AUTOCHROME PROCESS A process of screen-plate colour photography invented by MM. ere [Eng. Pat. 22,077, 1904 ; 25,718, 1904 ; 9100, 'goo], based on the use of starch grains, as far as possible of a uniform size, dyed to the necessary colours, red, green and blue violet, mixed and sifted on to glass coated with a tacky surface. The grains are then rolled and any white interspaces filled with a black pigment. The screen-plate thus formed is coated with a panchromatic emulsion. (See "Screen-plate Processes.") The plate is placed in the dark-slide so that the glass side faces the lens, and a black, smooth card should be placed in contact with the sensi tive emulsion to prevent any damage to the latter. A yellow screen must be used to cut down the excessive action of the blue violet and blue, and the makers provide special screens for this purpose, though the following, sug gested by Von Hfibl, has proved satisfactory in practice : A. Tartrazine . . 15i grs. I g.
Distilled water . 17i oz. soo ccs.
B. Phenosaffranine . 1.5 grs. oI g.
Distilled water . 24/ oz. goo ccs.
C. Gelatine . . 93 grs. 6 g.
Distilled water . 3 oz. 85 ccs.
64o minims or 38 ccs. of C should be mixed with 168 minims or io ccs. each of A and B. Immediately before use, 62 grs. or o4 g. of masculine dissolved in 338 minims or 20 ccs. of water with 3 or 4 drops of ammonia should be added to the dyed gelatine ; it is important to make the msculine solution only just before use, as it rapidly discolours. Of this dyed gela tine, 14o minims or 7 ccs. should be coated on every 16 sq. in. or every ioo qcm. The exposure must be determined by meter, or the speed of the plate may be taken as approximately 2 H. & D. or 3 Watkins, but, unfortunately, the speed varies practically both in sunlight and shade, and in winter and summer, this variation being dependent on the spectral com position of the light.
After exposure the plate should be developed, and the developer first recommended was pyro ammonia, but a later recommendation is the following : Quinomet (metoquinone) 62 grs. 4 g.
Sodium sulphite (anhydrous) 278 18 Liquor aramonim (.88o) 4 mins. 0.2 CCS.
Potassium bromide . x5i grs. x g.
Distilled water to . 35 oz. I,000 ccs.
The duration of development should be 2i minutes at a temperature of 6o° P. (155° C.). Many other developers may be used, such as a normal metol-quinol developer or rodinal in 12 for six minutes, amidol, rytol, etc.
As the emulsion is very sensitive to red, the plates must be worked in total darkness or in a deep green light such as is obtained through the following filter, and even then the plate should be exposed to this light as little as possible : New Bordeau R 3 °/,, solution . . 2 oz. x25 ccs.
Tartrazine 4 solu tion . . . 3 ); Light green S 5 ° solution . . 31 Glycerine PP 50 Gelatine I° °/ solu tion to . . . 20 1,000 pp 2 oz. or 56 ccs. should be coated on every zoo sq. in. or 645 qcm. Or, in place of the above, fixed-out dry plates may be dyed in tartrazine and methyl violet to form a red screen, which should be placed in contact with one stained with malachite green, and if a brilliant light is used a sheet of tissue paper, stained with tartrazine or malachite green, should be placed between them.
At the conclusion of development the plate should be rinsed for fifteen to twenty seconds, and then immersed in the following reversing bath : Potassium permanganate 31 grs. 2 g.
Sulphuric acid . . 17o mins. 10 ccs.
Distilled water to . 35 oz. 1,000 The plate should remain in this for two or three minutes and then be examined by a weak white light, and if all the metallic silver has disappeared it should be washed for about a minute. It is advisable to keep the above reversing solution in two separate solutions and mix as required.
After dissolving out the primary image of silver and washing, the plate should be exposed, emulsion side up, to white light and then re developed, which may be done by the following amidol developer : Sodium sulphite (anhydrous)135 grs. 15 g.
Amidol . . . 45 5 IIDistilled water to . 20 oz. I,000 ccs.
The original developer, metol-quinol, rodinal etc., may also be used. When the second development has completely reduced the silver bromide, and the picture now shows up in colour, the plate is washed for three or four minutes and, without fixing, set to dry. If the plate is seen to lack brilliancy (due to over exposure) it may be intensified with the following, first immersing for not more than ten seconds in 192 minims of the permanganate solution given above, diluted with 20 oz. of water : A. Pyro . . 26 grs. 3 g.
Citric acid . . 26 3 11 Distilled water to 20 oz. 1,000 ccs.
B. Silver nitrate . 39 grs. 2'5 g.
Distilled water . 3- oz. zoo ccs.
For use mix 5o minims or i 1 ccs. of B with fluid oz. or zoo ccs. of A, and immediately apply to the plate and allow to act for three or four minutes, then rinse for a minute or two and clear in the following : Potassium permanganate 8 grs. I g.
Distilled water . 20 OZ. 1,000 CCS.
for about one minute, and wash. The plate can then be fixed in " Hypo " . . 3 oz. 15o g.
Sodium bisulphite lye I 5o ccs.
Water to . . 20 1,000 for two minutes and washed and dried by a gentle heat. When dry it should be varnished with Gum dammar . . 31 grs. 20 g.
Copal . , 77 ,, II Carbon tetrachloride to 31 oz. i,000 ccs.
or Gum dammar . . I oz. 20 g.
Benzole . . . 5 Zoo ccs.
When properly treated, and with correct selec tion of the subject, particularly if glaring con trasts of colours are avoided, very exquisite results can be obtained.
Autochromes have been successfully repro duced by the three-colour and four-colour block processes by illuminating the transparency with reflected light and copying through colour filters, the same as in copying from a painting or other coloured original. The frontispiece to this volume is an example of four-colour repro duction.