This is part of a series of blog posts – looking into the appearance and composition of commercially available sharpening stones. If you are interested in the previous episodes, check out the archive for them.

If you have some suggestion on what I should look at next, or want to share your super secret DIY stones, I could be persuaded to open the bag of analytical devices… hit me up on Instagram under @marvgro for that.

Disclaimer: I’m not for sale. Every review you see on this blog is bought with my own money. I have no affiliation to any manufacturer.

Review

Today we’re going to take a look at 3 different leathers, and what is considered the world’s finest stropping compound – Stroppy Stuff, in the 1 micron size.

The idea behind stropping is to refine a cutting edge. With sharpening stones, what you are doing is mostly grinding – your cutting abrasive is fixed in place. With stropping, your cutting abrasive (the compound) is able to move around, but preloaded with force – this would be considered lapping in the engineering world.

The leather I have here is a very flesh, cheap bovine leather (sold by Schleifjunkies, a German reseller of sharpening equipment), and a thin, high end Kangaroo leather. Because I find it hard to spell, we’re going to call it Mow-leather respectively Roo-leather for the rest of the blog. The mow-leather we’re going to look at two sides, the flesh and the grain side. Because I have very little clue about which is which, I asked Max from Stroppy Stuff to identify it for me, and Lars, a sharpening ninja from Canada helped me.

Let’s take a look at the leather under the microscope!

Optical micrograph of the Roo-leather. Notice the fine grain and marmoration. Scale bar is visible in the lower right of each picture. Instrument: Leica Emspira.

The Roo-Leather is very fine and thin. It flexes just barely, and has a nice, fine composition. Leather is mostly collagen fibres, which are heavily artificed via chemical processing. A good introduction to this is chapter 3 in the book “Chemical Testing of Textiles” by Q. Fan, published by Elsevier in 2005.

Next, we’re going to take a look at the Mow-leather, the flesh side:

Optical micrograph of the Mow-leather, flesh side. Notice the coarser overall structure, and black particles.

It is coarser than the Roo-leather. The German reseller of this piece of leather likes to produce large, soft, junky pieces. They give a lot, so extra care needs to be taken to not push down with the edge, or convexing will inevitably happen.

Lastly, let’s take a look at the grain side of the Mow-leather:

Optical micrograph of the Mow-leather, grain side. Notice the coarser overall structure, and black particles.

It is coarser than the Roo-leather, but finer than the flesh side. The black sprinkles are more visible here.

Typically, leather is treated with stropping compound to raise the abrasive effect. Widely considered the best compound is stroppy stuff – it’s ultra high concentration (we will later see that this is true in our SEM analysis!), finely dispersed (also true!) and it is a compound that doesn’t leave you with a slimy film after it dries. I think that property alone is worth it.

Let’s start by looking at these things under the SEM. If you are unsure about what you are seeing here, I wrote an introductory post about the SEM analysis, which explains things in depth.

SEM micrographs of the grain side of the Schleifjunkies leather. Note the density & large prismatic particles. Instrument: Zeiss GeminiSEM560.

The grain side of the schleifjunkies leather shows large, flat areas, intermixed with fibrous sections. In the fibrous sections, there are prismatic particles. These can be identified via EDS as calcium oxide, a chemical which is often used for treating leather. Funnily enough, some of these are a perfect representation of the crystal structure of CaO, compare the wikipedia article for a picture of that.

EDS analysis of the leather and the particles. Instrument: Oxford Ultim Max  ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

Let’s take a look at the flesh side, as it’s not sanded:

SEM micrographs of the flesh side of the Schleifjunkies leather. Note the density & large prismatic particles. Instrument: Zeiss GeminiSEM560.

This side is much coarser, with thick fibres. and large particles embedded in it. This is probably the black particles visible in the optical micrographs. I’d guess that this is SiC from the grinding processing of the leather during manufacturing.

Let’s take a look at the Roo-leather. It’s supposed to be a much better stropping base.

SEM micrographs of the Roo-leather. Note the overall thicker fibres. Instrument: Zeiss GeminiSEM 560.

I’m quite surprised. instead of many small fibres, we have larger fibres, and a stacked, layered build up. Really cool! This explains why it’s so soft and yet so strong.

A piece of this Roo-leather was treated just like Max from Stroppy Stuff is explaining in his youtube tutorial. Let’s check how well it distributes the diamonds!

SEM micrographs of the Roo-leather, treated with 1 micron diamond compound. Note the small diamond particles. Instrument: Zeiss GeminiSEM 560.

The distribution in the flat areas is superb. Near protrusions, we find a bit higher density. In voids, there is very little particles visible. I think his method is spot on – this is a very nice result for a mechanical distribution, if one aims for a monolayer (like one does in stropping). As the voids will not have contact with the knife edge, I do not see any deteriorate effect when the abrasive is missing here.

I was curious to see the grain shape and size distribution of the stroppy stuff diamond emulsion, but also the concentration. For this, I placed a single drop of emulsion on a 5×5 mm silicone wafer. The emulsion was then evaporated inside a vacuum chamber (0.93×10-3 bar).

SEM micrograph of the dried stroppy stuff emulsion. We are left with a thin layer of hydrocarbons on the silicon wafer, and the actual grains. Instrument: Zeiss GeminiSEM560.

This is actually a really nice concentration. I did not expect this many diamonds! Cool. The diamonds are more angular than blocky, which makes them sharper. Size distribution is pretty good, with about a single digit percentage of outliers. All of them appear to be slightly oversized. I work professionally a lot with diamond powder, this is not really surprising to me at these powder sizes. I’d say this is a high quality raw diamond material, that only with lab grade diamond powder could be improved – but that would probably tripple the cost of the emulsion.

I wanted to quickly check whether every particle is really diamond, or if there are foreign particles. But also, whether the emulsion or colour in this one left any residue that we would not want on our knife. For this, we will use EDS again.

EDS analysis of the dried stroppy stuff 1 micron emulsion. Instrument: Oxford Ultim Max  ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

No foreign particles or anything besides the Carbon, some oxygen (which is probably the contamination film) and the silicon wafer are detectable. This is pure goodness, and it explains why no smearing film is left after drying. Nice job!

I’ve a couple of points to make I noticed during the analysis of the materials.

First, the leather does not contain silicates. None. I was not able to find a single scientific source that identified silicates in the leather. Silicates are excessively used to alter the leather, make it softer, more supple and grind it. But as far as I can tell, leather contains no natural silicates. Instead, it is mostly amino acids in the form of collagen fibres. Raman laser spectroscopy supports this. (Source: Bienkiewicz, 1983 “Physical Chemistry of Leather Making”, Krieger Publishing)

Second: I prepared the stroppy emulsion on the leather in a well ventilated room. The rest of my roo-leather was lying on a second table in this room. Still, we can find diamond particles on it:

SEM micrograph, focusing on some diamond particle contamination on the “untreated” Roo-leather. Instrument: Zeiss GeminiSEM560.

I think it is absolutely imperative, to keep your strops VERY separate and clean. If you have a bunch of these lying on top of each other, you are very likely introducing scratches into your sharpening.

Third: Leather is a natural material. There seem to be massive differences in quality, and sourcing good leather also appears to be a major task.