Ultramarine is a
blue pigmentconsisting primarily of a double silicateof aluminiumand sodiumwith some sulfides or sulfates, and occurring in nature as a proximate component of lapis lazuli. In the past, it has also been known as "azzurrum ultramarine, azzurrum transmarinum, azzuro oltramarino, azur d'Acre, pierre d'azur, Lazurstein." Current terminology for ultramarine include natural ultramarine (English), "outremer lapis" (French), "Ultramarin echt" (German), "oltremare genuino" (Italian), and "ultramarino verdadero" (Spanish). The pigment color code is P. Blue 29 77007. Ultramarine is the most complex of the mineralpigments, a complex sulfur-containing sodio- silicate(Na8-10Al6Si6O24S2-4), essentially a mineralized limestonecontaining a blue cubic mineral called lazurite(the major component in lapis lazuli). Some chlorideis often present in the crystal lattice as well. The blue color of the pigment is due to the S3 −radical anion, which contains an unpaired electron.
The name derives from Middle
Latin'ultramarinus', literally "beyond the sea" because it was imported from Asia by sea. [ [http://www.etymonline.com/index.php?search=ultramarine&searchmode=none Online Etymology Dictionary ] ]
The first noted use of lapis lazuli as a pigment can be seen in the 6th- and 7th-century AD cave paintings in
Afghanistantemples, near the most famous source of the mineral. Lapis lazuli has also been identified in Chinese paintings from the 10th and 11th centuries, in Indian mural paintings from the 11th, 12th, and 17th centuries, and on Anglo-Saxon and Norman illuminated manuscripts from c.1100. Natural ultramarine is the most difficult pigment to grind by hand, and for all except the highest quality of mineral sheer grinding and washing produces only a pale grayish blue powder. At the beginning of the 13th century an improved method came into use, described by the 15th century artist Cennino Cennini. This process consisted of mixing the ground material with melted wax, resins, and oils, wrapping the resulting mass in a cloth, and then kneading it in a dilute lyesolution. The blue particles collect at the bottom of the pot, while the impurities and colorless crystals remain in the mass. This process was performed at least three times, with each successive extraction generating a lower quality material. The final extraction, consisting largely of colorless material as well as a few blue particles, brings forth ultramarine ash which is prized as a glaze for its pale blue transparency.
The pigment was most extensively used during the 14th through 15th centuries, as its brilliance complemented the
vermilionand goldof illuminated manuscripts and Italian panel paintings. It was valued chiefly on account of its brilliancy of tone and its inertness in opposition to sunlight, oil, and slaked lime. It is, however, extremely susceptible to even minute and dilute mineral acids and acid vapors. Dilute HCl, HNO3, and H2SO4 rapidly destroy the blue color, producing hydrogen sulfide(H2S) in the process. Acetic acidattacks the pigment at a much slower rate than mineral acids. Because of this susceptibility, ultramarine was only used for frescoes when it was applied "secco", in which the pigment was mixed with a binding medium and applied over dry plaster (such as Giotto di Bondone's frescos in the Cappella degli Scrovegnior Arena Chapelin Padua).
European artists used the pigment sparingly, reserving their highest quality blues for the robes of Mary and the Christ child. As a result of the high price, artists sometimes economized by using a cheaper blue,
azurite, for under painting. Most likely imported to Europe through Venice, the pigment was seldom seen in German art or art from countries north of Italy. Due to a shortage of azurite in the late 16th and 17th century the demand for the already-expensive ultramarine increased dramatically. In 1814Tassaert observed the spontaneous formation of a blue compound, very similar to ultramarine, if not identical with it, in a lime kilnat St. Gobain, which caused the Societé pour l'Encouragement d'Industrie to offer, in 1824, a prize for the artificial production of the precious color. Processes were devised by Jean Baptiste Guimet( 1826) and by Christian Gmelin( 1828), then professor of chemistry in Tübingen; but while Guimet kept his process a secret Gmelin published his, and thus became the originator of the "artificial ultramarine" industry.
Chemistry and manufacture
The raw materials used in the manufacture are: (1)
iron-free kaolin, or some other kind of pure clay, which should contain its silicaand aluminaas nearly as possible in the proportion of SiO2:Al2O3 demanded by the formula assigned to ideal kaolin (a deficit of silica, however, it appears can be made up for by addition of the calculated weight of finely divided silica); (2) anhydrous Na2SO4; (3) anhydrous Na2CO3; (4) powdered sulfur; and (5) powdered charcoalor relatively ash-free coal, or colophonyin lumps.
The materials are 'baked' together in a kiln, usually in brick sized amounts. The resultant solids are then ground and washed as per any other insoluble pigment manufacturing process. [http://www.york.ac.uk/org/seg/salters/chemistry/V_VisitHP/dryprocess2.html] The chemical reaction produces large amounts of
sulfur dioxidemeaning that Flue gas desulfurizationis an essential part of its manufacture to comply with pollution regulations. Large chimneys were used to disperse sulfur dioxide produced in the process, resulting in ultramarine tinting the surrounding ground surfaces and roof vents with a blue color. (Google Maps offers views of two such synthetic ultramarine manufacturing sites, one near [http://maps.google.com/?ie=UTF8&t=k&om=1&ll=53.763363,-0.33092&spn=0.003019,0.007735&z=17 Hull, England] (now defunct) and another in [http://maps.google.com/maps?f=q&hl=en&geocode=&q=comines+france+Route+de+Wervicq&sll=50.780652,3.011112&sspn=0.012916,0.031028&ie=UTF8&ll=50.768894,3.033246&spn=0.001615,0.003878&t=k&z=18 Comines, France] .)
"Ultramarine poor in silica" is obtained by fusing a mixture of soft clay,
sodium sulfate, charcoal, sodium carbonate and sulfur. The product is at first white, but soon turns green ("green ultramarine") when it is mixed with sulfur and heated. The sulfur burns, and a fine blue pigment is obtained. "Ultramarine rich in silica" is generally obtained by heating a mixture of pure clay, very fine white sand, sulfur and charcoal in a muffle-furnace. A blue product is obtained at once, but a red tinge often results. The different ultramarines—green, blue, red and violet—are finely ground and washed with water.
Synthetic ultramarine is not as vivid a blue as natural ultramarine, since the particles in synthetic ultramarine are smaller and more uniform than natural ultramarine and therefore diffuse light more evenly. Synthetic ultramarine is also not as permanent as natural ultramarine.
Artificial, like natural, ultramarine has a magnificent blue colour, which is not affected by light nor by contact with oil or lime as used in painting.
Hydrochloric acidimmediately bleaches it with liberation of hydrogen sulfide. It is remarkable that even a small addition of zinc-white (oxide of zinc) to the reddish varieties especially causes a considerable diminution in the intensity of the colour, while dilution with artificial precipitated sulfate of lime ("annalin") or sulfate of baryta ("blanc fix") acts pretty much as one would expectvague|date=March 2008 . Synthetic ultramarine being very cheap, it is largely used for wall painting, the printing of paperhangings and calico, etc., and also as a corrective for the yellowish tinge often present in things meant to be white, such as linen, paper, etc. Bluing or "Laundry blue" is a solution of synthetic ultramarine (sometimes, prussian blue) that is used for this purpose when washing white clothes. Large quantities are used in the manufacture of paper, and especially for producing a kind of pale blue writing paper which is popular in Britain.
Ultramarine is based on the
sodalitestructure which is a 3 dimensional aluminosilicate cage containing 3 sulfur atoms bonded together to form an ion. These ions are charge balanced by cations of sodium in the natural material. The sodium ions can be ion exchanged with lithium and potassium as described above. The modification of the ions has a dramatic effect on the structure of the cages. Lithium being smaller than sodium causes the cage to contract whilst potassium being large causes the cage to expand. The modification of the cage structure and the interaction of the different cations with the central sulfur species modifies the colouration of the final pigment.
By treating blue ultramarine with
silver nitratesolution, "silver-ultramarine" is obtained as a yellow powder. This compound gives a blue potassium- and lithium-ultramarine when treated with the corresponding chloride, and an ethyl-ultramarine when treated with ethyl iodide. Selenium- and tellurium-ultramarine, in which these elements replace the sulfur, have also been prepared.
* [http://aic.stanford.edu/jaic/articles/jaic30-02-001.html Discussion of ultramarine] in an article on blue pigments in early Sienese paintings from "The Journal of the American Institute for Conservation"
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