San-Mai: Steel Trinity

The Three-Layer Construction Of San-Mai Results In Sharp, Durable, Beautiful Blades

San-mai is an ancient blade construction technique that originated in Japan around 1300 A.D. Those who continue to use the technique harken back to those early days of craftsmanship and take pride in their finished product, a combination of the old and new that presents a dazzling aesthetic in three layers combining tensile strength and edge holding in a working knife blade.

“San-mai is a Japanese term describing a knife composed of three layers of steel that are forge welded together and ideally unite some different properties of the used steels within one knife,” explained Benjamin Kamon of Korneuberg, Austria. “Or, it serves another purpose as, for example, to ‘save up’ rare high-carbon steel used in the middle for the edge, which is what I believe started it originally.”

Jeremy Bartlett of Parkersburg, West Virginia, agrees. “Traditionally, two softer metal outer jackets and one high carbon core steel for the edge are used,” he said. “The main reason to use san-mai is so you have a soft steel jacket and a high-carbon core cutting edge. This will make the knife very durable with a great edge.”

The san-mai process is similar to the layering of damascus, a.k.a. pattern-welded steel, but retains a methodology all its own. In damascus, the layering and relayering of component steels typically result in numerous layers in the billet, while the object of damascus forging is often to create a pattern in the steel itself, along with the enhancement of blade performance.

“San-mai ends up taking much less time to make [than damascus],” added Noah Vachon of Mistissini, Quebec, Canada. “For one thing, you’re starting with fewer layers—three instead of 15 or 20 in my case. That’s fewer surfaces to grind clean when prepping the billet material. Also, unlike making pattern-welded steel, there’s no need to restack or further manipulate the billet. One thing that is a bit trickier is the core material has to be kept centered. I try to work a blade evenly from both sides when forging san-mai. With damascus, this is less of an issue.”

How To Make A Knife With The San-Mai Technique

Curt Zimmerman of Longview, Texas, uses two methods of san-mai construction. “I make it with a laminate billet and sandwich my core material between it, or I will use a canister. For the core I use 1084 because it is the most forgiving steel that I have been using as far as heat treating and workability. For the jacket or laminate billets I use 1080, 15N20 and pure nickel shim material because it is readily available and easy to work with. For the material in canister, anything from chainsaw chains to fishhooks, ball bearings, scrap damascus or other stuff lying around the shop will work. I like this because it’s great to experiment with.”

Zimmerman showcases his san-mai talent in a beautiful fixed-blade fighter with a ball-bearing san-mai blade and an ancient walrus ivory handle. With inch lengths of nine for the blade and 14.5 overall, the fighter flaunts contrast between light and dark. The san-mai blade’s distinctive pattern and the integrity of the cutting edge are well defined.

“The most visible sign of a well-made san-mai is the center core showing equally on both sides,” Zimmerman observed. “I’ve only been building knives for about a year, and it has become a true passion of mine.”

Vachon is familiar with 1084 and 1095 carbon steels for the core of his san-mai blades and intends to branch out to 26C3 and White Paper #1. “I try to choose materials that play well with each other,” he commented, “that is, that bond easily and move at comparable rates when worked.

“I’ve been using 410 stainless for the jacket. It stays bright when etched, which gives a nice visual contrast with the darker core, and the carbon migration that goes on where the layers meet can be very attractive. A stainless jacket with a carbon core is sort of the best of both worlds in terms of function. Antique wrought iron is also a fascinating and beautiful jacket material that tells a great story.”

When Noah completed a breathtaking chef’s knife with a 10.25-inch blade and overall length of 16.2 inches, he knew he had done something special—a tour de force in his cutlery journey. 

“This knife is the culmination of all the skills I’ve learned during my career as a maker, with a cherry on top,” he related, “everything I learned from cabinetmaking and lutherie*, as well as the years studying ergonomics as an industrial designer.” The knife combines the san-mai blade of antique wrought iron and 15N20 and 1095 core with a handle of stabilized redwood burl and denim. A copper Micarta® bolster completes the package.

“This knife was forged from my first- ever house-made billet of wrought-iron san-mai,” Vachon recalled. “The iron is from a piece I brought home from the New England School of Metalwork in Maine. The outer layer visible near the spine is the wrought iron. It’s full of impurities from the way it was manufactured over 100 years ago, and that gives it loads of character. The next layer that looks brighter than the rest is the 15N20. It has nickel in it and that prevents it from etching darkly. The layer at the edge is the core material. Having five layers would usually be referred to as go-mai I suppose, but it’s the same idea.”

Kamon identifies a departure from the time-honored in his san-mai construction. “The biggest difference to traditional san-mai is probably that my outside layers are made from stainless steel instead of mild steel, which is mostly used in traditional san-mai,” he noted. “The advantage of stainless steel is exactly that. It’s stainless but with that comes also the visual advantage of a strong contrast after etching the finished blade as the stainless steel stays silver, shining while the cutting layer gets very dark. The cutting layer is from a tungsten-alloyed high carbon steel called 1.2519 that is not stainless, and the reason for its etching quite strongly in an acid such as instant coffee or ferric chloride.”

Benjamin’s chef’s knife with a 10-inch blade of seven-layer stainless steel with the 1.2519 cutting edge presents a dazzling effect. At 15 inches overall, this issue’s cover knife features a handle of ringed gidgee wood and a bolster of dark gray titanium. 

“Behind the edge is the darkly etched cutting layer,” he related. “After that is the nickel layer that prevents carbon from diffusing into the outer jacket layers of the blade. After the nickel there is a thin layer of high-carbon manganese-alloyed steel. This steel only serves a visual purpose and produces a slightly cloudy and dark faded effect in the stainless steel jacket. Last but not least, there is the outer layer of stainless steel in a shiny silver color.

“The texture on the blade is what I call denty, my brand for it. The technical term would probably be tsuchime. The denty serves the purpose of benefiting food release as the texture in the blade will produce small air pockets between the blade and foods that like to stick. It also makes a given blade lighter in weight compared to the same blade without texture.”

Delam Safeguards

One of the biggest challenges in constructing san-mai is the potential delamination of the steels—when the layers literally start separating from each other—during the forging process. Bartlett has devised techniques to help guard against that.

“When delamination happens, not only is the structure and durability of the blade compromised,” Jeremy reasoned, “but now you also have a void where contaminants can enter and cause the blade to rust from the inside out. The key to not having any delaminations in your san-mai billet is to take your time and make sure your mating surfaces are clean. Also, you need to make sure that you have no oxygen getting into your billet while heating in the forge. Check for small pin holes in your MIG welds all around the billet. I’ve tried using flux core to weld around the billet, but now I use only MIG wire with argon/CO2 gas. The last thing is to make sure that you don’t try and draw the billet out too much at one time. The jacket [sides] material and the core material will want to move at different rates because of the different makeup of the steels.”

Bartlett’s 15.25-inch fighter with a 9.25-inch blade of 416 stainless clad around a 1095 core, and a handle of curly koa wood with black G-10 spacer presents a highly contrasted look.

“You can see a total of three distinct areas on the blade,” he said. “The black area down to the edge is the core steel. Moving toward the spine, the next area will be the carbon migration area. This is where during the forging process some carbon leaves the core steel and migrates to the low carbon stainless jacket. This leaves a distinct crystal-like line between the two layers. The carbon leaving the core steel only affects the area where it meets with the stainless. No carbon is leaving the edge, where you need all of it for a good blade. The third area of the blade is the stainless jacket material. This area will be shiny and mostly free of any effect from the etching process. You will sometimes see, as in this case, a ghost hamon that will follow the peaks and valleys of the san-mai pattern in the blade.”

What Does San-Mai Look Like?

The look of high quality san-mai is readily apparent. The finest san-mai exhibits core steel that is centered in the billet along with the presence of the carbon migration line in the blade. In poorly made san-mai, the jacket steel approaches the edge of the blade too closely. After sharpening a few times, the soft jacket steel may run directly into the cutting edge, rendering the blade less than useful.

The watchful eye, the practiced hand and the awareness of control in each stage of construction will bring san-mai to life and perpetuate this centuries-old artistry of form and function beautifully.

*Lutherie is the construction and repair of stringed instruments that have a neck and a sound box.