Using burnishing as a method to maintain the sharpness of a knife is a commonly promoted technique to both extend the time between sharpening on a stone and to prolong the life of a knife by minimizing metal loss.
When used properly, a steel will realign the edge of the blade such that the sharpened bit is all facing the right direction. You should steel your knife every time you use it to ensure that you're getting the best edge possible.
Of course no article on burnishing would be complete without mentioning one of the first people to advocate it in the West and be outspoken about experimental results :
The steeled edge has been smoothed out (top) into an even sharper cutting edge, as though someone had wiped the frosting on a cake with the side of a knife. Notice that there are no primary furrows (caused by the hone) left in the steeled area. Bottom photo shows a more heavily steeled edge. The metal has actually flowed back as though it were molten. This is an edge no hone will ever equal. But be careful not to over steel. In some microphotographs we've taken, you can see a thin hair of metal peeling away from such an over steeled edge. That ruins the edge and the blade has to be honed again.
However burnishing, because it doesn't abrade, leaves worn and fatigued metal on the apex of the knife and so there are obvious concerns about durability and edge retention. To examine these effects of burnishing two knives were used to cut synthetic rope with an as-sharpened edge vs a burnished edge to see if there was any difference in edge retention and durability.
The following knives were used to slice 3/8" used polypropleyne rope on a 2" draw :
All knives had the edge bevels at ~8 dps with a 15 dps micro-bevel. The edge bevels were set a 240X Sigma Power Select II stone which is designed specifically to cut HSS and other very hard to grind steels.
After the first run with the DMT micro-bevel, three more runs were completed with the edges burnished at 25 dps with the Razor Edge's Raz-R-Steel. Three rounds were completed for each trial and a weighted average and standard error were used to compile the data.
A few immediate observations on the results :
As there were only three trials done with each finish, this is a very small sample and so the above conclusion are tentative. However as there is nothing overly surprising about the observations it is likely the same pattern would continue with repeated trials. Given the scatter visible then it is likely that five to ten trials would be necessary to make the above conclusions strongly statistically supported.
In regards to the edge reset, the SPS-II 240X stone readily recut both edges quickly :
Even with very low pressure 2-4 PSI the stone readily released a slurry so it did not have to be reconditioned.
In regards to the scatter, as this was used (dirty) rope it was fairly inconsistent (as are all practical working materials being cut) and so the effect on the edge tended to be highly random.
Note in the images on the right showing the large chipping common in the 10V blade after repeated steeling and continued cutting. This is not surprising given the 15n20 is much softer and a much lower carbide steel it would be expected that it would tend to deform and the 10V would be expected to chip. However again, a longer sample with repeated trials would be required to really have a strong (statistical conclusion).
It is interesting to think about though in the long term, if both of these knives were used in this work the 10V blade would actually wear out sooner if those chips were constantly sharpened out. In fact it would wear out far quicker, as in 10:1, as those chips are FAR deeper than the general wear on the 15n20 blade.
As a final caveat, While there was a large loss in edge retention in burnishing as edge retention is also influenced by the apex angle and grit finish 1,2 then the increased angle and the higher polish from the burnishing would also in part be why the edge retention was decreased. To isolate the effect of the burnishing specifically then the starting finish should also be raised to a very high polish (Spyderco Fine) and the burnishing angle lower as close as possible to the apex angle.
More extended commentary can be seen in the forum thread :
1 : Micro-Bevel influence on edge retention slicing hemp (VG-10 and S30V)
2 : Influence of apex grit (finish) on edge retention slicing half inch hemp
|Written: 30/07/2015||Updated:||Copyright (c) 2015 : Cliff Stamp|