Acid stains are one of the oldest ways to color concrete. Most acid stains are a mixture of water, hydrochloric acid, and acid-soluble metallic salts. They work 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.