Particle Size Revisited

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particle shapes 1

 Particle size is only part of the story. 


We are accustomed to seeing and purchasing whetstones labeled as 10k, 1k, 20k, etc. But what does ‘k’ really mean? For example, have you ever compared the sharpening results of an 800x Naniwa Chosera/Pro stone to a King 1k red brick stone? You might be surprised if you use a quality microscope to compare striations. In variably, I have found the 800x stria to be finer, thinner, shallower than those from the 1k King stone.  While that specific variation in performance might not impact anyone’s finished results, the principals behind it are very important. 

So, what’s up with that?

Abrasive particles are one thing, sharpening stones are another thing. 

Stones need to be qualified so consumers can make a purchase they are comfortable with, so the fine-ness of the ‘grit’ is the standard used. The ‘k’ – relegates to particle size or the effective ‘grit’ rating; 1k is coarser than 10k, and so on. 

Everyone should try to understand that the concept of ‘k’ or grit is not a finite standardized system. One manufacturer’s ‘k’ is usually different than another manufacturer’s vision of the same ‘grit’. And what does the k really mean anyway… ? 


Hello. 

All abrasives are ‘graded’ using one of several averaging systems. 

Why? 

Because there is no possible way to obtain a 100% true or absolute result.  


Particle size is only part of the story. 


Particle Shape and the "Equivalent Sphere"

Every material’s particle is going to have its own shape; garnets, Aluminum oxide, diamonds, corundum, various types of silica, etc, they all have different shapes. A particle of one shape cannot cut the same as a particle of a different shape. Garnets do not cut the same as a diamond, or silica, or anything else. Every particle has its own unique ‘fingerprint’ on whatever it is they are abrading. 


Often, people ask me to compare the finished edge results from one abrasive to another. 

First, we have to realize that there are many different particles shapes possible for any type of material. Spessartine garnets in Coticules come in at least 3 different crystal shapes. Diamond powders are available in mono and poly crystalline types, and the shapes of the actual particles is all over the place. Aluminum oxide and ferric oxide can be natural or synthetic, and each is capable of being produces with many different particle shapes and sizes. 


Because of the significant diversity amongst all of these abrasive particles, combined with a need for a relative sizing system, modern abrasive particle sizing techniques convert each different particle’s actual dimensions into an ‘equivalent sphere’.


For example, let’s say the particle in question is a perfect cube measuring 1.0” on all sides. The volume of that cube is 1 cubic inch. A corresponding sphere that has the same volume would be 1.28” in diameter. So – if we have an abrasive with a cubic particle that is 1” on all sides, then the particle size would be referenced as being 1.28”.  


Looking at the shape of a polycrystalline diamond particle, we realize how this averaging system can only be used as a very general reference. We must also consider that every particle is not exactly the same size or shape as the others. Manufacturers have to sift the particles to get batches that contain, on average, the same size particles. 


Particle size is only part of the story. 


Particle Size Distribution, aka PSD

This is a critical concern because it is impossible to achieve perfection with the above-mentioned sifting and sorting systems; in every batch of abrasive particles that is said to be XYZ grit, there will always be larger and smaller particles present. Grading systems like JIS mandate that particle sizes adhere to certain percentages; e.g. an 8k abrasive can only have a certain number of particles over or under the set standard for what 8k means for that standard. It is important to note that whetstones do not adhere perfectly to this standard. And, alongside particle size, there are other important factors to consider.  

 


Particle Friability

Under pressure, and through general use, many particles break apart or wear; their dimensions and sizes change, and they cut differently. Friability is connected to hardness, but it is actually its own characteristic. A hard particle can shatter easily sometimes, and a softer one might be more resilient under the same amount of force or pressure. Friability is usually mostly related to the crystalline makeup and shape. What matters most though, is the result of a particle cracking or shearing. Once broken, a particle’s cutting pattern changes, and it can wear/dull differently also. If particle sizes are equal, worn alundum cuts slower and ‘softer’ then worn diamond. Different particles, even of the same chemical composition, can have extremely different properties here.  


Particle Density 

The concentration of abrasive particles per square inch in a stone changes friction quotient and cutting aspects of the abrasive. Two identically sized stones made of exactly the same components, but having different amounts of abrasive, will behave differently. Usually, the higher density stone will seem finer.   


Binder Type

The type of binder used to make a whetstone has a huge impact on the cutting/polishing characteristics of the abrasive content. The same goes for a fixed substrate – like a barber hone or sandpaper. A Waterstone’s binder will flow away easily to reveal fresh abrasive. Many Barber hones were designed to not wear so the exposed abrasive particles stay fixed in place. Sharpening on 1k wet dry sandpaper is not like honing on any 1k whetstone. Arkansas stones have particles that are, on average, 10 µm in size, but they hardest type of those stones can put a sublime wonderful edge on a razor that can rival the results from any high-grit finisher.     


The Takeaway

There is no such thing as an ‘8k edge’ or ‘any k’ edge. The King 1k red brick stone is not going work the same as a Naniwa Chosera 1k Pro stone. Using .25µm poly diamond spray on felt is going to yield a different result than .25µm mono diamond used on balsa. Someone claiming that their Primeval Lake hone stone is 30k has no idea what they are talking about. Those bars of 0.5 µm Chomium Oxide from an online body shop dealer are not going to perform the same as lab grade 0.5µm Chromium Oxide sold by a reputable dealer. And so on, and so forth.  




© Keith V Johnson 2014 - 2018