Construction of Japanese blades
Japanese knives are an exceptional example of over a 1000 years old tradition implemented into the modern world. They are often handmade, with outstanding properties such as exceptional sharpness and edge-holding capabilities connected with high durability and plasticity. How come Japanese knives stand out as a quality symbol for other makers and companies?
One of the secrets of their superior performance dates back to ancient swordsmiths, who developed a method of obtaining sharp and hard edge and core with ductile, extremely crack-resistant warp. The name of this technique is lamination. You may associate it with paper lamination, but the concept is much broader. Lamination is a process in which one material is covered with another one to create a composite that exhibits all the best properties of both materials. The same process is used in the manufacturing of Japanese knives coming from the techniques used for the production of samurai swords, in which (usually) harder steel is on the outside layer when the core of the sword is forge-welded with softer, more malleable steel. The most skilful swordsmiths were able to accomplish this effect with just one advanced technique, which is used nowadays – selective quenching. There are many ways of connecting two types of steel developed by Japanese masters.
What exactly is lamination?
The lamination process begins with the forging of the blade. This involves heating the steel to a high temperature and then shaping it using a hammer and anvil. The steel is then cooled and heated again several times to remove impurities and improve its grain structure. Once the blade has been forged, it is ready for lamination. In this process, the blade is constructed out of many types of steel, often harder steel, known as the "cutting edge and softer steel, known as the "supporting" steel. The layers are then forge-welded together using heat and pressure. The result is a blade with a hard, sharp edge that is able to retain it for a long time, and a flexible spine that is resistant to breaking. This combination of qualities makes Japanese knives some of the best in the world for cutting and slicing a wide range of foods, from delicate fish to tough meats.
Relying on the approach used by knifemakers following types of knives can be distinguished into up to four groups (counting Damascus versions of certain steels):
Monolayer blades
- Pure mono
- Honyaki
Two-layered blades
- Ni-mai
Three layered blades
- San-Mai
- Warikomi
Damascus
- Ni-Mai Damascus
- San Mai Damascus
- Mosaic Damascus
Monolayer blades
Pure mono is the most minimalist version of the Japanese knife, it is made from one type of high-quality, durable, hard and scratch-resistant steel. Usually, these knives are beautiful in their simplicity with highly mirror-like polished surface, they may not be inferior to their competition produced in a more sophisticated way.
Honyaki knives are a type of Japanese knives that are known for their exceptional quality and craftsmanship. Unlike other Japanese knives, which are made using the lamination process. Similarly to pure mono knives, they are also made from a single piece of high-quality steel. This allows the blade to have a sharp, fine edge and a strong, durable spine. The process of making a honyaki knife begins with the selection of the best quality steel. This steel is then heated to a high temperature and carefully forged using a hammer and anvil. The blade is then quenched, or cooled quickly, to harden the steel and give the blade its sharp edge. After the blade has been forged and quenched, it is carefully honed and sharpened to achieve a fine edge. This process is done by hand, using traditional Japanese sharpening stones and techniques. The result is a blade with a razor-sharp edge that can slice through even the toughest ingredients with ease. Honyaki knives are prized for their exceptional performance and durability, and are considered by many to be the finest knives available. They are typically more expensive than other types of Japanese knives, due to the labour-intensive process of forging and sharpening them by hand by only the finest knife-making masters. However, for those who appreciate the beauty and quality of a truly hand-crafted knife, a honyaki knife is a truly exceptional investment.
Selective quenching of katana swords
The selective quenching dates back to samurai times in which the spine of the sword was covered in a special mix of clay dust, carbon, natural sharpening stone pieces and water. The cutting edge was uncoated. The covered blade was then heated over 800 °C and cooled down usually in the river water (most of the smiths waited for a specific period of a year to perform quenching). Freshly poured river water was thought to be the best quenching medium, only a single quench was used. Depending on the individual swordsmith, the water could have contained certain secret additions ( for example soft, straw and rice husks).
Cross-section of selectively quenched samurai blade (source: Dodd, B. (1978). The making of old Japanese swords. Journal of Mechanical Working Technology, 2(1), 75-84)
After the process, the inside of the sword, which was cooled much slower, had a microstructure of pro-eutectoid ferrite plus a little pearlite, while the cutting edge was mostly martensite. After polishing the line between these two structures was called the Hamon. The traditional methods of sharpening and polishing exposed the Hamon which itself become a true work of art. The sharpener as well as swordsmith who were involved in the process are often named. The beauty of tradition used for making katana is today applied to create the most magnificent and remarkable knives.
Two-layer blades
Ni-Mai knives are the type of knives with lamination on just one side of the blade. This version of the knife is intended for knives which have only one bevel. In this case, the cutting-edge steel, which is hard, is placed under the softer more pliable layer. The softer layer makes the sharpening process easier.
Three-layer blades
San-Mai is translated as 'three layers'. This is the primary version of the laminating blade. This practice is the most common in commercial knife-making in Japan. The hard, tough and more fragile cutting edge is placed between two layers of soft, more malleable steel creating a 'sandwich' consisting of one inside layer and two outer layers, and then they are forge-welded together. The process improves the manufacturing of the tool, i.e. quenching and sharpening. Moreover, the outer layers are often made from stainless steel to protect the high carbon, hard and tender steel susceptible to corrosion from external influence and cracking. The legendary blacksmith Teruyasu Fujiwara is often ascribed as the one who invented and used the lamination process first.
Warikomi this method is a variation of San-Mai used for demanding steels in terms of forging, often named 'split and insert'. In this technique, the main cutting steel is placed between two layers of softer steel, but only extending to half of the whole blade. The incursion is done when the laminating steel is still red hot, then split and inserted with cutting steel. After that, the whole 'composite' is forged together. This way the smith obtains a knife with a malleable core and hard, razor-like edge. Knives of this type are often praised as very sharp, fine and with great edge retention. This technique is often applied for laminating advanced powder steels.
Damascus type blades
What exactly is Damascus steel?
Damascus steel is a type of steel that was traditionally used in different parts of the Earth simultaneously and independently to make advanced blades. They were known for distinctive pattern, which is created by forging together several layers of steel with different compositions, creating a unique pattern as the layers fuse. This was done by heating the steel to a high temperature and hammering it repeatedly to create the desired shape. The finished product was then quenched in water or oil to harden it, and then tempered to give it the desired level of flexibility. This technique gives the steel its strength and flexibility, making it ideal for sword making. The exact method for making Damascus steel has been lost to history, but modern metallurgists have been able to replicate the process to some degree. Today, Damascus steel is still prized for its beauty and strength, and is used in a variety of applications, including knife-making and other forms of weaponry.
Ni-Mai Damascus is a standard Ni-Mai lamination, but with the use of multi-layered steel as laminate. Damascus pattern is usually on only one side of the blade (the side of the pattern depends on the type of Ni-Mai, on knives for right-handed it is placed on the right side, and for left-handers the opposite).
San-Mai Damascus in this case the name might be a little confusing, because there are often more than five layers of steel used to create the knife. The number is always odd. This version is often concerned as one of the highest arts of bladesmithing, sometimes referred to as suminagashi due to the resemblance to the marble on paper. What is worth underlying is that in this case the pattern is not a functional feature, it is purely ornamental. Embellishment comes from years of hard work, knowledge and centuries of tradition. The obtainment of the pattern allows blacksmiths to display their skills, talent and artistic capabilities, due to that, this kind of knife is often more expensive, because of its aesthetic. As you can read above, the Damascus pattern is created as a result of many cycles of forge-welding various types of steel into one blade (piece of steel), because their microstructure differs they respond differently to distinctive methods of treatment after polishing.
The patterns can be revealed by:
- Sharpening and polishing of the blade, exposure is done by using the differences in mechanical properties between structures, during using whetstones,
- Acid etching, the different structures react diversely to acid. Structures which are rich in carbon is less corrosion-resistant and darkens, while other remain bright. The use of acid display the characteristic dark and light 'flowing' pattern.
- Bead blasting with the use of special sand also allows to achieve more subtle ornament of structures. After blasting softer steel often exhibits a 'muted' look, while harder remains shiny. This process improves the surface of the blade, allowing food to be easier removed and it reduces cutting drag.
- Combination of methods listed above often allows to obtain the most fascinating patterns.
Mosaic Damascus is obtained by the recovery of the primal Damascus steel production process, which is somewhat analogous to the lamination process. Blades created this way are among the most expensive and most wanted ones on the market due to the complexity and time consumption of the process. The blades are made by numerous repetitions of folding and forge-welding processes, just like original Damascus steel. These knives are exceptional in case of their performance.