GASLIGHT PAPERS FOR BLACK TONES 571. Suitable Light for Working. One of the factors which have tended most to popularize the use of these papers among amateur photo graphers is the fact that they may be worked in safety without a dark-room.' Any domestic room may he used simply by waiting until the evening and not working too near a light. An amateur can, therefore, make his photographic prints during the evening, in a sitting-room, in comfortable warmth, very different from the con.difons in an improvised dark-room. For example, a screen may be placed near the table on which the printing frames are loaded and the exposed prints are developed, in such a manner as to shield the table from the direct light from the lamp, all the manipulations being carried out by the light diffused from the ceiling and walls. It is, however, far preferable to use yellow light, which allows a much stronger illumination without any risk of fogging. For this, a light wooden frame may be used, covered with a translucent yellow paper, and made in such a form that it will stand upright on the table. Half the table can then be used for exposing the prints, preferably by means of a portable lamp, the other part being devoted to the working. The sensitive paper should be kept in a drawer in the table until required, so as to be protected from the general diffused light.
In a well-arranged workroom, and especially in professional and commercial establishments, gaslight papers are manipulated in ample bright yellow light.' 572. Developers for Gaslight Papers. In con sequence of the relatively high degree of solu bility of silver chloride in solutions of sodium sulphite, and also the rapidity with which silver chloride dissolves when in a state of division as great as that in emulsions of the " gaslight " type, the use of a developer rich in sulphite leads inevitably to yellow staining or veiling, of the same nature as dichroic fog (§ 433). It follows that though a developer specially prepared for chloride papers may be used without any loss of quality for the develop ment of bromide papers, the various developers for negative work can only be used for these papers by considerably reducing the proportion of sulphite. This does not apply to amidol
developers, in which the quantity of sulphite is generally sufficiently reduced. As a consequence developers specially prepared for gaslight papers are inferior in keeping qualities to those with a normal proportion of sulphite. On the other hand, the concentration of the solution should be so regulated that development is very rapid. Even in a developer containing a small propor tion of sulphite, sufficient silver chloride would be dissolved in time to give rise to a yellow stain.
Among others, the amidol developer recom mended for bromide papers (§ 563) may be used, or the following formula (Kodak)— Metol-hydroquinone Developer— Metol . . . . . 18 gr. (2 grm.) Soda sulphite, anhydrous . . oz. (25 grm.) Hydroquinone . . . . 53 gr. (6 grm.) Soda carbonate, anhydrous . . 264 gr. (30 grm.) Potassium bromide, ro% solution 45 min. (5 c.c.) Water, to make • . . 20 OZ. (1,000 C.c.) It is sometimes recommended to develop soft or normal gaslight papers in a solution of only half the strength of that used for the vigorous grades. The time of development must in that case be doubled. This method possesses neither advantage nor disadvantage. But dilution of the solution must be avoided when developing vigorous papers." A comparatively recent vogue has given rise to numerous researches on means for giving to prints on silver chloride paper a bluish black tone, similar to that obtained indirectly by cur tailed gold toning. This bluish tone can be obtained directly by development with amidol (§ 387) or metol-hydroquinone with a very slight bromide content (which leads to the formation of yellow or grey fog). It seems then to be due to the silver being less dispersed than in black tone images, and especially in warm tone images. Many substances, first used to protect the emulsions against fog, have been found to give this bluish tone, which is generally less modified by drying at a high temperature than are warm tones or neutral black. Several of these substances can be introduced either into the emulsion, baryta undercoating, anti-abrasion overcoating, or developer.