Metal blackening process

Claim 1

A METHOD FOR BLACKENING CHROMIUM-PLATED APERTURE MASKS FOR COLOR TELEVISION PICTURE TUBES WHICH INCLUDES THE STEPS OF IMMERSING ONE OF SAID APERTURE MASKS IN A CHROMIUM PLATING BATH, IMMERSING IN SAID BATH A PAIR OF ELECTRODES, EACH HAVING A SURFACE AREA COMPARABLE TO THAT OF SAID APERTURE MASK, SAID APERTURE MASK BEING DISPOSED SUBSTANTIALLY EQUIDISTANT FROM EACH OF SAID ELECTRODES, CONNECTING SAID ELECTRODES TOGETHER, APPLYING EXTERNAL VOLTAGE TO SAID APERTURE MASK AND TO SAID ELECTRODES TO MAKE SAID APERTURE MASK CATHODIC RELATIVE TO SAID ELECTRODES FOR A SUFFICIENT LENGTH OF TIME TO PLATE A THIN FILM OF CHROMIUM ON SAID APERTURE MASK, DISCONNECTING SAID EXTERNAL VOLTAGE FROM SAID APERTURE MASK, EXTERNALLY ELECTRICALLY CONNECTING SAID APERTURE MASK TO SAID ELECTRODES TO MAKE SAID APERTURE MASK THE ANODE OF A VOLATIC CELL FORMED IN SAID BATH FOR A SUFFICIENT LENGTH OF TIME TO OXIDIZE SAID CHROMIUM FILM, AND CYCLICALLY REPEATING SAID CHROMIUM PLATING AND OXIDIZING OPERATIONS UNTIL SAID APERTURE MASK BECOMES BLACKENED TO THE DESIRED DEGREE.