Many dyes have been suggested for panchromatizing and more formulas for sensitizing, but as simplicity and ease of attaining results are among the main features that have been kept in view throughout this work, but one formula will be given, which has proved to be entirely satisfactory in practice. The dye recommended is that known as sensitol violet, and this should be prepared in the form of a stock solution, which will keep in the dark indefinitely. The following directions, which might seem unnecessarily diffuse to the expert worker, are thus given for the benefit of those without experience. In a dry 500 ccm flask place 1 gram of the dye and add 100 ccm pure ethyl or methyl alcohol. Shake with a circular motion so as not to distribute particles of the dye on the upper parts of the flask; then place the flask in a water bath at about 30° C. (84° F.) and raise the temperature gradually to 65° C. (150° F.), giving the flask an occasional swirl round. Then add 400 ccm cold alcohol and after shaking gently pour into the stock bottle; measure another 200 ccm of cold alcohol into the flask and swirl round so as to wash off any adherent dye solution, then add this to the stock bottle; now pour 300 ccm distilled water into the flask and after shaking well add to the stock bottle. The result will be a 1:1000 stock solution of the dye, which must be kept in the dark. It may be as well to remark here that, owing to the difficulty of making up extremely dilute solutions to exact percentages by means of English weights and measures, the experimenter in color should accustom himself to work in the metric system, and acquire the necessary weights and measures. If this be impossible, a conversion table will be found at the end of the book.
Unfortunately all the isocyanin dyes, to which class this dye belongs, are more or less sensitive to acids and are more or less decolorized by them, and when in this bleached state they have little or no color-sensitizing power. They are so sensitive to acid that even the small amount of carbonic acid present in distilled water is sufficient to decolorize them, so that it is necessary to make the water for the actual bath alkaline. Ammonia is the alkali generally used, but while it is efficient it is dangerous, unless used in very minute quantities, that is, not more than one or two drops per 100 ccm of the bath, as it induces the rapid growth of fog. Borax is a far better alkali to use, as there is less likelihood of fog with keeping. The actual sensitizing bath is:
Stock dye solution 20 ccm.
Borax, pure 2 g.
Distilled water 1000 ccm.
Dissolve the borax in the water first, then add the dye solution with constant shaking. The above quantity of solution is sufficient to sensitize about 500 square inches of plate surface, or thirty 4x5 plates.
Slightly higher color-sensitiveness may be attained by using with the sensitol violet an admixture of orthochrom T. This dye should also be made up into a stock solution in the same way as the violet; but the sensitizing bath will then be:
Stock sensitol violet solution 10 ccm.
Stock orthochrom solution 10 ccm.
Borax 2 g.
Distilled water 1000 ccm.
The advantage of this bath is that a rather better green sensitiveness is secured, and also slightly general higher speed to white light, or, in other words, the exposures are slightly shorter. The difference, however, is but slight and in practice the plain sensitol violet bath will be found quite satisfactory.
From the data given above as to the area that may be sensitized with these baths, it will be seen that a dozen plates will require just 400 ccm of solution, so that the procedure is fairly obvious; place four plates in a dish and flood them with an even regular sweep with 133 ccm of the dye solution. Rock the dish gently first from side to side and then end for end for four minutes, pour off the solution into a waste bottle, set the dish up at an angle so as to allow the remains of the solution to drain down to one corner and off. Then pour on the plates 300 ccm alcohol (ethyl or methyl) and rock the dish for one minute. Remove the plates one by one, giving them a rinse up and down to make sure that the dye solution adherent to the back of the glass is rinsed off by the alcohol. Then place them in the draining rack, not more than four plates to the twelve-plate rack and push the rack into the farther end of the drying tunnel which has the hot air blowing through it. The rack should not be pushed more than about three inches into the tunnel.
The alcohol used for rinsing may be poured into a graduate, the dish stood on end to drain and four more plates treated in like manner with another 133 ccm of sensitizing solution. The same alcohol may be used for rinsing and again poured off for the third lot of plates, which are treated in like manner. The second batch of plates should be placed in another draining rack and pushed into the tunnel, thus pushing the first rack nearer the source of heat. The third lot of plates is treated in like manner and placed in the third rack, and by this time, the plates in the first rack should be quite dry.
The racks should be placed in the tunnel so that the surface of the plates is parallel with the current of air, and not at right angles to it; this ensures a complete change of air over the whole surface of the plates, whereas if they are placed at right angles to the air-travel, only the first plate gets the full benefit of the hot air and the others are more or less in dead air pockets.