TL;DR: Dr. Marv takes a look at the 40 µm stone of the “coarse” grit set. SEM images show a very fine apex, with a “toothy” edge – probably his favourite stone for an edge with a bite now!

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Review

Today’s sharpening stone is something very, very special. I humbly and very proudly present to you the result of spending a decade in the pursuit of REMOVING MATERIAL. The mad Dr. Marv bring’s you the fabulous:

Dr. Marv’s Scientific Sharpening Stones – Coarse Grit Set

This is my own design. I offer these stones in 3 sets – Medium (20 µm, 10 µm, 5 µm), Fine (2.5 µm, 1 µm, 0.5 µm) and now in a Coarse set (80 µm, 60 µm, 40 µm).

The coarse set features a different bond then on the medium and fine set. It is based on the same, scientific principles: rigorous testing, evolution and quality control. At the same time, I felt that a set that is used for re-profiling or setting an edge should be a bit harder. I changed the formulation of the bond and also developed a thermal tempering process that increases the hardness of these. It’s an additional manufacturing step, but oh so worth it. They are noticeably harder than the medium and fine grit set, and the concentration of diamond by weight is even higher.

(Note: the fine and medium set will remain on their high performance bond which is a bit softer than this one)

Optical micrographs of the 40 µm stone. Instrument: Leica Emspira

The stone is a slightly greenish colour, which stems from the diamond type used. We can see regular stripes going from left to right, with a low roughness. All particles are either sparkling diamond, or if oriented in a way that no refraction happens, pure greenish diamonds. The bond itself is colourless and off white. The diamonds are a bit harder to differentiate, showing that they are smaller than on the 80 and 60 micrometre stone.

In order to better show the diamond density and distribution, I took a stone and broke it in half. What you are looking at above is the “cleaved” surface through the stone. We can see that a) this is only and purely diamond! no foreign particles! and b) there is a MASSIVE amount of diamond in this stone. Compare this to any other diamond stone you have seen before on this blog.

SEM micrographs of a broken through stone. Instrument: Zeiss GeminiSEM560

The concentration visible is much higher than on similar grit stones we have had on this blog. It’s really hard to make out the resin between the individual grains – I don’t think a higher concentration has ever been achieved!

Because of my proprietary treatment of the diamond, fantastic grain retention is achieved. It is specifically adjusted for the new resin to allow for some self sharpening, but remain a solid, long lasting stone.

SEM micrograph of the used diamond powder. Instrument: Zeiss GeminiSEM560

This first batch has the following particle metrics:

Particle metrics for the 40 µm nominal sized stone.

In order to evaluate the sharpening performance and material removal mode of this stone, a blade was sharpened with it. I am using a standardised testing procedure, read about it here. For this blade, the edge was first set with a coarse F150 electroplated diamond stone, and then prepared with the Dr. Marv 80 µm sharpening stone. This was followed by the Dr. Marv 60 µm stone from this review:

SEM micrographs of the edge after preparation with Dr. Marv’s 40 µm stone. Instrument: Thermo Fischer PhenomXL SEM.

This stone is obviously leaving us with the finest surface, but also the best apex of the 3 stone “coarse” set. There are no deep scratches, and the bevel surface is really shiny already. Moreover, the apex is nicely refined and thin – but ever so slightly serrated.

SEM micrographs of the edge after preparation with a) Dr. Marv’s 80 µm stone b) Dr. Marv’s 60 µm stone c) Dr. Marv’s 40 µm stone. Instrument: Thermo Fischer PhenomXL SEM.

I think head to head, this really shows the difference between the coarse stones. The edge on the 40 µm stone is a very nice “working” edge – some tooth to it, with a super smooth and already very glossy bevel! I think this will be my favourite stone to retouch kitchen knifes.

Edge prepared with the 40 µm Dr. Marv stone. The surface is super smooth and homogenous!

This is also reflected in the optical micrographs under our Leica Emspira microscope: A very glossy, shiny surface, with a toothy edge. I think some people, especially for rugged outdoor knives or kitchen knifes are going to be over the moon with this set!

That’s it folks. I’m very proud to announce that these are now available in a very limited handmade batch. Link is here or find it in the shop the top of the blog!

The coarse grit set of Dr. Marv’s Scientific Sharpening Stones in their storage box. Proudly handmade by Dr. Marv in the German Alps.