Acid stain (reactive stain)
A solution of metallic salts in a mild acid that reacts chemically with lime in the concrete to produce permanent, variegated, translucent color.
What does acid stain (reactive stain) mean?
Because the color is a chemical reaction with the concrete itself, results vary with the slab's composition, age, moisture, and finish, producing the mottled look that is the product's main appeal and its main risk. The palette is limited to earth tones. Residue must be neutralized and thoroughly removed before sealing or the sealer will fail.
In practice
Acid stain results cannot be reliably sampled from a color chart, so test patches on the actual slab are not optional.