Chalk is a widely used stationery in daily life, generally used for writing on blackboards. Chalk typically comes in a long cylindrical shape and is available in white and various colors.
According to historical records, the earliest documentation of chalk dates back to the Middle Ages when people discovered that by mixing lime with water, they could create a solid object and record it on dark or hard surfaces using a method similar to charcoal pencils. At that time, the production cost of chalk was much lower than that of paper, and it did not have the issue of smudging like carbon pencils used on wooden or rocky surfaces.
When using chalk, the friction at the tip causes the particles on the pencil to break off, which mostly adheres to the surface being written on. Using a board eraser can remove most of these particles from the surface, while the eraser will absorb the scattered particles. In both cases, a small amount of particles may still break off and float away from the writing surface, resulting in a significant amount of powder scattered on the ground after using chalk. Using clean water can effectively remove most of the chalk particles, similar to how outdoor chalk writing is easily washed away by rain.
Chalk can be used for writing in dry and rough environments because it does not rely on chemical reactions. In addition to blackboards, floors and walls are also popular writing surfaces, and many artists use chalk to create stunning street art.
Ancient chalk was typically made from natural chalk, but nowadays it is often replaced by other materials. Chalk, also known as white earth powder, is a fine deposit of calcium carbonate, a variant of calcite. Chalk generally refers to deposits from the Cretaceous period found in Western Europe, from which its name is derived. As a mineral, chalk is often used to manufacture chalk-like products.
The traditional components of chalk commonly used are calcium sulfate (CaSO4), a white precipitate that is not easily decomposed and has larger particles than dust, which can cause discomfort in the nose, throat, and lungs, potentially leading to lung cancer. It is lightweight, generates a lot of dust, and is difficult to clean. Calcium sulfate exists as white monoclinic crystals or crystalline powder and is odorless. It is hygroscopic and loses one molecule of crystallization water at 128°C, losing all water at 163°C. It dissolves in acid, sodium thiosulfate, and ammonium salt solutions, and it dissolves in 400 parts of water, with slower dissolution in hot water and extremely slow dissolution in glycerin. It is insoluble in ethanol and most organic solvents. Its relative density is 2.32 and has irritant properties. Typically, it contains two molecules of crystallization water and exists in nature in the form of gypsum mines.
In light of the disadvantages of calcium sulfate chalk, many environmentally friendly chalks today use calcium carbonate (CaCO3) instead of traditional calcium sulfate. Calcium carbonate is non-toxic, produces less dust, and can be metabolized normally by the human body (commonly found in calcium tablets or powdered food). Calcium carbonate, commonly known as chalk, limestone, stone powder, marble, and calcite, is a compound with a chemical formula of CaCO3 and is alkaline. Its solubility in pure water is very low (Ksp = 4.8×10-9) and dissolves in acid. Calcium carbonate is abundantly available on Earth and exists in various forms in rocks, minerals, and biological entities, such as calcite, limestone, marble, and as the main component of animal bones or shells (e.g., reptiles and bivalves). Calcium carbonate is also the main component of water scale, which is usually formed by the combination of calcium ions and carbonate ions in water and often occurs in areas with hard water. Calcium carbonate is also one of the active ingredients in agricultural lime, which can be used to neutralize soil acidity but may pose a risk of soil salinization if overused. In medical applications, calcium carbonate is a commonly used source of calcium and is often an active ingredient in antacids.
Colored chalk is made from the above materials mixed with dyes, and some manufacturers may add talcum powder or white powder.
The main components of ordinary chalk are calcium carbonate (limestone) and calcium sulfate (gypsum), or it may contain small amounts of calcium oxide. Because calcium sulfate is poorly soluble in water and can irritate the skin, when sweat occurs in summer, it produces alkaline substances that can further irritate the skin. The dust produced from chalk can dry and roughen the skin, causing itching and discomfort. During teaching, chalk dust is easily inhaled into the lungs, most of which gets trapped in the alveoli and may cause lung damage after prolonged irritation.
Dust-free chalk is an improved product of ordinary chalk designed to eliminate chalk dust pollution in classrooms. However, dust-free chalk is not entirely dust-free; it simply adds oil or polyol materials as binding agents to ordinary chalk, along with heavier fillers such as clay, limestone, and cement, which increases the weight and volume of the chalk dust, making it less likely to scatter. In practical applications, however, its effectiveness in reducing chalk dust pollution has not been as significant as expected.
Dust-free chalk incorporates a small amount of smooth agents and binding agents, allowing the chalk not to stick to hands and clothes, leaving no white marks on teachers' fingers after class, eliminating the need for washing hands. It also adds a small amount of moisture-proof agent, making it less likely to deteriorate after long storage. Ordinary chalk is easily dampened after long storage, leading to decreased hardness and increased breakage while writing.
Currently, the blackboards used in schools include cement boards, wooden boards, and frosted glass boards. Due to varying smoothness on the surfaces of blackboards, the softness and hardness of chalk required also differ. The new generation of dust-free chalk can change its softness and hardness freely, allowing use on various blackboards without slipping or breaking easily, and provides smooth writing, is labor-saving, and feels great in hand.
Dust-free chalk is available not only in white but also in various colors such as bright red, yellow, green, blue, and pink, and can be stored for long periods without color fading, maintaining vivid and distinct colors.
- By adding trace chemical materials to the main ingredients, the weight is increased, allowing chalk to write on blackboards without dust flying, maintaining hygiene and cleanliness, avoiding occupational diseases caused by inhaling chalk dust.
- A small amount of smooth agents and binding agents are added, preventing chalk from sticking to hands and clothes, leaving no white marks on teachers' fingers after class, eliminating the need for washing hands.
- A small amount of moisture-proof agent is added, ensuring long-term storage without deterioration. Ordinary chalk easily absorbs moisture after long storage, leading to decreased hardness and increased breakage while writing.
- The blackboards currently used in schools include cement boards, wooden boards, and frosted glass boards. Due to the different smoothness of the blackboard surfaces, the required softness and hardness of chalk also vary. The new generation of dust-free chalk can change its softness and hardness freely, allowing use on various blackboards without slipping or breaking easily, and provides smooth writing, is labor-saving, and feels great in hand.
- In addition to producing white dust-free chalk, various colors such as bright red, yellow, green, blue, and pink can also be produced, and long-term storage will not cause color fading.
Water-soluble chalk is mainly composed of titanium white powder and plant oils. Its ingredients can dissolve in water, are 100% dust-free, and can maintain a moist state for a long time, ensuring that no dust is produced during writing or erasing. Water-soluble chalk can be used on various qualities of blackboards, whether metal, wood, glass, or cement. Because it uses a completely non-irritating formula, teachers will not experience skin peeling from using chalk. Although water-soluble chalk is more expensive, one piece can last as long as many pieces of traditional chalk, making the overall cost of use roughly the same as traditional chalk.
- Does not alter the existing blackboard. Environmentally friendly water-soluble chalk can be used on glass, plastic boards, plastic films, whiteboards, greenboards, steel boards, and cement boards.
- Writing with it feels more refined, easier, and less labor-intensive, providing a feeling similar to holding a crayon.
- Colors are more vivid, durable, and do not break easily.
- Non-irritating to the skin.
- Does not produce dust during writing and erasing.
- Can be written on wet or dry boards after using a damp board eraser.
- Completely non-irritating, non-toxic, and odorless.
Teachers often need to write and draw on the blackboard during lessons. In a 45-50 minute class, they have to write and erase several times, often leaving a layer of white chalk dust on their hands, sleeves, and even hair and shoulders. Doing this day after day, year after year, after 20-30 years of work, thousands of pieces of chalk are consumed. Inevitably, some chalk dust is inhaled through the nostrils. Whether inhaling chalk dust is harmful is a major concern for many teachers. Chalk is made from gypsum, which consists of calcium sulfate. Gypsum is stable and non-toxic.
Writing with chalk involves applying gypsum powder to the blackboard. When erasing the blackboard, the gypsum dust (chalk dust) briefly floats in the air before settling on surfaces near the blackboard and on the ground. Its particles are relatively large, mostly over 100 microns, heavier, and fall more quickly, floating in the air for a short time. Dust particles in the air of living environments, with diameters over 10-20 microns, are mostly blocked by the nasal hair and throat upon inhalation, attaching to the mucous membranes of the small airways and alveoli. If the inhaled particles are harmful substances (like silica or asbestos), they may accumulate more in the alveoli and small airways, causing harm to the body. From the above, it can be seen that chalk dust particles are relatively large and mostly do not enter the lower respiratory tract. Additionally, gypsum is non-toxic to humans, and there have not yet been any reported cases of lung disease caused by inhaling chalk dust. According to relevant government regulations, chalk dust is not considered a cause of lung disease.
Despite the lack of significant harm from chalk dust, millions of teachers use chalk to write on the board daily, living amidst the chalk dust that remains after erasing the blackboard, which can still lead to discomfort in the nose, throat, and lungs, and should be taken seriously.
蘊藏許多助人的知識與智慧。