578. Warm Tones on Black-tone Plates. Development for warm tones on plates intended normally for black tones was formerly a question of some interest, when making transparencies by enlargement or reduction, in order to avoid the unduly long exposures necessitated by the use of the slow plates specially made for warm tones. The numerous processes of after-toning have, however, greatly altered the situation.
With regard to obtaining warm tones, the following is a summary of the methods of work ing (L. P. Clerc, 1897). The rules for deter mining the correct exposure and for controlling development are the same as those given in the preceding paragraphs.
Among others, the following developer may be employed— Soda sulphite, anhydrous . 350 gr. (40 grin.) Hydroquinone . . . 90 gr. (to grin.) Caustic soda . . s oz. (5o grin.) Water, to make . . . 20 oz. (1,000 c.c.) to which should be added, for each ounce (or Ioo c.c.) the quantity of a io per cent solution of ammonium bromide specified in the following table— The images thus developed are white or yellow when viewed by reflected light. They frequently show a slight veiling which can be easily removed by treating with a surface reducer (§§ 459 and The addition of a suitable proportion of thiocarbamide (i.e. thio-urea) to a developer
for warm tones, allows blue and violet tones to be obtained (S. H. Wratten, 19I0). The action of these developers is, however, exceedingly slow at temperatures lower than 65° F. The quantity of thiocarbamide added must be very exact ; and it is not possible in this case to control development by the Watkins method. The only practicable method is to develop for a constant time, which necessitates solutions exactly pro portioned and used always at the same tem perature. The exposure must be found by preliminary tests, as previously described.
The working developer is prepared from three stock solutions— (A) Metol . . . . 22 gr. (2.5 grm.) Soda sulphite, anhydrous 130 gr. (15 grim) Hydroquinone . . 90 gr.. (lo grm.) Soda carbonate, anhydrous 130 gr. (15 gym.) Water, to make . 20 oz_ (1,000 c.c.) (B) Ammonium carbonate . 2 OZ. (Too grm.) Ammonium bromide . 2 oz. (moo grin.) Water (cold), to make . 20 oz. (l,000 c.c.) (C) Thioearbamide . 26 gr. (3 gran.) Ammonium bromide . 9 gr. (1 grm.) Water, to make . 20 oz. (1,000 C.C.) The colours obtained by suitable combination of exposure and development are very trans parent and excellent in quality.