I often get questions by guest on my walking tours about some of the discoloration in the stucco walls of historic buildings here in Charleston. Stucco is a mixture of sand and calcium hydroxide, the latter substance being essentially salt. This allows the stucco to breathe so as not to trap moisture under the wall surface, but can cause problems if there are cracks in the stucco. Exposed crystals of calcium hydroxide will either rect with carbon dioxide from the outside air or dissolve with intruding rain water to form calcium carbonate that creates a discoloration usually in the form of patches and streaks – what is known as efflorescence. Calcium carbonate is not soluble in water, so it will not contribute to any further damage, but just give the stucco exterior a mottled look as in the image below of the old Charleston District Jail. In this case, the salt and sand stucco having worn off over the years has allowed the efflorescence to intrude.