The following story is *not* an urban legend...
Dick, my Grandfather owned a paint and lacquer business for 5 decades.
As a youngster, he would take me to the factory on the weekend where
a dozen or so, various sized, ball-bearing and pigment laden mills were
churned various pigments. I don't recall if the machines ran for
several weeks or not, but for several days for sure. The mills looked
a lot like the drums on a cement truck but would would be rotating
a lot faster maybe 30 rpm or so.
One by one he would stop each machine, climb up a ladder, rotate the drum
by hand, and use a large wrench to open the mill up. The contents were
under a lot of pressure due to the heat build-up and he would be very
careful to unscrew the cap carefully.
After a few years the ball bearings were replaced. They weren't round
anymore but were cube-like with slight concave indentations on each side.
(I wonder now how much iron ended up in those paints?). I remember this was
an age when I played with marbles and alleys. At school I had the monopoly
on square ball-bearings. I made some great trades!
And now back to alt-photo...
I don't see why you couldn't use a rock tumbler to mill your pigments
in small quantities. Its exactly the same idea. One thing to remember, is
that you will probably need a few different drums so that you won't
contaminate any white or light coloured pigments with darker colours.
Also, to a slight degree, your pigments will become iron-enriched!
Robert Hudyma, Email: rhudyma@idirect.com,
Semi-Tech Microelectronics, 131 McNabb Street, Markham Ontario Canada, L3R 5V7