Types of Abrasives

There are several types of abrasives that we often use and this page is meant to classify them and explain strength and weakness of each one.
There are 2 major types of abrasives, loose grit and abrasive stones. This page is dedicated to stones, because information about stones is a superset of information about loose grit.

There are 3 types of sharpening stones to consider:

1. Soft binder with crushable abrasive

2. Soft binder with non-crushable abrasive

3. Hard binder with non-crushable abrasive.

1. Soft binder with crushable abrasive
This is the Japanese water stones. The binder is clay which has hardened during last couple million years. The abrasive is ash, which is somewhat flaky and therefore is easily crushable. Advantage of this type of sharpening stone is that it has a very long reach. The cutting speed is increased by building up slurry, because the concentration of abrasive particles in the slurry is higher than that in the stone. The abrasive particles in the slurry break down into smaller particles, hence the fraction of smaller particles in the slurry increases. This results in used slurry producing finer polish of the blade than a stone would have without slurry.
2. Soft binder  with hard abrasive
This category contains all synthetic stones plus Belgian coticule stones. The abrasive in Belgian stones is garnet which is not flaky and does not break down into smaller particles. In man made stones the abrasive is Aluminum Oxide, or Aluminum carbide, or Chromium Oxide, or even diamons. In these types of stones the slurry has higher concentration of abrasive particles than the stone does, hence sharpening with slurry on the stones is faster than with a bare stone. Because of this speed difference some people say that slurry cuts and stone polishes, which is accurate reflection of speed of abrasion, however in both cases the quality of the polish is very similar with and without slurry.
3. Hard binder with hard abrasive
This is a category which Arkansas stones fall into. The abrasive particles are silica and due to the hard binder there is no noticeable slurry being formed. The quality of polish as well as the cutting speed depends heavily on the roughness of the stones surface. The hard Arkansas stone has much more dense packing of the abrasive particles than soft Arkansas stone. This results in a potential of having much smoother surface of hard Arkansas stone and therefore finer finish from it than soft Arkansas stone. Hard Arkansas stone can be roughed up with a rough DMT plate to temporarily increase its roughness and therefore its cutting speed, however the sharp peaks on the stones surface will soon break off leaving a smooth surface once again. Oil is usually used on these stones to prevent loading of the stones with swarf. It is common mistake to use too much oil on hard stones, which causes knife to glide on the film of oil above abrasive surface of the stone. Oil's job is to wet the stone to prevent metal swarf from sticking to the stone. The swarf can then be removed with more oil.