A modern-style (post 1890’s) gennou with high-carbon steel faces forge-welded to a soft jigane (low-carbon/no-carbon) iron body. You can see the difference in color between the soft jigane body and the hard high-carbon steel faces. This laminated construction was commonly used in Japan before cheap imported steel from the West became available. The finish is hand-filed, and the blacksmith’s name, “Kosaburo,” meaning the “happy third son,” is hand-engraved. A high-quality gennou head is truly a lifetime tool. In fact, I have used this head hard as a professional and hobbyist for over 35 years, as you can tell from the dings and light corrosion. The handle is made from Japanese kurogaki wood (black persimmon, ebony genus), a rare variety valued for high-end cabinetry and casework in Japan. Kosaburo’s eyes are always as close to perfect as a man can forge them by hand, and better than all but a few expensive machines can manage.
In the previous post in this series we talked about the difference between mass-produced and hand-forged gennou heads. In this post we will take a look at a more antique style of gennou head.
A Laminated Gennou Head
Prior to the advent of cheap imported steel from Europe, gennou had bodies forged of soft low/no-carbon steel with wafers of hard, high-carbon steel forge-welded to each face. The shiny strips called “Hachimaki,” meaning “ headband,” polished onto the sides of the ends of genno heads sold nowadays, while purely decorative, are vestiges of this old-timey method.
The photos above are of a laminated gennou head hand-forged by Kosaburo which came to me long ago as payment for a debt. Laminated gennou heads made this way are still available today at exorbitant prices. I understand Hiroki occasionally makes a few.
Some believe the combination of hard face and soft body produces a softer impact and less vibration making the gennou less tiring to use. Others prefer the slightly different sound a laminated gennou head makes. I have used this laminated Kosaburo head for many years, and while I am very fond of it, I cannot detect any advantage to its laminated construction.
While laminated gennou are much more expensive, the blacksmiths I have spoken with have told me that they are significantly easier to make than one-piece high-carbon steel gennou since they do not require the more difficult differential hardening process. And they all agree that laminated construction provides no practical advantage to the end user. A curio in other words.
If you are just getting started in woodworking, or are on a tight budget, a quality mass-produced gennou head will do the job if you clean up the eye and replace the handle with one that fits your body.
Better yet, buy a hand-forged head by Hiroki or Kosaburo and make your own handle in the best craftsman tradition, and take pride in a custom tool of the highest possible quality that will serve you well for your entire life.
However, if you have the budget and enjoy collecting traditional tools, then by all means try a laminated gennou head. They are not easy to find nowadays.
YMHOS
The following link is to a folder containing pricelists and photos of most of our products. If you have questions or would like to learn more, please use the form located immediately below titled “Contact Us.”
Please share your insights and comments with everyone by using the form located further below labeled “Leave a Reply.” We aren’t evil Google, fascist facebook, thuggish Twitter, or a US Congressman’s Chinese girlfriend and so won’t sell, share, or profitably “misplace” your information. May ice cream forever taste like burnt rubber if I lie.
A “Classic” style 150monme/ 556gm/ 20oz gennou head hand-forged by Hasegawa Kosaburo
The gentle reader will never, never know what a consummate ass he can become until he goes abroad. I speak now, of course, in the supposition that the gentle reader has not been abroad, and therefore is not already a consummate ass. If the case be otherwise, I beg his pardon and extend to him the cordial hand of fellowship and call him brother. I shall always delight to meet an ass after my own heart when I have finished my travels.
Mark Twain, The Innocents Abroad
In the previous post in this series we discussed the various styles of gennou heads you might consider when planning a custom-made gennou that fits your body perfectly, as well as the range of weights for each application.
While one has few options besides either using a mass-produced head or making one yourself nowadays, in Japan there are still a few blacksmiths that specialize in hand-forging high-quality gennou hammer heads. In this post we will compare mass-produced heads and hand-forged heads to help you make an informed decision.
Comparing Mass-produced Heads to High-quality Hand-forged Heads
When selecting a gennou head you have the choice of a mass-produced gennou (usually with a handle attached) or a handmade gennou head without a handle, if you can find one. Lets examine the differences.
A modern-style 200monme/750gm gennou head hand-forged by Aida Hiroki. The lines are straightish, the skin is plain forge oxide, and each end has a teeny tiny, almost no, polished band.A “Modern” style 100monme/375gm gennou head from the Kohiyama factory in Niigata prefecture (old-stock), definitely the upper-level of factory gennou heads, but not in the same class as hand-forged heads by Kosaburo or Hiroki shown above, but still useable. “Munehomare” brand. It tapers from both faces towards the waist, the chamfers further accenting the taper. An appealing shape.A mass-produced “Modern” style 80monme/300gm gennou head from the Miyaguchi factory in Niigata prefecture (old-stock), This is a typical factory gennou head, perhaps better than most. While the eye is properly centered, the walls are far from clean, and the overall quality is nothing to brag about. Definitely not an heirloom tool.
Mass-produced genno are readily available, relatively inexpensive and for the most part, entirely useable. However, as with any product, it is wise to inspect a gennou carefully before purchasing it, as discussed in a previous post in this series, or to at least buy from a retailer with a solid return policy. When purchasing any gennou, check to make sure the eye is centered in the head and not skewed.
The practical advantages of a hand-forged gennou hammer head over a mass-produced factory head are as follows:
Proper differential hardness;
Precise eye placement;
Uniform interior dimensions of the eye, resulting in time saved and aggravation avoided when fitting/refitting a handle;
Faces are square with the long axis of the head;
Improved Performance due to more uniform distribution of mass providing improved stability during the swing with fewer wiggles, more solid strikes, and fewer off-center hits.
Unique appearance and pride of ownership.
Let’s examine these points in more detail next.
Heat Treatment
Most Gentle Readers will have never given this matter any thought, but the quality of the heat treatment matters… a lot. This is a chemical/crystalline property impossible to confirm long-distance, so we must rely on the manufacturer, be it factory or blacksmith, to get right. Therefore, the manufacturer’s QC reputation as well as the retailer’s guarantee also matters.
A good head will be hard at the faces, becoming gradually softer towards the the eye, certainly softer than a file, to make the head “tougher,” using the materials engineering term, and to reduce resonant harmonic vibrations. If you have used hammers with uniformly hard heads and integral steel handles you have felt this tiring vibration before.
The faces must be hard, around 50~55Rc, to prevent deformation, but not so hard the hammer will chip or crack. You may think that wooden chisel handles could not deform a steel hammer face, but if the striking faces are not properly hardened, they will mushroom sure as eggses is eggses, something your humble servant learned when he made a gennou head for himself from unhardened octagonal steel bar stock back when dinosaurs roamed the earth.
It’s interesting to note that the handle I made in days of yore was straight, unlike the handles your humble servant currently advocates, and that lower 1/3rd of the face (closest to the hand) mushroomed, while the upper 2/3rds did not deform at all. Curious…. This was the first time I really noticed that the head of hammer with a straight handle attached perpendicular to the head’s centerline tends to hit the chisel/nail cocked and not in-line with the long axis of the chisel/nail. I also realized that the direction of flow of this steel indicated a lot of energy was being wasted by my straight handle. I hope you can see why the solution the anaya carpenters of Japan developed so long ago resonated with me so strongly.
But what happens if the heat treatment results in a head that is too hard? In this case, while the head and faces may not deform, the face may chip and the entire head may fracture, sometimes even causing embarrassing leakage of sticky red stuff (or at least that’s what the product liability lawyers assert).
A word to the wise: always use properly heat-treated hammers and wear safety glasses when pounding nails, mixing acid, or writing your name on your forehead with ballpoint pen after losing a tequila shot contest.
Precision of the Eye
Gentle Readers may think it passing strange that your most humble and obedient servant is seemingly fixated on the precision of something as boring as the hole in a hammer’s head. Rest assured, this concern is not a fever dream resulting from an obsessive/compulsive over-analysis of negative space, but was born of bitter experience. My new therapist concurs.
Of all handtools, the hammer is one of the simplest. But is also the one that moves the fastest and is most influenced by dynamic physics. Things like concentricity, center of gravity, and moment of inertia make a difference to an object moving this quickly, then suddenly slowed by an impact with a chisel or nail, and then returned to battery for another blow faster than you can say “ouch.” Negative spaces like holes influence the physics, and therefore the stability, of hammer heads. Don’t make me inflict you with differential equations, just take my word the hole needs to be centered, not skewed, and of uniform size or the hammer will be skittish resulting in twisted mortise holes, bent nails, and maybe even smashed thumbs.
Another advantage of a well-made gennou head is that its eye will have uniform interior dimensions. We talked about this in an earlier post. In summary, all four walls will be straight, clean, flat, free of wind, parallel and square to each other. This matters because a sloppy eye will not apply uniform pressure on the handle’s tenon, ensuring it will come loose sooner than later. In addition, this uneven pressure may also induce unwanted vibrations in the handle.
If a gennou head has a sloppy eye, it needs to be trued, a task that is difficult to accomplish using files. Depending on the degree of error, correcting the inside walls of that tiny hole in a steel head with skinny files can be frustrating work and may even help your own therapist buy that new bimini top he wants for his fishing boat. You see, most people make the problems worse the first time they try to correct them. I know I did. If you count your time worth anything, the extra cost of a precision eye is money well spent, and certainly cheaper than a Rolex for your therapist’s girlfriend.
A “Modern”style hand-forged gennou head by Hasegawa Kosaburo. This head is of antique construction with high-carbon steel wafers forge-welded to both striking faces of the softish iron body. Prior to the importation from Europe of cheap steel to Japan, all gennou had this laminated construction. I have been using this head for over 35 years. I think this is its third handle.A standard hardware store mass-produced gennou made by a large corporation and sold everywhere in Japan. The eye is sloppy, so the head is secured with steel wedges. The handle is tapered towards the eye to compensate for and conceal dimensional irregularities. Of course, this hammer can hit stuff just fine, but it will never feel right, the head will loosen much sooner than later, and it will be impossible to adequately repair. A decidedly unprofessional, disposable tool.
Precision of the Face
Let’s move on to item 4 “Faces are square with the long axis of the head.” The striking faces of mass-produced hammer heads are often not square with the head’s axis (i.e. manufactured 90˚ to to the head’s longitudinal axis) resulting most obviously in bent nails, but the less obvious consequence is that energy we intended to transmit directly to the chisel and into the workpiece ends up being wasted as heat and violent movement of the chisel out of alignment. It also wastes time and undermines the user’s confidence, once again providing a windfall to head doctors everywhere. A precisely manufactured face square to the tool’s centerline will yield better static performance and more precise chisel work and nail bashing than a mass-produced head.
A “Yamakichi” style 80monme/312gm gennou head hand-forged by Hiroki Aida
The hammer that is stable during the swing and makes a solid impact exactly where you aimed it without producing strange vibrations or wasting energy will help you make every strike with confidence and greater control, and will make your woodworking more enjoyable and profitable. Not insignificant benefits.
Improved Dynamic Performance
In addition to the advantages described in the previous paragraph, a hand-forged head, if made correctly, will be more stable during the swing, and wiggle less upon impact because its mass is more uniformly distributed. This means fewer misses and more energy transmission on impact. And that’s the whole purpose of a hammer, right?
A hammer made using a good head and a handle made to fit both that head and your body will perform better than any tool you have experienced before. You won’t need to look when you pick it up to tell which face (or claw/vs face) is oriented towards the nail/chisel because your fingers will know instantly. The handle will fit your hand perfectly without slipping or causing blisters or needing to be “choked-up on.” You will be able to sense beforehand exactly where the center of the hammer’s face will strike, and will be able to hit the target dead-center and squarely even with your eyes closed.
Appearance and Pride of Ownership
And finally, let’s examine the more ephemeral item 5: “Unique appearance and pride of ownership.” I have mentioned it elsewhere before, but it is a fact that, no matter how much they may deny it, humans are competitive beasties in all endeavors.
As a day job your humble servant manages construction projects in Japan and spends a lot of time inspecting jobsites, meeting with construction companies and speaking with craftsmen. Indeed, there was a time when I was labor instead of management on construction jobsites, so I have enough experience to know that, while some professional woodworkers may not read fashion magazines with titles like “Amazing New Toolbag Design that Will Lift and Shape Your Butt” during coffee breaks, or wear sleek, handmade calf-skin steel-toe Italian footwear during the workday, they are conscious of their personal performance, and the performance of their tools, in comparison to those of their peers. It’s a guy thing, something women think they understand because they have similar tendencies in other areas, but rarely do. This performance is difficult to ascertain unless one is working side by side with one’s peers, so it is human nature to make indirect judgements about performance by observing the quality and condition of a fellow worker’s tools.
Of course, one casually notices the brand of chisel or plane a fellow craftsman is using. Next, if it can be done without being rude (sometimes a difficult thing to accomplish), one examines the fellow workman’s tool’s cutting edge and its sharpness because this tells much about his character and skill (chisels are terrible gossips, you see). Such a close examination may not be possible, but one can usually examine the finish left by his plane and crispness and precision of his chisel cuts.
But more than plane or chisel, the tool that is always visible to others during the workday is the craftsman’s hammer comprised of the head forged by the blacksmith, and the handle made by the craftsman himself. The quality of this combination tells Japanese craftsmen much about the man that owns and uses the tool. A hand-forged head by Kosaburo or Hiroki with a graceful, professional-grade handle made by the owner, even if it is dinged through years of hard use, will be noticed by those with eyes to see for decades, even generations.
Among the Japanese professional woodworkers your humble servant knows well, most of whom ceased getting older long ago, a highly-decorated expensive gennou head is thought to be a gaudy, obvious plea for attention, and in poor taste, like wearing a purple sequined silk suit and Jimmy Choo 8″ gold lame stilettos to the hardware store (best saved for the New Years party anyway, you sexy beast). A plain head by Hiroki or Kosaburo, on the other hand, is understated tasteful evidence of a serious craftsman that appreciates tradition and places quality first. Very much a wabi sabi kinda thing.
And finally, a quality hand-forged head is a lifetime tool, one that won’t wear out or break. The handle may be damaged and wear out over time and need to be replaced, but such wear and tear is unavoidable if the tool is actually used, like a good pair of workboots. But the handle made for a quality head will last longer and be much much easier and quicker to replace. To the professional focused on turning his time into money, this is not an insignificant consideration.
A mass-produced factory head, by comparison, simply won’t work as well. It is of course neither high-quality, unique, nor a lifetime heirloom tool. It isn’t pleasing to the owner’s eye nor impressive to his fellow workers. It is a consumable, expedient tool, the sort of thing a craftsman does not want himself to become.
Conclusion
A high-quality head combined with a handle that fits your body will accomplish more work more precisely with less mental concentration and physical effort. It will become a lifetime friend and a source of satisfaction boosting your confidence. It will be relatively easy to make replacement handles for during your lifetime, and those handles will stay tighter longer. It will tell those who see it that you are serious, no-nonsense craftsman who understands true quality and prefers understated elegance.
If you take pride in your tools, appreciate heirloom value, don’t want to waste time trying to rework sow’s ears into silk purses that smell piggy, and are not inclined to invite psychiatrists to rummage around in your head, then a hand-forged gennou head is worth owning, in the opinion of your most humble and obedient servant.
In the next post in this swashbuckling adventure on the high seas we will try to dispel some rumors about laminated gennou heads.
YMHOS
My newest safety shoes. I must say they go really well with my dual-reel safety harness! What do you think about the steel-toe safety shoes below? Too severe?
The following link is to a folder containing pricelists and photos of most of our products. If you have questions or would like to learn more, please use the form located immediately below titled “Contact Us.”
Please share your insights and comments with everyone by using the form located further below labeled “Leave a Reply.” We aren’t evil Google, facist facebook, thuggish Twitter, or a US Congressman’s Chinese girlfriend and so won’t sell, share, or profitably “misplace” your information. May puppies all hate me if I lie.
The handle that is the focus of this series of posts is an interpretation of the gennou handle developed over several centuries by the anaya carpenters of Japan. In this post I would like to touch on some of their history and the ergonomic factors that drove their subtle innovations.
Historical Background
The word Anaya (穴屋) translates to “hole maker,” a type of carpenter that was common in Japan before the general availability of portable electrical mortisers. These craftsmen had their own guilds in major urban areas and specialized in cutting mortises in beams and columns for wooden structures. They didn’t do layout. They didn’t dimension timbers. They didn’t saw tenons. They didn’t do assembly or erection. Their only tools were the chisels and hammers they used from sunrise to sunset to cut mortises as quickly and accurately as they could.
Anaya did piecework, meaning they were paid according to the number of mortises they completed each day, not by the job or an hourly rate. Each individual Anaya was in direct competition with his fellows for speed and efficiency, so they were serious about the performance of their tools.
Consistent with the Japanese obsession with constantly making minor improvements to their tools, Anaya were forever asking blacksmiths to make them custom chisels and hammer heads reflecting their latest opinions. There are records of more than one chisel blacksmith, including the famous Chiyozuru Korehide, refusing to make chisels for Anaya because of their persistent, obsessive demands.
The gentleman that taught me how to make gennou handles 30 something years ago is now in his late 90’s. He was a young man back when the anaya trade in Tokyo was still burgeoning, and he learned from the best in the business.
Ergonomic Factors
Following are four ergonomic principles related to hammers in general and gennou in particular you should keep in mind when planning your handle. These principles are applicable to not just Japanese gennou, but to all varieties of hammers swung with a single hand. You need to understand them before you design your gennou handle.
Handle Length: Every person’s combination of bones, tendons, muscles and work habits is different. Therefore one size of handle does not fit all; There is a handle length that best fits your body, the way you work, and the type of work you do.
The Grip: For the reasons stated in No.1 above, one grip style does not fit all; There is a handle shape with dimensions that best fits your hand, the way you work, and the type of work you do.
The Knuckles: The human body operates a hammer or gennou most effectively when the plane of the head’s striking face at the instant of impact is oriented in line with the surface of the finger knuckles, particularly the pinkie finger, of the hand holding the hammer.
Head Angle: When swinging a hammer, the arc of the hand naturally moves ahead of the hammer’s striking face. Therefore, instead of being in line with the arc of the swing, the centerline of a typical hammer head will typically end up cocked out and away from the arc of the swing, assuming the handle is straight and hung (installed) with its centerline perpendicular to the head’s centerline. As a result:
The hammer’s face is unlikely to strike the nail or chisel squarely;
The center of mass of the head will most likely not be in alignment with the actual (versus intended) axis of travel of the nail or chisel on impact;
The nail or chisel will therefore be kicked out of the intended axis of travel;
Precision will suffer, and;
Time and energy will be wasted.
Before you design your handle, I highly recommend you thoroughly understand these four essential principles. If you doubt their validity, investigate them yourself. Google will not suffice. There are a couple of tests described in future posts in this series you can perform to verify them. In the meantime, here is a homework assignment: Figure out a way to determine if your hammer’s face is striking the handle of your chisel squarely, or if it is cocked. Let me know your conclusions in the comments below.
The positive impact of incorporating these ergonomic principles into your handle design, as well as the negative impacts of ignoring them, can make a big difference in your performance and work efficiency. In future posts we will show you how to deal with these ergonomic factors to design and make a gennou handle perfectly suited to your body and the way you work.
But before our tumble ass-over-teakettle down this particular rabbit hole loses every semblance of dignity, in the next post in this series we need to examine a critical but oft-ignored part of any hammer : The Unblinking Eye.
YMHOS
The following link is to a folder containing pricelists and photos of most of our products. If you have questions or would like to learn more, please use the form located immediately below titled “Contact Us.”
Please share your insights and comments with everyone by using the form located further below labeled “Leave a Reply.” We aren’t evil Google, facist facebook, thuggish Twitter, or a US Congressman’s Chinese girlfriend and so won’t sell, share, or profitably “misplace” your information. May the fleas of a thousand camels infest my crotch if I lie.
A Japanese shipwright using a hammer to perform “kigoroshi” on the edges of planks for a traditional boat. The planks are joined using long nails “toenailed” from the upper plank into the lower plank. The pilot holes for these nails are made using a “tsubanomi.” When the boat’s hull is later wetted the fibers crushed during kigoroshi will swell back to near their original size filling gaps and tightly locking the planks together, even when the planks are once again dry.
The difference between something good and something great is attention to detail.
Charles R. Swindoll
In previous articles in this series about the Japanese hammer known as the gennou, we examined the background, history and general varieties commonly available nowadays. In this article, we will expand our analysis of the gennou to include a function not well known outside Japan. We hope Beloved Customers and Gentle Readers find it diverting.
Kigoroshi 木殺し
As mentioned in Part 4 of this series, the standard ryoguchi gennou hammer has a flat striking face on one end and a domed striking face on the opposite end. The flat face is well suited to striking chisels, driving nails and the ceremonial wacking of thumbs, while the domed striking face excels at setting nails below the surface of a wooden board, just as Western hammers are. It can also be used for a task called “kigoroshi.” Indeed, this is a technique that can be employed with any hammer having a domed face, although the domed face on many Western claw hammers may be too drastic in some cases. It is a technique worth knowing.
The term Kigoroshi (木殺し）translates to “wood killing” meaning to use a hammer to temporarily crush wood cells. It is achieved by judiciously striking the wood with the hammer or gennou’s domed face. Easy peezy.
When a piece of wood is subjected to successful kigoroshi, the wood cells are deformed reducing their internal volume, but if the pressure is later relieved and some moisture added, over time the cells of many (but not all) species of wood will swell back to near their original volume.
So how is kigoroshi used? For instance, in the case of a mortise and tenon joint, the tenon is cut oversized, and then struck with the convex face of a gennou to deform the wood cells to the point the tenon will fit into the mortise. With time, the tenon absorbs moisture from its surroundings and naturally tries to swell back close to its original size locking it tightly into the mortise. I’m sure you can see the possibilities.
In this short video, the carpenter is performing kigoroshi with the convex face of his gennou to the shoulders of an Akita Sugi (Cryptomeria japonica) beam to enable it to fit inside a housed dovetail mortise. The shoulders will later swell back to close their original dimension closing any minor gaps, hopefully locking the beam tightly into the mortise hole.
Another application of kigoroshi is seen in traditional Japanese boat building where the edge joints between planks forming the hull are hammered, effectively making the planks narrower. After the planks are attached to the ship’s ribs, their crushed cells gradually swell and attempt to return to their original volume, tightly pressing the planks against each other and closing any gaps to create a waterproof joint. In this way, a joint that might otherwise loosen with time and changes in moisture content can be made to remain tight and waterproof. This boat building technique is not unique to Japan, of course.
A Japanese shipwright performing kigoroshi on the edges of planking prior to joining them together.
One more example. When making a rectangular wooden cask or bathtub from hinoki-wood boards (not staves) in the Japanese style, grooves are cut in the bottom board to receive tongues from the vertical side boards. If these tongues are planed oversize and then their sides are pounded judiciously with a hammer with a slightly rounded face like that of a ryouguchi gennou to reduce their thickness to fit into the groove, when assembled and then wet with water the crushed wood cells in the tongue will rebound and will expand to close its original thickness not only locking the tongue and groove tightly together, but also creating a watertight connection. If done properly, the joint will remain tight even after all the boards are no longer wet, same as the ship’s planking mentioned above.
Many people’s understanding of kigoroshi is too shallow to use the technique effectively and consistently without some practical experience. The opinions of inexperienced people therefore should be scrupulously ignored, but the Beloved Customers of C&S Tools are expected to meet a higher standard of woodworking, so I share this advanced technique with you.
There are a few points you should be aware of before attempting kigoroshi in a professional situation, in other words, a situation where cost, schedule, or reputation are at risk.
First, make no mistake: kigoroshi works reliably only on long-grain, not end-grain.
Second, please remember that if the flat face of the genno is used for kigoroshi, or the domed face is cocked so its corners dig in too far, or is used with too much force, the striking face’s perimeter edges may crush cells and sever fibers permanently so that they cannot return to anywhere near their original volume thereby defeating the purpose of kigoroshi and simply weakening the wood. Not good.
Third, be aware that if used in fine cabinetry and joinery work, kigoroshi can create unpredictable tolerance shifts at joints, making, for instance what should be a flush joint offset, so caution and experimentation may be necessary to avoid embarrassing snafus.
And fourth, kigorishi does not work well with some woods, especially hard, stiff woods, and can cause permanent cell damage in some cases. We will discuss this further below. But first, let’s examine the mechanics of kigoroshi.
Nuts and Bolts
Most commercial varieties of wood grow in climates with seasonal changes of winter and summer. A tree is essentially a big water pump that pulls (not pushes) water and some nutrients up from the ground through the pressure differential created by water evaporation at stomata in its leaves. The highest volume of water pumped, and cellular growth, occurs when the weather is warm, water is moving, and the sun is shining. Without liquid water, sunlight, and functioning leaves, the pump stops. In the case of freezing weather, evergreen trees stop pumping water to prevent freezing and the resulting expansion that would destroy the tree.
During the colder months, beginning when leaves fall and the sun fades in Autumn, the pump as well as the tree’s growth slows and then stops. The pump starts up again during the spring thaw when water moves, the sun again shines, and leaves bud.
The stained cross-section of oak below is an excellent illustration of this point. The photo is bifurcated by a a nearly solid band of tight fibers bordered above and below by larger cells, some are rather large white voids. This nearly solid band of cells forms during late Autumn and early spring and is called “late wood” or “Autumn wood.” The areas of less density and larger voids is formed during warmer months of high-growth and is called “early Wood or “Spring wood.” These voids form branching and merging tubes leading from the tree’s roots to the tiny holes in the leaves where the water they carry evaporates powering the pump.
The difference in appearance between these bands of cells (aka growth rings”) can be seen on the surface of a board as its “grain.”
Every type of wood, indeed every piece of wood, is different and will react differently to kigoroshi attempts. Let’s review the physical properties of wood relevant to kigoroshi by examining a cross-section of a tree. For instance summer wood is carefully designed to transmit large amounts of water and nutrients, and so is comprised of large cells with thin walls. After the tree is felled and as the moisture content of the wood decreases, the cells shrink, the cell walls become thinner, harder, stronger and wrinkled and crinkled.
A cross-sectional slice of White Oak dyed red for clarity.
Winter wood in most commercial varieties is designed to resist wind and winter storms. It is comprised of much smaller cells with thicker, stronger walls.
Effective kigoroshi temporarily squashes the cells of summer wood in what is called “elastic deformation,” meaning the deformation is temporary so that the cells rebound to near their original volume when the pressure is removed, depending on the nature of the wood and the elapsed time.
The cell walls of winter wood, on the other hand, instead of squashing and then rebounding, are often shattered by kigoroshi and can rebound little. This permanent damage is due to “plastic deformation.”
Why does this matter? Consider a cube of quartersawn Douglas fir, a wood with soft summer wood, and strong winter wood. If we strike this cube perpendicular to the parallel rings, the larger, weaker cells of summer wood will squash down while the harder lines of winter wood will just be pressed closer together as the layer of summer wood squashes. An application of moisture to this block of wood will cause the summer wood to return to near its original volume and the cube of wood may retain any apparent damage.
Doug Fir
Now what happens when we wack an identical cube in-line with the layer of harder winter wood? Some of the winter wood cells are squashed elastically and will rebound. But the rebound will be less and some of deformation will be permanent.
Oak wood, on the other hand is more dense and the cell walls are stiffer than a softwood like pine, so crushing the cells in kigoroshi will result in even less rebound, and may greatly weaken the wood permanently.
The point is to be aware of the nature of the wood you plan to do kigoroshi to beforehand.
Kigoroshi for Gennou Tenons, and Chisel Handles
There are those who advocate using a hammer to perform kigoroshi on the tenon of gennou handles, the idea being that an oversized tenon can then be crushed a little allowing it to fit into the eye, and that the wood will rebound later locking it into the eye tightly. This sounds like a great idea, but it has problems that stem from the fact that gennou handles are typically made of dense hardwoods like white oak, and not softwoods like cedar.
We need the extra toughness and density that hardwoods provide when making a gennou handle because tenons cut in softer woods will loosen over time. Hard woods like white oak, for instance, do not submit well to kigoroshi because the more rigid cell walls are broken in plastic deformation instead of elastic deformation and won’t rebound enough. In other words, kigoroshi on hardwoods like oak, hickory or persimmon may decrease the cellular volume, but it will also physically weaken the wood. Why would you want to do that?
Instead of kigoroshi, a better solution is to use a good dense hardwood and to precisely cut the tenon just enough oversize so that a lot of force is required to overcome friction driving it into the eye under high pressure. In this way, you will have a tight tenon without compromising it’s cellular strength, a better long-term solution and a more craftsman-like technique.
Another option especially effective when making a gennou handle in humid months is to cut the tenon oversized and shrink it by removing water from the cells using gradual heat. Placing the handle in a more-or-less sealed container with a dry heat source such as an incandescent light bulb will do the job. Silica gel desiccant is another method, but slower. I do not recommend putting the handle in an oven of any kind to accomplish this, however. You have been warned.
Still others advocate performing kigoroshi on the ends of chisel handles to make the crown (hoop) fit better. They then insist one must soak the end of the handle in water to make it swell back to shape and lock the crown in place. While popular, this is poppycock that wastes your time and weakens a properly-sized handle. Please do not do this to C&S Tool’s chisels.
If the handle is in fact too big to accept the crown (unlikely if you purchased the chisel with a handle and crown already attached), first deburr the inside of the crown and chamfer the inside edges. If that is not enough, please shave or file the end of the handle down to a dimension where it takes a number of hard hammer blows from a steel hammer to drive the crown onto the handle. The crown will thereby automatically perform all the kigoroshi necessary. This method is more professional and will provide better service.
Kigoroshi is a useful technique in some applications and with some types of wood. You may not need it but it’s worth understanding, especially if you have a gennou.
In the next post in this series we will examine the ancient ergonomic roots of the gennou handle we advocate and the unusual Japanese carpentry guild that codified them.
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