Forge Charcoal Types and Quality Control: How Swordsmiths Select White Charcoal, Black Charcoal, and Binchotan for Blade Forging

Carbon is a Sword Material—The Often-Overlooked Star of Forging

While tamahagane is often highlighted as the material in Japanese sword production, the recognition that "charcoal quality" determines sword quality at actual forging sites is an unwavering common knowledge among sword smiths. Charcoal is not merely fuel—it plays a role in adjusting the carbon content of steel, as the carbon in charcoal dissolves again into the steel at high temperatures.

If the type, quality, or origin of charcoal used is wrong, temperature control becomes unstable, the composition of steel becomes distorted, and the quality of hamon is affected. Charcoal selection is one of the particularly important technical decisions made by sword smiths.

The Three Major Classifications of Forging Charcoal

Japanese forging charcoal is broadly divided into three types.

White charcoal (shirotzumi): Created by carbonizing at high temperature (over 1000–1200°C) and then rapidly cooling. It is characterized by a whitish ash coating on the surface, high density, and high burning temperature. The representative example is "binchotan," the highest-quality white charcoal made from ubame oak from Wakayama Prefecture. White charcoal contains few impurities and can maintain stable high temperatures for extended periods.

Black charcoal (kurotzumi): Made by slowly carbonizing at relatively low temperature (400–700°C) and naturally cooling inside a sealed kiln. The surface is black, and compared to white charcoal, it is softer and more prone to crumbling. The burning temperature is lower than white charcoal, but it ignites quickly and is easy to adjust for heat control.

Rice-husk charcoal and bamboo charcoal: Some sword smiths use these for special purposes. They are sometimes used for removing the "black skin" (kurokawa) from finished blades or surface preparation, and their use differs from charcoal used in main forging.

Comparison of White Charcoal and Black Charcoal

Organizing the differences between the representative white charcoal and black charcoal from the three major classifications, we get the following.

Using these two types appropriately according to the purpose is itself part of a sword smith's skill.

Why Binchotan is Regarded as the King of Forging Charcoal

There are multiple reasons why binchotan is respected as the "king of forging charcoal" in the world of sword making.

First is high burning temperature and stability. Binchotan can stably produce temperatures exceeding 1300°C and is suitable for maintaining tamahagane at forging temperature (white-hot state, approximately 1000–1200°C). Temperature variations are minimal, making kiln temperature control easier.

Next is ash characteristics. Binchotan ash has fine particles that spread uniformly in the kiln. This produces a secondary effect of stabilizing the thermal environment inside the kiln. Inferior charcoal that produces coarse ash results in irregular heat distribution within the kiln.

And carbon dissolution control. As mentioned earlier, carbon monoxide (CO) generated during charcoal combustion fine-tunes the carbon content of steel. Because binchotan has few impurities, this carbon supply is stable, making unintended carbon increase (carburizing) or carbon decrease (decarburizing) unlikely during forging.

Regional Charcoal Characteristics and Sword Smith Preferences

The main production areas for binchotan are Wakayama (Kishu), Mie, Kochi, and Miyazaki, but charcoal quality varies subtly by region. Kishu binchotan is highly trusted by sword smiths because the quality of ubame oak is stable.

On the other hand, binchotan is expensive, and the amount of charcoal used per forging session (per blade) can reach tens of kilograms. Therefore, some sword smiths use binchotan in combination with high-quality black charcoal. A division of labor such as "black charcoal for the rough-forging stage, binchotan for the finishing stage of fine-forging" is considered rational.

Charcoal Quality Management—Sword Smith Evaluation Expertise

The main items that sword smiths verify when selecting high-quality forging charcoal are as follows.

Color and appearance: Uniform black (or ash-white for white charcoal), free of cracks or chips, with no visible knots or rot. Charcoal with large amounts of "shirata" (unevenly carbonized sections) is considered inferior.

Sound when struck: High-quality binchotan produces a dry, high-pitched "kan-kan" sound similar to a metallic tone. If it contains moisture or has internal voids, it produces a duller "kotsu-kotsu" sound.

Weight and density: High-quality white charcoal has high density and is heavy for its size. Charcoal that is too light has a rough interior and short burning time.

Verification of origin certificate: In recent years, charcoal with false origin labels has circulated. It is important to purchase from reputable charcoal merchants and verify origin certificates.

Points to Verify When Purchasing or Sourcing

Organizing the verification items mentioned so far for practical use at sourcing sites, we get the following.

• Are there no irregularities or dullness in color and appearance?
• Is the sound when struck high and dry?
• Is the weight and density sufficient when held in hand?
• Is the origin certificate clear and is it purchased from a reliable sourcing partner?

These are not individual criteria but are comprehensively assessed by combining multiple perspectives.

Contemporary Charcoal Supply Issues and the Search for Alternative Fuels

The decline in charcoal burners and reduction in domestic charcoal production affect sword smiths' procurement of forging charcoal. The price of high-quality binchotan rises year by year, becoming a factor that pressures the fixed costs of sword smith businesses.

Some sword smiths have attempted partial adoption of "gas kilns" or "electric kilns," but fully reproducing the "carbon supply and temperature characteristics" unique to charcoal fire is currently considered difficult. The reason charcoal fire is indispensable in traditional Japanese sword forging is not mere custom but derives from these carbon control and temperature characteristics.

Support for developing charcoal burners and promotion of domestic timber utilization for sustainable charcoal supply have emerged as issues for the entire sword-making industry, and coordination with the Forestry Agency and local governments is being explored. Protecting charcoal is directly linked to protecting the future of Japanese swords.

How Does Knowledge of Charcoal Connect to Choosing Swords?

For modern sword enthusiasts and purchasers, knowledge of forging charcoal is not merely background information. The more careful the charcoal management during forging, the more stable the steel structure tends to be, and this care is more likely to be reflected in the appearance of jihada and hamon. Conversely, forging with disrupted temperature control due to charcoal problems tends to result in uneven steel composition.

However, for existing swords, particularly those made in ancient times, it is difficult to accurately determine what type of charcoal was actually used after the fact. In purchasing or appreciation contexts, judging based on the appearance of jihada and the condition of hamon on the blade, along with the findings of professional appraisals, is practically the most reliable method, rather than attempting to deduce the type of charcoal used.