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Polishing with oxides


#1

I am a gem cutter. I have been told that polishing with diamond powder for my harder stones causes a “mechanical sheering” that allows the stone to polish. I understand this. I have also been told that polishing with oxides for softer stones causes a “chemical activity” that polishes the stone. My question is, what is the chemical activity that is taking place? I understand basic chemistry (protrons, nuetrons, electrons, balance, ions, etc.), so if someone can explain what is happening, I would appreciate it.


#2
 I believe it's all"mechanical shearing". It's just knocking off molecules and atoms till you get a polish.  Nothing is perfect though..... your polished facet will look like a mountain range through the eye of a scanning electron microscope.

#3

I agree with Davewoo. The polishing is mechanical.

The surface of the gem is not changing composition. It is simply becoming smoother. The smooth surface allows more light hitting the gem at an angle greater than the critical angle to enter the gem. The smooth surface also reflects the light hitting at less than the critical angle instead of scattering it.


#4

The chemical activity would depend upon the composition of the stone being polished, and the composition of the oxide compound. And the reaction present would be relatively minute if present at all.

In a nutshell, the oxide compound would react with the surface of the stone, altering the chemical properties, and therefore the physical properties of the surface molecules, allowing those particular molecules to be removed easier.

The reaction rate would be rather “slow” allowing for very precise control over how much material affected.
If you want to learn more about reaction speeds, google an explanation on how an iodine clock works.


#5

It’s not a chemical reaction, the oxides are just a finer form of polishing, with oxides like cerium you are polishing with I believe it’s 3 microns. Which is equivalent to about 50k diamond powder. The oxides are softer than diamonds to allow us to polish without getting the deep scratches from diamonds.


#6

John Gearloose answered this on Facebook but I wanted to share here because this concept is often misunderstood:

Since cerium oxide has a Mohs hardness of 6, it SHOULDN’T polish quartz well mechanically, since quartz’s hardness is 7.

Some oxides on some minerals use a chemomechanical action. Quartz undergoes hydroxyl depletlon with zirconium or cerium oxide. These are both reactive to oxygen and are used as catalysts.(The self-cleaning oven uses cerium oxide in its catalytic coating to transfer oxygen to the crud that it oxidizes to a fine grey ash. Zirconia is used in O2 sensors,) So silicates polish both by mechanical and chemical action on stones like quartz, iolite, scapolite, etc. Since cerium oxide has a Mohs hardness of 6, it SHOULDN’T polish quartz well mechanically, since quartz’s hardness is 7, but the chemical effect is more important. "The primary defects of concern are oxygen vacancies and small polarons (electrons localized on cerium cations). Increases in the number of oxygen defects increases the diffusion rate of oxide in the lattice as reflected in an increase in ionic conductivity. These factors recommend ceria as a solid electrolyte in solid-oxide fuel cells. Undoped and doped ceria also exhibit high electronic conductivity at low partial pressures of oxygen due to reduction of the cerium ion leading to the formation of small polarons. Since the oxygen atoms in a ceria crystal occur in planes, diffusion of these anions is facile. The diffusion rate increases as the defect concentration increases. "