Calotype
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The calotype was an early photographic process introduced in 1841 by William Fox Talbot, using paper sheets covered with silver chloride. The image was fixed in strong salt solution - potassium iodide of hypo.
It may be briefly described as the application of silver iodide to a paper support. Carefully selected paper was brushed over with a solution of silver nitrate (100 grains to a fluid ounce of distilled water, or 5 g/L), and dried by the fire. It was then dipped into a solution of potassium iodide (500 grains being dissolved in an imperial pint of water, or 57 g/L ), where it was allowed to stay two or three minutes until silver iodide was formed. In this state the iodide is scarcely sensitive to light, but is sensitized by brushing "gallo-nitrate of silver" over the surface to which the silver nitrate had been first applied. This "gallonitrate" is merely a mixture, consisting of 100 grains of silver nitrate dissolved in 2 fluid ounces of water (114 g/L), to which is added one-sixth of its volume of acetic acid, and immediately before applying to the paper an equal bulk of a saturated solution of gallic acid in water. The prepared surface is then ready for exposure in the camera, and, after a short insolation, develops itself in the dark, or the development may be hastened by a fresh application of the "gallo-nitrate of silver." The picture is then fixed by washing it in clean water and drying slightly in blotting paper, after which it is treated with a solution of potassium bromide, and again washed and dried. Here there is no mention made of hyposulphite of soda as a fixing agent, that having been first used by John Herschel in February 1840.
This process was the first to use a negative image that can be reused to produce several positive prints. Its primary weakness was in the reliance on a paper surface, as the fiber patterns and other imperfections were inevitably reproduced in prints. One available solution was to use a glass plate negative, but first it was necessary to find a way to bind the chemicals to the glass. This was accomplished in the early 1850s with the development of albumen prints and the collodion process, after which the calotype became obsolete.ja:カロタイプ zh-cn:卡罗法