This severe, unpleasant wine defect is usually caused by improper dry preservation of empty wooden barrels, but also by over-sulphurisation of the wine. However, sulphuric acid firn must under no circumstances be confused with firn. When the sulphur cuts are burnt, sulphur dioxide is produced which, in combination with moisture, becomes sulphurous acid, but can also partially oxidise to sulphuric acid. Before such a barrel can be refilled, it must be filled with water for several days to dissolve out the sulphur residues. The wine has a harsh sulphurous smell and an edgy, sharp-sour taste, which is also noticeable by a "dulling" of the teeth. The defect can only be combated or remedied with difficulty; at best, blending with a young wine helps. See also under wet preservation.
For me, Lexicon from wein.plus is the most comprehensive and best source of information about wine currently available.
Egon Mark
Diplom-Sommelier, Weinakademiker und Weinberater, Volders (Österreich)