Sharpening Part 13 – Nitty Gritty

“The true mystery of the world is the visible, not the invisible.” 

Oscar Wilde

In this post we will dig into a few important nitty gritty points about sharpening stones everyone needs to know. Perhaps you already know all these points, but please ready your shovel because there may be at least one buried surprise.

A Flea’s-Eye View

When seen under high-magnification, the surface of a sharpening stone looks like millions of densely-packed stones embedded in a flat field. The smaller the stones, the finer the grit.

As the blade is pushed and pulled over these stones, they scratch and tear metal from the blade’s surface leaving behind scratches corresponding to the size of these small stones. This violence continues until the blade’s ura and bevel form a clean intersection of two planes.

A view of a blade sharpened with 1200 grit diamond plate showing the furrows left by individual pieces of grit

Seen under high-magnification, the cutting edge is jagged where these furrow-like scratches terminate at the cutting edge. To some degree, it may even look like a serrated sawblade. Some blades, like kitchen knives and swords, are used in a slicing motion to cut soft materials like meat and vegetables and enemy arms, and their performance benefits from a serrated cutting edge more than a highly-polished edge, and so do not need to be highly polished on fine-grit sharpening stones. 

Plane and chisel blades, however, are used to cut wood, a material typically harder than foodstuffs, in a straight-on direction, not in a slicing motion, for the most part. In this situation, a rough, serrated cutting edge is weaker than a highly polished edge because the jagged edges are projecting out into space like the teeth of a handsaw blade, and are relatively unsupported and more easily damaged than a highly-polished blade with smaller, more uniform scratches terminating more cleanly at the cutting edge. 

Therefore, in order to produce a sharp durable blade, we must make the microscopic cutting edge smoother and more uniform by using progressively finer grit stones to produce shallower and narrower scratches, and a thin, uniform cutting edge.

But how fine is fine enough? There is a curious phenomenon related to friction that is applicable to cutting edges, and is useful to understand. 

The Friction Paradox

Imagine a cube of heavy stone with its downward flat face resting on the level, flat surface of a larger slab of similar stone. Let’s say it takes some specific measure of force pushing horizontally on the stone cube to overcome the static force of friction between the two stone surfaces in order to make the cube start moving. 

If we gradually increase the degree of polish between the two contact surfaces and measure the force required to start the cube moving at each progressively higher level of polish, we will find the force decreases with each increment of increased polish, for a time. This is at least partially because the irregularities between the two surfaces (asperities) do not interlock as deeply when the surfaces become more polished. 

However, at some point, more polishing brings the surfaces of the two stones into such intimate contact that the molecular attraction between them, and therefore the force necessary to move the cube, actually increases. 

The Inflection Point

The same phenomenon occurs with tool blades. If you sharpen and polish your blades past a particular point, the friction and heat produced between blade and wood will increase, as will the energy that must be expended, while the resulting quality of the cut and durability of the cutting edge will not improve significantly. Of course, the time and money invested in stones spent sharpening past this point will be mostly wasted.

The inflection point where additional polishing yields increased friction with little improvement in cut quality will depend on your tool and the wood you are cutting, but you can gain a pretty good idea of where it is if you pay attention over time. While the sharpening stone manufacturers hate my saying it, in my well-informed opinion there is little practical gain, beyond self-satisfaction, to be had from sharpening chisels or planes past 6,000~8,000 grit, making this range of grit an inflection point in my mind. What about you?

Conclusion

I encourage you to conduct your own experiments to determine the inflection point in the case of your planes and wood you cut. Many who figure this out save themselves significant amounts of time and money sharpening over the long-term.

To those of our Gentle Readers that love sharpening more than woodworking, and enjoy putting money in the pockets of sharpening stone manufacturers more than keeping it for themselves, I apologize for pointing out the floater in the punch bowl. But you probably would have it noticed it eventually anyway, if only from the taste difference.

I will touch more on this important point in the next exciting installment in this scientificish adventure.

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 below.

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.