This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
This compound is met with in nature in several forms; the most familiar is gypsum. On adding sulphuric acid or a sulphate to a solution of a calcium salt the same compound is precipitated. Sulphate of calcium is white, crystalline or amorphous, insoluble in alcohol, soluble in about 380 parts of cold water, and in 450 parts of boiling water; an aqueous solution, saturated at the ordinary temperature, becomes turbid on being heated to boiling. When heated to between 100° and 200° 0. (212° and 392° F.) the salt becomes anhydrous. The nearly anhydrous salt is calcii sulphas, Br. Plaster of Paris is calcium sulfuricum ustum, and has no application in the manufacture of mineral waters. Where calcium sulphate is neeessary in the manufacture of mineral waters, it should be produced by decomposition of chloride of calcium with sulphate of potassium or sodium or sulphate of magnesium within the mixture. The by-product is chloride of potassium or sodium or magnesium. If these by-products are not components of the combination of the artificial mineral water, then calcium sulphate should be produced for immediate use in a freshly precipitated and moistened state, when it is best soluble in carbonated waters and more especially under pressure.
Take chloride of calcium 1 part by weight and dissolve in 10 parts of distilled water. Prepare a separate solution of 3 parts by weight of sulphate of sodium in 30 parts of distilled water. Mix the solutions while stirring and allow time for the precipitate to subside, which will be calcium sulphate, a mutual decomposition having taken place. Add the precipitate while fresh and in a moistened condition to the fountain. When prepared for stock, but only in limited quantities, it must be well stoppered to keep the moisture (in wide-mouth bottles); but it is better prepared for immediate use. Aqua gypsum or calcium water, solution of sulphate of lime, should not be confounded with lime water, for which directions are given on another page (lime-water means a solution of slacked lime, or calcium oxide). Calcium water will be a convenient preparation where smaller quantities of sulphate of calcium (lime) are required. Calcium sulphate, as we know already, is soluble in 380 parts of cold water. To make a standard solution we propose to dissolve it in 400 parts of distilled water by shaking, and afterwards dilute the mixture to make it 500 parts. If this aqua calcium be employed the proportion is then 600 parts to one part of calcium sulphate praecip.