Anhydrite has not been a widely popular stone, but its availability on the west coast of North America and the beauty of its finish are likely to result in more sittings in sculpture exhibits in the future. It has the pearly luster of alabaster and yet carves and finishes more like harder stone.
Anhydrite is both a mineral and a sedimentary rock. Its name comes from the Greek for “without water:” essentially it is the anhydrous form of calcium sulfate (CaSO4). Anhydrite is the sister or brother of alabaster (gypsum), which is the hydrated form of calcium sulfate (CaSO4: 2H2O). Two molecules of water are attached to each molecule of calcium sulfate in gypsum.
Anhydrite commonly is found in white, gray, brown, and light red, or pink. It is heavier than alabaster, weighing in at about 187 pounds per cubic foot with a specific gravity of 3.0. One of the key differences between alabaster and anhydrite is the hardness: alabaster is 2 and anhydrite ranges from 3 to 3.5. It is insoluble in hydrochloric acid and sulfuric acid; however, it is easily attacked by water. In the earth, anhydrite converts to gypsum when it absorbs water, and in your yard it will break down with facility, if exposed to the elements.
Anhydrite is one of a group of rock-forming minerals called evaporites, so called because they form by the precipitation of the minerals from evaporating brines or salt water. The other evaporites are gypsum and halite (salt). They form in shallow salt water seas that are alternately submerged and dewatered. Beds of the rock can be found in thicknesses of a few feet to hundreds of feet. Which mineral is precipitated at any time depends on the temperature, pressure, and salinity of the water. At higher temperatures, anhydrite is the first one to precipitate, followed by gypsum; however, at lower temperatures, the opposite is the case. Halite is normally the last to form. In the laboratory, and presumably in nature, gypsum and anhydrite can be made to convert to one another by the changing of pressure and temperature. It is also known that anhydrite can be changed to gypsum by the addition of water, such as during the weathering process. Conversely, the reverse can be achieved by the baking or drying of gypsum.
In North America, anhydrite is found in the salt domes of Louisiana and Texas and in stratified deposits in Nova Scotia, New York, New Mexico, and British Columbia. It is also in large layered salt deposits of Poland, Germany, Austria, France, and India. Anhydrite is not considered an important industrial mineral and is treated as a contaminant in large deposits of gypsum. It is used as a source of sulfuric acid, as a retardant for concrete curing, for a filler in paper, and as a soil conditioner.
Although anhydrite is relatively hard stone, it is somewhat easy to mine because it is naturally broken by numerous joints. This makes mining anhydrite, for sculpture, easily done without blasting, which would shatter and make unusable much of the stone. It is removed from the ground or rock face by hand tools or with a backhoe. The best method for determining the integrity of the stone is to tap the stone with a hammer and listen carefully for changes in the ring.
Anhydrite is brittle and hand tool working is not recommended. Although a hardness of 3 to 3.5 is not extreme, sculptors who have worked the stone definitely prefer power tools. Hand chisels tend to create small chips in the surface, and sharp corners are commonly lost when unexpected breaks occur. The good part is that anhydrite is unlikely to bruise, in the manner of alabaster. It can be worked with a hand-held grinder as well as a stationary wheel, and the stone cuts very evenly and easily with such tools. Anhydrite takes well to a grinder and carbide disks will show little wear after working the stone.
Anhydrite can be taken to a 70 to 600 grit finish, depending on the color and pattern of the stone, and buffed with tin oxide using a wet rag or a mechanical buffer. Of all the colors, the translucent white produces the most lustrous finish although, the brown may have the most interesting patterns.
Cracks and joints are not common flaws in anhydrite. In general, it is a solid stone. The white variety tends to be the most consistent in color and hardness, containing scattered soft spots. Sometimes, small white spots may be present in the stone.
Although not exactly a flaw of the stone, a major weakness of all anhydrite is its susceptibility to water. It must be kept indoors before and after carving. If left outdoors, it will absorb water and deteriorate.
No particular safety hazards are reported for anhydrite however, we all need to remember to protect our lungs, eyes, and ears at all times. Be a sage and safe sculptor.
Thanks to Randy Zieber of Vancouver, British Columbia for his quarrying and carving expertise and to Carol Way of Seattle, Washington and Vic Picou of Burbank, California for sharing their carving experiences with me.