What are Grinding Stones Made From?

I'm going to go into significant depth about ceramics, and really, the composition of most ionic solid materials, hopefully without dragging on forever. So, let’s start with the basics. Grinding stones are made from ceramics. Ceramics are a class of materials (just like pure elemental metals, metal alloys, non-metals (such as oxygen, nitrogen, carbon), and polymers (plastics and rubbers) are all classes of materials) that are composed of metal elements that are combined with non-metal elements. There is a large difference in the electronegativities (the strength at which they hold onto electrons) of the elements, so one element (the metal) give up an electron  (or a few electrons in some cases) to the other element (the non-metal). This is called ionic bonding, and ceramics have ionic bonds. The particular ceramic that grinding stones are typically made from is a mineral (minerals are generally ceramics) called corundum, which is chemically known as aluminum oxide (meaning it's metallic aluminum atoms and non-metallic oxygen atoms combined together) and tends to have impurities - atoms of elements other than aluminum or oxygen - in small concentrations. Corundum is literally the same stuff that composes sapphires and rubies - its the impurities that give the gemstones their colors.

The chemical formula for aluminum oxide is Al2O3, so there are 2 aluminum atoms for every 3 oxygen atoms. Al2O3 has a neutral electronic charge, but because the oxygen has a higher electronegativity (likes electron more) than aluminum it will steal some electrons from aluminum. It can’t take all of aluminums electrons because for every electron that oxygen takes away, the next electron gets harder to steal and oxygen only has so much space to store the electrons. So, aluminum is actually able to give up 3 electrons, while oxygen is only allowed to take 2. This boils down to a concept called the valence band of electrons, and it basically says that these atoms are happy when they have 8 valence electrons. Aluminum starts out with 3 extra electrons, and oxygen is missing 2 electrons, so they strike a deal. Since electrons have a negative charge, when aluminum gives up its electrons, it become positively charge, and oxygen gains electrons and then becomes negatively charged. Since they’re opposite charges, the aluminum and oxygen stick to each other…this is ionic bonding.

Okay, that’s great and all, but why is corundum used for grinding stones? Well, these ionic bonds are very strong, which translates to it being difficult to move atoms around in a bulk material, which leads to high hardness. High hardness is great, but it often comes at the cost of toughness, which means that corundum is very brittle, like most ceramics. The other practical reason for using corundum, is that it’s widely available, inexpensive, and available as a powder that can be sintered (a process to heat particles so they stick together into a specific shape).

Okay, but who cares about all that science, when I just wanted to know that corundum is hard and cheap? I CARE… and if you made it to this point, you either skipped a lot or you care just a little bit, too.