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Polishing a slab by hand


#1

I had this idea for displaying my jewelry using slabs of rock and so
I bought a few. But they were not polished. Can anyone walk me
through how to hand polish these few slabs effectively and
semi-efficiently. My equipment consists of a Foredom and hand tools.
And please, tell me if it cannot be done.

Thanks. MaryAnn


#2

Can be done, but it’s going to be quite a bit of work.

You will need to get a selection of carbide grits (80 through 1200
at a minimum in roughly doubling steps: 80-180/220-400-800-1200) and
a piece of window glass ~1/4" thick (or thicker) two or three times
the size of the biggest slab you want to grind.

Now wet the glass and add ~tsp of 80 grit lay the slab on top of the
grit and in a big figure of eight begin to grind out the saw marks.
When the grit doesn’t seem to be cutting well (noise dies down) add
more, when the saw marks are gone you are ready to go onto the next
grit. Wash EVERYTHING down well and repeat with the next grit, but to
tell when you’re done examine the ground surface against a strong
light with the light at a grazing angle, to start with you will need
to get your eye rather close to the plain of the slab to see any
reflected light, but as you progress through the grits you will be
able to get the angle of incidence up to ~40 degrees, and if you’re
lucky even closer to 45 degrees.

At this point your ready to polish, this is a bit tricky without
power tools (a car “sheep skin” buff might work), but assuming you
have silica based slabs you need a slurry of cerium oxide which you
wet the buff with and running it at a moderate speed push the fine
ground face of the slab against the buff with EXTREAM CARe: one slip
and the slab will go flying! Hold it there for a bit and then examine
it again the light, a polish should be forming, repeat til you’re
happy with the surface.

Other kinds of rock may require different polishing mediums like
Chrome oxide (green powder), tin oxide (white) or even rouge (opals
like this one as it’s less aggressive than cerium oxide).

Now there are probably lots of things I’ve forgotten to mention, but
it has been years since I last did this and then I had a big 24"
powered polishing lap, with heavy duty rubber guard, for that
occasional slab that escaped my grip.

The other way is to just varnish the slabs, not as nice looking, but
it will get you there, and you won’t have your nicely polished metals
bumping against hard rock.

Cheers, Thomas Janstrom.
Little Gems.
http://tjlittlegems.com


#3
My equipment consists of a Foredom and hand tools. 

Sorry, wrong tools. Slabs need to be polished with a flat tool that
is larger than the slab. Progressively finer series of laps, then a
polishing lap.

If you’re not experienced at this, find a rock club near you, maybe
someone there has a flat lap machine.

Wayne Emery
The Gemcutter
www.thelittlecameras.com


#4
My equipment consists of a Foredom and hand tools. And please, tell
me if it cannot be done. 

What’s the material? How big are they?

Al Balmer
Sun City, AZ


#5
I had this idea for displaying my jewelry using slabs of rock and
so I bought a few. But they were not polished. Can anyone walk me
through how to hand polish these few slabs effectively and
semi-efficiently. My equipment consists of a Foredom and hand
tools. And please, tell me if it cannot be done. 

Your fordom is useless for this with any sort of slab bigger than
about an inch and a half or so in size. And even then, won’t do a
good job, since usually one want’s such things reasonably flat.

So you’re doing it by hand. Use successive grits of silicon carbide
wet/dry paper or cloth to sand them smooth, then smoother, then
smoother still. The grade you start with will depend on how flat the
saw cuts are to start. Most likely, you’ll need to start with a 120
or 220 grit. Tape the sheets down to a flat surface to work on. Use a
little water as lubrication, which washes the grindings from the
paper surface keeping it cutting. You’ll need to use successive grits
(ie, 120, 220,320,400,600, etc) all the way down to perhaps a 600
grit. Worn 600 grit is better, and finer grits still, would be even
better than that, if you’ve got it available. Note that this is time
consuming to do by hand. I’d guess that to fully hand sand a normal
sized slab to a reasonably smooth pre-polish might take at least an
hour. Maybe more. Each sanding stage should be done in a direction 90
degrees or so to the previous one, so you can be sure all traces of
the previous step have been sanded out.

Polishing is done on a felt, leather, or even canvas surface, also
affixed to a flat surface. You may need to use contact cement to keep
it stuck down. Use a slurry of water and cerium or Tin Oxide. Not too
wet. Just damp. Again, it can take some time and effort.

Somewhat faster polishing might be achieved with diamond compounds.
Use a canvas surface for the paste polishing compounds. You’ll have
to, as with the wet/dry paper, use successive grit sizes to work up
to a polish. 8000 grit will give an acceptable polish. 14,000 will be
better. Once you’ve got a halfway decent polish started with the
lower grits, finer and finer ones take less time, so it’s worth it.

Now, after you’ve read all the above, and considered that you’ll
have most likely spent two or three vigorous hours on each slab, you
just might wish to consider a much faster, and perhaps acceptable
alternative.

Go to the hardware store and buy a can of spray clear plastic
coating like Krylon spray. Get the gloss finish, not the matte. Be
sure the slabs are clean, oil free, and dry before you spray them.
The end result will be hard to tell, at least from display case
viewing distances, from an actual polished surface, and it takes
minutes, not hours, to do.

Oh, and in case the above isn’t clear, I’d strongly recommend the
spray. But you asked how to actually polish the slabs, so if you
want, there’s the basic procedure. It does work. Just time consuming
and calorie burning. The larger the flat surface, the harder it is to
polish, so any by small slabs would be a royal pain in the backside
to polish fully by hand. But it IS possible if you want.

The final method might also be worth considering. Find a local rock
shop or lapidary hobby group or some individual lapidary artist or
hobby worker who has a vibratory flat lap polisher. These, like
vibratory tumblers for jewelry items or tumbled stones, polish flat
surfaces with a succession of finer and finer grits, and then a
polishing agent. Every bit as slow, or slower, than doing it by
hand. But the machine can do several slabs at a time (depending on
size), and you don’t need to be there wearing out your arms and
fingers… If you’ve got lots and lots of these things to polish, and
anticipate doing yet more on an ongoing basis, then spend the couple
hundred dollars to buy your own machine. Small slabs (1 to 2 inch
sort of sizes) can be polished in lapidary vibratory or rotary
tumblers as well, if they fit the barrel or tub of the machine. That
then polishes and rounds off the edges and corners as well as the
flat faces.

Cheers
Peter Rowe


#6

Hi MaryAnn,

A rockhoud would want a real polish on the slabs. But, the equipment
and time needed to polish your slabs might be more than you want to
invest in. You are a jewelry and all you want is a display. So the
quick and easy way to polish your slabs is to put a coating of
Future floor polish on them. Use a sponge brush so to avoid brush
marks from your normal brush. The good thing, if you do not like it
you can clean it off and go the other route of polishing them with a
course grit in stages to a fine grit (100, 220, 600, 2000, 3000,
final polish) then to a polish.

Just an idea. I am sure you will get others.

I hope I was of some help.

John
Grand Junction, Colorado


#7
I had this idea for displaying my jewelry using slabs of rock and
so I bought a few. But they were not polished. Can anyone walk me
through how to hand polish these few slabs effectively and
semi-efficiently. My equipment consists of a Foredom and hand
tools. And please, tell me if it cannot be done. 

Well, it can be done with a lot of elbow grease. Use a piece of 1/4"
plate glass, silicon carbide grit, and water. If the slabs are in
pretty good shape, meaning there are no saw marks you can start out
with 120/220 grit. Put about a tablespoon in the middle of the glass
and a few drops of water. Start rubbing the slab in the grit in a
circular motion. Keep it wet enough so it develops a nice slurry.
Progress from 120/220 grit to 600, then 1000. Polishing maybe more of
a problem. You generally need a polish like tin oxide on leather. You
might be able to use your Foredom here if you can find a leather
buffing wheel. Polish with the leather damp and the tin oxide as a
paste.

  • Or -

You can take the cheap way out and coat the slabs with urethane or
some other clear varnish type product.

  • Or -

You can contact me offline and I can polish them for you.

Rick Copeland
Silversmith and Lapidary Artisan
rockymountainwonders.com


#8

My equipment consists of a Foredom and hand tools.

Best thing, if you not got the right polishing equipment is to spray
the slabs with a couple of coats of clear lacquer. Good enough for
displays.

Hans.
http://www.meevis.com


#9

MaryAnn, the Chinese made huge jade carvings using bamboo and sand.
But that was a different world, too. Depending on what you mean by
"rocks", it could take you anywhere from a week to six months to
polish a slab truly by hand - rubbing away with sandpaper and grit.
By comparison, it takes several weeks to hand grind a small
telescope mirror, which many people undertake as a project. Glass
nowhere near as hard, in practical terms, as agate can be… Some rock
shops will throw a slab on a viibrator for $10…

Maybe less…


#10

Small slabs can be polished with a foredom like tool, but it is a bit
of a pain. And of course it depends on the specific material to some
degree. If youre working with smaller, thin slabs you might consider
attaching the slab to a piece of plywood or something as a backing
with dop wax to secure it. Use diamond or silicon carbide with a belt
or drum sander if you have one or you can buy abrasive cloth, diamond
or, silicon carbide, the wet-dry kind, in 220, 400, 600, and I think
maybe 1200 grit and possibly use it with an orbital sander or maybe
even a sanding block to rough and fine sand it to prepare it for
polishing.( Honestly, Im just thinking of how I might approach doing
this if I didnt have a Druam or belt sander, or larger flat lap to
work with.

Ive only hand polished small slabs a couple times. If I need a
polished slab for something, Ill buy it already polished. That is my
advice. I used to buy polished agate slabs and cut them into round to
make tops out of them. I have a good supply of diamond smoothing
material I bought from HIS Glassworks a few years ago i use for
sanding gem materials. Ill attach it to small flat wood blocks or cut
disks of it to make mini laps that I can use with my foredom tool.)
Anyway, when sanding its best to keep the material wet. Be careful to
be thorough in each stage of sanding and dont leave any large
scratches from one grit stage to the next. You can use hard flat
felt, leather, wood, or special adhesive backed polishing cloth and
cerium oxide or aluminum oxide and water to finish polish. Youll have
to figure out how to make a flat lap on a mandrel that can fit in
your foredom.

Probably it would be much more convenient for you to buy a mini lap
kit that comes with pre-cut diamond disks and mandrels here

http://www.hisglassworks.com/cart/cart.php?m=product_detail&p=34

or from other lapidary supply companies like Crystalite, rather than
making your own. Even so, polishing say 3"or 4" slabs with 1" flat
laps isnt ideal and its hard to get good results like this. It can
be done but it can be tedious and frustrating.

After all that, Id really recommend buying your slabs already
polished.

August


#11

I’d definitely recommend finding a rock club and using their
equipment. Hand polishing slabs can be done (I’ve done it) but it’s a
pain.

RC


#12

Hi Mary Anne, Yes, its possible to polish a slab by hand, but it
takes time and you wont get a great polish. I use a piece of plate
glass and loose silicon carbide grit - you can use emery paper but
it tends to round the edges. Its a lapping procedure, the same as
polishing a flat piece of metal, you start with coarse grit, about
220 grit, and grind until the marks are gone, then finer and finer
until about 600 grit - I lap with a figure eight motion, seems to
work best. For a polish I get some soft leather and stick it to a
piece of wood, I use tin oxide as the polish, mix it with water to a
paste, put some on the leather and rub the stone on it - the same
motion as lapping - I use more pressure for this part. As I say, its
a fairly drawn out process, but it works !

Good luck ! Best wishes,
Philip Wells in New Zealand