Rock polishing
Alex…
Silicon carbide in screened grits is the most common "grinding" compound…I
most often used three: a combination 60/90 for initial grinding; 220 for a
secondary grinding; and 500/600 for "prepolish".
There are a great many polishing approaches depending on what you're tumbling.
I've tumbled agate and jasper for the most part.
Cerium oxide together with some liwuid dish soap is a typical polishing
substance, most often used in conjunction with plastic spheres or other types
of "buffering" objects (walnut shells). All these compounds are available at
lapidary supply stores.
There are a number of books about tumbling which go into some detail about
tumbling speeds and types of rock which can be mixed in the tumbler (they must
all be of nearly the same hardness). One that I particularly liked is buried in
the garage at the moment and the title escapes me; I just saw it listed in LJ
or Rock & Gem but for the life of me I can't find the listing right now. If you
ask at a lapidary supply store I think it has a white cover with a variety of
tumbled stones depicted.
Typically, cooling isn't an issue with tumbling because the movement inside
isn't fast enough to generate much frictional heat.
There are nearly as many tumblers as there are backyard engineers. Everything
from old automobile tires set on turning rollers to cement mixers have been
successfully used.
Be aware that the tumbling process does produce gases that can build up
pressure and in large (commercial) operations have been know to burst the
tumbling barrel.
-Russ
PS What I ended up with was a "system" of three tumblers. A 10# unit for rough
grind. A 6# unit for medium; and a 3# unit for polish…I found that I was
sorting a 10# batch and getting about a full 6# "load" that I wanted to advance
for the next phase. Then I'd refill the 10# barrel with the remaining stones
from the old batch plus new rocks to fill it back up…When the 6# batch was
done it usually yielded about one 3# load for polish. Eventually, I had all
three going most all the time.