Selenite (Gypseous White)
Common Names: English: gypsum, mineral white, terra alba, lady’s ice, specular stone, selenite, moonstone, gypsum spar, atlas spar, silk spar; Italian: scagliola; Latin: selenites, lapis specularis, glacie divae maria, diaphanes, aphroselenus, gypsum lamellare Color: White Colour Index: Not Listed ASTM Lightfastness: Not Tested Refringency Index: Birefringent e = 1.53 v = 1.62 Hardness: 2 Specific Gravity: 2.3+ Chemical Formula: CaSO4-2(H2O)
Selenite is a variety of gypsum that shows a pearl-like luster and has been described as having a moon-like glow. It is basic calcium sulfate formed into transparent and translucent scales of tabular, bladed or blocky crystals with a slanted parallelogram outline. It is found in nature as massive, crusty, granular, earthy and fibrous deposits with halite, calcite, sulfur, pyrite, borax and many others minerals. Some samples of selenite are fluorescent. It has a very low thermal conductivity, so that a crystal of selenite will feel noticeably warmer than a like crystal of quartz. The word selenite comes from Greek for Moon and literally means moon rock.
Color: The powdered forms of all varieties of calcium sulfate, if reasonably pure, are white. They do not possess the high scattering or hiding power in drying oil-base paints as do the heavy-metal whites, such as cerussite. White paint made from selenite and aqueous medium has a translucent matte quality pleasing in appearance.
Origin: Gypsum is one of the more common minerals in sedimentary environments. It is a major rock-forming mineral that produces massive beds, usually from precipitation out of highly saline waters. Since it forms easily from saline water, gypsum can have many inclusions of other minerals and even trapped bubbles of air and water. Gypsum has several other variety names that are widely used in the mineral trade. One variety of gypsum is a compact fibrous aggregate called satin spar. This variety has a very satin-like look that gives a play of light up and down the fibrous crystals. A fine-grained massive form of gypsum is called alabaster and is an ornamental stone used in carvings since Antiquity.
Where Found: Notable deposits of selenite include those found in Naica, Mexico; Russia; Sicily; Utah and Colorado, USA; and many other localities throughout the world.
Optical Properties: Selenite crystals retard light a quarter wavelength. Because of this unusual optical property, selenite is used in light compensators to produce an interference pattern so that positive and negative birefringence can be seen in a microscope.
Preparation and Use: Selenite was put in an oven at 300°C until it was broken and could be ground with mortar and pestle into powder that was then sieved. Today, the mineral is usually ground in a powered mill and sieved into medium-sized or fine grains. The processing of the crystals to produce a paintable powder should really be carried out only by pounding, since milling causes the reflective fracture surfaces in the pigment grains to turn matte. To obtain the characteristically strong light reflection of selenite and the depth effect that the pigment can create it should be coarsely milled and then pulverized in a mortar. Iconofile selenite is available as a coarsely milled pigment for that very purpose.
History of Use:This mineral was known and used since Antiquity under the name of lapis specularis because of its translucent, mirror-like surface. The thin flakes of gypsum were used for a long period for sparkling decorations on devotional images and music boxes: "The virgins in the nunnery make all sorts of ornaments out of the flakes, and commonly lay these over their pictures and shrines." Since Medieval times, it has principally been used in coating and casting. De Mayerne mentions "unburnt alabaster" as a base for "verditer"; 140 years later, the German translation of Dossie's Handmaid to the Arts describes Marienglas (selenite) to render his "powdered talc." Selenite and sericolite, both occurring in the Appennino Emiliano in Italy, were called scagliola in 17th century Italy. Water and animal glue was added to it to produce a thick paste that could be colored for use in fresco or distemper paintings. It was rediscovered in 1983 by Wolfgang Fünders after 200 years of obscurity, during the reconstruction of polychrome coatings on the walls and ceilings of the palace and hunting lodge of Clemenswerth, and the role it played has yet to be fully investigated.
Permanence and Compatibility: Selenite is considered to be among the most permanent pigments in the artist's palette. It can be safely used in all techniques and combines well with all other pigments.
Oil Absorption and Grinding: Selenite does not have the hiding power of heavy-metal white pigment in oils, because of its low refractive index compared to that of linseed oil. However, the crystal structure of the pigment particles give oil paint a pearl-like luster when mixed with other pigments and applied in thin glazes. It is an effective whiting in aqueous mediums, such as egg tempera, distemper and casein.
Toxicity: Selenite is not considered to be toxic but care should be used in handling the dry powder pigment so as not to inhale the dust.
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