CAL'OTYPE PROCESS (Gr. kalos, beautiful, typos, impression), a title comprehending a variety of methods for the production of negative photographs on paper; and so named by the inventor, the late Dr. 11. Fox Talbot, who exhibited the result of his experiments in the year 1840. The principle involved in the C. P. depends on the sus ceptibility- to the action of daylight of a surface chemically prepared, and the practice consists in the preparation, and exposure in the camera, of a sheet of paper, having on one surface an even and finely divided layer of iodide of silver, nitrate of silver, and an organic acid; the image obtained on this surface being subsequently developed with gallo-nitrate of silver. It will be unnecessary here to describe the various modifications which have been introduced with the object of imparting a high degree of sensibility to the paper; one process—the best—will suffice to describe the manipulation.
Good English paper, sized with gelatine, should be chosen. the foreign starch-sized papers being unsuitable, on account of the solutions sinking in too deeply, and thus impairing that sharpness of outline which should be possessed by a good negative. The paper is then floated on one side, and for a moment only, on a solution of iodide of silver in iodide of potassium; prepared by adding freshly precipitated iodide of silver to a strong solution of iodide of potassium. It is then dried, and plunged into a dish containing distilled water, which, by removing the soluble iodide of potassium, precipi tates the iodide of silver in an even and finely divided condition over the whole surface of the paper, which in this state will keep good for twelve months. It is now ready to receive the sensitive coating; this operation, which is called exciting the paper, is per formed in the following manner: Two solutions are prepared—one, a saturated solution of gallic acid in cold distilled water, called solution A; the.other, a solution of 50 grains of nitrate of silver in 1 oz. of distilled water, to which 1 dram of glacial acetic acid has
been added; this is called solution B. The iodized paper obtained as above is now laid on a board having a piece of clean blotting-paper on it a little larger than the paper to be excited, and the following solution brushed over it with a clean Buckle's brush: distilled water. 1 oz.; solution A, 15 drops; solution B, 15 drops. This mixture, pre pared in a chemically clean glass vessel, should be freely applied, and the excess alisorhed by clean blotting-paper. The paper is now ready for exposure in the camera, and may he at once placed in the dark slide; or a stock may be thus sensitized and preserved between folds of blotting-paper until required for use. The time if oposare —varyiwr from three minutes to a quarter of an hour—is determined by the diameter and foeal length of the lens employed, the aperture of the diaphragm or stop, and the amount of light prevailing at the time. The development of the latent image, an operation which, like the preceding, is, of course, conducted in a room illuminated only by yellow light, is accomplished by applying freely and uniformly over the whole surface solution A; and when the image begins to appear, applying a second quantity, to which a few drops of solution B have been previously added, to increase the intensity. The whole operation of development occupies about a quarter of an hour; and when the details are fully out, the picture should be washed with water, and fixed, by immersion in a solution of 1 part of hyposulphite of soda to 4 parts of water: it is then again freely washed in frequent changes of water during several hours; it is lastly dried and waxed; when it may he regarded as a finished negative, from which positive prints may be obtained, having the lights and shadowt: as hi uattire, See POSITIVE PRLNTINO.