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Various Processes 691

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VARIOUS PROCESSES 691. Ozotype. A curious variation of the carbon process was devised in 1899 by T. Manly. This process, however, has been abandoned since the introduction of the Ozobrome process described in Chapter XLIII.

A sheet of single-transfer paper (§ 652) is sensitized in a solution of potassium dried, and exposed under a negative until all the details in the image are visible. A sheet of carbon tissue is immersed for about a minute in a bath of— Acetic acid, glacial . . dr. (3 c.c.) Hydroquinone . . 9 gr. (i grin.) Water . . 20 OZ. (Lon° c.c.) The print on bichromated paper is rapidly placed in this bath, and applied to the carbon tissue. The two are withdrawn together, the liquid between them is drained off, and the two sheets are left between blotting-paper to dry. After drying, they are together placed in cold water for about half an hour. after which development is done as in the single-transfer carbon process.

The chromium chromate, formed in the single transfer paper during its exposure to light, is de composed by the acetic acid, liberating chromic acid. This latter, absorbed by the incompletely swelled carbon tissue, is reduced in the coating of pigmented gelatine on contact with the hydroquinone, the products of this reduction giving rise to insolubility of the gelatine ; the latter, after development, remains adhering to the single-transfer paper (A. Haddon, 1901).

692. Dye Prints by Photo-mordants. A piece of fabric, well washed and dried, is impregnated with a solution of potassium or ammonium bichromate (with or without addition of am monium metavanadate), drained, and dried at a low temperature. It is exposed under a negative till a brown image appears. It is washed until the ground is completely colourless. The image, nearly invisible, is then formed of chromium hydroxide, which can act as a mordant towards numerous dyes (see also § 624, footnote). For example, the mordanted tissue may be placed for from ten to twenty minutes in a boiling solu tion of alizarine, for red or violet ; or of alizarine blue, orange, or black, or antliracine blue or brown, or gallein, coerulein, galloflavine, etc. (Persoz, 1857 ; E. Kopp, 1863; A. Villain, 1892). After dyeing, the fabric is rinsed, and the whites are brightened by washing in hot soap and water and, if necessary, cleared in a solution of sodium hypochlorite.

Instead of using ready-prepared dye, a dye may be formed by oxidation of suitable organic substances by means of the chromic acid available in the chromo-chromate, formed during exposure to light. The operation may be carried out in the cold, and this process is, therefore, applicable to sized or unsized paper. The paper or fabric is sensitized with bichromate and, after exposure under a negative, is washed in several changes of water acidified with o-i per cent of sulphuric acid. It is then im mersed in a very dilute solution of one of the following substances, together with a little sodium bisulphite (10 to 20 gr. per 20 oz. = I tO 2 grm. per litre of each constituent), parapheny lene-diamine, paraminophenol, pyrogallol, and other polyphenols and polyamines (E. Kopp, 1863; Andresen and Gusseron, 693. Prints with Diaz° Compounds. Several printing processes, known under the general name of diazotype, are based on the destruction by light of diazo compounds' (products of the reaction of an iced and slightly acid solution of sodium nitrite on an aromatic amine), and on the property possessed by these bodies of pro ducing, by coupling (in an alkaline medium) with a phenol or an aromatic amine, dyes (called azo dyes) that become fixed on cellulose (paper, cotton, etc.) without a preliminary mordanting.

These processes may be classed as follows (D. A. Spencer, 1928)— (i) The diazo compound A produces when decomposing in light a substance B. Develop ment is effected in an alkaline solution by a coupler C which gives with A a coloured com pound that is insoluble and does not react with B (A. Green, C. F. Cross, and E. Bevan, 1890). For instance, a fabric dyed with primu line is diazotized (its colour changes from yellow to reddish) and dried in darkness. After expo sure to light under a positive the image is fixed and intensified by coupling in a solution of fl-naphthol alkalinized with caustic soda. A red image is thus obtained.' (2) The diazo compound A decomposes and produces a substance B. Development is effected by applying a substance C which gives with B a coloured product, but does not react with (G. Kogel, 1926). For instance, a paper sensi tized by a diazo compound is exposed under a negative and then treated by an ammoniacal solution of silver nitrate which develops a posi tive image of reduced silver.

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