1. How Chalk Is Made and How It Leaves Powder on the Blackboard
Chalk is a writing tool made by mixing powdered gypsum or calcium carbonate with water, then hardening it into a stick shape in a mold [1]. For blackboards, chalk is typically made from limestone powder (calcium carbonate) held together with a binder, or from gypsum [2]. In Japan, chalk was first imported from France in 1873, and domestic production began in 1875 [1].
When you write, the chalk wears away bit by bit, leaving powder on the board's surface. The visible writing is thought to be this trail of powder. Development documents for shell-based chalk also discuss how powder adhesion relates to the clarity of the lines [2]. Therefore, when you use an eraser, you are removing powder from the blackboard surface. If erasing means removing powder, that powder must go somewhere. Let’s look at where it goes.
2. Why Erasing and Tapping Erasers Sends Chalk Dust into the Air
After use, a blackboard eraser is full of powder. In the past, people often tapped the eraser against a stick to remove the dust. However, patent documents note that this method had the drawback of scattering dust and dirtying the surroundings [7]. Another patent lists the scattering of white powder, which dirties hands and clothes, and the potential impact on the lungs if inhaled, as traditional problems [8].
According to Wikipedia, chalk is said to produce floating dust particles of 10 micrometers or less [5]. To put that in perspective, 10 micrometers is 0.01 mm, or one-hundredth of a millimeter. Every time you erase, some of this dust may enter the air. Efforts to reduce this dust begin with the chalk itself.
3. Why Low-Dust Chalk Still Makes Some Dust
Commercial chalk comes in calcium carbonate types and calcium sulfate (gypsum) types. A research project by the Science and Mathematics Department of Inanokita High School suggested that calcium carbonate chalk is hard and has vivid color, while calcium sulfate chalk is more viscous. Their study indicated that the components may influence friction, or how the chalk feels when writing [4]. Since this was a high school project, the conclusion remains a possibility rather than a definitive fact.
The "Dustless Chalk" by Nippon Rikagaku Kogyo is introduced as a type of chalk that is less likely to scatter dust. However, it is not completely dust-free; it is designed to reduce the amount of dust [6]. One innovation in its raw materials involved using discarded scallop shells.
4. Chalk Made with Powdered Scallop Shells
Nippon Rikagaku Kogyo and the Hokkaido Industrial Research Institute developed chalk by replacing part of the limestone-derived powder with powdered scallop shells. The main component of shell powder is calcium carbonate, and its particles are rod-shaped [2]. Compared to traditional calcium carbonate chalk, this new chalk is less likely to break, adheres better to the blackboard, creates clearer lines, and writes more smoothly [2]. Good powder adhesion and clear lines are noted together in the development materials.
Commercialized in 2005, this chalk uses about 80 tons of scallop shells annually as raw material [3]. In Hokkaido, approximately 200,000 tons of scallop shells are generated each year. This represents about 1/2,500, or 0.04%, of the generated amount. While this is a small percentage, it helps utilize what was once fishery waste. However, there is no data stating that shell chalk completely prevents dust from flying. Next, we look at tools that remove dust that has already flown.
5. How Vacuum Cleaning Tools Collect Chalk Dust
To address the problem of dust flying when erasers are tapped, patents have proposed cleaning tools that use a vacuum to suck up and collect the dust [7]. A patent for an electric blackboard cleaner describes a specific structure [8]. First, a suction fan draws air inside the housing. This air passes through a circulating belt-shaped filter (a rotating filter), which traps the white powder. The clean air exits the housing, and the powder is collected in a drawer.
You can think of the air as water and the filter as a strainer. The water passes through, but the trash stays in the strainer. However, unlike a stationary strainer, a real rotating filter differs in that the belt keeps moving. This is just one example of a patented structure; not all products may use the same mechanism.
6. The Classroom Standard for Floating Dust in the Air
The School Environment Sanitation Standards (Ministry of Education, Culture, Sports, Science and Technology) set the limit for floating dust in classrooms at 0.10 mg/m³ or less [9]. This means no more than 0.10 milligrams of dust in one cubic meter of air, which is one ten-thousandth of a gram. Measurements are typically taken by collecting dust particles with a relative settling diameter of 10 μm or less on a filter paper and calculating the mass.
This standard does not target only chalk dust; it applies to all floating dust in the classroom. The standard value alone does not reveal the actual concentration in a specific room. The same standards also cover blackboards: achromatic blackboard surfaces must not have a lightness exceeding 3, the illuminance on the blackboard should preferably be 500 lx or more, and the surface must not have gloss that hinders visibility [9]. Standards for reducing dust and standards for visibility are listed in the same document.
7. How to Observe the Blackboard After Erasing
Do not perform demonstrations that deliberately stir up dust. Instead, after a normal class ends, look at the blackboard from an angle together with your teacher. Are the words completely gone? Are there faint white traces left behind?
Also, look at the surface of the eraser. If it has become white, that is evidence that powder has transferred to it. Record whether your own eyes can verify the idea that "the writing is made of chalk dust." For further reading, the Ministry of Education, Culture, Sports, Science and Technology’s School Environment Sanitation Standards and the Hokkaido Industrial Research Institute’s materials on scallop shell chalk are available in Japanese [9][3].