M. Poitevin, and published in the bulletin de la Societe Francaise de la Photographie: -
"In the new principles of permanent printing in carbon or other inert pigment, which I submit, the pigment remains imprisoned in an organic material, originalky insoluble, and remaining so in those portions not acted upo ny light, or coagulated in certain parts only the impressed surface. The first pricinple, that which I have most followed up to the present time, rests upon a well-known reaction . the insolubility communicated to organic matters, such as gum, albumen, gelatine, &c., by salts of iron, the perchloride, for example, and upon a new fact which I have observed, which is, that this matter, coagulated and rendered insoluble in cold or warm water, becomes soluble under the influence of light, in presence of tartaric acid, which, reducing the ferric compound, restores the organic matter to is natural state. Gelatine is the substance with which I have succeeded best. The following is my mode of operating. I dissolve 5 to 6 grammes of gelatine in 100 grammes of water, and add sufficient quantity of carbon or other inert pigment to obtain the intensity of tone I desire to produce. I pour this solution into a flat dish, and keep it warm so as to prevent the gelatine solidifying. Each sheet of paper is foated on one side only on this solution, and a uniform coat of colored gelatine adheres to it; I then place the sheet of paper on a flat surface and leave it to dry spontaneously. To sensitize these sheets, I impregnate them on both sides with a solution of perchloride of iron and tartaric acid in the proportion of 3 to 1. The quantities which have appeared to me most suitable being 10 grames of perchloride to 100 cubic centimetres of water, and 3 grammes of tartaric acid. I leave the thus-prepared sheets to dry in the dark; then the coating of gelatine has become completely insoluble, even in boiling water. I rint these surfaces from positives on glass or on paper, and in all those portions upon which the light acts, the coating becomes soluble in warm water; this solubility, be it understood, commencing from the surface. After a few minutes' exposure to the sun, if the positive cliché be not very dense, which is preferable for this kind of printing, I remove the paper from the printing frame, and immerse it in warm water; thereupon all the parts which have been modified by light dissolve in proportion to the quantity of light which may have passed through the various portions of the positive cliché. In the parts corresponding to the lights of the cliché, the black or colored coating will be dissolved down to the surface of the paper, leaving perfect whites, while in the half tines a part only of the coating will dissolve, commencing with the surface, and these half-tones will be rendered upon the greater or lesser thickness of the coating of gelatine remaining insoluble; and as this part is in immediate contact with the paper, it cannot be removed by washing; as to the portions of the negative which are entirely black, they will be rendered by the entire thickness of the primitive coating. To complete the proof, it is only necessary to dry it in the air, or treat it with water acidulated with hydrochloric acid, which removes the stain and salt of iron, then to wash it freely in water, and dry it again spontaeously. It is now unchangeable, but a tanning of the gelatine, accomplished by known methods, with alum, bichloride of mercury, &c., will give it greater solidity. Before this fixing we can make whites wherever they may be required, by means of a pencil dipped in warm water. We do not encounter such dangers in this method as presented themselves in that I proposed in 855, in which I employed a coating of gelatine mixed with an alkaline bichromate and carbon, and which I printed by eans of negatives; for in that method the gelatine was rendered insoluble by light, commencing at the surface, and the half-tones were removed in the washing, undermined from beneath by a portion of the coating remaining soluble. The method I now propose does not possess this inconcenience, and to obtain perfect proofs by it requires only suitable paper with a glazed surface, uniformly coated with a film of the colored preparation, which will be found easy to realize in practice."
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