Concrete acid staining works by penetrating the surface and reacting chemically with the hydrated lime (calcium hydroxide) in the concrete. The acid in the stain lightly etches the surface, allowing the metallic salts to penetrate more easily. Once the stain reacts, it becomes a permanent part of the concrete and won’t fade, chip off, or peel away. The palette for acid-etch staining is generally limited to earthy tones, such as tans, browns, terracottas, and soft blue-greens.