Manufacturing Process of Japanese Knife

  • To begin with, Japanese knife manufacturing processes are broadly categorized into "forging" and "casting." This page aims to explain these methods in detail and help you understand their differences.

  • Forging involves shaping metal by hammering it to achieve the desired form. The metal is heated and hammered, which aligns and compresses the internal crystal structure. This process results in a more durable and resilient material. The toughness gained from forging is known as "toughness," which refers to the material's resistance to impact and sudden force.

  • On the other hand, casting involves melting the metal and pouring it into a mold, where it solidifies as it cools. While cast products may look similar to forged ones, their internal structure differs. Casting allows for mass production and uniform shapes quickly and cost-effectively, making it suitable for producing large quantities of products.

  • Differences Between Forging and Casting

    • Appearance: Forged and cast products may look similar, but there is a significant difference in their internal quality.
    • Strength: Forging compresses and aligns the metal's crystal structure, resulting in enhanced strength and toughness. Casting, however, has a coarser internal structure, leading to lower strength and toughness.
    • Cost and Production Efficiency: Casting is ideal for mass production and cost reduction, but forging involves more time and effort, resulting in higher quality products.

  • At KIREAJI, we offer Japanese kitchen knives forged by craftsmen in Sakai City. This page will delve deeper into the details of the forging process and highlight its techniques and appeal.

Ⅰ. Forging Process: Traditional Techniques to Shape and Strengthen the Blade

  • Forging is the process of shaping metal by hammering it. In this process,soft iron and steel, which serve as the base materials, are heated until they are glowing red. By heating the metal to make it malleable, it becomes easier to process and shape into the desired form. Hammering the metal not only extends its shape but also densifies the internal structure, enhancing its strength and toughness. This method creates a blade with exceptional sharpness,durability, and remarkable beauty.

  • 1. Forge Welding (Wakashitsuke)- High-Temperature Metal Bonding

  • Forge welding involves heating metal to approximately 1100-1200 degrees Celsius, making it pliable for bonding through hammering. This technique, traditionally known as "Wakashitsuke" in Sakai City, is used to heat soft iron and bond it with steel, creating a durable blade. This high heat allows the metals to fuse effectively, ensuring both strength and sharpness in the final product.

  • Temperature Control in Wakashitsuke - The Art of Enhancing Sharpness

    In the Wakashitsuke method, precise temperature control is critical to the quality of the knife. If the temperature is too low, the metals won’t bond sufficiently, while excessive heat can damage the steel. This meticulous temperature adjustment enhances the blade’s sharpness, allowing artisans to fine-tune each knife’s edge. Working at lower temperatures helps maximize the knife’s sharpness, and the craftsman’s ability to assess the ideal temperature by sight is key. A slight temperature miscalculation can lead to defects in the blade, so artisans carefully monitor the temperature and handle each blade with exceptional attention to detail.

  • Combining Soft Iron and Steel - A Legacy of Japanese Sword Crafting

    Japanese knives often feature layers of soft iron and steel, combining different materials for optimal flexibility and strength. This technique is derived from traditional Japanese swordmaking, imparting superior durability and sharpness to the blade. Sakai artisans strongly believe that "using the finest materials is essential" in knife making and select each element with utmost care.

  • The Pinnacle of Blacksmithing: Pursuing the Ideal Knife – The Art of Temperature Control and Fine Grain Structure

    In the art of knife-making, the key to crafting the perfect blade lies in the fineness of the metal’s grain structure. This fine grain determines the precision of the edge and its cutting performance. At the same time, precise temperature control during forging is indispensable. If the temperature is too low, shaping the metal becomes difficult; if it’s too high, the grain structure coarsens, compromising the blade's sharpness and durability.

  • Ancient blacksmiths mastered the delicate balance of maintaining the metal’s fine grain by forging it meticulously at low temperatures. They observed the metal’s color and texture to determine the right moment to strike. This intricate temperature management is a testament to their expertise, ensuring the internal structure of the metal remains optimized to maximize its potential.

  • A knife is not merely a tool—it serves as an extension of the chef’s hand, unlocking the full potential of ingredients. When crafted with fine-grained metal and the artisan’s skill, the blade’s edge becomes sharp and uniform, cutting effortlessly without damaging the food. This ensures that the flavor, texture, and natural beauty of the ingredients are preserved, elevating the culinary experience.

  • The harmony between craftsmanship and natural materials, honed over centuries, surpasses even the most advanced modern technologies and synthetic materials. Knife-making is a process that requires delicate control of heat and pressure, minimizing stress on the metal. Through careful low-temperature forging, the internal structure of the blade is refined, producing a flawless edge when polished.

  • Each knife is imbued with the passion and dedication of the artisans who create it. The ideal blade not only supports the craft of the chef but also embodies the harmony between nature and human skill. Through their work, artisans deliver a sense of reverence and inspiration, connecting the hands of the maker to those of the user. This is the essence of knife-making—a blend of functionality, artistry, and timeless beauty.

  • Yasuki Steel - Known for Its High Purity and Superior Sharpness

    Sakai knives use Yasuki steel from Shimane Prefecture, a high-purity steel containing minimal impurities like phosphorus and sulfur, which can affect sharpness. Known for its exceptional cutting performance, Yasuki steel demands delicate temperature management, a skill limited to only the most experienced craftsmen.
Yasuki Steel

  • 2. Preliminary Forging

  • In this stage, the craftsman skillfully alternates between a mechanical spring hammer and a hand hammer, carefully shaping the heated soft iron and steel. This traditional forge-welding process, called wakashitsuke, ensures the metals are thoroughly bonded at high temperatures, creating a strong, unified blade structure. After bonding, the blade is reheated and shaped to its final form. Through this cycle of heating and hammering, the steel's internal grain structure is refined, resulting in exceptional sharpness and cutting performance.

  • For example, forging a 300mm Yanagiba knife made of Shirogami steel takes about 20 minutes with a hammer. However, performing the same process with Aogami steel requires roughly twice as long because Aogami steel is significantly harder. Shirogami steel visibly stretches under hammering, while Aogami steel initially shows almost no change. This difference demonstrates the unique handling challenges of each type of steel, as well as the time and skill required from the craftsman to bring out their best qualities.

  • 3. Core Taking and Shaping

  • From the pre-cut steel material, a single knife is cut out, forming what is known as the han-gata, which serves as the base of the knife. Next, the core that will be placed inside the handle of the knife is made. Since cutting ability is not required, it is created from base metal.

  • Once the core is made, the blade tip is shaped. This process is called shaping. During this time, the base is left thick while tapering down to a thinner edge (known as a taper structure or tapering). This technique sharpens the cutting edge and reduces weight, resulting in a knife that is less tiring to use.
Japanese Knife Structure

  • 4. Annealing (Yaki-namashi)

  • Annealing is a crucial step in knife production, aimed at removing internal stress after forging and softening the steel structure to improve workability. In the production of Sakai knives, traditional methods are employed, with the steel being slowly cooled over a full 24-hour period. This process adjusts the hardness and resilience of the material, enhancing the knife's overall quality and sharpness. While many craftsmen use mechanical furnaces to manage the temperature, there is also a method where the steel is placed in ash after burning straw, which helps prevent carbon evaporation and creates a more robust metal structure. The craftsmanship of the artisans ensures that this annealing technique maintains the high reputation of Sakai knives.

  • Ⅱ. Heat Treatment Process: Refining the Steel's Properties for Optimal Performance

    A precise process of temperature control and cooling to enhance hardness, strength, and toughness

  • 1. Rough Forging

  • Rough forging is one of the essential processes in the production of knives. During this stage, a hammer is used to strike the surface of the blade, removing the oxide film. Subsequently, a spring hammer is employed for further forging. This effectively eliminates the oxide film formed during the fire forging process and results in a smoother blade surface.

  • This smooth surface makes the subsequent work of the sharpener much easier and significantly impacts the final quality of the blade. By undergoing the rough forging process, the knife's edge evolves into a more precise and sharper cutting tool.

  • 2. Urasuki

  • Urasuki is the process of creating subtle indentations on the back surface of a knife, a task performed by skilled artisans. The craftsman uses a hammer to carefully strike the back of the blade. This delicate indentation allows for reduced contact area between the food and the blade, resulting in a smoother cutting experience.

  • By applying urasuki, food is able to release easily from the knife, enhancing the efficiency of food preparation. This effect can be particularly appreciated when cutting various ingredients such as meat, fish, and vegetables, making it an essential process for chefs. Thus, urasuki plays a crucial role in enhancing the performance of the knife.
Urasuki

  • 3. Quenching (Yaki-ire)- The Moment Life is Breathed into the Knife

  • Quenching is the most crucial and tension-filled process in knife making. It wouldn't be an overstatement to say that the success of all previous work depends on this single process.

  • Meticulous Preparation and Careful Techniques

    Before quenching, a special clay mixture is applied to the blade. This clay serves several vital purposes:

    • Prevents uneven heating
    • Enhances cooling efficiency when submerged in water
    • Ensures uniform hardness throughout the blade

    The workshop is intentionally darkened. This allows the master craftsmen to precisely observe the subtle color changes in the steel - a skill that only comes with years of experience.

    The Critical Moment

    The high-purity Yasuki steel (Yasuki-hagane), due to its exceptional purity, demands incredibly precise temperature control. At this crucial moment, the craftsman must identify the exact instant when the steel reaches its ideal temperature. Then, without a moment's hesitation, the blade is plunged into water.

    The Science Behind the Art

    During the quenching process, several transformations occur:

    1. The heated steel rapidly absorbs carbon
    2. Rapid cooling fixes the carbon in place
    3. Through this process, the steel achieves its ultimate hardness

    Master Craftsman's Wisdom

    The hardened steel has a tendency to warp. Drawing from generations of experience, craftsmen employ a clever technique: they pre-bend the blade in the opposite direction to ensure it straightens properly during quenching. This represents another crucial aspect of the master craftsman's expertise.

    Words from the Master

    "The quenching process determines the blade's cutting edge. A single mistake during this stage renders all previous work meaningless. Working with high-purity Yasuki steel is particularly challenging, demanding our utmost attention to temperature control. A split-second decision breathes life into the blade - or destroys it."

    This crystallization of skill and experience - the quenching process - truly represents the moment when the soul is breathed into a Sakai knife.

  • Phase Transformation Points

    Austenite:

    When metal reaches a certain temperature (approximately 800 to 850 degrees Celsius), iron, carbon, and trace elements dissolve and mix uniformly. This state is known as austenite. This phase, also referred to as austenitic transformation, makes the metal softer and easier to work with.

    Martensite:

    By rapidly cooling the austenite, a harder microstructure known as martensite is formed. Rapid cooling is crucial for achieving this structure, which results in a significantly harder material. This phase transformation is known as martensitic transformation, and the greater the amount of carbon in the iron, the harder the material tends to become.

    The Secrets Behind Knife Warping: A Master Craftsman's Perspective 

  • From the perspective of a traditional craftsman

  • Breathing Life into Metal: The Soul of a Blade Born Through Cooling

    Cooling is not just a process; it is the very act of transforming the essence of metal. By applying stress through precise cooling techniques, we alter its structure and awaken its potential. This critical step unlocks the hardness and durability essential for crafting fine blades.

  • In the world of traditional craftsmanship, cooling is seen as more than science—it’s a moment of artistry where the material’s spirit is shaped. By controlling the speed and method of cooling, whether rapid quenching or gradual air cooling, we dictate the character of the blade.

  • This process embodies the philosophy of balance and precision. Without proper cooling, the metal reverts to softness, losing its ability to hold an edge. It’s in this moment, as the heat dissipates and the structure tightens, that the blade’s true nature is born.

  • Importance of Quenching

    Quenching is essential for dramatically improving the hardness and durability of metal. The transformation from austenite to martensite through this process significantly enhances the metal's performance. As a result, quenching produces a metal that is stronger and more durable.

  • Let's dive into a more intriguing aspect of quenching: the methods used for rapid cooling. There are primarily two methods for quenching: water and oil. The process of quenching with water is called water quenching, and the process using oil is called oil quenching.

  • Water quenching cools the metal extremely rapidly, resulting in a very high quenching rate. In contrast, oil quenching cools the metal more gradually, with a slower quenching rate compared to water quenching.

  • The speed of cooling significantly affects the formation of martensite. Martensitic transformation occurs when the metal transitions from austenite to martensite. The faster the cooling rate, the more robust the martensite structure becomes. In other words, the quicker the quenching process, the more effectively martensite forms. Thus, water quenching, with its rapid cooling capability, tends to produce a superior martensite structure and is generally considered to provide better quality.

  • However, some people prefer oil quenching for its specific advantages. While water quenching is often seen as superior in terms of the microstructure of martensite, oil quenching has its own set of benefits, which can be a matter of personal preference. Therefore, while water quenching is typically favored for its performance in creating a strong martensite structure, oil quenching also has unique advantages that may be preferred by some.

  • From the perspective of a traditional craftsman

  • The Destiny of a Blade Lies in the Art of Cooling

    The speed and method of cooling are what determine the fate of a blade. This delicate balance between heat and cold decides whether the metal will transform into a sharp, durable tool or return to its soft, pliable state.

  • In crafting, every step is intentional. Cooling is not just about temperature—it’s about shaping the future of the blade. A rapid quench can create extraordinary hardness, while a slower air cool allows for a more balanced toughness. The choice depends on the purpose of the blade and the skill of the craftsman.

  • Pair text with an image to focus on your chosen product, colleThrough careful cooling, we breathe life into the metal, forging not only a tool but a piece of art. It’s a reminder that even the smallest decisions can have profound impacts, turning raw materials into something exceptional. This is the soul of traditional craftsmanship.ction, or blog post. Add details on availability, style, or even provide a review.

  • 4. Tempering (Yaki-modoshi)

  • Tempering is a crucial process that imparts a unique toughness to the steel. This process combines the opposing properties of hardness and softness, resulting in a blade that is less prone to chipping.

  • Specifically, the knife is placed in a furnace or machine at a temperature of about 180 degrees Celsius for tempering. This procedure enhances the blade's toughness (resilience), making it less likely to chip. This characteristic is essential for achieving high performance in culinary applications.

  • The low temperature of 180 degrees Celsius helps to adjust the internal structure of the steel, maintaining the appropriate balance. As a result, the knife is finished as an ideal cutting tool that is flexible yet possesses sufficient hardness.
Japanese Knife Materials 4 points

Sub-Zero Processing

KIREAJI's Sub-Zero processing technology rapidly cools steel to sub-zero temperatures, significantly enhancing knife hardness and durability. This process, rooted in the craftsmanship and expertise of skilled artisans, results in knives with superior sharpness and lasting performance.

Sub-Zero Processing

Discover the Allure of Forged Knives

Forged knives represent the pinnacle of craftsmanship, where strength, durability, and beauty converge.

Discover the Allure of Forged Knives

Technical Explanation of Aike in Japanese Traditional Knives

In traditional Japanese knives, "Aike" is a unique phenomenon that can significantly impact the blade's quality. Are the staff at the store you're considering aware of Aike? Many shops sell knives without completing the final sharpening process, leading customers to unknowingly purchase knives with Aike. This page delves into Aike's technical aspects and why it matters for your knife selection.

Aike

  • Ⅲ. Sharpening: Mastering the Edge for Ultimate Sharpness and Precision

    The process in which skilled sharpeners give the knife its sharp edge is called "togi" (blade sharpening).
    This sharpening process consists of approximately 20 individual steps.

  • Sharpening

  • Starting with a coarse whetstone and gradually moving to finer ones, the blade is carefully sharpened. The sharpening process involves approximately 20 steps, with the main stages being Rough Grinding, Hon-togi, and Back Sharpening. By sharpening while running cold water, the rise in temperature, which can cause tarnishing and reduce hardness, is prevented.

    Through this detailed process, a beautiful, distortion-free knife is completed.

  • 1. Rough Grinding

    Rough sharpening is the first grinding process in knife manufacturing. During this stage, the knife is fitted into a wooden mold and ground roughly against a rotating grindstone. The primary goal of rough sharpening is to reduce the thickness of the blade's tip.

  • This initial work lays the foundation for shaping the knife, ensuring that subsequent sharpening processes can be carried out smoothly. Through rough sharpening, the knife's shape is defined, and the preparation for achieving a sharper edge is established. Furthermore, rough sharpening is a critical moment that tests the skills of the knife artisan, requiring a high level of craftsmanship.

  • 2. Correcting Warp

  • During the grinding process, the frictional heat generated by the rotating grinding stone can cause the knife to warp. If this distortion is overlooked, it will prevent the blade from making uniform contact with the grinding stone, resulting in ineffective sharpening. Therefore, a crucial step in the crafting process is dedicated to correcting this warp.

    Craftsman's Insight: "When sharpening, temperatures can rise to between 300 and 400 degrees Celsius, causing the carefully tempered steel to lose its hardness. If the heat treatment is reversed, the steel softens significantly, rendering the blade ineffective."

  • 3. Hon-togi

    Hon-togi is a crucial process that gives the knife its true shape. During this stage, the entire knife is pressed against a rotating grindstone, first to flatten the surface and reduce its thickness. After that, the blade’s edge is carefully honed to enhance its sharpness.

  • Craftsman's Note: "Shinogi is the most challenging part of the process. When performing this task, I hold my breath while sharpening the knife."

    This stage determines the knife's final performance, requiring the craftsman's skilled technique and focused concentration.

  • 4. Back Sharpening

    Back sharpening is an essential step in the knife manufacturing process. In this stage, the back bevel created during the forging process is further thinned and shaped. The back bevel refers to the area on the reverse side of the blade, and its finishing significantly contributes to the knife's cutting performance.

  • By performing back sharpening, the thickness of the blade is properly adjusted, achieving a sharper cutting edge. This process requires skilled techniques, showcasing the craftsmanship of the knife artisans. A knife refined through back sharpening can greatly influence the outcome of the dish, maximizing its performance.

  • Ⅳ. Handle Attachment: Crafting Balance and Comfort for the Perfect Fit

    At KIREAJI, we do not cut corners in the final stages of knife-making, including handle attachment. It is our commitment to meticulous attention to detail that enables us to create knives that our customers can use for a long time.

  • Handle Attachment

  • The part called the "nakago" (tang) is heated and then inserted into the handle. By tapping the end of the handle with a wooden mallet, the tang is firmly secured all the way in. Once any distortions are corrected, ensuring the blade is perfectly straight, the Sakai forged knife is complete.

  • Instead, the handle is secured by expanding the tang with heat. This method enhances safety and durability, as it can withstand long-term use and contact with water without loosening. Natural wood is typically used for the handle, ensuring stability and aesthetic appeal. Additionally, maintenance is straightforward; if the handle loosens, it can be easily re-secured. The handle’s material and shape add individuality, allowing customization based on the user’s preferences. This craftsmanship and aesthetic sensibility make handle fitting an essential element of Japanese blade culture.

The Process of Making a Knife

The process of making a Japanese knife involves numerous steps and significant artisanal handiwork, taking one to two months per knife. The primary methods in Japan are traditional "forging," where steel is hammered and shaped, and "stamping," where blades are cut from steel sheets. These techniques, rooted in Japanese sword-making history, showcase rare technical prowess and high quality in knife manufacturing.

The Process of Making a Knife

The Process of Making Japanese Knives

The Origin of Craftsmanship

  • The quenching process requires absolute focus—even a single moment of carelessness can determine the sharpness of the blade.

  • Quenching is one of the most crucial steps in breathing life into swords and tools. Heating the steel in fire and cooling it at the exact right moment—this split-second decision profoundly impacts the sharpness and durability of the final product. Even for seasoned artisans, the tension in this moment never fades.

  • It demands not only technical skill but also a craftsman’s keen intuition and unwavering concentration. The slightest change in temperature or the subtle behavior of the steel must be observed and judged precisely. Each attempt is unique, and no matter how many times we repeat this process, there is no room for compromise.

  • When someone holds a finely sharpened tool and feels its precision, or when they cherish it for years to come, that is the reward we strive for as craftsmen. We pour our hearts into ensuring that each tool serves its purpose flawlessly, imagining the impact it will have on someone’s daily life or work.

  • Quenching is where the “soul of craftsmanship” is truly tested. It is for this reason that we approach every single piece with the utmost seriousness and dedication.
Sakai Traditional Craftsmen Collection

KIREAJI's Three Promises to You

  • japanese_knife_made_in_Sakai

    1. High-quality Japanese Knive

    We offer knives crafted by craftsmen from Sakai City.

  • Honbazuke

    2. Genuine Sharpness

    To ensure you experience sharpness, we provide a free Honbazuke by our skilled craftsmen.

  • 3. Lifetime Knife Use

    Every knife comes with a free Saya(Sheath) for durability. We also provide paid after-sales services with Sakai City Japanese knife workshop.