Chalk



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 (Fe 2 O 3 ) 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 (CaCO 3 ), 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.

To make chalk, limestone is first quarried, generally by an open pit quarry method. Next, the limestone must be crushed. Primary crushing, such as in a jaw crusher, breaks down large boulders; secondary crushing pulverizes smaller chunks into pebbles. The limestone is then wet-milled with water in a ball mill—a rotating steel drum with steel balls inside to further pulverize the chalk. This step washes away impurities and leaves a fine powder.
To make chalk, limestone is first quarried, generally by an open pit quarry method. Next, the limestone must be crushed. Primary crushing, such as in a jaw crusher, breaks down large boulders; secondary crushing pulverizes smaller chunks into pebbles. The limestone is then wet-milled with water in a ball mill—a rotating steel drum with steel balls inside to further pulverize the chalk. This step washes away impurities and leaves a fine powder.

The base of pastel chalks is calcium sulfate (CaSO 4 ), which is derived from gypsum (CaSO 4 -2H 2 O), 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

Pulverizing the chalk

After grinding, the chalk particles are sifted over vibrating screens to separate the finer particles. The particles are then mixed with water, extruded through a die of the proper size, and cut to the proper length. Finally, the chalk is cured in an oven for four days.
After grinding, the chalk particles are sifted over vibrating screens to separate the finer particles. The particles are then mixed with water, extruded through a die of the proper size, and cut to the proper length. Finally, the chalk is cured in an oven for four days.

Dehydrating gypsum

Sifting, cleaning, and shipping
the chalk

Making white classroom chalk

Making colored classroom chalk

Making pastels

Boxing the chalk

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.

Rose Secrest



Also read article about Chalk from Wikipedia

User Contributions:

1
Jacqueline Nyathi
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Jul 30, 2007 @ 4:04 am
This document was well documented and simple though relevant language used. It is very helpful to me and has enhanced my knowledge of chalk in particular, and limestone in general.
2
felix
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Aug 6, 2009 @ 2:02 am
TELL ME THE GOOD RATIO FOR CHALK MAKING WHEN YOU GET THIS MESSAGE
3
osuobeni fidelia
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Aug 7, 2009 @ 12:12 pm
This article is very interesting and good for those who r interested in learning and knowing the origin of things around them. Am glad i was part of those who benefited from this article.
4
Samantha Livermore
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Jan 27, 2010 @ 3:15 pm
Currently I am a student at the Fashion Institute of Design and Merchandising(FIDM) in San Fransisco and a class I am taking requires us to research the most green home products both hard and soft. Using chalk for displays is common but with the goal of sustainability and Staying as close to home as possible (San Fransisco)I was wondering if there are any known chalk companys within the U.S who sell/produce chalk. It sounds a bit crazy, but be eliminating "made in china" our carbon foot print drops dramatically.
5
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Jun 24, 2011 @ 5:05 am
how do we recycle a chalk dust?what is the materials needed to stick together the chalk particles?
6
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Aug 18, 2012 @ 9:21 pm
This article has really helped me to understand the factors of making chalk, i very much liked the step-by-step orientation and looking forward to learn more in this website.
7
Not Telling
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Oct 17, 2012 @ 6:18 pm
Most Teachers use white boards and expo markers today
8
Michael oluseto
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Jul 17, 2014 @ 11:11 am
The article is good and helpful. Pls how can i get the materials like mould,drier,bis manual etc to buy?
9
girish
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Aug 16, 2014 @ 2:02 am
How to make a snooker chalk , can i get detail about row material with picture

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