Acid Staining gives concrete a variegated or marbled look. It also creates reddish brown and green colors on the concrete. Mixing and matching available colors, or applying them at different rates will create new shades. Most acid stains are a mixture of water, hydrochloric acid and acid-soluble metallic salts. This method works by penetrating the surface and reacting chemically with the hydrated lime in concrete. It lightly etches the surface, allowing the metallic salts to penetrate more easily. Acid-based stains are translucent and the earth-toned colors they produce vary depending on the color of the substrate they are applied to. No two acid stained slabs of concrete will have the same exact color tones, creating a very natural look.