A nagura is a separate stone you apply to the sharpening surface of a whetstone after adding water. This produces a granular and abrasive slurry on the whetstone’s surface helping to or changing how you sharpen a blade. There are many combinations and reasons that someone would use a nagura.
A good opening question to this subject is whether you even need a nagura or not. Some stones are soft enough to “self-slurry”, where the act of sharpening or polishing will pull enough abrasive off the base stone to not need a nagura. This is the ideal circumstance and at that point a nagura may not help you much. Such a soft stone will also likely not be a great host for nagura slurry and may burn through the stone faster than necessary. Often stones that self-slurry will not be very hard or fine, and as such may not be a great finishing stone for razors. The average stone though can benefit from a nagara which we will cover below.
One common way to use a nagura is to have a softer nagura than the whetstone you are using. Applying the nagura to a wet whetstone and rubbing will result in the generation of Slurry. In this case the slurry is formed by the particles from the nagura transferring into the water and which sits upon the surface of the whetstone.
Using soft nagura like this is done where a hard whetstone is desired to give you a better working surface which will not easily dish out after sharpening an edge. This will provide the benefits of the softer particles coming off the nagura stone and allow different naguras to be used in a progression. Using successively finer naguras will reuslt in a better edge then the whetstone alone would produce for blades such as razors. This method will still pull particles from the whetstone via the rubbing of the nagura, but very few will be mixed in.
Another way to use a nagura is in the opposite manner. A harder nagura is rubbed onto a softer whetstone resulting in the slurry coming primarily from the whetstone. This is commonly done on what are rather hard whetstones which will not self-slurry. Self-slurrying is the term given to a slurry produced by only rubbing the blade’s edge on a whetstone’s surface. This method can still be used on a softer whetstone which will self-slurry to speed up slurry generation. This method will still result in a small particles from the hard nagura being mixed into the slurry.
An alternative method of nagura is to use what is called a tomo nagura or a partner nagura. Traditionally, tomo nagura are a piece of the whetstone itself which has been cut separately. This provides a palm-sized chunk or smaller which can be used on the larger whetstone’s surface. A traditional tomo nagura is ideal as the the slurry generated between the whetstone and nagura will consist of all the same particles since they are essentially the same material. This is the only natural nagura type where you do not pull particles which are different from the whetstone into the slurry mix.
It is rare however to be able to get a piece of the same stone you are using as your whetstone. This has lead to the use of the term tomo nagura to reference a small piece of stone cut from a different whetstone which can be used as a nagura. Having one of these tomo nagura which are not of the same stone still provides the ability to raise slurry and change the way the whetstone is sharpening by adding in new and different particles.
Because it was cut from a whetstone itself, it still possesses very desirable fine particals which can often act like a traditional tomo nagura. Tomo nagura that are not of the same stock require experimentation to see how the two stones will act together. Nagura in this fashion are commonly able to be found for cheap as it is a way to utilize less than desirable whetstones. This also offers a fun way to explore new options inexpensively while keeping your original base whetstone.
In theory, nothing is stopping someone from rubbing two whetstones together as nagura. This experience though is often fairly unpleasant. The two large stones create a suction between one another which make separating them from one another unwieldy. It does still work though, just be careful should you attempt it.
Diamond nagura or diamond plates refers to aluminum plates coated on one side with a diamond grit. These are most often used to flatten a whetstone or nagura. It is not uncommon though for some people to use them to generate slurry from their whetstones. Using a diamond nagura generates slurry extremely fast even from the hardest stones. It has the added benefit of acting similar to a “true” tomo nagura, where only base stone slurry exists on your sharpening surface. Diamond nagura should be used specially on Suita stones which have Su holes. These honeycomb looking holes on the surface of the whetstone are liable to hold onto material from naguras you use. This can be an issue of that material is not breaking down at the same speed as the rest of your slurry. If it comes out of that Su hole while you are sharpening or polishing, it may introduce that unworked abrasive and leave less than desirable results.
The downside to these nagura is they can be very wasteful. Often the amount of slurry generated is unnecessary for sharpening the blade. An additional danger of this nagura is slurry polution. When using a diamond nagura there is always a risk that one of the diamond grits releases from the nagura into the slurry. In such a circumstance, you may seriously scratch the blade you are working on. It is worth it though to give it a try yourself and see if you prefer this method over other nagura. I have grown to prefer this method personally.
This process is where you progress through a series of nagura with a graduated grit range from coarse to fine, simulating using many stones. The most common nagura for a nagura progression are called Mikawa Asano nagura. You do not need to use these particular nagura for a nagura progression, but if you do not, you will have to evaluate nagura on your own to determine coarseness/fineness levels and place them in a progression yourself. This can be very difficult to do for inexperienced users, which is where the pre-evaluated Mikawa Asano nagura are valuable. Check out the link before for information on the different Mikawa Asano nagura.