chalk

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Background

Chalk used in school classrooms comes in slender sticks approximately .35 of an inch (nine millimeters) in diameter and 3.15 inches (80 millimeters) long. Lessons are often presented to entire classes on chalk-boards (or blackboards, as they were originally called) using sticks of chalk because this method has proven cheap and easy.

As found in nature, chalk has been used for drawing since prehistoric times, when, according to archaeologists, it helped to create some of the earliest cave drawings. Later, artists of different countries and styles used chalk mainly for sketches, and some such drawings, protected with shellac or a similar substance, have survived. Chalk was first formed into sticks for the convenience of artists. The method was to grind natural chalk to a fine powder, then add water, clay as a binder, and various dry colors. The resultant putty was then rolled into cylinders and dried. Although impurities produce natural chalk in many colors, when artists made their own chalk they usually added pigments to render these colors more vivid. Carbon, for example, was used to enhance black, and ferric oxide (Fe₂O₃) created a more vivid red.

Chalk did not become standard in schoolrooms until the nineteenth century, when class sizes began to increase and teachers needed a convenient way of conveying information to many students at one time. Not only did instructors use large blackboards, but students also worked with individual chalkboards, complete with chalk sticks and a sponge or cloth to use as an eraser. These small chalkboards were used for practice, especially among the younger students. Pens dipped in ink wells were the preferred tool for writing final copy, but these were reserved for older students who could be trusted not to make a mess: paper—made solely from rags at this time—was expensive.

An important change in the nature of classroom chalk paralleled a change in chalk-boards. Blackboards used to be black, because they were made from true slate. While some experts advocated a change to yellow chalkboards and dark blue or purple chalk to simulate writing on paper, when manufacturers began to fashion chalkboards from synthetic materials during the twentieth century, they chose the color green, arguing that it was easier on the eyes. Yellow became the preferred color for chalk.

Almost all chalk produced today is dustless. Earlier, softer chalk tended to produce a cloud of dust that some feared might contribute to respiratory problems. Dustless chalk still produces dust; it's just that the dust settles faster. Manufacturers accomplish this by baking their chalk longer to harden it more. Another method, used by a French company, is to dip eighty percent of each dustless chalk stick in shellac to prevent the chalk from rubbing off onto the hands.

Raw Materials

The main component of chalk is calcium carbonate (CaCO₃), a form of limestone. Limestone deposits develop as coccoliths(minute calcareous plates created by the decomposition of plankton skeletons) accumulate, forming sedimentary layers. Plankton, a tiny marine organism, concentrates the calcium found naturally in seawater from .04 percent to 40 percent, which is then precipitated when the plankton dies.

The base of pastel chalks is calcium sulfate (CaSO₄), which is derived from gypsum (CaSO₄-2H₂O), an evaporite mineral formed by the deposition of ocean brine; it also occurs disseminated in limestone. Chalk and dehydrated gypsum thus have similar origins and properties. Pastels also contain clays and oils for binding, and strong pigments. This mixture produces sticks that write smoothly without smearing and draw better on paper than on chalkboards. Although great care is taken to eliminate contaminants when chalk is manufactured, some impurities inherent to the mineral remain. Chief among these are silica, alumina, iron, phosphorus, and sulfur. In less significant, amounts, manganese, copper, titanium, sodium oxide, potassium oxide, fluorine, arsenic, and strontium may also occur.

The Manufacturing
Process

Quarrying limestone

• Approximately 95 percent of the limestone produced in the United States is quarried. After a sufficient reserve (twenty-five years' worth is recommended) has been prospected, the land that covers the deposit is removed with bulldozers and scrapers. If the chalk is close to the surface, an open shelf quarry method can be used; however, this is very rare. Usually an open pit quarry method is used instead. In this method, holes are drilled into the rock, explosives are placed inside, and the rock is blown apart. Depending on the nature of the deposit, a pit can be enlarged laterally or vertically.

Pulverizing the chalk

• Once comparatively large chunks of limestone have been quarried, they need to be transported to crushing machines, where they are pulverized to meet the demands of the chalk industry. The first step is primary crushing. Various crushers exist, but the principle is the same: all compress the stone with jaws or a cone, or shatter it through impact. Secondary crushing is accomplished by smaller crushers that work at higher speeds, producing pebbles which are then ground and pulverized.
• The next phase, wet grinding, washes away impurities. It is used to make the fine grade of limestone necessary to make chalk suitable for writing purposes. Wet grinding is carried out in ball mills—rotating steel drums with steel balls inside that pulverize the chalk until it is very fine.

Dehydrating gypsum

• Gypsum, like limestone, is also quarried and pulverized. The major difference in processing gypsum is that it must be dehydrated to form calcium sulfate, the major component of colored chalk. This is done in a kettle, a large combustion chamber in which the gypsum is heated to between 244 and 253 degrees Fahrenheit (116-121 degrees Celsius). It is allowed to boil until it has been reduced by twelve to fifteen percent, at which point its water content will have been reduced from 20.9 percent to between 5 and 6 percent. To further reduce the water, the gypsum is reheated to about 402 degrees Fahrenheit (204 degrees Celsius), at which point it is removed from the kettle. By now, almost all of the water has evaporated, leaving calcium sulfate.

Sifting, cleaning, and shipping
the chalk

• The particles of chalk or calcium sulfate are now conveyed to vibrating screens that sift out the finer material. The ensuing fine chalk is then washed, dried, packed in bags, and shipped to the manufacturer. Upon receiving chalk or calcium sulfate, the chalk factory usually grinds the materials again to render them smooth and uniformly fine.

Making white classroom chalk

• To make white classroom chalk, the manufacturer adds water to form a thick slurry with the consistency of clay. The slurry is then placed into and extruded from a die—an orifice of the desired long, thin shape. Cut into lengths of approximately 24.43 inches (62 centimeters), the sticks are next placed on a sheet that contains places for five such sticks. The sheet is then placed in an oven, where the chalk cures for four days at 188 degrees Fahrenheit (85 degrees Celsius). After it has cured, the sticks are cut into 80 millimeters lengths.

Making colored classroom chalk

• Pigments (dry, natural, colored materials) are mixed in with the calcium carbonate while both are dry (the procedure is similar to sifting flour and baking powder together before adding liquid, as in a cake recipe). Water is then added to the mixture, which is then baked in the same manner as white classroom chalk.

Making pastels

• Another manufacturing method is used for pastels, the chalks used for art drawing. The procedure resembles that used for colored classroom chalk, but calcium sulfate is used instead of calcium carbonate. In addition, the dry material is mixed with clay and oils, and more pigments are added to produce a slurry that has the consistency of toothpaste. Because the final products must be relatively moist, pastels are usually air-dried rather than baked.

Boxing the chalk

• Placed in small boxes, the completed chalk sticks are stacked in large boxes to be shipped to supply stores.

Quality Control

Chalk that is intended for the classroom must undergo stringent tests in order to perform well and be labeled nontoxic. All incoming materials are tested for purity before being used. After the chalk has been made into sticks, one stick from each batch is selected for tests. The density and break strength of the sample stick are determined. The sample is then used to write with, and the quality of the mark is studied. Erasability is also studied. First, the chalk mark is erased using a dry eraser, and the quality of erasure is examined. Then, the chalkboard is washed, and again the amount of chalk left on the board is examined. Furthermore, a sample from each batch is kept for five years so that it can be inspected if in the future its quality is questioned.

Chalk for classroom use adheres to the American National Standards Institute performance standards. Written specifications state the proper length of the chalk stick, as well as how many sticks should go in a box. On November 18, 1990, a Federal Act(Public Law 100-695) went into effect, mandating that all art materials sold in the United States must be evaluated by a qualified toxicologist who must then issue a label explaining their toxicity. Toxicologists are concerned not with cost but with safety, and they must consider many factors before granting approval. Each ingredient, the quantity in which it is used, and its possible adverse reactions with other ingredients are studied. The product's size and packaging, its potential harm to humans, and its tendency to produce allergic reactions are also considered. Toxicologists also take into account the products use and potential mis-use, as well as all federal and state regulations. Formulas for every color and every formula change must meet approval.

Classroom chalk is labeled "CP [certified product] nontoxic" if it meets the standards of the Art and Craft Materials Institute, a nonprofit manufacturers' association. This label certifies that art materials for children are nontoxic and meet voluntary standards of quality and performance, and that the toxicity of art materials for adults has been correctly labeled. The CP seal also indicates that the product meets standards of material, workmanship, working qualities, and color developed by the Art and Craft Materials Institute and others such as the American National Standards Institute and the American Society for Testing and Materials (ASTM). To ensure honesty, most chalk manufacturers are tested at random by an independent toxicologist, who checks to see that they are meeting nontoxic standards. Most manufacturers conform to such exacting standards because knowledgeable schools will not purchase chalk that is not properly labeled.

The Future

Many people consider using chalk and chalk-boards to present material outdated. Some experts claim that teachers have stubbornly resisted new technologies that could improve teaching—and eliminate the chalkboard entirely. A study which recently investigated whether teaching with overhead projectors was more effective than using chalkboards concluded that chalkboards were more interactive, progressive, and fruitful.

A development much in the educational news lately is the electronic chalkboard. In place of a regular chalkboard, a teacher uses a large TV screen, inputting materials from a computer terminal. In a more advanced scenario, each student uses a terminal, to which the teacher sends information from a master computer. Experts claim that such set-ups are more visually exciting to students, more versatile than the old-fashioned chalkboards, cleaner than dusty chalk, easier for the teacher to use, and better able to present more complex material through the use of graphics and animation. Many studies on the feasibility of electronic chalkboards have been made, however, and most seem to favor keeping the traditional chalkboard, at least for now. Electronic chalkboards that are sophisticated and easily readable lie beyond the budget constraints and technological capabilities of most schools. Further, studies of the electronic system's effectiveness report that teachers who use it spent more time preparing their lessons, teachers and students were less interactive, students were dissatisfied with the electronic chalkboards, and the new devices divided the students' attention between the screen and the teacher conveying the information. Although the enthusiasm for electronic blackboards in some areas remains high, chalk use in the classroom is guaranteed for some time to come.

Where To Learn More

Books

Boynton, Robert. Chemistry and Technology of Lime and Limestone. John Wiley & Sons, 1980.

Cobb, Vicki. The Secret Life of School Supplies. J. B. Lippincott, 1981.

Institution of Civil Engineers Staff, eds. Chalk. American Society of Civil Engineers, 1990.

Periodicals

Toth, Beth. "Jeanne Otis: A Color Dialogue," Ceramics Monthly. January, 1988, p. 40.

[Article by: Rose Secrest]

The term sometimes used in a broad sense for any soft, friable, or weathered fine-grained limestone; however the term is mostly restricted to pelagic (biogenic) limestones. Chalk is a uniformly fine-grained, typically light-colored marine limestone primarily composed of the remains of calcareous nannofossils and microfossils. These minute pelagic organisms live in surface and near-surface oceanic waters and include coccolithophores (algae) and planktic foraminifers (Protozoa). Larger fossil constituents may be present, but only in subordinate amounts. The dominant pelagic skeletal remains are composed of low-magnesium calcite and have accumulated where the sea floor lies at a depth of less than about 4 km or 13,000 ft (the carbonate is redissolved at greater depths). Typical chalk sedimentation rates are 30 m (100 ft) per million years, so chalk accumulation is also dependent on the exclusion of diluting materials such as reefal detritus or terrigenous debris (clay, silt, or sand) transported from land areas by rivers. Chalks therefore form mainly in isolated outer shelf or deeper-water settings that are far from land areas. See also Calcite; Limestone.

The unique combination of light color, compositional purity, softness, and fine texture led to many of the early uses of chalk for writing on blackboards. Chalks are also widely used in the manufacture of portland cement, as lime for fertilizers, and in powders, abrasives, and coatings.

Definition: pale
Antonyms: flushed, rosy

adj

Definition: powdery
Antonyms: smooth

A pure form of limestone composed of the shells of minute marine organisms together with spherical or egg-shaped particles of calcium carbonate.

A soft limestone, usually white, gray, or buff in color, composed chiefly of the calcareous remains of marine organisms.

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Chalk Farm is an area in the London borough of Camden.

An abrasive agent made from compact calcite.

• Sickly, out of shape, dull, or weak
• Ugly, unattractive, or malformed

For other uses, see Chalk (disambiguation).

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Chalk

Sedimentary rock
The Needles, situated on the Isle of Wight, are part of the extensive Southern England Chalk Formation.
Composition
calcite (calcium carbonate)

Chalk ( /ˈtʃɔːk/) is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO₃. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates (coccoliths) shed from micro-organisms called coccolithophores. It is common to find chert or flint nodules embedded in chalk. Chalk can also refer to other compounds including magnesium silicate and calcium sulfate.

"Nitzana Chalk curves" situated at Western Negev, Israel are chalk deposit formed at the Mesozoic era's Tethys Ocean

Chalk has greater resistance to weathering and slumping than the clays with which it is usually associated, thus forming tall steep cliffs where chalk ridges meet the sea. Chalk hills, known as chalk downland, usually form where bands of chalk reach the surface at an angle, so forming a scarp slope. Because chalk is porous it can hold a large volume of ground water, providing a natural reservoir that releases water slowly through dry seasons.

Contents 1 Deposits 2 Formation 3 Composition 4 Uses 5 See also 6 References 7 External links

Deposits

The Chalk Group is a European stratigraphic unit deposited during the late Cretaceous Period. It forms the famous White Cliffs of Dover in Kent, England, as well as their counterparts of the Cap Blanc Nez on the other side of the Dover Strait. The Champagne region of France is mostly underlain by chalk deposits, which contain artificial caves used for wine storage. Some of the highest chalk cliffs in the world occur at Møns Klint in Denmark.

Formation

Ninety million years ago the chalk downland of Northern Europe was ooze accumulating at the bottom of a great sea. Protozoans such as foraminifera lived on the marine debris that showered down from the upper layers of the ocean. Their shells were made of calcite extracted from the rich sea-water. As they died a deep layer gradually built up and eventually, through the weight of overlying sediments, became consolidated into rock. Later earth movements related to the formation of the Alps raised these former sea-floor deposits above sea level.

Composition

Chalk is composed mostly of calcium carbonate with minor amounts of silt and clay. It is normally formed underwater, commonly on the sea bed, then consolidated and compressed during diagenesis into the form commonly seen today. During diagenesis silica accumulates to form chert or flint nodules within the carbonate rock.

Uses

Former underground chalk quarry in Meudon, France.

Chalk is used to make quicklime and slaked lime, mainly used as lime mortar in buildings. In southeast England, Deneholes are a notable example of ancient chalk pits. Such bell pits may also mark the sites of ancient flint mines, where the prime object was to remove flint nodules for stone tool manufacture. The surface remains at Cissbury are one such example, but perhaps the most famous is the extensive complex at Grimes Graves in Norfolk.

The traditional uses of chalk have in some cases been replaced by other substances, although the word "chalk" is often still applied to the usual replacements.

• Blackboard chalk is a substance used for drawing on rough surfaces, as it readily crumbles leaving particles that stick loosely to these surfaces. Although traditionally composed of natural chalk, modern blackboard chalk is generally made from the mineral gypsum (calcium sulfate), often supplied in sticks of compressed powder about 4 in (10 cm) long.
• Sidewalk chalk is similar to blackboard chalk, except that it is formed into larger sticks and often colored. It is used to draw on sidewalks, streets, and driveways, mostly by children, but also by adult artists.
• In agriculture chalk is used for raising pH in soils with high acidity. The most common forms are CaCO₃ (calcium carbonate) and CaO (calcium oxide).
• In field sports, including grass tennis courts, powdered chalk was used to mark the boundary lines of the playing field or court. This gives the advantage that, if the ball hits the line, a cloud of chalk or pigment dust can be seen. Nowadays the substance used is mostly titanium dioxide.^{[1][not in citation given]}
• In gymnastics, rock-climbing, weight-lifting and tug of war, chalk — now usually magnesium carbonate — is applied to the hands to remove perspiration and reduce slipping.
• Tailor's chalk is traditionally a hard chalk used to make temporary markings on cloth, mainly by tailors. Nowadays it is usually made from talc (magnesium silicate).
• Toothpaste also commonly contains small amounts of chalk, to serve as a mild abrasive.
• Polishing chalk is chalk prepared with a carefully controlled grain size, for very fine polishing of metals.^[2]
• Chalk is a source of quicklime by thermal decomposition, or slaked lime following quenching with water.
• Builder's putty also mainly contains chalk as a filler in linseed oil.
• Woodworking joints may be fitted by chalking one of the mating surfaces. A trial fit will leave a chalk mark on the high spots of the corresponding surface. Chalk transferring to cover the complete surface indicates a good fit.
• Fingerprint powder

See also

• Chalk carving
• Chalk line
• Flint
• French chalk
• Hill figure
• Pastel
• Sanguine
• Sidewalk chalk
• List of types of limestone

References

1. ^ "Light spells doom for bacteria". http://www.photonics.com/Content/ReadArticle.aspx?ArticleID=35722. 
2. ^ Information on polishing powders, from the 1879 book "The Workshop Companion"

External links

Wikimedia Commons has media related to: Chalk

• Chalk cliffs in the North of Ireland
• Chalk: Sedimentary rocks

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - kridt, kridtstreg
v. tr. - kridte, skrive med kridt

idioms:

• chalk and cheese    ild og vand
• chalk out    ridse op
• chalk up    skrive op

Nederlands (Dutch)
krijten, krijt(je)

Français (French)
n. - craie, (Hort) sol crayeux
v. tr. - écrire (qch) à la craie, frotter (qch) avec de la craie

idioms:

• chalk and cheese    (être) comme le jour et la nuit
• chalk out    tracer (qch) à la craie
• chalk up    écrire qch à la craie, mettre qch sur l'ardoise (de qn), remporter (une victoire), mettre qch au compte de

Deutsch (German)
n. - Kreide
v. - mit Kreide schreiben/malen

idioms:

• chalk and cheese    Tag und Nacht
• chalk out    skizzieren, umreißen
• chalk up    anschreiben, verbuchen

Ελληνική (Greek)
n. - κιμωλία, (ορυκτολ.) κρητίδα
v. - γράφω ή τρίβω με κιμωλία

idioms:

• chalk and cheese    (διαφέρουν όσο) η μέρα με τη νύχτα
• chalk out    διαγράφω, σκιαγραφώ (σχέδιο)
• chalk up    σημειώνω (βαθμολογία ή σκορ), δίνω με πίστωση

Italiano (Italian)
gesso

idioms:

• by a long chalk    di gran lunga
• chalk and cheese    del tutto dissimili
• chalk out    abbozzare
• chalk up    mietere successi

Português (Portuguese)
n. - giz (m), conta (f), favorito (m)
v. - marcar com giz

idioms:

• by a long chalk    por margem ampla
• chalk and cheese    não haver nenhuma semelhança
• chalk out    traçar planos
• chalk up    obter vitória, jogar na conta

Русский (Russian)
мел, известняк

idioms:

• by a long chalk    ничего подобного
• chalk and cheese    небо и земля
• chalk out    делать набросок
• chalk up    отмечать

Español (Spanish)
n. - tiza, crayón, creta
v. tr. - enyesar, dibujar con tiza

idioms:

• chalk and cheese    como el día y la noche
• chalk out    esbozar, bosquejar, trazar
• chalk up    apuntar, escribir, anotar

Svenska (Swedish)
n. - krita, kritstreck
v. - krita, skriva med krita

中文（简体）(Chinese (Simplified))
粉笔, 白垩, 用粉笔写, 和以白垩

idioms:

• chalk and cheese    本质上完全不同, 截然不同
• chalk out    计划, 制图
• chalk up    记下, 取得

中文（繁體）(Chinese (Traditional))
n. - 粉筆, 白堊
v. tr. - 用粉筆寫, 和以白堊

idioms:

• chalk and cheese    本質上完全不同, 截然不同
• chalk out    計劃, 製圖
• chalk up    記下, 取得

한국어 (Korean)
n. - 백악, 분필, 득점
v. tr. - 분필로 표를 하다, 표백하다

idioms:

• chalk out    초크로 윤곽을 그리다, 계획하다
• chalk up    기록하다, 마음에 새기다, 탓으로 하다

日本語 (Japanese)
n. - チョーク, 白亜
v. - チョークで書く

idioms:

• chalk and cheese    正反対
• chalk out    輪郭を描く, 計画する
• chalk up    書き記す, 記録する, 挙げる, せいにする

العربيه (Arabic)
‏(الاسم) طباشير, كربونات الكلس, جص (فعل) كتب أو رسم بالطباشير‏

עברית (Hebrew)
n. - ‮גיר‬
v. tr. - ‮סימן בגיר, כתב בגיר‬

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