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Color Filters or Color Screens 49
may dip a strip of gelatin into a dye solution and at once tell whether it will stain or not, and in the latter case it may be assumed to be basic, and the confirmatory test with tannin may be applied. Or it may be further tried with a collodion film, for basic dyes have a strong affinity for cellulose.
It may happen, of course, that excess of one or other of the dyes will redissolve the precipitate first formed in the above case, but it is hardly worth while to play such tricks, when it is so easy to coat two separate glasses with the dyes. Frequently also the tinctorial power, or the velocity of dyeing gelatin, is increased by the addition of some weak acid, such as acetic ; while it may, on the other hand, be markedly decreased by the addition of a base, so as to make the solution alkaline. Basic dyes generally dye more rapidly in a faintly alkaline solution, such as that of borax.
The fugitiveness of a dye is another point, which, however, need practically only be considered in connection with dark-room screens, for the actual exposure of selective or compensating filters is, or should be, reduced merely to the duration of the exposures themselves. Naturally if a careless operator leaves a filter exposed for hours to strong sunlight or even full daylight, it may fade, and also if it be used in front of a powerful arc light, one may expect trouble. Von Hubl's test was to cover half the filter with black paper and expose to bright sunlight and then compare the color of the exposed with the protected part. Those that showed no change after 100 hours exposure were considered fast.
The light-sensitiveness of a dye can only be of interest photographically when examined in the medium in which it is to be used, and as in nearly all cases this is gelatin, this alone should be used in such tests. The addition of glycerol has been recommended, but as pointed out by von Hiibl this may act as an energetic sensitizer, especially with basic dyes, and if it be thought desirable to make any addition to render the gelatin more supple a little syrup should be used. It is not without interest to note that the author has used golden syrup for dark-room light screens for more than 20 years, in place of glycerol, as it dries better and has less hygroscopic properties.
Some have advised the use of hard gelatin and others that of soft. As a matter of fact for dark-room screens it is almost immaterial, though the soft is less likely to split under the heat, which is frequently a factor in badly designed lanterns.
Of great importance are the chemical properties of the gelatin, and von Hiibl4 specifically called attention to the presence of sulfites. The gelatin, therefore, should not be looked upon as an inert vehicle. The most striking instance is that of fast red D (echt-rot D), and if a solution of this be added to gelatin, containing sulfites, a brownish-yellow film is obtained; while if the gelatin is pure a purple-red color is formed, which dries a rose-red. Crystal ponceau behaves in the same way; pheno-safranin, naphthol green, filter blue and tartrazin are bleached ; filter yellow