That's goood to know James, I never knew why AO was used for the final stage(s).
If media was made out of pure SiC it would shatter in hours in a rotary.
Case in hand, Washington Mills salesman gave me some large SiC grit, like #4. Particles about 1/8".
After 12 hours in the rotary it was broken down to at least 30-40 grit.
After 2 weeks in a rotary it is probably broken down to 500 grit or finer.
Aluminum oxide is a ceramic. Hard but tough, made to stay together and not fracture into sharp pieces.
Therefore imparting a fine finish as it smooths.
They do make sharper aluminum oxide for some applications like grinding wheels.
Aluminum oxide variations. I have 4 different aluminum oxide 500's. Maybe Rock Shed has the roundest 500.
Or Rock Shed's is one of the best AO 500's for tumbling in a vibe for whatever reason.
I have the best results with Rock Shed AO 500. One tumbling company calls theirs pre-polish, it has a range of 200-1200. It gave me trouble getting a polish.
I think Rock Shed bought a large quantity of expensive high grade AO in a mass purchase. That stuff is miracle AO 500.
Any way, AO not created equal.
Sharp aluminum oxide(X 1000):
Smooth aluminum oxide, can be very expensive. Not sure magnification:
You can look at images at 'aluminum oxide particles' and see a plethora of different shapes and compositions.
It comes in 100's of different forms.
When applying a finish to a helicopter rotor shaft for instance, a few over size abrasive particles can create scratches that would propagate fractures due to extreme stresses.
So the final finish has to be near perfect or human life is at risk. Just the sifting process to remove larger particles is very expensive.
Aluminum oxide that is +/- 1% in size probably $100's per pound. That would be AO 495-505. Trust me, the stuff we buy from tumbling companies ain't near that range.
Probably the best 500 for tumbling is full of particles from 300-600.
The regulators allow abrasive companies to market single grade abrasives with a surprisingly large range.