GELATINE, BICHROMATED Gelatine treated with an alkaline bichromate forms the basis of the carbon process and of all photo-mechanical printing methods. Fish-glue is employed in some processes, but that is a sub stance closely allied to gelatine ; and other colloids are sometimes substituted, as in the gum-bichromate process. Gelatine, in its nor mal condition, will absorb cold water readily, and dissolve easily in hot water. Gelatine treated with an alkaline bichromate retains these qualities if kept in the dark ; but if exposed to light, it no longer absorbs cold water, or swells in consequence, and it also becomes insoluble. These properties are utilised in different printing methods. In the carbon process, and in some photo-mechanical methods, an image in gelatine is produced by exposing a film of bichromated gelatine under a negative, and then dissolving away those parts on which the light has not acted by means of hot water. In others, 4
gelatine relief is produced by means of the unequal swelling of a gelatine film that has been exposed under a negative when soaked in water. Details of the various processes in which these qualities are utilised are given under their respective headings.
In process work, many processes depend on the properties of bichromated gelatine. Among these may be mentioned collotype, photogravure, photo-lithography, and photo-relief. For the half-tone process gelatine has not been found so satisfactory as fish-glue, the latter having no setting property, and being easily developed with cold water. An attempt was made of late to utilise for half-tone a gelatine in which the setting property had been destroyed, but it was found to be more subject than fish-glue to changes of temperature, and also liable to become putrid very readily.