CERAMIC PROCESS The art of obtaining a burnt-in impression of a photograph on earthenware, china, or porcelain. Such pictures are permanent because the image, formed by a vitrifiable powder, is protected by an imperishable glaze. The material of which the picture is composed must obviously be of a special nature. The process is based on the dusting-on or powder process, in which a bichro mated film loses its tackiness in different degrees by exposure to light under a negative, so that on brushing it over with a suitable powder an image is obtained ; but in ceramic photography the powder used is a vitrifiable colour which stands firing without injury.
The transparency used must be bright, vigor ous, and of the highest quality. The image for firing is not produced at first on its final support. A polished glass plate forms a temporary sup port, the print obtained on it being afterwards coated with collodion, stripped and transferred. The glass plate is cleaned and coated with a mix ture of the following two solutions :— Organifier Le Page's fish-glue . 2 oz. 62 g.
Glucose . . . 8„ ,, Glycerine . . . 20 mins. ccs.
Water . . . . 21 oz. 600 „ Sensitiser Ammonium bichromate 90o grs. 58 g.
Water . . . . 21 oz. 60o ccs.
If a smaller quantity is desired, keep to the same proportions. Mix together in equal parts and filter as required. Such solutions may be purchased ready prepared. The coated plate is dried by gentle heat, not greater than that which can be borne comfortably by the back of the hand. A whirler is useful for rapid drying. A thin, even coating should be aimed at, and the film ought to be quite glossy when dry. The exposure under the transparency varies with the quality of the latter and the light, and it is better to use an actinometer ; it may range from forty seconds to three minutes in the sun, or to as much as fifteen minutes in diffused light. With a correct exposure the image will be faintly visible on the film. Development is performed as in the dusting-on process (which see), but the plate is held in an inclined position resting on a sheet of white paper, sensitive side towards the light, so that progress may be viewed by both transmitted and reflected light ; and vitri fiable powders must be employed. These con sist of metallic oxides and other fusible sub stances, and they are obtainable in a number of different tints. Sift the powder through silk, but it may be necessary, before doing this, to re-grind it with water, using a muller on a glass slab. The powder must be thoroughly dried
before sifting. Firing alters the colours of the powders to an extent that only experience will show. When the image possesses about the same vigour and transparency as a good lantern slide, the surplus powder is dusted off, and the picture coated with collodion (either plain or a special preparation), which is flowed over the plate like a varnish. When the collodion has set, a sharp penknife is pressed downward through the edges of the film on three sides of the plate, cutting through to the glass ; the plate is then immersed in several changes of water, preferably filtered, to remove the bichromate salt. The film will separate from the plate, except on one side. When the bichromate seems nearly all removed, the film is placed in a solu tion of fused borax for about ten minutes, finally washing in a fresh bath of filtered water for an equal period. To prepare the fused borax solution, place 2 oz. of the fused borax in an enamelled saucepan with water, boil rapidly for five minutes, decant, add fresh water, and repeat until all the borax is dissolved. For use, take three parts of the saturated solution (cold) and add one part of water. For trans ferring, fill a large basin or dish with the borax solution. The plate, after its final washing, is stood in a rack for a few minutes to drain off the surplus moisture. The penknife is then very carefully passed along the fourth edge of the film, which must not be cut through by drawing the blade along, as that would pucker or tear the film. The plate is now placed in the basin, and the film will float off the glass, which can then be withdrawn. With a camel-hair brush turn over the film in the solution so that the collodion side is downward ; if the fingers were used there would be a risk of damaging the loose film or of abrading the powdered side. The plaque or other support is next introduced under the floating film, which is guided into its correct final position. The support is then slowly, and by degrees, lifted out of the water with the film adhering, powder side uppermost, and placed on blotting-paper to dry, after which loose pieces of film may be removed with a damp sponge, and the image should be very carefully examined for any black spots or imperfections, the former being removed with a fine needle-point set in a wooden handle. The delicate powder is next protected by flowing over it— Patty oil of turpen e . . . so rains. 20 ccs.