Calotype Process

Calotype Process, a photographic process by means of which a negative is obtained upon paper. It was patented by Dr. Fox Talbot in 1841. The process depends on the sensitiveness to light of silver salts. Good paper is first brushed over with a solution of silver nitrate (100 grains to the ounce), and dried. It is then floated on a solution of potassium iodide, by means of which silver iodide is formed on the paper. It is then made more sensitive by brushing over with, first, a saturated solution of gallic acid, second, a solution of silver nitrate (50 grains to the ounce), with a little acetic acid added. The paper is then ready for exposure in a camera, in the same manner as ordinary plates. After exposure it may be left to develop in the dark, or the development hastened by means of the gallic acid and silver nitrate used in sensitising. It is then washed, fixed by hyposulphite of soda, again washed and dried. It is finally waxed, to make the paper translucent, when it can be used as a negative to produce positive prints. It is needless to state that all the operations of sensitising and development should be performed by red or yellow light only.