Printing From Screen Plates PracticalColorPhotography 22

Fig. 20.

The required length of magnesium ribbon is cut off, and this will naturally depend on the density of the picture; with ribbon of 2.6 mm width, which is about the average, from 10 to 20 cm will be needed. It should be folded end to end and then placed edgewise in the spiral. An opaque card or something that will act as a shutter must be placed over the filter aperture; a good sized book that will stand up on its edge is handy. The ends of the ribbon can then be ignited by means of a candle or spirit lamp, the shutter removed from in front of the filter, and the exposure made. The plate is then treated exactly as though it had been exposed in the camera.

No definite data can be given as to the length of ribbon required, but one can tell from the results as to whether under or overexposure has been given, and this will be a guide in future work. It is important to have the ribbon standing on its edge in the spiral, as if lying flat it may go out.

Obviously one can use the camera and either enlarge or reduce the size of the picture; but as the color elements are also enlarged at the same time, there is obviously a limit beyond which one should not go. Precisely the same arrangement as just described may be used for the illumination. The lens of the camera takes the place of the printing frame and the connection between the illuminating box and the lens should be made light-tight, which can be easily done with a cloth.

It is possible to reproduce an autochrome by means of a Paget screen-plate, and thus obtain a negative from which any number of transparencies can subsequently be made; one may proceed exactly as described above, only substituting the Paget taking screen and panchromatic plate for the autochrome.

It will be found most convenient to carry out these operations in the dark room, and further it may be said that in no case should daylight be used as the illuminant, as its composition varies so much that false color rendering is almost certain. Other illuminants than magnesium may be used, but in every case the special filter must be used. Magnesium is cheap and handy and its composition always constant, so that it is by far the most convenient.

As regards the reproduction of screen-plates in colors direct on paper, the only method of doing this is by the bleach-out process, which is described later, and it can not be recommended. The paper has to be prepared and the results are not worth the trouble involved, especially as they are not permanent and rapidly fade in light.

To prepare permanent prints from screen-plate pictures, one must have recourse to one of the tri-color sub tractive methods already described; to utilize these it is obvious that we must have the three constituent negatives, but these are not difficult to make. The first requisite is a set of sharp-cutting filters, the purpose of which is to isolate each individual color, that is all the red, all the green, and all the blue. These filters must be of such a nature that they transmit only the light of one color. They can be made without much trouble and as they are not used except as light screens, they need not be made with such careful attention to parallelism of surfaces, or cementing, as is necessary in the case of lens filters. It will in fact be more convenient to make them the full size of the pictures.

The red filter can be made from two glasses, one coated with methyl violet and the other with rose Bengal and tartrazin, or crystal violet and tartrazin. The rose Bengal filter is made from:

Tartrazin 10 g.

Rose Bengal 5 g.

Gelatine, 8 per cent solution 700 ccm.

The methyl violet is:

Methyl violet 0.7 g.

Gelatine solution 700 ccm.

The crystal violet is:

Crystal violet 0.4 g.

Tartrazin 5.0 g.

Gelatine solution 700 ccm.

The above quantities are sufficient for 1 square meter. It is not advisable to mix the violet with the rose Bengal. A few drops of glacial acetic acid may be used with both the violets to facilitate solution. Unless the dyes are used in the form of a solution previously made, care must be exercised that they are actually in solution. There should be no difficulty in this, as they are readily soluble in the hot gelatine, only, if added all at once, small lumps of the dyes may become coated on the outside with chilled gelatine and may therefore not dissolve well. It is preferable to make the gelatine solution double strength, use half of the water to dissolve the dyes, and then mix. The dyed gelatine should be filtered through linen that has been well washed and wrung out of hot water.

The green filter must be made with two glasses; one should be coated with:

Tartrazin 2 g.

Naphthol green 1 g.

Gelatine, 8 per cent solution 700 ccm and the other is coated with:

Acid green JE 0.5 g.

Gelatine solution 700 ccm.

In place of the acid green, 3.0 g of brilliant green may be used. The blue filter is prepared with:

Yellowish eosin 4.0 g.

Gelatine, 8 per cent solution 700 ccm.

3.0 g bluish eosin may be used instead of the yellowish. The second glass is coated with:

Methylene blue 4B 1 g.

Gelatine solution 700 ccm.

For those who do not want the trouble of making the filters, the Wratten & Wainwright gelatine film filters of the requisite size may be bound up between glasses. The numbers of their filters are, for the red No. 29 or F, for the green No. 61 or N, and for the blue No. 50 or L. A moment's consideration will show us that as we want to reproduce the red, we must use a red-sensitive plate, and it will be found more satisfactory to use panchromatic plates for all three negatives, as by doing so the gradation in the three negatives will be more alike than when we use different kinds. There is some latitude here, as we are able to modify the final result, and one might choose a panchromatic for the red, an isochromatic for the green and an ordinary plate for the blue filter exposures; but it cannot be advised. Whether a fast or slow panchromatic plate be used is of no particular moment; the slow kind will give as good results as the fast and is less likely to fog. At the same time it must be borne in mind that we are making negatives, not transparencies, and this must be kept in view in developing. Brilliant-looking plates with clear glass shadows are not the desideratum; but soft negatives, rather thin in the high-lights, with fully exposed shadows, should be aimed at. It is immaterial what developer is used, and the beginner should use that to which he is accustomed. Naturally the desensitizing process may be adopted, and the plates should be backed.