CHAPTER VII.
Negatives Suitable for Collotype.
A COLLOTYPE plate may be readily produced from any negative, but the highest results are naturally only obtained when the negative is prepared with a view to its special use. First should be considered the question—May the negative, without detriment to the finished print, be utilised without a reversal? In the not very frequent case of this important point being immaterial, it is simply necessary to produce the negative on patent plate, and it is at once available for printing from. As the negative may easily sustain damage in any of the various operations, it should be thinly and evenly varnished; this, while affording protection, does not mar the sharpness of the resulting print to any appreciable extent.
Reversed Negatives.—In the majority of cases, the resulting copy has to exactly correspond with the original as regards the position left and right, and in such cases the negatives will have to be reversed. This is usually effected in one of four different ways—
1. Reversal of the image by reflection with a prism or mirror, or reversing the plate.
2. Stripping the negative from the glass.
3. By the dusting on or powder process.
4. By the use of negative films.
The second is that usually adopted. With regard to the special features desirable in negatives for Collotype, one giving a good silver print will answer equally well for Collotype—they should incline rather to softness than the other extreme. It has been remarked that they should be as free from veil or fog as possible, and may be intensified or not according to the judgment of the operator.
Portraits from Nature, with their fine half-tone, and at the same time necessary contrast of light and shade, are the most difficult to reproduce in Collotype. The negatives should be perfect in gradation, and such as are usually only obtainable from a talented operator who has the lighting of his studio entirely under his control.
Black and White subjects, on the contrary, for their satisfactory reproduction, require a negative of the utmost density, but perfectly clear in the lines. Such are better produced by the collodion process, as directed for photo-lithography. For subjects in lines or dots entirely free from half-tone, photo-lithography is more suitable for reproduction than Collotype, where price is a consideration.
Reversal by Reflection, by means of a mirror or prism, necessitates a special and rather expensive apparatus, and the exposure being somewhat lengthened, it is, although quite successful, more suitable for lifeless objects.
Stripping the Collodion Film is a safe and cheap procedure for producing the necessary reversal, and in the subsequent printing operations the danger of breakage, when compared with glass, is reduced to a minimum. The glass plate, free from defects, must be well cleaned, and it is safer, and renders the stripping more certain, if it be covered with a fine film of pure beeswax. This is best applied by rubbing over the surface a few drops of a solution of beeswax in benzole and carefully polishing, at the same time avoiding the entire removal of the wax. The piece of linen used in the operation should be rubbed upon a lump of wax previous to the polishing. The plate must on no account be gelatinised or albumenised, except round the edge only, as a precaution against premature slipping.[J] Use a tough collodion, and otherwise proceed as usual for the wet process. After the negative is finally washed and dried it should be varnished with a thin solution of gum arabic, to which has been added a few drops of a solution of chrome alum. After complete drying the margin or edges of the plate are rubbed with a little tallow. To prevent overflow the plate is accurately levelled and flooded with a solution of 10 parts gelatine soaked in 60 parts of water and dissolved by heat, to which is added 20 parts of alcohol and 6 parts of glycerine, using 1 dram of the solution to each four square inches. A few drops of phenyl, carbolic or salicylic acid may be added to the solution as a preservative, and the whole before use well filtered, while hot, through linen. The hotter the solution during the pouring on the more evenly the coating will run upon the glass, and it may be assisted in its flow by careful tilting of the plate. The use of paper strips is dangerous, and, notwithstanding the gum, extremely liable to damage the negative.
[J] Instead of wax the plate may be dusted over with powdered French chalk, all traces of which are removed before applying the collodion. After such treatment the film is scarcely likely to leave the plate before required. But as a further precaution, if the edges of two plates be drawn across each other a rough edge will be produced, to which the collodion coating will adhere tenaciously.—Trans.
Reversal of the Plate, either collodion or gelatine, is another method of securing reversed negatives, and consists simply in inserting the sensitised plate in the dark slide with the uncoated side towards the interior of the camera. A modification of the dark slide may be necessary, and it will be obvious that the ground glass focussing screen must be reversed, or the focus corrected after the insertion of the dark slide by shortening the camera a distance equal to the thickness of the sensitive plate. Dry plates present great advantages in this method, but, in development, density must be judged entirely by transmitted light, as the image is formed in, not upon, the film. As the rays of light pass through the glass to the sensitive surface, it is necessary that the uncoated surface be quite clean, and the glass itself should be perfectly free from bubbles, scratches, or other marks.
The plate should remain in its levelled position for about fifteen minutes to set and then be carefully removed to a safe and airy position, gelatine side outwards, and allowed to dry. This operation will probably take two days; the hot gelatine layer should be about 3 mm. thick. The negative may then be coated with a toughened collodion (one or two per cent. of castor oil added to plain collodion). The plate is placed aside, and in an hour will probably be dry and ready for the next and final operation. Cut with a knife through the film, take it by one corner, and, without pause or hesitation, strip it from the glass.
Any necessary retouching should be done upon the gelatine surface after roughing it sufficiently with pumice powder to enable it to take the pencil, and all such work must be completed before removing the film, particularly if it has been damaged, as in such places the colour used in retouching would cause an adherence, and the film would almost assuredly tear in stripping.
Upon the quantity of gelatine solution used the nature of the film depends. It should be entirely free from air bubbles. Too thick a layer almost always cockles, and does not lie flat during printing, in consequence of which portions of the picture will not be sharp in the print. On the other hand, too thin a layer is extremely awkward to handle in the larger sizes, and it is preferable to transfer it in a reversed position to a sheet of plate glass coated with indiarubber solution.
Gelatine Plates for Stripping are in the market. In their preparation it is necessary to first coat the glass with a solution of yellow wax in benzole, or an indiarubber solution, before coating with emulsion, such plates being known as strippers in America and Germany. When finished, the negative is placed, together with a gelatine film—or “skin”—in a solution of glycerine and water, both are withdrawn together, a squeegee applied to remove the superfluous solution, and the negative, with its adhering skin, placed aside to dry. Afterwards the edges of the film are cut through, and the negative removed from the glass.