Blackboard chalk

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White chalk
Colored blackboard chalk

Blackboard chalk , school chalk or writing chalk is a material for writing on rough surfaces, primarily blackboards . It leaves behind small particles that stick loosely to the board and can be easily wiped off again. Blackboard chalk is typically supplied as a pen 8–9 cm long with a square or circular cross-section approx. 1.2 cm in diameter, sometimes with a paper sleeve so as not to rub off on the hand.

Chalk for school use, at least in Germany, is one of the goods that are still traded in the old unit Gros (144 pieces = 12 dozen = 12 * 12 pieces): Typical packaging sizes are large, half-large and dozen boxes. (This does not apply to imported products, for example French products.)

history

Chalk has been used for writing on blackboards for centuries ; for example, in Johann Amos Comenius ' Orbis sensualium pictus from 1653 there is an illustration of a blackboard inscribed with chalk. The invention of colored blackboard chalk is attributed to the Scot James Pillans (1778–1864), who in his book Physical and Classical Geography (1854) describes in detail the manufacturing process and its use in geography lessons.

material

School chalk made from plaster
Street chalk for children

Blackboard chalk originally consisted exclusively of natural chalk ( calcium carbonate ), a particularly pure, fine-grained and soft form of limestone. Since real chalk is relatively expensive, blackboard chalk is mostly made from plaster of paris ( calcium sulfate ) or magnesium oxide, but mixed forms also occur (for example the so-called "Bolognese chalk", plaster of paris with chalk content). Colorful chalks are made by adding dyes .

Blackboard chalk made from natural chalk available in Germany often comes from France, where there are extensive chalk deposits in Champagne (the central Champagne is also known as Champagne crayeuse , chalky Champagne), in particular from the Omey plant of Omya AG near Châlons-en-Champagne . This chalk is often sold under the name champagne chalk and sometimes costs more than twice as much as normal gypsum chalk.

Whether a piece of chalk is made of real chalk can be determined by testing it with an acid (e.g. lemon juice or vinegar): Lime decomposes in a foaming manner to form CO 2 and the calcium salt of the acid (calcium citrate or calcium acetate). Magnesium oxide and gypsum do not change in these acids.

ergonomics

Writing with chalk and especially wiping the blackboard dry creates chalk dust, which is annoying for allergy sufferers and is also suspected of causing problems with the respiratory tract . Most school chalk available today is therefore treated to be particularly low-dust or "dust-free". The general limit values ​​for workplaces for A dust and E dust according to TRGS 900 apply .

When running the chalk on a blackboard, there are occasional squeaking noises that many people find unpleasant. The noise arises because the chalk keeps getting stuck when sliding on the board due to surface friction ( stick-slip effect ) and vibrations stimulate the chalk to resonate in its natural frequency . To eliminate the squeak, it helps to break the piece of chalk to a shorter length so that the natural frequency increases and is in the inaudible range.

Popular belief and superstition

Markings and demarcations made with chalk play a role in the belief in warding off evil spells and powers. Chalk crosses, for example in the hat, under the soles of the shoes or under the milk pail, are supposed to offer protection against bewitching . A chalk line between the joints of two floorboards is supposed to keep changelings away from the bed of a woman who has recently given birth . In the Catholic Church, too, which consecrates pieces of chalk for this purpose, chalk signs are considered protective, such as those written on the doors of the Epiphany with the letters C, M and B.

Web links

Individual evidence

  1. ↑ http://archive.org/details/elementsphysica01pillgoog
  2. ↑ Technical rule for hazardous substances, occupational exposure limit values, TRGS 900. (pdf) Federal Institute for Occupational Safety and Health, March 3, 2015, p. 5 , accessed on April 10, 2015 .
  3. ^ Eduard Hoffmann-Krayer (ed.): Concise dictionary of German superstition . tape 5 . Walter de Gruyter, Berlin / New York 2000, ISBN 3-11-016860-X , p. 460-462 .