LITMIR.BIZ

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plate, and so protected it in those parts from the etching mordant employed, usually chloride of potassium or perchloride of iron. His successor in these researches was a Frenchman, Poitevin, who invested these processes with even greater practical importance. He, however, worked chiefly on paper with the bichromate mixed with albumen or gelatine, and attained results which were the first practical steps towards photo-lithography and carbon or pigment printing. Further on we shall return with more detail to these subjects. In the hands of Tessié du Motay and Marechal, of Metz, the discoveries of Poitevin derived further practical value, and to them we are indebted for the first advances in collotype, they in their practice using metal plates as a support. Joseph Albert, of Munich, followed, and introduced further and considerable modifications, amongst others utilising glass plates as a foundation. The want of adhesiveness of the chromated gelatine layer to its support was a source of great trouble to the earlier experimentalists, and was a matter giving rise to innumerable difficulties, which were, however, finally surmounted by the researches of Husnik, Albert, and Obernetter.

[A] For the production of an intaglio plate the impression on the chromated gelatine must be printed from a positive, which will naturally give a negative on the copper plate.

CHAPTER II.

       Table of Contents

      The Bichromates in Conjunction with Organic Substances.

      CHROMIC Acid, now usually known as chromium-trioxide—CrO₃—is found in beautiful red needles, which are slightly deliquescent. According to the present accepted theory, water containing such crystals in solution contains the true chromic acid, H₂CrO₄, which cannot be produced as crystals. Both the crystals and a concentrated solution in water give up with facility a part of their oxygen, sometimes with explosions accompanied by the appearance of fire, and green chromium oxide, now called sesquioxide, Cr₂O₃, is formed. Bichromates of alkalies act in a similar manner, but in a lesser degree. If, for instance, a few drops of sulphuric acid be added to a solution of potassium bichromate, and then a little alcohol, or, instead of that, a piece of sugar be added and moderate heat applied, the solution will be violently agitated, its yellow-red colour will disappear, the bichromate be decomposed, and chromium oxide be formed, which, combining with the sulphuric acid, forms a solution of a violet green colour. If a sufficient quantity of sulphuric acid be present, the chromium sulphate and the potassium sulphate combine, forming chrome alum crystals, octahedron in form, similar to those of ordinary alum, but of a beautiful dark violet colour. The same are used in both the carbon and collotype processes. Bichromate of ammonium is easily decomposed by fire by heating it in a dry state. It will be understood that in consequence of the decomposition, or rather reduction, of chromic acid by organic matter, the latter is also decomposed and oxidised in a corresponding degree. The agent producing this effect may be either heat or light, or both together; but of special interest to the collotypist is the action of light upon different combinations of chromic acid, in the shape of alkali bichromates in conjunction with organic matters.

      Glue or Gelatine, in a solution containing about five per cent. of one of the bichromates spread upon glass plates, and exposed to light, will, where exposed, become dark brown, at the same time become insoluble in water, and also lose its power of absorbing the same. According to the researches of Dr. Eder, this brown substance consists of chromium chromate and unaltered gelatine. The oxidised portion of the latter remains soluble and may be entirely removed by treating with warm water. During the exposure to light formic acid is undoubtedly formed. Ammonium bichromate, with gelatine, is in the dry state much more sensitive than the potassium salt; it also decomposes more readily when heated in the gelatine solution, a matter which must not be lost sight of in the preparation of the collotype plate.

       Isinglass—a tolerably pure glutin. In conjunction with the bichromates it presents the same characteristics as gelatine.

Albumen, when mixed with a solution of a bichromate, is not thereby coagulated, as occurs upon the addition of pure chromic acid. If poured upon glass and dried it also becomes insoluble in water after a sufficient exposure to light. This solution is used with great advantage in photo-lithography, as the unexposed parts are readily soluble in cold water. The older method, which consisted of using gelatine in conjunction with bichromate, rendered hot water necessary in the development of the fatty ink image upon the transfer paper, and as a consequence of the rubbing at times necessary, the ink, already softened by the warm water, became smeared, the resulting image considerably impaired, and the transfer thereby rendered altogether useless. In collotype a bichromated albumen substratum is still used by many operators, this coating when dry being exposed through the glass plate, the latter placed coated side downwards upon a flat black support, preferably of velvet. After this exposure the gelatine layer—from which the prints are produced—is applied. This method was first published by Albert, who originated it. The chromated albumen solution will not keep long even in the dark,[B] as the albumen coagulates, and on this account photo-lithographic papers coated with this solution must be used within a few hours of their preparation. Such papers are now conveniently prepared with the albumen,[C] and sensitised as required by floating upon an aqueous solution of one of the bichromates, to which has been added twenty-five per cent. of methylated spirit.

[B] I have found this solution keep a considerable time—a month at least.—Trans.

[C] I believe Husnik has now discontinued to prepare his photo-litho transfer paper with an albumen coating. Recent issues of his paper have been simply coated with gelatine.—Trans.

      The chromated gelatine used in collotype may be kept as a jelly in the dark room, but is better prepared fresh.

      Of carbon hydrates only gum chromate is of interest to the collotypist, its employment being restricted to the “dusting on” process, and the occasional addition of titles, &c., to the otherwise finished plate. As with all mixtures of the bichromates and organic substances, it is only sensitive to light when in a dry condition, and solutions of it may, without harm, be exposed for some time to diffused daylight. Aqueous solutions of gum arabic soon become sour and mouldy, and in this condition are useless for the “dusting on,” or, in fact, any other photographic process. As a preventive a few grains of sulphate of quinine may be added, which will preserve it effectually, and is quite inert in photographic operations.

      This solution applied to glass and dried is, in its dry condition, extremely sensitive to light. If printed under a negative, and then exposed to a damp atmosphere, it absorbs moisture, and becomes “tacky” upon the portions fully protected by the densest portions of the negative, while the parts exposed under the clear glass of the negative will be free from any such tendency. The gradations of the negative between these two extremes will be represented by the surface of the gum being more or less adhesive, and it is this property which renders it valuable in various processes. After exposure under a negative finely-ground colour—for instance, graphite—is carefully dusted over the surface of the plate by means of a camel-hair brush, and the powder will adhere in exact proportion to the protection afforded by the negative. A picture is thus produced in graphite powder possessing all the gradations of the original. From a negative a second negative is produced, while from an exposure under a positive a positive will result, each, however, reversed as regards left and right when compared with the original. Advantage is taken of this process to produce reversed negatives, a subject more fully dealt with in another chapter.

      Bichromate and Sugar are never used alone, but as an addition to heighten the sensitiveness and increase the hygroscopic properties of the gelatine or gum-chromate solutions. By the addition of sugar and a little ammonia to chromated gelatine the solubility of the gelatine in hot water is increased. The

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