Ordinary chalk a head of coarse, about two inches long, a head of thin frustum of a cone shape, very well-balanced, very hard brittle. Commonly used white chalk.

Now, the domestic use chalk main common chalk and clean chalk two kinds, its main ingredients are calcium carbonate (limestone) and calcium sulfate (plaster), or contain a small amount of lime. Edit this section clean chalk what is clean chalk
Clean chalk belong to common chalk improvement products, refers to destroy the classroom chalk dust pollution. It is only in ordinary chalk to add fat class or polyalcohols material as bonding agent, and add the proportion of large packing, such as clay, marl, cement such as can make chalk dust weight and volume are increased, not easy off, but in practical application, it to chalk dust pollution reduction effects is not very obvious.
Clean the main advantages of chalk
(1) the main ingredient in adding trace chemical raw materials, increase the specific gravity, chalk to write on the blackboard, clean fly phenomenon, clean chalk
Clean, to avoid the for chalk foam through breath into visceral and occupational disease. (2) with a small amount of smooth agent and binder, chalk don't touch with one's hand and clothes, the teacher after a lesson fingers without white mark, do not need to wash their hands. (3) to add a little moistureproof agent, long time storage not metamorphism. Ordinary chalk long time storage extremely easy absorption of moisture, hardness drop, write break easily. (4) the university now use the blackboard have floor slab, board and wool glass plate, etc., for the blackboard surface smooth degree, request the chalk level is not the same. A new generation of clean chalk can be arbitrary change its hardness, in all kinds of the blackboard use won't appear skid or dry phenomenon, and fluent in written, energy, and feel very good. (5) clean chalk in addition to production white outside, still can produce bright red, yellow, green, blue, pink and so on the many kinds of colour, and long term storage, chrominance won't drop, always protect thunder-and-lightning, color clear. To edit this period of chalk chemical composition chalk is made of calcium sulfate hydrate (commonly known as plaster stone) is made. Also can join all kinds of pigment made color chalk. In the production process the plaster stone heated to a certain temperature, make its part of the dehydration into plaster of Paris, then the plaster of Paris water mixing into paste, penetration model set and become. Plaster stone into the reaction of plaster of Paris for (requires heating) : 2 caso4 · 2 h2o = 2 caso4 · 0.5 h2o + 3 h2o plaster of Paris adding water into plaster stone reaction for (room temperature can) : 2 caso4 · 0.5 h2o + 3 h2o = 2 caso4 · 2 h2o edit this section health effects liquid chalk
Teachers often need to write on the blackboard, drawing in the lecture. In 45 ~ 50 minutes of a class, will integration.it several times, the end of the class, often on the hands, sleeve, and even hair, shoulders leave a layer of white chalk. So every day, year after year, the work 20 ~ 30 years, kill the tens of thousands of chalk. Inevitably from the nose suction some chalk. Suction chalk is sick, it is broad teacher is very concern. Chalk is made by gypsum. The composition of gypsum is calcium sulfate. Gypsum stable performance, Environmental friendly. For thousands of years, the doctor of traditional Chinese medicine with plaster stone (contain water of crystallization of calcium sulfate caso - 4 · 2 h2o) Fried uniform, exterior calcined gypsum (plaster of Paris) applied to the affected part. Think can cool and refreshing antipyretic, fluids, thirst, give birth to the muscle gathered boils. Use chalk writing blackboard writing, is gypsum powder coating on the blackboard. Wipe the blackboard, gypsum dust (chalk) in the air flying on the blackboard after a fall near the surface and the things. The grain is bigger, more than 100 microns above, heavier, fall faster, floating in the air time is short. According to the labor health related material, production and living environment of dust particles in the air, diameter in 10 ~ 20 microns above, the nose suction timely most be nose hair, nose, pharynx and throat blocked, adhere to the mucosa small airway and alveolar. If for the body harmful substances (such as silica, asbestos), in the alveolar and small airway stored up in more, will be caused to human body damage. By above knowable, chalk dust particles is bigger, it most likely will not suction to lower respiratory tract, and add gypsum itself to human Environmental friendly, the medical profession has been there for suction chalk cause lung disease report. China labor department of health, in the professional the relevant provisions of the pneumoconiosis, also didn't will chalk as the etiology of lung. Although chalk on the human body to be harmless, but China's millions of teachers, everyday want to use chalk as blackboard writing, every day live in to wipe the blackboard after the chalk dust, still can cause nose, pharynx, laryngeal is unwell. To edit this period of chalk magical effect 1. Take a piece of chalk, let the students looked at from the intermediate broken (note section don't grinding), then the two paragraphs chalk butt, in butt in the original some butt, don't turn, connecting with after his forefinger and thumb hold chalk ends and extrusion, make some place produce certain pressure, slowly loosen, two paragraphs can come together, took the end can show students the. The shaking gently also won't break, this serves to illustrate that molecular exists between gravity (because chalk is dry, and is a viscous materials, not to make the students produce like plasticine that misunderstanding). Write on the blackboard with chalk, indicating that the blackboard and chalk intermolecular also have gravity exists, so that the intermolecular attraction exists not only in the same intermolecular, different kind of intermolecular also exists attraction. 2. Will chalk turn an Angle and press, it is not easy to catch, the reason is that most molecular spacing is more than 10 r0, close to the distance between molecules within 10 r0 few, therefore, gravity is insufficient to make it together, so that the distance between molecules and more than 10 r0 when molecular force can be neglected. With simple, easy to find at the same time the chalk that exist between the molecular attraction and molecular force under what circumstances can ignore problems. In addition, the experiment can and my classmates, edge experimental side summary, a abstract boring class can in the relaxed and active atmosphere complete. Edit this section praise chalk articles on west. Chalk praise ignorant even pick up chalk, wild graffiti walls. Ten years learning the blackboard side, dyeing time teacher hair cream. Forty the spring and autumn period and the twin, intellect sprinkle candle. Tumbling help brilliant, all admire the peaches and plums fragrance.

Now, China's domestic use chalk main common chalk and clean chalk two