CALOTYPE, OR TALBOTYPE, PROCESS A negative process upon paper, invented by Pox Talbot and patented by him on September 20, 184r. It was the third British patent for photography, the two previous ones being for the Daguerreotype process. The patent was afterwards disputed by the Rev. J. B. Reade, but Talbot's claim was upheld in the law courts, mainly for the reason that Reade's previous dis covery was not properly published or made known. Fox Talbot's process was afterwards considerably improved by C. Cundall. The original process is briefly as follows : Paper of close texture was washed over with a solution of too grs. of silver nitrate in 6 oz. of water. When dry, the paper was immersed in a solution of potassium iodide, 25 grs. to each ounce of water, for two or three minutes, then rinsed in water and dried. Paper in this condition was called " iodised paper," and could be stored in a port folio for use as required. Sometimes the double operation referred to above was performed at one time by brushing a solution of iodide of silver and potassium over the paper with ct Buckle brush. In order to prepare the paper for exposure in the camera two solutions were necessary : A. too grs. of silver nitrate dis solved in 2 oz. of water, to which is added one sixth of its volume of strong acetic acid. B.
A saturated solution of crystallised gallic acid. Equal parts of A and B were mixed together, the mixture being called gallo-nitrate of silver. The iodised paper was brushed over with this solution, or the paper floated upon it for half a minute, then rinsed in water or blotted off, the operations being carried out in the dark-room. The paper was then placed, either wet or dry, in the dark slide and exposed in the camera, the exposure necessary being, under good conditions, about six minutes. The paper was developed by washing over with gallo-nitrate of silver (as above), and was fixed, after washing in water, by a minute's immersion in ct solution of roo grs. of potassium bromide in 8 oz. of water. Finally it was washed, dried, and printed from.
Modifications of the process consisted in slight alterations in the sensitising bath, the use of ferric protosulphate as a developer, and of sodium hyposulphite as a fixer, and the making of the paper negatives more easily printable by waxing or by immersion in almond oil. The calo type process was popular between r841 and 1851, but was superseded by the collodion process