Calcium Oxalate: CaC2O4
Beer stone is the calcium salt of oxalic acid. It often precipitates on containers that are used for brewing and fermenting beer by means of a reaction that occurs with the interaction of alkaline cleaners, hard water minerals, and amino acids.
Beer stone is an essential organic compound used for identifying the production of beer with residue analysis. The recent finds at Mijiaya, the famous prehistoric Chinese brewery, preserved beer stone residues on many of its tested vessels. The presence of beer stone alone, however, is not necessarily indicative of beer because calcium oxalate is found elsewhere in nature, such as in spinach, rhubarb, and the soil. Even kidney stones are composed primarily of calcium oxalate. For identifying beer production and consumption archaeologically, any detected presence of beer stone must be considered within the context of the site, the vessels in which it is discovered, and the other residues accompanying it.
Beer stone is not a benign compound; in fact, it is poisonous in large enough doses. For brewers, the greatest danger of beer stone is its propensity to foster the growth of microorganisms that can lead to off-flavors, shorten the shelf life of beer, and, most dangerously, spoil the beer. Beer stone is not very soluble in hot water and requires special cleaning treatments.
Funnel for beer making, Mijiaya. Source: © Jiajing Wang/http://on.wsj.com/1RsVroN