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
) 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 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
), 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 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
), which is derived from gypsum (CaSO
O), an evaporite mineral formed by the depos 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
Quarrying limestone
1 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 quarr
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
2 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.
3 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.
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
4 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
5 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
Making white classroom chalk
6 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
7 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
8 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
9 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
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
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.
American Society of Civil Engineers, 1990.
Periodicals
Toth, Beth. "Jeanne Otis: A Color Dialogue,"
Ceramics Monthly.
January, 1988, p. 40.
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