Gas Black and Boiled Oil.

An attempt is made to appraise theoretically the basic principle of lithography. The differences in dispersions produced by gas black in boiled linseed oil and in mineral oil are considered. These differences are ascribed to the polarity of the linseed oil acids which results in multimolecular adsorption layers of them on the pigment particles, thereby producing a gel structure in concentrated suspensions. The affinity of the polar heads of drying-oil molecules for water is next considered. It is claimed that mono-molecular films of oil molecules produced during printing are transferred into globules of oil dispersed in the plate moisture. These oil globules eventually wet the desensitizing gum film and cause scumming. This wetting is prevented by including some inorganic material that swells the gum film and provides acidity. Acidity causes repulsion of the polar heads of the oil molecules and inhibits their transfer.