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Knife dulls overnight after sharpening
What an interesting mixture of posts. I often find that the best time to read bessex posts is in the early morning quiet hours when only I and my coffee are stirring. I like the mixture of different observations. The predawn quiet gives them the opportunity to sink in which they deserve. 

Athletes routinely warm up before competing. Does it seem strange that knives (and knife users) benefit from similar warm ups? It would be interesting to compare the accuracy and efficiency of a butcher's first morning cut with those made ten minutes later.

Some inventive fellow invented a product which is ideal for removing overnight oxidation. I think he called it the Sharp Pad.......

Excellent topic.

(Yesterday, 06:55 PM)grepper Wrote:
So, the Victorinox spend another 24 hours relaxing in the drawer. I pulled it out and tested from handle to tip:
135, 140,130, 105

After one pass each side of blade with the leather belt on the Kally:
120, 100, 100, 105

In summary over two days: (I have add some numbers to Greppers list below...)

First sharpening: 110, 130, 110, 125 = 475 / 4 = 118,75
After overnight : 160, 150, 150, 175 = 635 / 4 = 158,75
After stropping : 125, 110, 105, 100 = 440 / 4 = 110,0. (Second sharpest result)
Overnight #2 : 135, 140, 130, 105 = 510 / 4 = 127,5
After stropping : 120, 100, 100, 105 = 425 / 4 = 106,25 (sharpest result)

You can se what I did above.
It shows me (I think) that after 1 days resting and then stropping, the edge is sharper then the first sharpening - and when it is repeated 24 houers later again - we have the sharpest result.

So, not only have Grepper take away the oxidation, he have also remove some small amount of metal.

It would be interesting if Grepper could sharpen an edge, meassure it and then as fast as possible, put the blade in a place without air.. And meassure it again 24 hour later.
If the edge are duller - then perhaps it is not oxidation, or, have it oxidate in another way? Wink

Gas is heavyer then air - so keep the blade in a glas filled with gas can be a possibillitie?

I am not good on this - but I think that you are Grepper Wink

Mr. Ken eloquently uttered, "I often find that the best time to read bessex posts is in the early morning quiet hours when only I and my coffee are stirring. I like the mixture of different observations. The predawn quiet gives them the opportunity to sink in which they deserve."

I love the peaceful mental image your writing invokes.  Coffee

Mr. Edgepal, I think your idea of isolating the blade from oxygen by placing it in an inert gas is similar to Mr. KG's edge sealing test that he referenced a couple of posts back in this thread.  He stated:

Initial Sharpness (BESS) - EDGE SEALING - Sharpness in 24 hours (BESS)

95 - none - 97
103 - Lanox MX4 spray - 99
93 - Tuf-Glide - 98

Lanox MX4 is an Australian lanonlin-based heavy duty anti-corrosion lubricant, made by INOX, they have a sales branch in the USA.

I wonder why these blades did not dull more due to relaxation after the stress of sharpening?

Mr. Jan.  Thanks for the explanation.  That makes sense to me.  Your thoughts and observations are always helpful and really appreciated!  Many times you come up with points of view I had not considered and it's always enlightening.
This is the final set of measurements I will take on the Victorinox.  

First sharpening: 110, 130, 110, 125
After overnight : 160, 150, 150, 175
After stropping : 125, 110, 105, 100
Overnight #2   : 135, 140, 130, 105
After stropping : 120, 100, 100, 105
Overnight #3    : 140, 145, 135, 140
After stropping : 90, 110,100, 110

So far the overnight increase of sharpness values seems pretty consistent and stropping returned the edge to its original sharpness. To this point there seems to be little if any variation or tendency to change.  What this means is that if the knife is used tomorrow sharpness will be in the 140 range.  Plenty sharp for broccoli.

I have seen many times via microscopy that repeated use of the powered leather belt smoothes or burnishes the bevel and reduces the toothy quality of the edge.  I want a toothy edge so I took probably two minutes and reground the edge forming a very small burr.  Then I did a two pass deburring with the SB belt and one pass each side with the leather belt and measured:

110, 145, 115, 160

I had very quickly and with little care slopped out the sharpening and those numbers indicate that either I didn’t grind perfectly evenly or missed a bit of burr whilst deburring.  I’m about 99% sure that one more pass on each side of the blade with the leather belt would even out the edge, but I really just don’t care.  I’m going to use that blade, not marry it, and that variation in sharpness will not be noticeable in normal use.  As soon as it is used the sharpness readings will be in the 200-250 range anyway so back in the drawer it goes… nice and toothy again. Wink

I wish to thank Mr. Mike.  Without a sharpness tester none of this could have been documented or even be possible to understand let alone accurately communicated.  A sharpness tester is essential for understanding knife sharpening.  If there is anyone out there reading this that sharpens knives and does not have a sharpness tester or is on the fence about getting one, all I can say is pull the trigger.  You will be amazed at how instantly instructive they are, and how they enable accurately sharing edge sharpness results.  So, thanks Mr. Mike!
(08-12-2018, 01:07 PM)grepper Wrote: ...

I wonder why these blades did not dull more due to relaxation after the stress of sharpening?


My best guess is that deburring with 5 microns diamonds at a circa 1 degree higher angle cuts off the stressed top of the edge apex; by the 10 BESS drop in sharpness that we saw the apex becomes 20 nm (0.02 micron) wider.
It seems this edge loss problem is not limited to knives only. I just reshaped a turning skew. With no special attention paid to deburring, it showed a BESS reading of 200. After using it to round one billet, the BESS reading had more than doubled to 420. Apparently, there is something in wood which causes edges to deteriorate.

Excellent use and application of your edge sharpness tester Ken. We're going to guess that you've rolled the edge. Can you improve the edge by simple stropping with your Sharp Pad or other means? We don't really know if a turning skew can be stropped.
Thanks, Mike. 

I tried the Sharp Pad. It brought the BESS numbers down to 350. I don't think that even the Sharp Pad can totally overcome a clumsy rookie. I have no doubt that my BESS scores will improve as I become more fluent with turning and sharpening turning tools; it just won't happen today. "No se ganó Zamora en una hora." (Zamora, the last Moorish stronghold in Spain, was not conquered in an hour.)

I believe it is healthy to occasionally be a rookie in a new area. It keeps us humble, and it keeps us learning.

We just woke up Jan and wanted to add some confirmation to your theory expressed earlier in this thread:

"My hypothesis interprets the sharpness drop over the first night as an edge misalignment caused by the relaxation of major part of residual stresses in the edge after sharpening. Stropping after the first night probably does not generate additional residual stress in the edge and so the smaller sharpness drop over the second night can be interpreted as relaxation of the minor part of the remained residual stresses generated by the initial sharpening. "

This reduction in overnight sharpness level does decrease after subsequent stroppings. We've noted this over a period of years now.
EOU, thanks for your confirmation that you observed similar behaviour also. Smile

Initially I expected a monotonous decrease of overnight sharpness drop, but additional data revealed to me that the behaviour is more complicated. Sad

Even more it surely to some degree depends on steel microstructure and also the way how we sharpened the edge. Aggressive sharpening may generate more residual stresses.

I also think about whether gentle elastic bending of a knife blade can speed up the relaxation of internal stresses.



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