The Japanese Gennou & Handle Part 3 – What is a Gennou?

Kiyomizu Temple, Kyoto, Japan

What we have is given by God; To teach it to others is to return it to him.

Gian Lorenzo Bernini

There are as many varieties of hammers in Japan as there are in western countries. With one notable exception, and in one specific application, Japanese hammers are not especially superior to their western counterparts. That exception is the gennou (pronounced gen/noh), a hammer intended specifically for striking chisels, adjusting plane blades, and crushing wood (i.e. “kigoroshi” or “wood killing”). This article will provide a further introduction to the gennou hammer.

What Is a Gennou?

A box-stock, garden-variety, economy Japanese gennou hammer with a one-size-fits-somebody handle

The Japanese have different terms for different hammers, of course. A hammer used strictly for driving nails, or banging sheet metal, or driving stakes is called a “kanazuchi” meaning “steel mallet.” The gennou, on the other hand, can be used to drive nails, but it is also suited to striking chisels and adjusting planes.

The word “gennou” was borrowed from the name of a Buddhist priest who lived, or so the story goes, in the 1300’s and used a steel hammer to destroy a poisonous rock that was troubling the common folk. I’m not sure what one has to do with the other, but there you are.

The Attraction of the Gennou

Many Japanese craftsmen often have an emotional attachment to their gennou. Perhaps this is because, unlike saws, chisels, and planes that are gradually but inevitably sharpened away until almost nothing remains, or squares or marking gauges that loose tolerance or wear out, a quality gennou will last for a lifetime relatively unchanged other than the occasional replacement handle. A good gennou is a simple, reliable, hardworking friend that never complains. It doesn’t have a pigtail; It doesn’t need to be sharpened. And most importantly, it will never ask you a dangerous question like “do these pants make my butt look huge?”

Technical Matters

The gennou is a simple tool consisting of a steel body of one shape or another attached to a wooden handle. The head has a rectangular hole called the “eye” in English and “hitsu” in Japanese to receive the handle’s tenon.

The steel used for modern Japanese gennou is typically designated SK, a standard high-carbon tool steel made in Japan used for making hammers, axes, and many other tools. Chemically, it is very similar to 01 steel in the Americas; Not as pure as Hitachi Metal’s Shirogami or Aogami steels, but still completely adequate for hammers. I wouldn’t pay extra for a gennou head made from Shirogami steel, much less unobtanium and you shouldn’t either

Mass-produced gennou are either cast or drop-forged very inexpensively. The eyes are rough and the handles are secured with wedges. Indeed, the eyes are typically so irregular that the head will not stay on the handle without wedges, but a high-quality gennou with a good eye and a handle made by a skilled craftsman doesn’t need wedges or other silly contrivances compensating for sloppy tolerances to connect the two.

A gennou head with rough and/or irregular eyes can create unnecessary problems for the user. “Irregular” has several connotations in this context. One obvious irregularity is an eye that is not truly rectangular. For instance, it may have curved, twisted walls, wonky interior dimensions, or interior corners that are not square. Not only will it be a right pain in the tuckus to make a handle to fit such an eye, but you can bet your sweet bippy it will cause the handle tenon to loosen up sooner.

Another irregularity commonly seen in the eyes of poor-quality gennou is rough interior walls. You would think that rough walls would hold onto the tenon better, and perhaps they do compared to highly-polished walls, but rough, uneven walls tend to wear-out the wooden tenon of the handle causing it to loosen over time. Imagine the vibrations the tenon is forced to absorb through those walls and the grinding action between wall surface and handle that results.

An intentional irregularity frequently seen is end walls (versus the longer side walls) that are sloped from each opening towards the center of the eye, essentially making the eye bulge inwards at its center. The purpose of these bulges is to crush the wood of the tenon when it is forced into the eye, increasing friction, while also providing a dovetail-like area for the steel wedge to expand the eye back into. It’s a reasonable solution for rough, irregular eyes in low-cost hammers to be used by amateurs, but one that the craftsman that truly understands gennou and wants a lifetime tool finds undesirable. We will touch on this detail more in future posts.

Still another irregularity the careful craftsman must watch out for is an eye that is not perfectly centered in both axis in the head. You might think that an eye that is a little skewampus wouldn’t make a big difference, but it does because, not only is the balance and center of mass of such a head also skewampus so that the head tends to twist during the swing and wiggle on impact, but because making handles for such a head is unnecessarily troublesome. A clean, uniform, straight, properly-centered eye is worth every penny it costs, especially if you are a professional and consider your time and sanity of any value.

A difficult question I am frequently asked is “how much irregularity is acceptable?” The answer is simple: If you think it is too irregular, then it is, because the work to correct the defect or compensate for it will all be on you.

Please understand that correcting major defects in hammer eyes is hard work. It takes time, concentration, a good eye, a flashlight, and a deft hand with skinny files and rulers to remove just the right amount of metal in just the right places inside that narrow eye, a task that is much more difficult than removing metal on an exposed surface because the files are thin and bendy, it’s hard to see what you’re doing, you don’t have much leverage, and consistently making a straight pass is not easy. Blisters will bloom. Patience will be tried. Sanity may quiver. Try it yourself and you will quickly see why.

This is the whole point of high-quality heads like those made by Kosaburo and now Hiroki and why they are worth the high cost: Their eyes are true when new, no adjustment necessary, saving the purchaser many hours of tedious work and blisters. Every time you make a handle for a high-quality head it saves time and leaves you with a good feeling. It’s a friend.

On the other hand, a poorly-made head is a curse, a money-pit (if your time is worth anything), and a frequent source of irritation (especially when it inexplicably loosens and soars free as a hardened-steel bird (ツ)。)

I hate to say it, but Beloved Customers and Gentle Readers should beware of one last defect when purchasing an expensive handmade gennou head. A perfect eye is truly a difficult thing to make, certainly more difficult than making a head cosmetically beautiful. Unfortunately, one or two famous blacksmiths (who shall remain unnamed in this series of articles, so don’t ask) have earned a reputation among knowledgeable professional woodworkers in Japan for occasionally making gennou with skewampus, eyes. Caveat emptor, baby. She may wear high-heels, a short skirt and be beautifully made-up, but if she has a curly tail and oinks she’s probably a pig, unless she’s a trans-boor.

If you cannot hold and inspect an expensive gennou head in-person before concluding the transaction, at least make sure you purchase from someone with a solid guarantee, one without weasel-words and that reimburses you for return shipping, like C&S Tools’ guarantee does. A guarantee that you must argue about and then spend more money to benefit from is less than half a guarantee IMO.

We will delve further into the tempering and differential hardening of gennou, as well as laminated gennou heads in future posts in this series, same bat time same bat channel.

Why Use a Gennou for Chiselwork?

This is a questions we addressed in a previous post, but which we also examine further here.

Almost any striking tool, from steel hammer to leather mallet, can be used to strike a chisel. The problem is that, unless one is either gentle or the handle of the chisel is reinforced, a steel or even brass hammer will eventually destroy the chisel’s handle. The solution in the West in the last century has been to use a mallet made of wood, leather, rubber, or plastic instead to cushion the blow and preserve the handle. Let’s consider this for a moment. 

The purpose of striking a chisel with a hammer is to drive the chisel into and through the wood by cutting it, right? But a soft-faced wooden mallet deforms when it impacts the chisel cushioning the blow and wasting energy through deformation and heat generation. It may also unnecessarily waste energy through air drag, as we discussed in the Part 2 of this series. Since energy is lost, more mallet strikes are necessary, wasting time. This is demonstrably counter-productive.

Besides being relatively soft, a fat-faced mallet is bulkier, slower to swing, and is therefore less precise than a slimmer steel hammer. While there may be some that are thrilled with cutting slowly and expending extra time and energy in the process of cutting a joint, most people want to cut as much wood as possible, as precisely as possible, in the shortest amount of time as possible, and with the least energy expenditure possible. But if a chisel handle is so fragile that one must sacrifice time and energy to keep it intact, then it is only logical to conclude that there is something wrong with the design of the chisel.

Ise Jingu Shrine, Mie Prefecture, Japan

The Japanese are very serious about woodworking, as anyone who has gone to Kyoto or Nara and seen the ancient wooden temples there can attest. When it comes to chisel work, Japanese carpenters don’t tolerate such silly nonsense as a chisel that must be coddled, and so early-on Japanese craftsmen developed a wooden-handled chisel that can be struck hard with a steel hammer all day long without breaking. 

When using a steel hammer to strike a Japanese chisel (versus a push or paring chisel), the maximum amount possible of the user’s energy and time goes into actually cutting wood. The same cannot be said of mallets made of wood, rawhide, or plastic.

The excellent design of the Japanese chisel combined with the quality of steel, and the forging, lamination, and heat treatment techniques used in manufacturing most Japanese chisels provides a tough cutting edge that stays sharper, longer, placing Japanese chisels at the very top of the evolutionary pyramid of chisels, IMHO. As the Japanese are wont to do, they developed a hammer specifically for striking chisels.

Most hammers intended for driving nails have a domed face which does not work well with Japanese chisels with lots of use because it tends to dish in the end of the handle causing the hoop to loosen. This can even result in the handle cracking or splitting. A flat-faced hammer is much better. The Japanese “ryouguchi” double-faced gennou has one face that is forged flat, for striking chisels, and an opposing domed face for driving nails or performing “kigoroshi.”

The simplicity of the design combined with these two types of faces are the primary reasons we recommend using gennou for motivating our chisels.

And while one absolutely could grind the face of a Western claw hammer flat and use it to strike Japanese chisels without any problems, the gennou is a hammer that is designed specifically for striking chisels. In your humble servant’s opinion, it is a superior tool for the intended purpose.

In the next post in this series we will examine three varieties of gennou to help you decide which is best for you.

YMHOS

The Pagoda at Horyuji Temple, registered as one of Japan’s National Treasures. Yessireebob, a lot of hammer and chisel work went making this structure.

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