Abstract A coherent silver image which is bonded to the grained surface of an aluminum lithographic plate having an aluminum oxide layer thereon is transformed into a hydrophilic non-image area by contacting the silver image with a viscous composition comprising an aqueous nitric acid solution of sufficient concentration to rapidly effect the removal of the silver image and thereby expose the aluminum oxide under this image and wherein the viscous composition has a viscosity of at least about 500 centipoises. The preferred viscous composition is a thixotropic composition consisting essentially of (1) an aqueous solution of nitric acid present in an amount between about 35% and 70% by volume based upon the total volume of nitric acid solution, and (2) finely divided inert metal oxide capable of imparting thixotropic properties to the solution.
In the method of transforming portions of an oleophilic coherent silver image containing between about 1 and about 12 grams of silver per square meter and bonded to a grained metal surface of an aluminum lithographic plate substrate having a metal oxide barrier layer thereon into hydrophilic areas by contacting said portions with a deletion fluid, the improvement wherein said deletion fluid comprises an aqueous nitric acid solution containing from about 35% to about 70% by volume nitric acid and a finely divided inert metal oxide thickening agent present in an amount sufficient to provide said deletion fluid with a viscosity of at least about 500 centipoises.
Claim 2
Dependent Claim
107 words
In the method of transforming portions of an oleophilic coherent silver image containing between about 1 and about 12 grams of silver per square meter and bonded to a grained metal surface of an aluminum lithographic plate substrate having a metal oxide barrier layer thereon into hydrophilic areas by contacting said portions with a deletion fluid, the improvement wherein said deletion fluid comprises an aqueous nitric acid solution containing from about 35% to about 70% by volume nitric acid and a finely divided inert metal oxide thickening agent present in an amount sufficient to provide said deletion fluid with a viscosity of at least about 500 centipoises.
Claim 3
Dependent Claim
18 words
The improvement according to claim 1 wherein said deletion fluid also contains a surfactant for the thickening agent.
Claim 4
Dependent Claim
23 words
The improvement according to claim 2 wherein said aqueous nitric acid solution contains from about 40% to about 60% by volume nitric acid.
Claim 5
Dependent Claim
21 words
The improvement according to claim 3 wherein said deletion fluid has a viscosity of between about 20,000 and about 80,000 centipoises.
Claim 6
Dependent Claim
27 words
The improvement according to claim 4 wherein said finely divided inert metal oxide comprises silicon dioxide having an average particle size of less than about 1 micron.
Claim 7
Dependent Claim
19 words
The improvement according to claim 5 wherein said aqueous nitric acid solution contains about 50% by volume nitric acid.