Chalk

== Description ==
A soft, porous white mineral composed of fine-grain limestone. Also called whiting, pure varieties of chalk contain up to 99 percent calcium carbonate as the mineral calcite. Chalk was formed in the Cretaceous period and occurs naturally in thick beds in many parts of the world, such as the chalk cliffs along the English Channel. Chalk beds are collections of the shells of such tiny marine organisms as foraminifera, coccoliths, diatoms, and rhabdoliths. Ground chalk has been used as a pigment since ancient times. When mixed with glue, it was the most common ground for northern European paintings from medieval times well into the 18th century. Chalk was also commonly used for making lime, portland cement, putty, and polishing powders. Industrially, it is still used as a filler, extender, or pigment in a wide variety of materials, including ceramics, putty, cosmetics, crayons, plastics, rubber, paper, paints, and linoleum. Chalk is stable and inert, but has poor covering power in oil. It helps control gloss, add texture, absorb oil, and provide plasticity. Chalk is made synthetically by precipitating fine particles of calcium carbonate. 
Note: Blackboard chalk is made from gypsum and tailors chalk is made from talc
== Synonyms and Related Terms ==
calcium carbonate; Pigment White 18; CI 77220; whiting; Kreide (Deut.); craie (Fr.); calcare (It.); creta (Esp., It.); calcita (Esp.); tiza (Esp.); kimolia (Gr.); krijt (Ned.); kreda (Pol.); kalk (Sven.); cré (Port.); English white; Paris white; gilder's whiting; Champagne chalk; calcite; limestone; marble white;
[[[SliderGallery rightalign|Chalk-glue.TIF~FTIR (MFA)|ChalkUCL.jpg~Raman (UCL)]]]
== Physical and Chemical Properties ==
* Reacts with acids to evolve carbon dioxide. 
* High birefringence with strong interference colors 
* Microscopically, coccoliths may be seen at about 500x in natural chalk. 
* Precipitated chalk has fine, uniform particles. 
Composition 	CaCO3
Mohs Hardness 	3.0
Density 	1.8 - 2.7 g/ml
Refractive Index	e =1.486, w =1.64 -1.66
== Comparisons ==
== Additional Images ==
File:ChalkEnglandKES.jpg|Chalk
File:The Cobra_Niobrarachalk.jpg|The Cobra Niobrara chalk
File:image4_chalk.jpg|Chalk
File:Red chalk - 500x.jpg|Red chalk
== Resources Checked for Data in Record ==
* R. Gettens, E. West Fitzhugh, R.Feller, "Calcium Carbonate Whites", Artists Pigments, Vol. 2., A. Roy ed. Oxford University Press, Oxford, 1993.
* Nicholas Eastaugh, Valentine Walsh, Tracey Chaplin, Ruth Siddall, Pigment Compendium, Elsevier Butterworth-Heinemann, Oxford, 2004 Comment: Refractive index: e =1.486, w =1.64 -1.66
* Encyclopedia Britannica, http://www.britannica.com Comment: Chalk. Accessed May 25, 2003.
* Wikipedia: http://en.wikipedia.org/wiki/chalk (Accessed Sept 2 2005)
* The Dictionary of Art, Grove's Dictionaries Inc., New York, 1996 Comment: "Chalk"
* R. J. Gettens, G.L. Stout, Painting Materials, A Short Encyclopaedia, Dover Publications, New York, 1966 Comment: density = 2.70 and ref. index = 1.510; 1.645
* G.S.Brady, Materials Handbook, McGraw-Hill Book Co., New York, 1971 Comment: p. 181
* Ralph Mayer, A Dictionary of Art Terms and Techniques, Harper and Row Publishers, New York, 1969 (also 1945 printing)
* Dictionary of Building Preservation, Ward Bucher, ed., John Wiley & Sons, Inc., New York City, 1996
* M. Doerner, The Materials of the Artist, Harcourt, Brace & Co., 1934
* Richard S. Lewis, Hawley's Condensed Chemical Dictionary, Van Nostrand Reinhold, New York, 10th ed., 1993
* R.M.Organ, Design for Scientific Conservation of Antiquities, Smithsonian Institution, Washington DC, 1968
* Van Nostrand's Scientific Encyclopedia, Douglas M. Considine (ed.), Van Nostrand Reinhold, New York, 1976
* Random House, Webster's Encyclopedic Unabridged Dictionary of the English Language, Grammercy Book, New York, 1997
* The American Heritage Dictionary or Encarta, via Microsoft Bookshelf 98, Microsoft Corp., 1998
* Thomas B. Brill, Light Its Interaction with Art and Antiquities, Plenum Press, New York City, 1980 Comment: ref. index = 1.66; 1.44
* Teri Hensick, contributed information, 1998
* Susan E. Schur, Conservation Terminology: A review of Past & Current Nomenclature of Materials, Technology and Conservation, Spring (p.34-39); Summer (p.35-38); Fall (p.25-36), 1985
* Art and Architecture Thesaurus Online, http://www.getty.edu/research/tools/vocabulary/aat/, J. Paul Getty Trust, Los Angeles, 2000
* Website: http://webexhibits.org/pigments/indiv/overview/chalk.html e =1.486, w =1.64 -1.66
* CRC Handbook of Chemistry and Physics, Robert Weast (ed.), CRC Press, Boca Raton, Florida, v. 61, 1980 Comment: density=1.9-2.8
Category:Materials database