description: <p style="text-align: justify;">The gypsum (calcium sulfate hexahydrate) crystals are water clear, as lustrous as polished glass, sharply terminated and without scratch marks on them. Since gypsum has a hardness of only 2 on the Moh's hardness scale, high quality and pristine gypsum crystals are highly susceptible to scratching so if dusted, minor abrasion to the crystal faces will normally cause scratch marks easily visible under a 10 x loop but also often clearly visible to the naked eye of most people as well. Such damage is off putting to any discriminating mineral collector. <br /><br />Anhydrous calcium sulfate and all of the hydrous derivatives of it are also water soluble. At 20 degrees centigrade, 2.40 grams of calcium sulfate dihydrate will dissolve in 1000 mL of pure water. When cleaning fine quality gypsum crystals, use Sterile Water For Irrigation USP or an equivalent grade of purified water which is sterile and free of metallic cations.  An appropriate grade of purified water is available through almost all retail pharmacies and is normally supplied in 1000 mL or 2000 mL plastic bottles.  The author would not recommend distilled water for cleaning fine quality gypsum crystals.  This is because although distilled water is free of metallic cations, it may not be free of volatile compounds that may damage the faces of high quality gypsum crystals.</p> <p style="text-align: justify;"> </p> <p style="text-align: justify;">To make the pharmaceutical grade water suitable for cleaning gypsum crystals, add 3.5 grams of anhydrous calcium sulfate per 1000 mL of water.  Do not allow the temperature of the solution to exceed 20 degrees centigrade.  This is because the solubility of calcium sulfate increases when water is heated above 20 degrees centigrade.</p> <p style="text-align: justify;"> </p> <p style="text-align: justify;">The hardening of water and maintainence of a constant temperature of 20 degrees centigrade are necessary if you would like to clean high quality gypsum crystals without causing frosting of the crystal faces.  This type of damage must be avoided when one works with gypsum specimens worth in the six figures.</p> <p style="text-align: justify;"> </p> <p style="text-align: justify;">Photo by Matthew Webb of Melbourne, Victoria, Australia    </p> <p style="text-align: justify;"> </p>
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