What is aluminum oxide stone for?

The market of abrasives for manual knife sharpening is changing before our eyes and as fast as possible. Two decades ago, in the era of the first angle-adjustable sharpeners, owners could only count on a minimal selection of electroplated diamond stones and a narrow pool of synthetic apex stones no longer than 6”.

We are considering knife sharpening in the first place. Yes, these devices, with the use of special accessories, allow you to sharpen various scissors, carpentry tools, ice-drill blades, etc. However, it is the knives that form the basis. Which, in practice, have a completely different composition, hardness, strength and other qualities of steel. If with modern “powders” from the world's largest manufacturers much is clear in advance, their quality is generally predictable and working with them is quite easy, all other blade materials require careful and thoughtful approach.

They can have very brittle “over-dried” steel, or very soft metal that can hardly be sharpened at all. Low-quality steel on such products can have extreme ductility, it is not cut off by the abrasive during sharpening, forming a so-called “foil” burr. It goes from one side of the grip to the other, often preventing the knife from being sharpened for hours. This steel forces the use of special methods: step sharpening with an increased angle or the so-called “technological barrier”. For sharpening these knives just at the finishing stage, light, no-pressure work with aluminum oxide stones can help. The oxide is less damaging to the knife's substrate than the sharp edges of a diamond and cuts the metal more gently, without provoking hidden small defects in the steel. The ceramic-bonded block is more suitable than diamond for “tech.barrier”, when its plane runs along the line of the cutting edge, removing a stable burr. Of course, in the case of sharpening metal of such poor quality, aluminum oxide is not a panacea and does not always help, but often its use gives good results.

The other extreme on cheap knives can be excessively “dry” steel, which scratches out with every abrasive movement, forcing the sharpener to work very slowly and constantly resharpen the knife, trying to find the optimal “finish”.

It is in such cases that “super abrasives” may fail and an alternative must be found.It becomes stones made of aluminum oxide.The specificity of the work of its grains allows you to sharpen even the most capricious steel, prone to chipping.This is due to the fact that in the process of sharpening, the grains, rolling in oil or water suspension, do not split into pieces, as in silicon carbide, but roll around, gradually decreasing in size.Thus, aluminum oxide cutting the metal minimally “traumatizes” the substrate, creating a shallow sharpening risk, without forming additional microcracks and can be used even in the most “severe” cases.

Similarly, these bars are also used when carbon or stainless steel is “honed” to the maximum hardness threshold. This often applies to author's products from steels 95x18, VG10, CVG, 9XS, P6M5, Japanese laminates, etc. Heat treatment at the upper limit of permissible values “squeezes” the maximum out of the metal, increasing its ability to retain sharpness. But at the same time, it creates preconditions for excessive brittleness. It is possible to work on such steels with diamond stones on organic or hybrid bond (e.g. PDT Expert / Hapstone Premium), but with great care and constant optical control. In case diamond stones start to stain the cutting edge, it is recommended to switch to aluminum oxide. In this way, oxide becomes a reliable alternative for “non-powder” steels with hardness up to 63 HRC.

It is important to note that on ceramic bond, aluminum oxide works on hard steels somewhat slower than diamond or elbor. However, on magnesia bond, for example, Naniwa or Sharton bars, it often demonstrates high and quite competitive with superabrasives speed of metal processing. At the same time, these stones still work as gently as possible to prevent chipping.

The main disadvantage of aluminum oxide abrasives is their rapid loss of shape and the appearance of “saddle”. This is true for roughing stones and stones for basic sharpening. However, finishing stones, especially ceramic-bonded ones, demonstrate quite good resistance to workmanship. Abrasives made of vacuum-dense ceramics (e.g. ruby ceramic) are even better at maintaining surface flatness. They have a very hard bond and can compete with the bonds of superabrasives, for example, copper-tin.

To summarize, we can confidently state that abrasives based on electrocorundum have not lost their relevance so far. On the contrary, they have become the most suitable, effective and reliable abrasives for the treatment of “difficult” carbon and stainless steels. They can be recommended to every sharpener who wants to work with the full range of knives on the market.