If the industrial research chemists who began concentrated work on silicones less than two decades ago could have looked into the future and seen the many ways silicones would be used, they would have been astonished. The researchers would have been surprised because the use of silicones in masonry water repellents was one of the applications for these materials which they completely overlooked. Yet the use of silicones in masonry water repellents is today one of the fastest growing applications of these versatile materials. This is because silicones can provide: long-lived protection of masonry against the continual assault of weather; easier maintenance of the natural beauty of masonry; and increased highway safety. Silicones do their protective job by making surfaces water repellant, not water proof. They function by lining the capillary pores of masonry and creating a huge hydrophobic or water hating area which repels moisture. And because they line the pores rather than fill them, they permit moisture formed or trapped behind or within a masonry structure to escape in vapor form. It is this distinctive behavior which explains the successful use of silicones in textile finishes, cosmetics, and pharmaceuticals. Two different types of silicone materials are available for use in water repellant formulations. One is soluble in water, the other in mineral spirits or other hydrocarbon solvents. One fundamental rule is of utmost importance: neither type will perform as desired unless the formulation contains a sufficient silicone concentration. The general silicone industry recommendation for the solvent based materials is a silicone content of not less than 5 percent; for the water soluble repellents, not less than 2 percent.