Hematite polishing compound???
aFe2O3
polish my samples to an atomically flat finish. Currently I have used
SiC paper in stages to 2400 followed by alumina paste(0.05 micron) or
"Syton"(colloidal silica 0.03 micron) with mixed results. To the naked
eye the surfaces are mirror shiny but when examined under an optical
microscope they appear mottled. I am using nap polishing cloths
designed for metallography work. Clearly my particle size is quite
small (even diamond paste generally does not go below 0.5 micron) so
that is not the problem.
I think the problem is that i need to find a polish that will shine
not only mechanically but chemically as well (i.e. a classical CMP or
chemical mechanical polish). I read on this newsgroup some people
have recommended various polishes made specifically for gemstones and
minerals in jewelery and I was wondering what the success of these is.
Also, if someone could tell me the chemical makeup of some of these
polishes that would be useful (i.e. I think that what you call "Linde
A" is actually 0.3 micron alumina). Some "buffers" that I have seen
recommended include tripoli and Zam. I am curious what these are made
of (what is the abrasive and size?) and if they have any "chemical"
action as well as mechanical. I have also seen a post recommending
cerium oxide which I have not been able to try.
Any help would be appreciated.
Peter W. Rowe
(aFe2O3) wrote:
>>
>> I read on this newsgroup some people
>>have recommended various polishes made specifically for gemstones and
>>minerals in jewelery and I was wondering what the success of these is.
Hematite can actually be a little tricky to polish for gem use, but Sinkankas
recommends Linde A, primarily, or diamond compound. Note, though, that the
polish required for lapidary use is probably not high enough for your needs.
>>Also, if someone could tell me the chemical makeup of some of these
>>polishes that would be useful (i.e. I think that what you call "Linde
>>A" is actually 0.3 micron alumina).
Correct.
>>: Some "buffers" that I have seen
>>recommended include tripoli and Zam. I am curious what these are made
>>of (what is the abrasive and size?)
Tripolis (there are several grades) are generally silica, from diatomacious
earth. Quite impure. Probably, judging by the typical brown color, with some
iron oxide too. Zam is a proprietary commercial compound, but judging from
it's green color, I'd assume it's mostly chromium oxide. I have no idea of the
partical size.
>> and if they have any "chemical"
>>action as well as mechanical.
I doubt it. I'd guess you'd need something with an acidic ph to get chemical
activity. Perhaps a small amount of something like oxalic acid might help? I
don't know if it will. that's a wild shot in the dark. but it does help with
some other materials, such as certain fossil corals and other calcite based
materials. And oxalic acid is sometimes used, in stronger mixes, to help remove
rust stains. so perhaps small amounts added to an otherwise mechanical polish
might help you here. Worth a try at any rate.
>> I have also seen a post recommending
>>cerium oxide which I have not been able to try.
>>
Cerium oxide is widely used as a lapidary polish, especially for quartz based
stones. It's also widely used with glass, in lens polishing. I don't know how
effective it is with hematite.
Another often effective polish is tin oxide. It used to be the preferred polish
for quartz, and is faster than cerium oxide. I still prefer it. However great
increases in price compared to the much cheaper cerium oxide, have considerably
reduced it's popularity. It, like any polish, doesn't work on everything. But
on those materials its effective on, it's very good.
You may also wish to surf on over to the ganoksin.com web site. There you'll
find the availability of the Orchid mail list, which you can subscribe to, or
read/search on the site, including the archives of the mail list. It's MUCH
more active than rec.crafts.jewelry, with often upwards of 50 posts per day, so
it can take some time to participate in that one. But amongst it's wide
readership there are quite a number of highly skilled professional gem cutters
who may be better able to help you with your quest than I am.
Hope this helps.
Peter Rowe
moderator, rec.crafts.jewelry
Curt Gates
polish itself. This includes things like the type of lap, speed of lap,
the pressure of stone on the lap, the amount of water/lubricant, amount of
polish, sequence of abrasives leading to polish, all sorts of things that
tend to make polishing more of an art rather than a science.
The mottled area might be a form of undercutting where there are
differences in hardness or orientation of the hematite crystals, and those
different areas, naturally, respond differently. One way to deal with that
is to use a hard lap , like a ceramic lap, and light pressure. That way,
the lap can't conform to the different areas of hardness of the material
being polished. And, if you use light pressure, it helps prevent
undercutting, too. Excessive polish can build up and scour out the softer
areas. There are a lot of variables. If you're using things like paper and
cloth as laps, you're inviting an uneven surface.
Bottom line: I suggest that you try different laps and techniques, and
check each one with the microscope as you go. The answer may be more in
technique than in the polishing compound. Diamond or alumina should be
okay. Maybe even the colloidal silica, but I'd go for the harder, more
agressive, even-cutting abrasives. Forget the tripoli and zam.
aFe2O3
the polish, and Sn would be quite bad because of its low vapor
pressure.
The alumina and silica I use are quite pure (unlike the tripoli) so I
will stick with those, maybe trying to modify the pH of the solution
used.
aFe2O3
oriented differently thus wearing at a different rate, although I can
see how that would be an issue in a polycrystalline sample.
Thanks for your advice, I will attempt to use a "harder" surface to
polish on and see what happens. I have several different wheels that I
can use.