Ever since man discovered the possibilities in heating and hammering on a chunk of meteorite, he has been making iron tools. But meteorites were so rare that iron was more valuable than gold. Plentiful and uniform quality steel was a long time in coming. Steel smiths experienced many failures on their way to producing tool-quality materials. And their successes resulted as much from luck as from actual metallurgy know-how. Today, as toolmakers introduce ever more exotic and better-performing alloys, we almost take for granted the high quality in edge tools.
Steel production has become a very complex science. Metallurgists can make an almost infinite variety of steels; some tough, some incredibly hard, and some rust-resistant. I've spent years studying the subtleties of what makes a certain type of steel ideal for a plane blade and another better for a chisel. I've wondered why some edges stay sharp a lot longer than others do. To understand that, we need to back up and look at how we got here.
Iron ore hematite is common throughout the world. Heat it enough to release the iron, mix in just the right amount of carbon, and you have steel. Simple maybe, but the problems are many. Hematite has impurities like sulfur that weaken steel. There's also the problem of adding just the right amount of carbon and mixing it in uniformly. Steel containing 1 percent carbon has an almost magical ability to accept and retain an edge. Too little carbon yields softer wrought iron, and too much produces brittle cast iron.
Edge toolmakers had far better success making steel from a richer and purer iron ore magnetite. A huge reserve in northern Sweden explains the allure of Swedish steel. For hundreds of years, Sheffield, England, imported Swedish wrought iron, forged it into the highest quality edge tools, shipped them throughout the world, and became famous for it. But this steel was so costly and hard to produce it was reserved just for tools' cutting edges. I have a few such tools -- plane blades and an axe with a distinct line where the steel edge is forge-welded to a softer wrought iron body. Today's Japanese tools are made in a similar way.
You might also see "Cast Steel" prominently stamped on an older edge tool. However, it wasn't cast in a mold as the name implies. Cast steel evolved from the work of a British clockmaker, Benjamin Huntsman, who in 1740 rediscovered a way to make small batches of high-quality steel in a clay crucible. (India produced wootz steel this way years earlier and even exported it to England.) Thanks to the age-old problem of getting the carbon/iron mix just right, Huntsman was having a hard time making steel consistent. He found that bits of charcoal and chunks of wrought iron brought to a melting heat yielded good steel.
A century after Huntsman's discovery, Henry Bessemer built the first blast furnace to purify molten iron by forcing air through it. Steel then became more consistent in quality and less expensive to make. Later, gas and electric furnaces (often used for making steel today) paved the way for alloys and steels with very specific properties.
Add a little chromium, vanadium, molybdenum, tungsten, manganese, or nickel, and steel becomes harder, tougher, wear-resistant, and more shock-resistant. It can also hold an edge better. Tungsten and vanadium, among other elements, form micro carbide particles in steel -- the same sort of hard carbides found in router bits and saw teeth.
New-Age New-Edge Alloys
Knife makers were the first to experiment with new alloys. Toolmakers typically made plane blades and chisels from common O1 and W1 alloys while knife makers tried 440C and D2 alloys with amazing results. But an alloy called A2 takes edge tools into new territory. Edges made from A2 can last twice as long as those from other alloys.
New planes made by Lie-Nielsen, Lee Valley, and other makers all have blades made of A2 alloy. Replacement blades from Ron Hock also are made of A2. For the past few years, I've used A2 blades from English toolmaker Karl Holtey in my bench planes and other tools. The performance difference is stunning.
At first, sharpening these blades to a keen edge took me longer. That's the price paid for a hard alloy (full of carbides) that holds this kind of an edge. My oil stones just didn't cut the steel fast enough -- until I learned how to enhance the stones with diamonds. Now I can keep my A2 blades going, even while planing tough ebony or rosewood.
Praising an alloy's magical mix is only part of the picture. Toolmakers are quite savvy about heat treating steel. The most wonderful alloy is useless unless it's very carefully hardened and tempered. For centuries, heat treating was done in forges. Today, it involves double or triple tempering in special ovens and cryogenically cooling tools to -320 degrees F.
Heat treating makes tool steel hard enough to take an edge and tough enough not to chip or break. Heat-treated metal is first heated red-hot, then plunged in water or oil -- or blasted with air, as in the case of A2 and other alloys. At this point the steel is as hard as it can possibly be, but too brittle to be useful. Hit a knot or a bit of tough grain and part of the edge might snap off.
Tempering takes care of that problem. This process toughens the steel by softening it slightly in an oven or forge set at 400 degrees F. Tempering is all about balancing the opposing qualities of hardness and toughness.
A2's particular alloy mix gives it other toolmaking virtues: it machines well and it's very stable during the thermal stresses of heat treating. Toolmakers can grind an A2 blade to size, heat treat it, and it won't curl up as W1 has been known to do.
A2 steel performs even better when you add another bit of metallurgy magic -- cryogenics. Supercooling steel by slowly lowering its temperature to around -320 degrees F, then warming it very slowly (over 20 hours or more) enhances its ability to hold an edge. Engineers can provide a technical explanation, but I liken cryogenics to shrinking steel's grain structure ever tighter, giving the metal even more density, strength, and wear resistance.
An alloy's ideal qualities still depend on consistently mixing the right ingredients. Today, metallurgists use powdered metal (PM) technology. They first atomize the various ingredients by shooting them in a molten state into an inert gas. They blend them well, apply high pressure and heat, and produce steel.
A2 is conventionally made in a furnace, but I have two PM plane blades from a toolmaker that is also testing a similar alloy for chisels. I sometimes disassemble my plane thinking it must be time to sharpen it, but the PM blade still has a respectable edge.
Chisels and planes that stay sharper far longer are just one of the many ways steel science is improving woodworking tools. A new handsaw or bandsaw blade likely has electrically or possibly laser-hardened teeth. The production process hardens the teeth surface but leaves the blade body soft enough to be flexible. New drill bits might have a titanium-nitrate coating for low friction and long life, and lathe-turning tools might be made of special high-speed M2 or M4 alloys.
I must admit that despite all the alloy innovations, I still love my cast steel tools. The other day I pulled out my very antique Spiers smoothing plane to sharpen it. The massive blade is still as black as the day it was forged and is proudly stamped with the maker's mark. The blade sharpened easily, maybe even to a keener edge than my A2 or more exotic alloy blades. But it won't hold for nearly as long as my newer A2 edges.
Try sharpening A2 or other high-performance alloys and you'll quickly understand why the edge lasts longer; the steel is very hard. You really notice the difference when you hone it on a water or oil stone, where even a new fresh surface cuts quite slowly. Diamonds, on the other hand, work beautifully.
Manufactured diamond stones like those made by DMT are good for flattening the backs of blades and removing nicks, but they aren't fine enough for honing a really sharp edge. My fairly unorthodox solution is to smear a tiny amount of diamond paste on my oil stones. About once a month I put a dab of 10-micron diamond paste on my medium India oil stone and 4-micron paste on my black Arkansas. I then strop the blades with a scrap of birch plywood smeared with 1-micron diamond paste.
Sources of Supply
Toolmakers Using A2 Steel:
Hock Handmade Knives
Holtey Classic Hand Planes (England)
Beta Diamond Products
Garrett Hack is a furniture maker in Vermont, and author of The Handplane Book and Classic Hand Tools, both published by Taunton Press.