The chemical CaSO₄, which occurs naturally as the mineral anhydrite. Gypsum is the dihydrate mineral form, CaSO₄路2H₂O. Anhydrite and gypsum (commonly called gyp) are found in the subsurface and drilling even small stringers of these minerals can upset a freshwater or seawater mud. Gyp muds, lime muds and oil muds tolerate these salts best. CaSO₄ is used as a mud treatment when no pH increase is needed to remove carbonate ion contamination in freshwater and seawater muds. (Lime increases pH when added for this purpose. ) Gypsum and lime treatments are often used together to keep pH in the proper range. The test for determining the dissolved and undissolved calcium sulfate in a gyp mud requires two titrations with the strong EDTA reagent and Calver II庐 indicator when performed to API standards. It also requires a retort analysis for water content in the mud in order to calculate CaSO₄ content, lbm/bbl.