In Shop Blog, Techniques, Tools

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Sometimes brand-new chisels and planes (even from the best manufacturers) don’t hold an edge well. I’ve seen some edges crumple like tin foil after two whacks with a mallet or two strokes on a board.

Weak edges aren’t as common a problem as weak chin lines, but they do happen. When I teach a class of 18 people, for example, there’s always one person with a spanking new tool that would crumble if you chopped a Moon Pie.

My solution to this problem has always been to take the tool to the grinder and create a new primary bevel. Then I grind off just a tad more. I take the tool back to the stones for honing and then (by magic) the tool holds its edge.

The strategy almost always works, but I’ve never known exactly why.

So I went to tool steel guru Ron Hock of Hock Tools looking for answers. As always, Ron set me straight. There could be two culprits: too much heat or too much oxygen during the manufacturing process.

“Should the blade be subjected to temperatures in excess of the steel’s critical temperature (the temperature at which the iron crystals transform from ferrite to austenite) the steel will tend to form large grains, which don’t stick to each other as well as we’d like,” Hock writes. “This will cause the resulting steel to be very brittle and crumbly, though it will test as properly hard with a Rockwell hardness test.”

If a tool breaks, you can see evidence of this problem, according to Hock. In a well-treated tool the fracture should look a very fine-grained gray (almost like gray primer paint).

“If you see sparklyness instead, it’s been overheated,” Hock writes, “Which is probably why it broke and you’re looking at it.”

Because the cutting edge of a tool is typically the thinnest part of the tool, it’s the easiest part to overheat, even if the overheating is brief.

The other culprit is oxygen. As steel approaches its critical temperature, the carbon is released and is free to migrate about the steel. If there is air present when it reaches the surface (such as when heat-treating in air with a torch or forge) the carbon atom will run off with the oxygen atom to become carbon monoxide or carbon dioxide and the carbon is lost from the steel, according to Hock.

Most professional heat treaters use furnaces with atmosphere control (vacuum or inert or carbonaceous gas) to minimize this problem, which is called “decarburization.”

“This creates a low-carbon skin on the steel,” Hock writes. “This would not be a big deal except for the fact that the flat back of the tool is the cutting edge, and any loss of carbon results in a loss of hardness. Here again, the edge takes it in the shorts with the most to lose and the least to lose it from.”

Both of these problems can completely ruin a piece of steel through-and-through. But usually the damage is localized, and you can get to the good stuff by grinding away some of the bad stuff.

Just tell your spouse you’re exfoliating.

– Christopher Schwarz

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Showing 7 comments
  • Christopher Schwarz

    Who are you calling Yankee?

    I was born in St. Louis (a border state in the War of Northern Aggression), but grew up in Arkansas and now live in Kentucky.

    In college I was constantly asked if I knew how to work the flush toilets…..


  • Jay Simmons


    How does a Yankee like you know about a Moon Pie?

  • James Watriss

    I think that falls under the "don’t feed him after midnight, and keep him away from water" category.

  • Tony Francis

    It must be time for a Chris Schwarz Sharpening post?!

  • James Watriss

    One other theory I heard was that some manufacturers polish their tools a little too highly. Pfeil and Two Cherries are a couple of examples of what comes to mind. The theory was that they polished a little too much, or too hard, and overheated the steel that way.

    My real problem isn’t with those chisels, though. In general, that was always good steel. My issue is that I’ve seen the Irwin/Marples chisels fracture a few too many times. I’m only one guy, I’ve only seen so many of these things. But I’ve seen at least two examples that I can recall, and heard of a few more, where the tool actually broke, and a large chunk came away… large being something roughly 1/4" square.

  • AAAndrew

    P.S. (and OT)
    If you follow this link, you’ll see a picture of a famous person and a famous bench. (and the not-so-famous me)

    I saw a thread on another blog that was calling for people to post pictures of themselves with famous people. This was the closest I could get except the picts of me with Frank Klausz and Roy Underhill, also from the same venue.


  • AAAndrew

    How very timely. I received a big ol’ paring chisel from Lee Valley for Christmas. I honed it up, which seemed to go well, and it sat in my shop until yesterday when I needed it to trim a tiny (1/4" x 1/4") square of walnut end grain. Of course, being a paring chisel, I was just using the force of my hands to push the chisel. I pushed and it seemed like the end grain was made of iron. I found myself trying too hard and that’s always a bad sign, so I pulled the chisel away and looked at it.

    The edge had folded over like damp tissue paper. I couldn’t believe it! I put it away and was going to look into sending it back. Now I’ll try and grind away carefully some of the end and see if it holds an edge better. I never would have believed it of even Sorby chisels to fold under standard walnut. If this doesn’t work then I’ll assume I got a bad batch of steel and will return it.

    Thanks! Again you’re blog is eerily right on time.

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