Process for preparation of lithographic printing plates

Claim 1

A process of preparing lithographic aluminum sheet having a grained surface, comprising the steps of: a. anodizing said grained sheet in an aqueous solution containing up to 250 grams per liter of a polybasic aromatic sulfonic acid having at least one nuclear acidic constituent selected from the group consisting of a carboxylic acid and a phenolic hydroxyl, and a lesser concentration of a polybasic mineral acid, said anodizing solution being maintained at a temperature of 10° to 80° C and effecting the anodization within 1 to 30 minutes, at a potential of 20 to 150 volts and a current density of 5 to 100 amperes per square foot, whereby to produce a porous anodic oxide surface coating on said grained sheet; b. coating the anodized sheet with a photosensitive layer; c. exposing said photosensitive layer to light through a stencil or a photographic negative to develop hardened areas on said photosensitive layer at points which the light passing through said stencil or negative contacts said layer; and d. washing said sensitized layer to remove those areas not contacted by light, leaving the light-hardened areas as oelophilic receptors for subsequently applied oleaginous printing ink.