1. How Tiny Gaps Between Brush Hairs Hold Ink
Capillary action is a phenomenon where the liquid level inside a narrow tube or gap becomes higher (or lower) than the outside level. This happens due to surface tension, causing liquids that wet the tube walls, like water, to rise [4]. In the tip of a brush, the spaces between the hairs act as these narrow gaps. A single hair cannot create enough space to hold ink. By bundling hairs together, the brush creates places where ink can be kept [4].
2. Why Brushes Mix Soft Hairs and Hard Hairs
Brush hairs are made from various animals. Soft hairs come from sheep, while hard hairs come from horses. Other materials include weasel, raccoon dog, and rabbit hair [1]. Brushes that mix hard and soft hairs are called "jianhao" (mixed-hair brushes). This combination is used to balance elasticity and ease of writing within a single brush [1]. According to the English encyclopedia, longer hairs are said to hold ink for longer, and hard hairs are suited to keeping the shape of the tip. However, this is noted as a tendency rather than a strict rule [5].
3. How Hair Shape Changes the Way a Brush Holds Ink
Brush hairs are cone-shaped, being thick at the root and thinning towards the tip. A 2014 study reported that the surface of Chinese brush hairs has small and large structures that vary depending on direction [3]. The study describes the balance of ink volume in terms of three forces: a difference in Laplace pressure, different holding forces depending on direction, and gravity [3]. The title of the study uses the word "ratchet," which refers to a gear mechanism that turns in only one direction. This word can be read as a metaphor for the mechanism where the direction of the hair affects how ink is held [3].
4. How Kumano Brush Makers Mix Different Hairs into One Tip
Kumano Town in Hiroshima Prefecture is known for brush making. It is said that during the Edo period, people from families with little farmland would buy brushes and ink in the Kinki region during the farming off-season and sell them on their way home. This traveling sales is believed to have connected Kumano with brush making, though the exact starting time is not clear. It is estimated that this practice was happening around the Tenmei era (1781–1789) [2]. In Kumano brush making, techniques like "nerimaze" (mixing hairs) and "bonmaze" are used. Bonmaze involves mixing dry hairs in a tray and then wetting them for work. This technique developed in Kumano during the Meiji era and allows for efficient mixing of large amounts of hair [2]. Kumano brushes were designated as a traditional craft in 1975 [2].
5. What Scientists Still Do Not Know About Ink in Brushes
What happens to the movement of ink inside the hairs when there is a lot of ink? How does ink spread when it touches the paper? In the research, the relationship between hair shape and ink is being investigated. Current research focuses on Chinese brushes. Whether these findings apply directly to Japanese brushes is not addressed in the papers [3].
6. How You Can Investigate Brush Hairs and Ink Yourself
Try looking at the tip of a brush with a magnifying glass. Observe the thickness of the hairs and the gaps between them. Place a brush wet with water gently on white paper and observe how far the water spreads. It is safer to do this with water only, without ink. Explanations of the production area in Kumano Town can serve as an entry point to verify brush-making methods through text [2]. The characteristics of different hair types can be checked in encyclopedia entries on brushes [1].