Wicked Edge diamond pastes – what the heck are they doing?


When reviewing the results of my stropping experiments with 14 micron and 10 micron diamond pastes, something began to dawn on me.

I realized that I was trying to explain what I saw through the microscope in terms of assumptions I held about why stropping with the diamond compounds worked.

Basically these assumptions were:

  1. Leather has a stiction (thanks for the word, Clay) that causes it to burnish the edge, i.e. smear out metal over a larger area that causes scratches to become less wide and less deep or sometimes even to disappear.
  2. The diamond pastes are abrasive because they contain diamond particles of a specified (micron) size. These diamond particles abrade the edge much like the diamond stones, i.e. by removing metal by making lots of scratches with a width and a depth of the specified micron size (or a little lower, since only a part of every diamond particle would touch the edge).
  3. The diamond pastes work on leather due to the dual effects of burnishing (1) and abrasion (2).
  4. The diamond pastes also work on balsa due to abrasion (2).

I have shown in my previous posts that assumption (2) is likely not true. The pastes may still contain diamond particles, but these do not leave scratches of approximately the size that stones with such diamond particles leave.

But if assumption (2) is not true, assumption (4) cannot be true either and assumption (3) is not true completely. My house of cards was starting to collapse…

I needed to do some more experiments to see what was going on.

Some more experiments

So if assumption 4 was not true and the diamond pastes did not work on balsa due to abrasion, could there be another way in which the balsa strops caused scratches on an edge to disappear or at least to get less wide and deep? It seemed unlikely, but does balsa perhaps have a similar stiction like leather that causes it to burnish an edge?

I decided to test this by stropping with plain balsa. Below is the edge before and after stropping.

Edge before stropping with plain balsa.

Edge after 500 stropping strokes with plain balsa.

The balsa must have picked up some contamination. But the pictures show that plain balsa does not burnish the edge. It is not abrasive either.

Now I had seen that balsa did not burnish the edge, was assumption (1) true? Does leather have a stiction that causes it to burnish the edge? I decided to do some stropping with plain leather as well. Below is the edge of the knife before stropping.

Edge after 1000 grit stones before stropping on plain leather.

I did a total of 500 stropping strokes.

Edge after 50 stropping strokes with plain leather.

Edge after 100 stropping strokes with plain leather.

Edge after 250 stropping strokes with plain leather.

Edge after 500 stropping strokes with plain leather.

A big surprise! The leather hardly burnished the edge of the knife! If it has a burnishing effect, this is much less than I expected. When I saw this after 250 strokes I tried to increase the burnishing by applying more pressure to the strops. But as you can see, this did not have much effect.

I tried whether I could increase the burnishing of the leather by making it wet with plain water from the tap and then doing 250 stropping strokes.

Edge after 500 stropping strokes with plain leather and then 50 with wet leather.

Edge after 500 stropping strokes with plain leather and then 100 with wet leather.

Edge after 500 stropping strokes with plain leather and then 250 with wet leather.

As you can see, making the leather wet did not increase its burnishing power.

By now all four assumptions I held about the way in which the strops and the diamond stropping compounds worked had had been invalidated.

An alternative theory

Plain leather (at least the top grain cow leather used for the Wicked Edge strops) hardly burnishes an edge. But we have already seen that leather loaded with a diamond stropping paste burnishes an edge very well. There can be only one explanation: the diamond paste itself causes the burnishing, perhaps aided only a little by the leather!

When I took a look again at the pictures of the edges after stropping with the loaded balsa strops, the pieces fell together. What was going on in those pictures was not abrasion, it was burnishing! It was a surface rubbed smoother, rather than scratches replacing scratches. The diamond pastes cause burnishing on balsa as well!

Nearly all of the smoothing of the edges during stropping was due to burnishing and not due to abrasion. And the burnishing is caused by the pastes and not (or only a little bit) by the strops.

This is perhaps not so surprising if you simply feel the strops with a finger. Both a plain leather strop and a plain balsa strop feel very smooth. The balsa strop feels only a little smoother than the leather strop. And when loaded with diamond paste, you can feel the stiction on both strops quite well. (In fact, I once overloaded a leather strop with paste and I could only move it along the edge of the knife with a stick-slip effect – forward jumps alternating with sudden steps.)

Differences between pastes

My initial assumptions about the way stropping pastes would work were guided by the specification of a micron size on every stropping paste. I thought this would say something about its abrasive power. And maybe it does. The tiny scratches caused by the 14 micron paste are perhaps a little wider than those caused by the 10 micron diamond paste. And, as I will show in a future post, the 3.5 micron paste hardly creates any visible scratches anymore. That said, these tiny scratches are so tiny that they hardly remove any material compared to the amount of material the 1000 grit (or even 1600 grit) stones remove.

When you look at the pictures of the edges after, say, 250 or 500 stropping strokes, the edge of the knife after the 14 micron paste on leather is quite similar to the edge after the 10 micron paste on leather. Sure, the edge after the 10 micron paste has less scratches, but if you look at the initial edge before stropping, you see that the edge used for the 14 micron paste was rougher than the one used for the 10 micron paste to start with. And I can already reveal that the edge after 500 stropping strokes with 5 micron paste on leather looks remarkably similar to the 10 micron edge after 500 strokes on leather (even a little better, since I stropped after the 1600 grit stones rather than the 1000 grit ones).

So it seems that all of the diamond pastes I tested (I don’t have the 1 micron and 0.5 micron pastes) are quite similar in their effects. Perhaps this is not so strange if you consider that their differences should be (at least according to my initial assumptions) in their abrasive power. But now we have concluded their burnishing power far outweighs their abrasive power, these abrasive qualities are not very relevant anymore.

2 thoughts on “Wicked Edge diamond pastes – what the heck are they doing?

  1. What if it is not the leather or the balsa that burnish. What if it is the knife material being removed from the blade embedding in the stroll next to the abrasive that burnishes the blade. When the material is being sheared off by the abrasive it is also being work hardened. That removed material goes somewhere. It turns the strop black. When the strop is run across the surface again the sheared knife material contacts the existing knife material and burnishes the blade.

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