Assessing and comparing whetstone quality has become very important recently as the supply and availability of different whetstones is ever changing. This particularly for natural stones as their quarries struggle to remain viable. To be able to objectively compare the different alternative stones I decided that I first needed to devise a series of bench tests. This post describes the process of my detailed investigations.
I also have an interest in being able to assess the quality of natural stone that I have collected myself; in particular greywacke stone which is the subject of a Windrow article due to be published soon. This post is written in part to support this (hopefully more readable) windrow article with details on my methodology. For a shorter review of whetstone choices see the blog post on my own website
Introduction
Before devising a practical assessment method I asked other scythe instructors what they looked for when assessing whetstones. The consensus seemed to focus on three main criteria:
- What effective grade is the grit: fine/ medium/ coarse
- How quickly does it remove metal: fast or slow?
- How hard/soft/durable is the stone (the body or matrix)
The general shape and size of the whetstone is also important: how it feels in the hand and how it fits in the whetstone holder.
A well-equipped scythe set I think needs to contain at least two basic whetstones.
- A coarse/medium stone for reprofiling the cutting edge bevel
- A finer stone used out in the field to produce and maintain a keen sharp edge
The coarse/medium stone is needed in the first instance. Its main job is to quickly and efficiently remove metal to form, or to restore, the secondary/micro -bevel which is the actual cutting edge of the blade; the very last millimetre.
The secondary micro-bevel is formed using a coarse stone on the very edge of the primary bevel (the profiled taper which, in Austrian blades, is created and maintained by peening (hammering)). So, in practice the coarse stone is most often employed immediately after peening to reinstate the micro-bevel as preparation for progressing to using a finer stone.
A finer stone is then needed to refine the secondary micro-bevel and enhance its sharpness. Once a keen edge is attained, the fine stone is used to hone in the field. Frequent honing with a fine stone maintains the keen sharp edge.
Using finer stones not only delivers a sharper edge, they remove less metal with each hone so extend the mowing time before the primary bevel is worn back to the point where it needs reprofiling by peening
The use of an additional third extra-fine-stone, such as a Rozsutec, can take a fine edge up one level of sharpness and then be used to hold it there.
What makes a good whetstone?
There are two main types of whetstone: those cut from quarried natural stone, and synthetic stones manufactured by moulding and bonding carborundum or other hard grit. The functional requirements from each are essentially the same.
To perform well a whetstone needs to contain sharp angular grains of a sufficiently hard mineral to cut metal. In natural stones this is mainly quartz (sand grains). Many of the best natural stone whetstones (e.g. Rozsutec and Mailander) are types of sandstones. In synthetic stones the grit is carborundum which is harder than quartz (but not quite as hard as diamond).
Like sandpaper these grains need to be of the right grade: (coarse to fine) to achieve the desired outcome and fineness of edge finish.
These cutting grains need to be held in place securely by a matrix ‘glue’ of just the right strength. If this bond is too weak the stone will crumble. If the bond is too strong (as in granite or millstone grit) the quartz grains at the surface will become blunt and rounded and will cease to function unless the surface is dressed like a grindstone to expose fresh angular grains.
The ideal matrix for a whetstone is strong enough to hold the quartz grains in position and keep the stone together in one piece, but soft enough that surface progressively erodes shedding dulled grains as they wear to expose fresh ones beneath. A bit like how shark’s teeth renew their bite.
Methods
Over the winter of 2022/23 I devised in my scythe ‘laboratory’ three comparative bench tests to assess the key criteria identified as
- What effective grade is the grit: fine/ medium/ coarse
- How quickly does it remove metal: fast or slow?
- How hard/soft/durable is the stone (the body or matrix)
Grit size/grade
For synthetically manufactured stones the grit size is determined and controlled by the manufacturing process and is thus a known quantity that can be compared to grit sizes in sand paper. With natural stones hewn mainly from rocks of sedimentary origin, the grit size is less uniform. The grit size and variation in particle sizes in natural rock is determined by the sedimentary and subsequent metamorphic geological processes that created it. Examination of natural stone either by eye or by feel gives only limited clues as to the effective average grit size, even closer examination under a microscope does not really help very much. The surface feel can be as much about the way the stones surface was cut and dressed as to its grit composition: a new Rozsutec feels quite different to a well-used one.
A better indirect way to assess as stone is to examine the scratch pattern it makes on a plain polished metal surface. Henk Boss in his appraisal of whetstones of Europe abraded new stainless teaspoons with each stone he tested to compare scratch patterns. I followed his lead and created my scratch patterns on the edge of a cheap stainless carving knife blade using a simple jig and a single pass with each stone in turn to get a series of sample scratch patterns that I could compare and assess.
To grade the stones I produce a second series of reference scratch patterns of familiar stone on a separate blade that I could rank all stones against by matching them side by side under a stereo microscope (x20 magnification).
My reference stones were : Styria (a synthetic carborundum), Mailander, Rozsutec and Welsh slate. From this ranking I was able to categorise a variety of stone’s scratch patterns as being either: coarse, medium, fine, very fine or ultra-fine.
Cutting speed
For this test I simply honed/filed a 60mm oval wire nail held in a vice for two minutes. I filed in the same spot one whetstone’s width of nail, in one direction only (pulling as in honing) and using as consistent pressure and stance as possible. I included a flat chain saw file in this test for comparison. I weighed each nail before and after filing and recorded the weight of steel removed after 2 minutes on a set of digital scales (in milligrams).
Hardness/softness (durability)
Rather than assessing hardness per se (as on the Mohs scale) the objective of this assessment was to try to predict the durability and wear quality of each stone. This is a composite character but has more to do with the matrix bond strength than the hardness of the cutting grains.
The quick objective accelerated wear method I came up with was to subject the flat side of each stone to ten seconds with a 9mm tungsten tipped masonry bit in a battery drill applying only light pressure and medium-slow drill speed. I then measured the diameter of the circular depressions created. I did include the synthetic carborundum stones in this test but did them last as carborundum, being harder than tungsten, quickly blunted my test drill’s cutting faces.
Results and discussion
The results of my comparative assessment of whetstones can be seen in the attached table (to be added). This includes whetstones in my collection, including some novel stones from Austria and Sweden, two ‘designer’ synthetic whetstones from the US, plus some homemade by myself from rock samples I have collected over the years.
(in same order as in table)
Scratch Pattern/ effective grit grade
From observing and comparing pairwise scratch patterns under the microscope I felt fairly confident that I could consistently rank and group the stones into four main categories:
- Coarse (2 stones), like Silicar (Blaumarke)
- Medium (5), like Styria synthetic and La Bergamasca
- Fine (5), like Mailander and La Saurat natural stones
- Extra-fine (4). Like Rozsutec
(numbers in brackets = number of stones in each category)
One of my stones, the Welsh slate, was even finer than the Rozsutec and in a class of its own which I added as ultra-fine.
In general, as one would expect, the scratch patterns do reflect the grit size description for the familiar whetstones.
The basic low cost styria carborundum stones surprised me as they produced a finer (medium) scratch pattern than I had expected as compared to the coarse silicar. From using both stones I would have predicted these to be the other way around. This however could just show the limitations of my bench test.
Of the newer stones I tested, the 2023 Fux Natural stone produces a ‘fine’ result similar to the Mailander (which it is replacing for 2023 due to quarry supply problems in Italy). The Swedish stone sent to me by Jenny Nilsson produced an ‘extra fine’ scratch pattern comparable to a Rozsutec. The two synthetic stones from Baryonx both produced a scratch pattern a bit coarser than I had predicted from the description and feel of the stones, especially the finer of the two, the Arctic fox, which was not in the Mailander ‘fine’ category as I had expected.
Cutting Speed
The cutting speed of the stones as one would expect proportional to the grit size and scratch pattern.
Coarse stones like the Silicar and Bull thistle remove metal the fastest, and quickly create a bevel (with a somewhat toothy edge).
Fine and ultra-fine stones remove metal at fraction of the speed which is why they are used as finishing stones to follow use of coarser grades. The metal chain saw file was faster than any of the stones.
A few stones in my test stood out from the general trend. Most noticeably Jenny Nilsson’s Orsasten stone which produced an extra-fine scratch pattern, but unusually, this was combined with a faster metal removal rate equivalent to a coarser stone. Potentially a good stone for getting a fine result quickly?
The Styria delivered one of the highest speeds of metal removal, but was not as coarse as the two whetstones in the coarse category – perhaps an underestimated stone because it is so cheap?
Softness and potential durability
The softness of the stones in my tests varied considerably and this was not directly related to ether grit size or speed of cutting, being more a measure of the matrix bond strength than the cutting grains.
These accelerated wear test results are not the same as real use tests so have to be interpreted cautiously. They do however produce some comparative figures which are interesting particularly for novel stones.
Synthetic carborundum stones are generally quite wear resistant in my tests. The carborundum stones in my tests are made of pure carborundum with grains fused together as the bond (no matrix). As mentioned above, my drill test results for these may underestimate the softness of this bond as the drill tip was rapidly blunted by the carborundum – its crystals being harder than tungsten. From experience both Styria and Silicar stones do wear back in use and grains are progressively released from the stone’s surface in use
The two synthetic Baryonyx stones have synthetic gemstone grit held in a vitrified ceramic bond. These are softer than synthetic carborundum stone. They are of a hardness designed to progressively release fresh grit as described in the introduction.
Softer natural stones generally produce a slurry in use, although this I think also depends on the nature of the matrix: a fine clay-based matrix producing more slurry. This slurry is considered useful with finest stones when lapping (polishing) the edges of knives and chisels, but perhaps of limited value with a scythe? The Mailander produces slurry, and can be quite messy to use as a result. It however doesn’t wear back too quickly. The new Fux natural stone was one of the softest stones in my test and produced quite a bit of slurry, so much so that in my nail test I had to rinse the stone of accumulated slurry more often to keep it cutting.
Of the stones I have made from rock samples over the years Welsh slate is one of the softest rocks in my tests. This is no surprise as slate as a composite rock material has a Mohs hardness index of around 3.5 as compared to carborundum 9, and diamond 10. The Mohs for slate is actually less than steel (at around 5) So, how can slate (3.5) remove metal and sharpen a steel blade? Slate is a composite material containing mainly softer metamorphised clay minerals, but including within this matrix some ultra-fine-grained silica grains (quartz -Mohs 7) which do the cutting (slowly). More accurately one might describe this as polishing the edge rather than cutting, and so only of practical use on an already very keen edge resulting from progressively honing through the grades from coarse to fine.
Greywacke produced an extra-fine scratch pattern of similar quality to a Rozsutec, and in my tests removed metal faster than a Rozsutec. It was however the most wear resistant rock in my drill tests. This quality might prove a limitation if the stone dulls in use and it does not wear back to expose fresh grit. This does happen to some extent even with the softer Rozsutec whose surface can become dull and glazed over so needs periodic refreshing by cleaning and scouring.
Concluding observations
For a coarse stone for reprofiling I generally turn to the Silicar. I also frequently use this stone for field honing ditch blades. The considerably cheaper Styria synthetic stone looks as if it should be as good in this role, and I perhaps have over looked it. A part of my preference for the silicar is for its size shape and feel, being a bit longer. I have based many of my homemade stones on its shape.
The two Baryonx stones both look and feel very useful in these roles. The Artic Fox in particular is a more refined and efficient choice for field honing ditch blades which I find myself turning to more often now.
Of the fine whetstone group for field honing grass blades, they all do a similar job, the choice has much more to do with availability, price and preference.
The old Bregenzer was a good stone to use but its size and shape very unpredictable, especially at the end of their production They were very prone to breaking, but were cheap.
Mailanders came in to replace them and produce a similar result. Their main drawback is that they are usually too chunky to fit well in whetstone holders without jamming. They are bit messy to use (slurry) as compared to the Bregenzer.
The new Natural stone which Fux offer as a replacement for the Mailander (since its quarry has now also closed) produces a similar fine result. It is a nicely shaped and finished stone and feels good to use. Probably better in this regard than the Mailander. It is however also soft and a messy stone (slurry) and currently very expensive.
The La Saurat gives good results as a fine stone, is a good shape and for me wins in this category on value for money and availability.
In the extra fine category the Rozsutec remains the go to standard for me. I use them all the time when mowing meadows and lawns with a well peened grass blade. They are clean to use but do tend to glaze or dull from time to time and need periodic refreshing (I clean them with detergent and scourer or wet-and-dry emery paper.
I particularly liked Jenny Nilsson’s Orsasten Stone (the “Pink Oyster” as I call it being pink and as I am not confident of the pronunciation of the Swedish quarry from where it came). It stood out as being different in my tests. If this stone was available in quantity at a good price it could be a winner.
On a final rather trivial observation that pink and white coloured stones (pink Orsasten, the pink Bull thistle, and white Arctic fox) look lovely, fresh and new in their packets but quickly look rather grubby once used. Grey coloured stones do not have this problem.