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Dusting-On Processes 677

image, powder, light, coating, method, positive and glass

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DUSTING-ON PROCESSES 677. General. Gamier and Salmon, who, since 1858, had used a solution of ferric citrate for the same purpose, showed in 1859 that a, thin layer formed of albumen, sugar, and bi chromate, coated on glass, dried, and exposed under a positive transparency and then kept for a few moments in a damp atmosphere, will retain, on those parts protected from the action of light, mineral substances in powder form dusted on the surface. The powder cannot adhere, however, to parts sufficiently modified by the action of the light ; there is very satis factory rendering in the tones.

With some minor modifications, this method has been regularly used since then for the production of photographic enamels, and for the photographic decoration of ceramic goods, powdering being done with vitrifiable glazed colours.' The method is also most suitable for making duplicate negatives or positives direct, and also provides the means for the intensifying of negatives or positives without affecting the original image in the least. It is for these latter purposes that it is described below.

Some years ago R. Namias (1922) resuscitated a process used in 1879 by A. Sohacchi for copying tracings, and applied it to pictorial photographic printing (resinopigmentypc); a similar process was brought out by E. Buri (1924).

In connection with these processes may be mentioned the " color " process of J. Sury (1913), constituting, in a way, a method for preparing photographic pastels.

678. Direct Reproductions by the Powder Process. Dissolve in the cold— Gum arab ic . u oz. (50 grin.) Sugar . . 2 oz. (ioo grim) Water, to make . 20 OZ. (1000 c.c.) to which may be added an antiseptic, e.g. 18 gr. (2 grin.) of salicylic acid. At the time of use, the required quantity of this liquid, which must be quite clear, is mixed with a quarter of its volume of a 20 per cent solution of ammonium bidiromate. This mixture is coated in a thin film on a perfectly clean glass. Excess of liquid is drained off, and the plate is dried by holding it, bare face downwards, over a gas ring,' until the coating is no longer sticky to the linger.

Before the glass has quite cooled down, it is placed in a frame under the negative (or positive transparency), which should be perfectly dry, and exposed to light.

As soon as exposure is complete, the plate is supported above a sheet of white paper, against which the development of the image is followed, taking care not to breathe on the sensitive coating ; to prevent this, a of glass is held in front of the operator.

A large, very soft, and perfectly dry badger hair brush is dipped in the finest powdered graphite, which must have been dried for some minutes in a metal container on a stove. The brush is then drawn lightly in all directions over the sensitive surface.

The image gradually appears, according as the portions of the layer protected from the action of light absorb the moisture from the air. The contrasts of the image will not increase beyond a certain point, the powder then begin ning to adhere uniformly all over the surface. Powdering must be stopped before this point is reached, and the image must then at once be given a coating of collodion.

The same method is used in intensifying a negative, the sensitive coating being applied on the surface of the image, which has been previously varnished.

The method may also be used for obtaining positive images by printing (from a negative) on a black support (or on clear glass which is given a coating of collodion and then of black varnish). In this case, powdering is done with a white or very light powder, e.g. aluminium powder or bronze powder.

679. Resinopigmentype. The material used is a smooth paper base, heavily coated with har dened gelatine. This is sensitized by immersion for two or three minutes in a 5 per cent solution of ammonium bichromate.

The sensitized paper is exposed under a positive transparency (or under a very vigorous positive print, the contrast of which is naturally less when used by transmitted light) in the shade or in sun,' until a weak negative image in brown on a yellow ground is seen on the paper.

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