Method of etching printing plates

Claim 1

IN A METHOD OF ETCHING A COMBINATION PRINTING PLATE OF AN ACID SOLUBLE METAL SELECTED FROM THE GROUP CONSISTING OF MAGNESIUM, HIGH MAGNESIUM-BASE ALLOYS WHEREIN THE SURFACE OF THE PLATE IS CONTACTED WITH TWO ETCHING COMPOSITIONS IN SEPARATE STEPS, EACH OF SAID ETCHING COMPOSITIONS COMPRISING AN AQUEOUS SOLUTION OF HNO3, SAID SOLUTION HAVING IN ADMIXTURE THEREWITH AN ESTER OF SULFOSUCCINIC ACID AND AN ALIPHATIC ALCOHOL HAVING FROM 4 TO 12 CARBON ATOMS AND A WATER-IMMISCIBLE PETROLEUM FRACTION HAVING AN ALIPHATIC SIDE CHAIN, THE STEP OF REMOVING FROM THE SULFACE OF THE PLATE THE RESIDUE REMAINING THEREON FROM THE FIRST OF SAID TWO ETCHING COMPOSITIONS BY WETTING THE SURFACE OF THE PLATE WITH AN AQUEOUS SOLUTION OF A MILD ALKALINE MATERIAL SELECTED FROM THE CLASS CONSISTING OF TRISODIUM PHOSPHATE, BORAX, SODIUM SILICATE, AND A MIXTURE OF AT LEAST TWO OF THE FOREGOING MATERIALS, AND THEREAFTER SCRUBBING THE SURFACE OF THE PLATE WITH AN AQUEOUS SOLUTION CONTAINING A DETERGENT HAVING WETTING AGENT PROPERITIES BEFORE CONTACTING THE SURFACE OF THE PLATE WITH THE SECOND OF SAID TWO ETCHING COMPOSITIONS, SAID DETERGENT BEING SELECTED FROM THE GROUPS CONSISTING OF FATTY ALCOHOL SULFATES, ALKYL ARYL SULFONATES, AND DIALKYL SULFOSUCCINATES, EACH ALKYL GROUP OF SAID DIALKYL SULFOSUCCINATES HAVING 6 TO 18 CARBON ATOMS.