Pigment Information: Aragonite (Crystal Whiting)
Common Names: chalk, drop chalk, prepared chalk, whiting, English white, Paris white Commercial Names: Precipitated calcium carbonate, precipitated chalk, Aeromatt, Albacar, Purecal Refractive Index: 1.7 Specific Gravity: 2.95 Hardness: 3.5 to 4 Chemical Formula: CaCO3
Aragonite is basic calcium carbonate that occurs naturally as a white to pale yellow orthorhombic mineral with a vitreous luster found in limestone caverns and near hot springs and geysers. Aragonite is also a constituent of the shells of many sea creatures. Most bivalve marine animals and corals secrete aragonite for their shells, and pearls are composed of mostly aragonite. The pearly luster and iridescence of seashells such as abalone are made possible by minute layers of aragonite. Calcium carbonate is found in nature as the minerals calcite, aragonite and vaterite -- all different types of limestone. Aragonite is a polymorph of calcite, which means that it has the same chemistry as calcite but a different structure, and more importantly, different symmetry and crystal shapes.
Where Found: Aragon, Spain is where aragonite was first discovered and subsequently named. Other notable occurrences of the mineral include Morocco; Bastennes, France; Girgenti, Sicily; Alston Moor and Cleator Moor, Cumberland, England; Baja California, Mexico (Mexican Onyx); Tsumeb, Namibia; Carinthia, Austria; Leadhills, Scotland; Harz Mountains, Germany; Russia and in several localities in the Southwestern United States. The formation of aragonite in sedimentary layers is not clearly understood. It is thought that under certain conditions the crystallization of calcite is somehow discouraged and aragonite forms instead. This may be caused by the magnesium and salt content of the crystallizing fluid, its turbidity, and the crystallization time. Aragonite is often found with gypsum, barite, malachite, calcite, quartz, clays, dolomite and limonite among many other minerals.
Optical Properties: Aragonite is strongly birefringent giving creamy high-order interference colors. Aragonite is fluorescent and intensely reacts to both short wave and long wave ultraviolet light. Portable lamps that emit long wave ultraviolet light (common know as black light) are sometimes used to identify aragonite. The ultraviolet light from these lamps reacts with chemicals of the mineral and causes the mineral to glow. Aragonite emits a bluish white glow when exposed to ultraviolet light. This property of aragonite makes whites appear brighter and more intense in normal light. Hence the use of fluorescent materials by manufacturers of laundry detergents and the origin of the advertising slogan, "Brighter than white." This quality also lends itself to special effects in artwork when used in color fields surrounded by dark, non-fluorescent pigments. One Australian artist, Eduardo Deleon Aragon, adapted aragonite as a pigment to create "invisible paintings." He uses aragonite pigment mixed with acrylic polymer paints to create dual-image artworks. For example, a portrait of an individual under normal lighting is transformed into a group portrait under ultraviolet light.
History of Use: Aragonite, dolomite and chalk were widely used by the Romans for backgrounds, motives and in mixtures with other pigments, according to the Proceedings of the International Workshop on Roman Wall Painting: Materials, Techniques, Analysis and Conservation. In motives, aragonite was the dominate white pigment used. Aragonite, dolomite, cerussite and diatomite were generally found in rich paintings associated with expensive pigments such as vermilion, purpure, Egyptian blue, red lead minium, etc. "Quartz, gypsum or calcite crystals (aragonite), transparent in oil medium, alter the play of light without interrupting the color tone," Pliny wrote in his Historis Naturalis. Jan van Eyck created unusual depth effects in his paint layers with the use of thin transparent glazes. Whether mineral whites were mixed with his lead white has not yet been fully investigated. It is known however, that he enhanced the visual beauty of rose madder, from the root of the madder plant, by mixing it with aragonite.
Color: The whiter native calcium carbonate is the greater its value as a pigment in paints and grounds. For this reason French chalks are considered to be the best whiting. While our aragonite is not whiter than the best French chalk, its fluorescent properties adds a luster and brilliance to color mixtures not possible with the whitest French chalk.
Permanence and Compatibility: Aragonite is a permanent white pigment that has useful properties as a fluorescent additive to artists' paint. Unlike "Day-Glow" poster colors that fade after long exposure to direct light, aragonite retains its natural fluorescent properties. It can be safely used in all techniques and combines well with all other pigments.
Oil Absorption and Grinding: Aragonite is not effective as a white pigment in oil mediums, because of its low refractive index compared to that of drying linseed oil. However, the crystal structure of the pigment particles and the natural fluorescence of the mineral adds optical properties to oil paints that impart a pearl-like luster when mixed with other pigments and applied in thin glazes. It is has good covering power as a white pigment in aqueous mediums, such as egg tempera, distemper and casein, and exhibits the same fluorescent properties.
Toxicity: Aragonite is not considered to be toxic but care should be used in handling the dry powder pigment so as not to inhale the dust.
 Read cautions about handling pigments |
