DUSTING-ON (POWDER) PROCESS (Pr., Procede aux poudres ; Ger., Einstaubver fahrung) Known also as the " Powder Process," based, as are the other carbon processes, upon the oxidising action of chromic salts upon organic matters. In the present instance, the organic matter loses its tackiness or stickiness under the action of light and refuses to retain dust. The process is largely used in the production of photo-ceramics and for the intensification and doctoring of faulty negatives when these are of great value. Various substances are available for the support of the powder picture, but none is better than ground opal or pot opal glass. As the effect of the light in the process is to cause the film to refuse to take powder, the parts acted upon remain light ; this being the case, a transparency (positive) must be used for printing from and not a negative, as a print from the latter in this process would be a nega tive. The requisites are sensitising mixture, opals, powder, and a transparency of excellent quality. Although opal has been named and is recommended, glass, ferrotype, and other supports can be used. In any case, it must provide a proper contrast to the powder. Thus, if the dust or powder used is silver or gold bronze, the image may be produced on a dark background, for which a ferrotype tinplate will serve.
The support must be cleaned and sensitised in a solution consisting of a gum, a hygroscopic substance, and a light-sensitive medium. The following formula is one of a great number between which there is but little to choose : White sugar . . zoo grs. 20 g.
Gum arabic . . zso >, Ammonium bichromate zoo 20 Methylated spirit . i oz. 5o ccs.
Water . . . 20 I,000 An old, and possibly the original, formula, still in use, is as follows : Water . . . ro OZ. r,000 ccs.
Dextrine . . . 24o grs. 48 g.
Grape sugar . p 48 Potassium bichromate 240 I, 48 The first of these formulse is said to possess many advantages. The gum and sugar should be covered by the water and dissolved by heat ; when cool, the other ingredients are added. The spirit is used simply to make the solution flow over the plate more evenly, and more or less may be used as desired. After washing, the wet plate is flowed over with the sensitive mix ture, drained for a few minutes, and then baked in an oven until bone dry. The operations up
to baking may be carried on in daylight, but as the plate dries it becomes sensitive and should therefore be removed and examined in a dull light. The plate is then ready for exposure under a positive, and, as in many other cases, this is the most difficult part of the process, for so much depends upon atmospheric conditions.
The duration of exposure can be found only by experiment, but as a rough guide it may be said to be one minute in bright sunlight on a summer's day, when a transparency of average density is used. When exposure is complete, the plate is " developed " by dusting on a powder. The choice of a powder is almost unlimited, but it must be exceptionally fine, to ensure which it should be sifted through a muslin bag. Ordinary powder colours from an oil-shop may be used. Ivory black and Indian red are good colours, used alone or mixed in varying proportions, while some use levigated graphite.
For printing, the sensitive plate should be warmed, or it may be warmed by being printed in strong sunlight. An image will be seen faintly upon the sensitive plate when removed from the frame. The plate is held in the fingers or on a pneumatic holder, and some powder sprinkled over the surface, immediately spread with a soft camel-hair brush, and kept on the move until sufficient detail and density have been secured.
Development proceeds rapidly as the plate cools, and it should be assisted by a gentle stream of air blown from the mouth across the plate, the current not being strong enough to dis turb the powder or cause excessive moisture ; the air must not be absolutely dry, and the powder must be kept on the move with the soft camel hair brush. Almost any depth can be obtained in any part of the picture by patient re-applica tion of the powder rather than by attempting to make too much powder adhere at one time. As the progress of development is visible and under perfect control, it is not difficult to bring out or keep back certain parts, and in this way very artistic results may often be easily obtained.