DIAZOTYPE There are several printing processes based on the light-sensitiveness of the diazo compounds, which, although not much used, are of con siderable interest, as they give a great range of colours. The diazo compounds are extremely rich in oxygen, and are formed by the action of nitrous acid on the aromatic amines, amido sulphonic acids, amidocarbonic acids, etc., and readily combine with certain phenols and amines to form azo dyes. If a paper or material impregnated with a diazo compound is exposed under a negative, the diazo compound is decom posed by the action of light, and on immersion in a solution which forms a dye with the diazo compound a negative image is obtained.
The primuline process invented by Green, Cross and Bevan in 1890 is based on the light sensitiveness of the diazo compound of primuline, a yellow water-soluble dye which dyes material or paper without a mordant. Paper or material is immersed in a solution of :— Primuline . 32o grs. 33 g• Hot water to . . 20 OZ. 1,000 ccs.
and then washed and immersed in— Sodium nitrite . . 64 grs. 6.6 g.
Hydrochloric acid . 15o rains. 15 ccs.
Water to . . . 20 oz. 1,000 „ It should then be dried in the dark and exposed under a vigorous positive till those parts under the bare glass are colourless. Then wash thoroughly with water, and treat with one of the following solutions : For Red 0-Naphthol . . 9.6 grs. to g.
Sodium hydrate . 128 „ , Water to . . . zo oz. I,000 ccs.
For Orange Resorcin . . . 64 grs. 6.6 g.
Sodium hydrate . Io6 „ II „ Water to . . . 20 oz. r,000 ccs.
For Purple a-Naphthylamine . i90 grs. 20 g.
Hydrochloric acid . 1 oz. 5o ccs.
Water to . . . 20 „ 1,000 „ For Black Eikonogen . . 125 grs. 13 g.
Water to . . . 20 OZ. 1,000 ccs.
For Brown Pyrogallol . . 113 grs. 12 g.
Water to . . . zo oz. i,000 ccs.
Wash well after development. Various col oured images can be obtained on the same print by local application of the above solutions with a brush. This process does not give pure whites.
Andresen suggested the following modifica tion : Pyridine base (pure) . 110 grs. 23 g.
Boiling water . . 10 oz. 50o ccs.
then add— Sulphuric acid (pure) . i8o mins. 37•5 ccs. Distilled water . i8o „ 37'5 „Benzidine sulphate is formed, and partially separates out. Cool the solution down to Doc)* to zo° F. to C.), and add— Sodium nitrite . . 86 grs. 18 g.
Water . . . I oz. so ccs.
in small quantities with continuous stirring. The benzidine sulphate is diazotised and dis solves. Filter the solution and pour into five times its volume of alcohol, which precipitates the diazo compound; filter out the precipitate, and dissolve (without drying, as it explodes when dry) in— Distilled water to . 20 oz. i,000 ccs.
The paper or material is sensitised in this cold solution by floating or immersion for two min utes, dried in the dark, and exposed under a positive, and then developed in a 2 per cent. solution of amidonaphthol sulphonic acid —5, or amidonaphtholsulphonic acid —9, containing 2 per cent. of sodium hydrate. Deep blue images with pure whites are thus obtained.
Feer's process uses aniline diazosulphonate, amidobenzols, etc., with •• phenolic alkalis, amines, and phenylamines, the following being typical sensitisers : I. Sodium toluoldiazo sulphon ate . 24o grs. 25 g.
0-Naphthol . . 24o „ 25 Sodium hydrate . 76 „ 8 „ Distilled water to zo oz. r,000 ccs.
2. Sodium ditolyltetrazo sulphonate . 240 grs. 25 g.
p-Phenylendiamin 190 „ 20 „ Distilled water to . zo oz. r,000 ccs.
3. Sodium ditolyltetrazo sulphonate . 240 grs. 25 g.
Resorcin . . 211 „ 22 „ Sodium hydrate . 154 „ 16 „ Distilled water to . zo oz. r,000 ccs.
The paper is immersed in these solutions, and after drying exposed for about five minutes to sunlight or electric light. On the exposed parts the insoluble azo dye is formed, whilst in the unexposed part the sensitiser remains colourless and washes out. The print should be fixed in hydrochloric acid.
Andresen discovered in 1894 another diazo printing process. The sensitiser is the diazo compound of a-naphthylamine or 0-naphthyla mine, the former giving brownish grey images and the latter brown-red. If the exposed paper is washed amd treated with tetrazo-diphenyl ether, violet images are obtained.
Distilled water . . 5 oz. 15o ccs.
heat to boiling, and add— a- or 0-Naphthyla mine . . . 220 grs. 14'3 g.
then add— Hydrochloric acid (sp. g. 1.19) . . 152 grs. I° g.
and as soon as the salt has dissolved Hydrochloric acid . 617 grs. 4o g.
with constant stirring, and cool the paste down to 4o° F. An evolution of gas takes place, and a yellowish solution is formed which must be filtered into an ice-cold dish. Float paper on this for fifteen seconds and dry in the dark. Expose for two or three minutes under a nega tive in the sun and develop in a io to 20 per cent. solution of twice fused acetate of soda, and wash well.
These processes give rather pleasing effects when applied to silks and other materials.