Patent ID: 7060658

Claim:
Algae-retardant roofing granules having an enhanced ability, over an extended period of time, to release algicidal metallic or bimetallic ions to inhibit the growth of algae and fungi upon atmospheric exposure of roofing surfaces coated with said granules comprising: a) a base material of crushed mineral or synthetic aggregates in the form of granules coated with a first or inner layer and a second or outer layer; b) said first or inner layer comprising a semi-ceramic composition consisting of a fired silicate-clay matrix containing: of from about 80 pounds to about 180 pounds of cuprous oxide per ton of said base material; or from about 5 pounds to about 40 pounds zinc sulfide admixed with of from about 80 pounds to about 180 pounds of cuprous oxide per ton of said base material; and c) said second or outer layer comprising a semi-ceramic composition comprising a fired silicate-clay matrix containing: of from about 0.25 to about 2.5 pounds of an internal gas forming compound per ton of said base materials selected from the group consisting of hydrogen peroxide, alkali metal perborates, alkali metal persulfates, alkali metal borohydrides, and alkali metal azides, wherein said internal gas forming compounds form microvoids in said second or outer layer thereby rendering the second or outer layer porous to allow passage of algicidal copper or zinc ions from said first or inner layer therethrough; and of from about 1 to about 20 pounds of a pigment per ton of said base material.