Glauber's Salt is sodium sulfate decahydrate, Na2SO4.10H2O, an identification that rests on unusually solid historical ground because Glauber's own seventeenth century descriptions of its preparation, its crystal form, and its efflorescence behavior match the modern compound closely and because the salt has been in continuous documented use, under his name, from his own time to the present.
Unlike many entries in this atlas, there is essentially no scholarly debate to report here: the substance, its discoverer, and its approximate date of discovery are all matters of clear historical record rather than reconstruction from ambiguous medieval sources. What later chemistry added was an explanation for the efflorescence Glauber observed, the loss of water of crystallization to the surrounding air, and a precise account of why the decahydrate form is the one that crystallizes from cool aqueous solution while other hydration states form under different conditions, refinements that filled in the mechanism behind a substance Glauber had already characterized correctly by observation alone. Glauber's own confidence in the salt's medicinal value went well beyond what later medicine would confirm, a useful reminder that a chemically sound identification of a substance does not, by itself, validate every claim its original discoverer made on its behalf.