Vignetting.Special smoked-glass negatives are obtainable which fit into the printing frame outside the ordinary negative, and give to the picture the delicate shading off of the vignette. The same effect is obtainable with the canvas or serrated tissue edged cards pinned or strapped loosely on the wooden upper surface of the frame. Fig. 52 shows a very simple form of card vignette. The aperture is shaped for a portrait (head and shoulders) and is cut out rather smaller than the final vignette is to be ; after wards it is notched as shown and the edges turned up. Fig. 53 is a rather more elaborate device ; the opening in the card is larger, and within it two thicknesses of old celluloid film (with the silver fixed out) project towards the centre. The inner thickness is serrated at the edge. For landscapes, vignettes may be made of oval, square, or arabesque shapes according to taste. A piece of flat cork about / in. thick might be glued at each corner of the printing frame to raise the vignetting card a little further from the negative and receive the fixing pins. The distance increases the soft delicacy of the vignette.
For this kind of vignettes direct sunlight must be avoided, and the frame shifted several times during printing, or slung by two loops of string from above, so that it will swing to and fro. Vignetting is also done in the camera, especially for enlarging, by placing a black card, with an oval opening of the proper size, about half-way distant between lens and plate. It may be made to fit in the folds of the bellows.
Masks and discs are also employed, especially in pro ducing postcards with ornamental borders. The border negative is usually printed first, and then replaced by the picture negative, with a mask of black paper protecting the portion that contains the border.
Toning.When the print is finally taken from the printing frame it must be placed (under pressure if possible) in a place secure from light and moisture to await toning, which process is sometimes delayed for weeks to suit con venience. This practice is a bad one as, however carefully protected from light and air, the whites have a tendency to deteriorate. The toning process consists in adding a thin layer of gold or platinum to the image, improving its chance of permanence and also its colour, which would otherwise be most disagreeable after fixing.
The components of chloride emulsions are not always the same in the different brands of P.O.P., and the directions supplied by the makers should be carefully followed. Some varieties must be placed directly in the toning bath, without any preliminary washing, or the loss of colour will be enor mous. Other kinds must be washed for ten minutes before toning. Nearly all are improved by immersion for five minutes in a solution of common salt to transform the free silver salts. Some use a little carbonate of soda to destroy the acidity of the paper.
Gold Toning.The toning bath will consist of chloride of gold, with some compound to prevent the chlorine from reducing the silver image while the gold is depositing. Two grains of gold will tone a full-sized sheet, 24i x 17 in., and it is well to place prints in regular batches in the bath with a certain quantity of solution in order to obviate the irregular action of baths of varying strength. A too strong bath tones very quickly, but the prints lose terribly while fixing ; a weak bath is too slow, and produces poor, dull pictures.
Acetate Bath. Stock Solution. Sodium Acetate . . . . i oz.
Gold Chloride . . . . . . . Is gr.
Water . . . . . . . . 4 oz.
Mix 24 hours before it will be required for use. To tone half a sheet of paper, take 2 dr. of stock solution to io oz. of water. A reddish brown colour is reached in about lc) minutes. This is a very economical bath, giving a good brown to blue-black colour in the finished print.
Sulpho-cyanide Bath. Gold Chloride . . . . . 2 gr.
Ammonium Sulpho-cyanide . . 24 ,, Water . . . . . . . . i6 oz.
Mix separately in boiling water, and then add the two solutions together. This bath takes 12 hours to ripen. Some papers will not tone well in the sulpho-cyanide bath ; others do better if the proportion of sulpho-cyanide is re duced by about one-half. The directions of the manufacturer should be consulted on this point.
Phosphate Bath (Kentmere). Gold Chloride . . . . . z gr.
Sodium Phosphate . . . . . . . 6o Water . . . . . . . . . zo oz.
Formate (Wellington), Gold Chloride . . . . . . . . z gr.
Sodium Formate . . . . . . . 6o Water . . . . . . . . 40 oz.
The two last mentioned should be made up an hour before using. They will not keep.
Borax Bath. Gold Chloride . . . . . . . . 2 gr.
Carbonate of Soda . . . . . . . 2 60 II Water . . . . . . . . 16 oz.
The borax must be dissolved first in hot water with the carbonate, and the gold added on cooling.
Chloride of gold frequently contains free acid, and should be neutralised with a very little chalk or magnesium carbonate.
Subject to what has been remarked above with regard to the special characteristics of some emulsions, the sulpho cyanide bath gives the most pleasing colour ; tungstate, formate, and phosphate being likely to produce a cold blue black after prolonged immersion. The toning bath does not work well when warm, and the best tones are usually found in cold weather. Keep the prints continually moving, and do not let the bath get too poor in gold, or the prints may turn yellow in the high lights, and may also give " double tones." As each print reaches its proper tone transfer it to a dish of clean water, replacing it with an untoned one. Five or six prints are enough for the toning bath at any one time.