For subtractive tinting a uniform yellowish brown coating is produced by bathing the negative for some time in a solution of potassium permanganate containing 9 gr. per 20 oz. (I grm. per litre), without any addition of acid. After the negative has been rinsed and dried, local decolorization is carried out by means of a dilute solution of bisulphite of soda (e.g. 5 parts of the commercial solution diluted to Too parts), thickened with glycerine or gum arabic (R. Namias, 1914). When the work is finished, the negative must be washed in a strong stream of water or by shaking it vigorously in a large dish of water.
468. Local Abrasion of the Negative. In order to diminish the density of small areas of the image, the surface of the gelatine may be rubbed down by means of the finger or a piece of wash-leather with a little powdered pumice or powdered cuttle-fish bone ; if a less vigorous treatment is required, the rubbing may be done with a wad of flannel or a skin stump soaked with metal-polish, or merely moistened with alcohol.' 469. Work on the Back of the Negative. A little carmine (moist water-colour) may be applied with the finger to the back of a glass negative, so as to cover uniformly those parts which are to be blocked out ; the boundaries of these parts may be overstepped, and in such cases the excess may be removed with a moist duster, followed by a moistened brush. Printing must be carried out in diffused light, so that the thickness of the glass will prevent the outline of the work from showing too clearly in the print.
The back of a glass negative may also be covered with colourless or tinted matt varnish. Those parts which are to be clearly printed are then cut away with a scraper or with a wad moistened with alcohol, according to the area of the part to be treated. Parts which need darkening are covered with colour, as above, or are worked up with blacklead (graphite, plum bago), put on with a dry brush or with a soft stump.
An excellent matt varnish of average grain may be prepared according to the formula given below. The resins are dissolved in the ether 1 in a flask fitted with a sound wood cork. When they have dissolved, a process which may take several days in spite of frequent shaking, the benzene is added slowly, and the mixture is allowed to stand at least a week before the clear liquid is decanted— Ether . . 14 oz. (70o c.c.) Sandarac . 570 gr. (65 grm.) Mastic (tears) . 130 gr. (15 grm.) Benzene, to make . . 20 oz. (1,000 c.c.) A finer grain is obtained by increasing con siderably the amount of ether ; on the other hand, a slight increase in the amount of benzene gives a much coarser grain. The matt varnish may be coloured yellow by dissolving a certain amount of aurantia in it.
The precautions necessary for spreading the matt varnish on the glass side of the negative are described later in the paragraph dealing with varnishing (§ 479). The chief thing to avoid is the application of any varnish to a place already dried. A negative on which the varnishing has not been successful may be cleaned by rubbing with a wad of cotton wool moistened with methylated spirit.
None of these methods is suitable for film. negatives, but equivalent results may be ob tained by working on a thin sheet of matt cellu loid or on a piece of tracing paper which is fixed to the back of the film by pieces of gummed paper.
470. Reduction of Contrasts by Soft Positive. It has often been suggested (A. Leitner, 1890)
that excessive contrast in a negative may be compensated by placing against its back a soft positive transparency taken from the same negative. A special " auto-retouching " printing frame was indeed constructed (E. Artigue, 1903) for this purpose ; with this apparatus a suitable transparency, which had been previously printed in the same frame, could be interposed in correct register.
471. Blocking-out. The object of blocking out is to obliterate all traces of background in a photograph which could not be taken against a white ground ; the shadows cast by an object standing on a white ground are also obliterated by this For this, one of the following methods may be used : The negative may be worked-up by tinting the gelatine in the way previously described (§ 467), or an opaque paint may be applied, either to the glass side (for a portrait in which the outlines are not usually very clearly marked) or to the gelatine surface (photographs of furniture, machines, etc., in which the outlines are sharply defined). Water colour body-paint or water colours (chrome yellow, vermilion, india red, etc.), or a solution of 20 per cent bitumen in turpentine thickened with a little wax, or special opaque paints may be applied with a brush or pen (at least for the outlines) to a breadth of about half an inch. Areas beyond this band may be masked by sticking-on opaque paper. Good water-colour brushes, giving fine points, must be used. Before starting work it is advisable to remove all traces of grease from the surface of the negative by rubbing lightly with a wad of linen or cotton soaked in methylated spirit. In order to dilute the colour to the desired strength and to make sure of rapid drying, alcohol, or a mixture of alcohol and water, may be used.
472. Spotting. Spotting is the process by which the clear spots on a negative (dust, air bubbles, scratches, etc.) are touched out. For this purpose very small sable brushes are used ; these, when moist, must give a very fine point. It is usually impossible to match the density of the spot exactly with that of the image, and thus it is more often attempted to produce a slightly greater density in the spot than exists in the surrounding image. The work is then finished by retouching the positive prints.
Spotting is carried out on a well-illuminated retouching desk. The ground glass of the desk should be masked with a sheet of black paper having an opening of small diameter. The different parts to be spotted are then brought one by one over this opening, so that attention is concentrated on the spot to be treated.
For this purpose black water-colour, more or less diluted, may be used, or indian ink thickened with gum arabic or strong liquid glue (usually obtained in tubes) ; or blacklead mixed in a little negative varnish (§ 478). Mistakes may then be cleaned off by means of alcohol, but negatives which have been retouched in this way cannot be varnished unless a varnish is chosen which has no action on the resins of that previously used.
The work is followed through a lens. It is essential not to allow the colour to go so far as to form a ring round the hole to be filled, otherwise the defect is made worse. For very small holes, the colour used must be fairly thick ; only a little must be placed on the brush, and the latter must be held perpendicularly to the surface of the negative.