Sidewalk chalk

Blackboard chalk, school chalk or chalk is a material for describing rough surfaces, primarily panels. She leaves behind small particles that stick loosely to the board and can be easily wiped off. Chalk is typically supplied as a pin of 8-9 cm in length with a square or circular cross-section of about 1.2 cm in diameter, some with paper sleeve, so as not to rub off on your hand.

Chalk for school use at least heard in Germany to the goods that are still in the old unit Gros ( 144 pieces ) are traded: Typical package sizes are the majority, semi- bulk and dozen box. ( For imported, for example French products shall not do that. )

History

Chalk will for centuries to label panels use; as found for example in Johann Amos Comenius ' Orbis pictus sensualium of 1653 the figure a label with chalkboard. The invention of colored chalk is the Scots attributed to James Pillans (1778-1864), who in his book Physical and Classical Geography (1854 ) detail the manufacturing process and the use in geography lessons describes.

Material

Chalk was originally exclusively from natural chalk (calcium carbonate), a particularly pure, fine-grained and soft form of limestone. Since real chalk is relatively expensive, blackboard chalk is now mostly made ​​of gypsum (calcium sulfate), or magnesium oxide, mixed forms occur ( for example, the so-called " Bologna chalk ", gypsum share with chalk ). Colorful chalks are produced by adding dyes.

In Germany available blackboard chalk made ​​from natural chalk often comes from France, where there are extensive chalk deposits in Champagne ( the Zentralchampagne is crayeuse as Champagne, chalky Champagne ) and in particular from the work Omey Omya AG in Chalons-en -Champagne. This chalk is " Champagne chalk " offered commercially under the name often and costs in some cases more than double of normal gypsum chalk.

Whether a piece of chalk is made of real chalk, (eg lemon juice or vinegar) can be ascertained through the sample with an acid: lime decomposes into CO2 foaming and the calcium salt of the acid ( calcium citrate or calcium acetate ). Magnesium oxide and gypsum do not change in these acids.

Ergonomics

By writing with chalk and especially during dry wiping the blackboard chalk dust is produced, which is troublesome for allergy sufferers and is also suspected to be causing problems with the respiratory tract. Most school chalk nowadays available is therefore treated to particularly low in dust or to be " dust-free".

When performing the chalk on a blackboard arise occasionally squeaking sounds that are perceived by many people as unpleasant. The sound is caused because the chalk getting stuck when sliding on the board due to the surface friction remains (stick- slip effect ), and so the piece of chalk is excited by vibrations at its natural frequency to the resonance. In order to eliminate the squeak, it helps to break the piece of chalk to a shorter length, so that the natural frequency increases and is no longer in the audible range.