Sharpening Part 8 – Soft Iron 地金

If you can’t explain it to a six year old, you don’t understand it yourself.

Albert Einstein

In the previous post on sharpening Japanese woodworking tool blades we looked primarily at the nature of the hard high-carbon steel used in making woodworking blades. In this post I will try to dispel some of the confusion that surrounds the other metal used in making most Japanese knives, axes and woodworking blades, namely the soft low-carbon steel called “Jigane” (地金). I hope this brief explanation will improve your understanding of some Japanese tools and aid your sharpening efforts.

Sources of Jigane

Most Japanese knives and woodworking blades are comprised of a thin piece of hard high-carbon steel, discussed in my previous post, forge-weld laminated to a piece of softer low-carbon steel or wrought iron called “Jigane” (地金) in Japanese, which translates directly to “ground metal.”

I will write more about this bi-metal lamination in the next post in this series, but for now take my word that it is essential to the performance of many types of Japanese cutting tools nowadays, and for many centuries was critical to manufacturing cutting tools in America and Europe as well.

The best jigane material for plane blade bodies is said to be scrap iron from the boilers of old trains, boats, and factories, etc.. Such boiler tanks were subjected to thousands of heating and cooling cycles during their years in service which drove out impurities, including carbon, making the iron very soft to the point of weakness.

The most desirable jigane for plane blades is called “tired” iron, named because it is not only soft, but looks weak and exhibits a visible grain along with cracks and imperfections which those familiar with Japanese plane blades covet.

A pile of jigane, probably old salvaged structural steel. Looks like boards of old wood, but it ain’t.

Wrought Iron Production

Nowadays, this very low-carbon steel, also known as “ wrought iron” is not produced in any volume for several reasons. First, demand is just too low to make it worthwhile to manufacture. Hand-forged ornamental iron is the only commercial usage besides Japanese tools, truly microscopic markets.

The second reason is that steel manufacturing processes have changed drastically in the last 100 years. For instance, it used to be that steel began as iron ore, basically rocks and dirt, which was melted and refined into low-carbon wrought iron, so wrought iron was an intermediate product of steel production. Indeed, this low-carbon product was much less expensive to produce than high-carbon steel and so was used for everything from the boilers, bridges, trains, ships and anchor chains mentioned above to axes, chisels, farming implements, machinery, what’s called “miscellaneous metals” in the construction industry, and of course plane blades. There are still a few surviving structures that were made using this archaic material.

Nowadays, things are very different. Carbon is incorporated into the steel early in the manufacturing process, so low-carbon wrought iron never becomes an intermediate product.

Also, scrap metal has become critical to steel manufacturing processes nowadays. Remember what happened to steel prices worldwide when China was buying up huge volumes of scrap metal worldwide for its Olympic infrastructure building projects?

I think we can agree that this energy-efficient cost-reducing recycling of natural materials, one that was hardly an option 150 years ago, is a very good thing. But it does have a tiny downside, namely that most commercially-available scrap metal available in any useful volume today has been through the modern steel-manufacturing process many times and already contains not only high levels of carbon, relatively speaking, but alloys such as chrome, molybdenum, and nickel from previous melting pots. Indeed, undesirable chemicals such as phosphorus, sulfur and silica tend to be high in general junkyard scrap metal. On the other hand, keeping these unintended alloys and impurities under control is a serious challenge for manufacturers of tool steel.

In summary, wrought iron simply isn’t made anymore, and it is not a sustainable, profitable product.

Japanese blacksmiths making high-quality plane blades nowadays mostly use wrought iron recycled from old anchor chains, old iron bridges, or other recycled iron structural components. If you see a hole in a plane blade, like the extra-wide plane blade pictured at the top, it once housed a rivet. Yes, structural steel was once connected with hot rivets instead of bolts. Hi-tensile modern bolts are better.

Plane Blades

A plane blade by Ogata-san in his “Nami no Hana” series using Swedish Asaab K-120 steel. Notice not only the fissures and defects, but also the striations and grain typical of soft, tired “wrought iron.”

Mr. Takeo Nakano (see his photo below) makes my plane blades. He is a kind, quite man with the outward appearance of a sedentary grandfather, but when using hammer and tongs at his forge within his dark smithy, his posture and visage reminds me of an intense Vulcan reinforcing the gates of Hades.

Like nearly all the plane blacksmiths in Niigata, he uses scrap iron obtained in a single lot many years ago from an iron bridge that was dismantled in Yokohama Japan.

Mr. Nakano at home

I am told that most of the jigane used for plane blades in Hyogo Prefecture is old recycled anchor chains.

The fissured and cracked jigane of a a 70mm plane blade by Usui Kengo, another Niigata blacksmith (RIP). Notice the rod which retains the chipbreaker is non-existent, replaced by two short stubs. An elegant detail in this plane body by Ito-san (Soh 宗).
The back of the same Usui plane blade. Notice the cracks and inclusions in this excellent jigane exposed at the polished bevel. Very wabi-sabi. This jigane was once part of an iron bridge in the city of Yokohama, Japan.

In the case of plane blades, structural strength is not critical, so laminating a thin layer of high-carbon steel to form the cutting edge to a soft iron body is adequate. Indeed, the thicker the hard steel layer, the more time and effort it takes to sharpen the blade, so in a high-quality blade the thicknesses of the high-carbon steel layer and the soft jigane body will be carefully balanced to ensure the blade’s bevel rides the sharpening stones nicely and can be quickly abraded.

Plane blade blacksmiths use the same strip jigane used for chisels for making less-expensive plane blades.

Chisel Blades

In the case of chisels, while ease of sharpening is still important, the body and neck must be harder/stiffer to prevent them from bending, so a different, stiffer variety of jigane with a higher carbon content and fewer defects is used, and the steel layer is typically made thicker.

The jigane used by my chisel blacksmiths is a commercial product not produced anymore (thank goodness they have stockpiles) called “gokunantetsu” 極軟鉄 which translates directly to “extremely soft iron.” With a carbon content of 0.04~0.07%, a better description would be “very low carbon steel.” When heated and quenched, it doesn’t harden much.

The adventure will continue in the next exciting episode where we will bring it all together into a blade. Don’t forget to have popcorn and jujubes ready.

YMHOS

Links to Other Posts in the “Sharpening” Series

Sharpening Japanese Woodworking Tools Part 1

Sharpening Part 2 – The Journey

Sharpening Part 3 – Philosophy

Sharpening Part 4 – ‘Nando and the Sword Sharpener

Sharpening Part 5 – The Sharp Edge

Sharpening Part 6 – The Mystery of Steel

Sharpening Part 7 – The Alchemy of Hard Steel 鋼

Sharpening Part 8 – Soft Iron 地金

Sharpening Part 9 – Hard Steel & Soft Iron 鍛接

Sharpening Part 10 – The Ura 浦

Sharpening Part 11 – Supernatural Bevel Angles

Sharpening Part 12 – Skewampus Blades, Curved Cutting Edges, and Monkeyshines

Sharpening Part 13 – Nitty Gritty

Sharpening Part 14 – Natural Sharpening Stones

Sharpening Part 15 – The Most Important Stone

Sharpening Part 16 – Pixie Dust

Sharpening Part 17 – Gear

Sharpening Part 18 – The Nagura Stone

Sharpening Part 19 – Maintaining Sharpening Stones

Sharpening Part 20 – Flattening and Polishing the Ura

Please share your insights and comments with everyone in the comments section below. If you have questions or would like to learn more about our tools, please use the questions form immediately below.

The Varieties of Japanese Chisels Part 6 – The Mortise Chisel (Mukomachi Nomi 向待鑿)

The best carpenters make the fewest chips.

~English proverb, c.1500s

Japanese mortise chisels are called “Mukomachi Nomi” 向待鑿. I am unsure of the origin of the name, but the Chinese characters can be read as meaning “wait over there.” A curious name, it may refer to the shape of the transition from blade to neck, called a “machi” which is unique in Japanese chisels. I will simply call them “mortise chisels.”

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12mm Mortise Chisel – Sukezane (Side View)
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12mm Mortise Chisel – Sukezane (Ura View)

DESCRIPTION

Mortise chisels are single-purpose tools for cutting rectangular holes in wood for mortise and tenon joints, the oldest recorded wood joint known.

Unlike other Japanese chisels, and even Western mortise chisels, the sides of the Japanese mortise chisel are shaped square to the “flat” instead of being angled slightly less than 90 degrees. The surfaces of the sides are of course straight along their length, but are either flat or slightly hollow across their width.

Other varieties of chisels have sides angled inwards to prevent the chisel from binding in the cut. This is less than ideal, however, when cutting small mortises because it allows the chisel to twist inside the mortise scoring the sides and reducing precision. The Japanese philosophy is that the blade’s sides should shave and clean the mortise at the same time it is cutting it so the sides don’t require additional cleanup with a paring chisel. Its a matter of precision and efficiency.

The straight flat sides of the mortise chisel have a relatively larger surface area that can create a lot of friction in the cut making extraction difficult in some cases, so the standard maximum width is 15mm.

Many advocate using double bevel cutting edges for Western mortise chisels. I have no problem with double bevels for atsunomi used to cut wide, deep mortises because the double bevel tends to kick more waste out of the mortise hole than a single flat bevel, although double bevels are more trouble to sharpen. But in the case of the standard Japanese mortise chisel, I recommend using a simple flat bevel for two reasons:

The first reason is that, since sharpness is critical for precise work, and a flat bevel is quicker and easier to sharpen, a flat bevel is more precise.

The second reason is that a flat bevel tends to stabilize the chisel in the cut more than a double bevel blade can, keeping it from twisting out of alignment and gouging the sides.

The lubrication provide by an oilpot makes using a mortise chisel quicker and the final product cleaner and more precise. Please see my previous post on the subject. https://covingtonsons.home.blog/2019/05/09/the-essential-oilpot/

APPLICATIONS

The mortise chisel is a specialist chisel for joinery, cabinetmaking and furniture work. It is not generally used by carpenters. Craftsmen that routinely use mortise chisels work to much tighter tolerances than most woodworkers, so a professional-grade mortise chisel must be forged and shaped to tighter tolerances than other chisels.

I only have one blacksmith with the skills and attention to detail required to make mortise chisels to my specifications. He thinks I’m a prissy pink princess. I think he’s a stubborn old fart. We’re like an old married couple(ツ).

If you need to cut lots of precise mortise holes quickly, then this tool will definitely improve your results and increase your satisfaction. It may not be the most handsome chisel in your toolchest, but you will come to rely on it more than any other for quality joinery work.

Standard widths for mortise chisels are 3mm, 4.5mm, 6mm, 7.5mm, 9mm, 12mm, and 15mm, but Sukezane won’t make 15mm mortise chisels for me anymore, dagnabit.

More than any other, mortise chisels are subtle, intelligent beasties, or at least they can be. I will talk more about what to look for in a good mortise chisel, as well as how to realize their Einstein-like focus to help you do better work, in future posts.

YMHOS

Links to Other Posts in this Series

The Varieties of Japanese Chisels Part 1 – The Main Categories

The Varieties of Japanese Chisels Part 2 – The Mentori Oiirenomi (面取追入鑿)

The Varieties of Japanese Chisels Part 3 – The Shinogi Oiirenomi (鎬追入鑿)

The Varieties of Japanese Chisels Part 4 – Kakuuchi Oiirenomi (角打追入鑿)

The Varieties of Japanese Chisels Part 5 – High-Speed Steel Oiirenomi (HSS 追入鑿)

The Varieties of Japanese Chisels Part 7 – The Nihon Mukomachi Nomi (二本向待鑿)

The Varieties of Japanese Chisels Part 8 – The Atsunomi (厚鑿)

The Varieties of Japanese Chisels Part 9 – The Uchimaru Nomi Gouge (内丸鑿)

The Varieties of Japanese Chisels Part 10 – The Sotomaru Nomi Incannel Gouge (外丸鑿)

The Varieties of Japanese Chisels Part 11 – The Tsuba Nomi Guard Chisel (鍔鑿)

The Varieties of Japanese Chisels Part 12 – The Usunomi Paring Chisel (薄鑿)

The Varieties of Japanese Chisels Part 13 – The Shinogi Usunomi 鎬薄鑿 Paring Chisel

The Varieties of Japanese Chisels Part 14 – Kote Nomi (鏝鑿Trowel Chisel)

The Varieties of Japanese Chisels Part 15 – Ootsuki Nomi 大突き鑿

Please share your insight, questions or comments in the comments section below. If you would like to learn more about our tools, please use the contact form located directly below.