How To Sand And Polish A Slab Or Geode Half .... Quickly

[bobby1]

Here is my method of sanding and polishing slabs and geode halves in just a few minutes. It involves sanding with a dry Silicon Carbide 8" by 3" belt on an Expando drum.
Here is a photo of the slab that I will demonstrate on. It is a Brazilian Agate and the saw cut very roughly and left rather significant saw marks. It is a slab that I got last Spring at the local rock pow wow.

Here is a photo of the belt. It is a 100 grit that has moderate wear. When you feel of the grit surface you can feel the grits rather easily but they are not sharp.

Here is another view. There is some buildup of rock dust on its surface.

Because the sanding is done dry you must use a high quality dust mask. I use these N95 rated masks that can be purchased at most hardware or paint stores.

Here is a photo of the begining of the sanding.

I always sand only in one direction, parallel to the long direction of the slab. I sand a few strokes (four or five) and feel of the slab. If it has gotten hot enough that you can't comfortably touch it , stop and wipe it with a damp cloth to cool it down. Don't wet the belt because it will wash the rock dust out of the belt. Your objective is to get a buildup of dust on the belt. Stop and look for progress in removing the saw scratches. Here is a photo after 3 or 4 minutes of sanding.

Another couple minutes of sanding.

A few more minutes. We are getting real close to being finished with the sanding.

There are a few scratches left.


Here we are finally finished sanding. I look for no diagonal saw marks (one of the reasons I always sand in only one direction, preferably across the saw marks) and for very shallow fine sanding marks.

Turn it a couple directions to check for the sanding marks.


Here is how the snding belt looks after the sanding process. More dust than when I started.

The time elapsed so far is 12 minutes including picture taking.
Next I go to my 12" diameter, carpet covered wheel with Cerium Oxide paste.

Here is a photo 15 minutes later before I washed it (and dropped it in the sink, breaking off the tip of the unsanded part of the slab).

Another unwashed view. You can see the polish streaks still on the top right corner.

Two views of the finished slab after it was washed off.

 I intentionally left one end of the slab in its original condition as a comparison.
If you are working a thicker slab (this one was 3/16" thick) or a geode half you can do the sanding steps faster because there is more mass to the piece and it won't heat up as fast or require more time to cool the slab when it over heats.
Bob

[Taogem]

Excellent Bob !!!

So, the only grit was the 100 grit.. That is amazing...

Would we consider something different than the 100 for Jasper's or maybe some softer minerals?

Also... I suppose we could use this same process on flat cabs? Sure would save a heck of a lot of work..

Just dry sand em ?

[bobby1]

George,
I used the 100 grit on this one because the saw marks were so deep. On most slabs with smooth saw cuts I use a well worn 220 grit.
On the backs of larger cabs, I use the 220 grit. smaller cabs (1") I use a 400 grit.
I never sand dry on the girdle of the cab or on the rounded edges of the dome. It will definitely burn the cab as evidenced by white spots on the surface.
Bob

[Bluesssman]

Bob, I am guessing the build up of rock material on the sanding belt is what makes for increasingly finer cutting. Is that correct?


Gary

[bobby1]

Gary,
Your guess is the same as my guess. If I wet the belt and wash the rock powder off it goes back to leaving very noticeable sanding scratches until the powder builds up again. A new belt definitely leaves significnt scratch marks until it is broken in.
This sanding process mirrors the Richardson's Ranch High Speed Sanding and polishing method. The difference is their sander uses disks in an inverted position on a 3450 rpm machine. They also use a large diameter slow turning polishing wheel with elk hide, rough side out. I did some geode halves on one of their sanding machines but it was very difficult to get the semicircular sanding scratch marks out. I also used elk hide on my polishing wheel for a while but I didn't like the fact that the Elk hide seemed to be either too wet or too dry, not that semi wet/dry condition for ideal polishing. I learned
about the carpet covered wheel many years ago from a rockhound that did a lot of work on Obsidian. He also taught me about the dry sanding method.  I've been using the carpet wheel ever since for nearly all my polishing activities. It has two main advantages: 1. The knap holds the polish embedded within it much better and longer  than an elk hide will. 2. It holds the moisture for a much longer time than the elk hide, thus the ideal wet/dry condition maintains itself much longer...... sometimes up to 5 minutes or more before I have to respray the wheel with water.
Bob

[Bluesssman]

Bob, what kind of carpet are you using and at what speed?


Gary

[mirkaba]

Good instructions Bob. I have used this method and 400 grit belt to do Obsidian. For some reason I have always been a little leery of Obsidian. But this seems to work for me.........Bob

[bobby1]

Bob,
It works great with Obsidian flats. One problem occurs if you sand on the edge closest to you. The edge chips and the chips get drug across the slab and cause scratches. That is why I always sand on the lower half away from you. Any chips that occur are drug away from the surface being sanded. Obsidian is not heat senstitve like most Agates are.
Bob

[bobby1]

Gary,
Here some details on my polisher.
An overall view. The bearing mounts are made from 2" angle iron.



The motor is a 1/3 hp, 1725 rpm.
The motor pulley is 2", the polisher pulley is 9", though a 10" would also work. I've always felt that mine runs a little too fast but I haven't been motivated enough to chang to the 10".
The wheel is made from a 12" by 3/4" plywood disk that I got at a local hardware store. I stacked some smaller, thinner disks in the center, filled the gaps with wood filler and turned the wheel on a lathe to get a domed, smooth surface to mount the polish pad on. The dome makes it easier to polish the center of larger cabs, flats or geodes. As I noted before I was using the elk hide pad but changed to the carpet. I just mounted the carpet over the leather pad. If you don't have the leather to put underneath just use a 1/8" foam rubber pad.
I soak the carpet in water for about a half hour so the jute backing will be soft and flexible. This allows me to bend the carpet over the curvature of the edge of the wheel. This is very important to do because it allows you to polish the inner curves on cabs when you grind and polish concave sides  like I frequently do.
Cut gussets in the outer edge of the carpet so that as you tightly pull the carpet over there won't be any lumpy puckers left in the surface of the wheel edges. It takes a while to work your way around the wheel but it is worth the time to get the carpet very smooth and tight. I use self drilling 1/2" oval head washer screws.
Wheel photos.



The carpet is the thin (1/4") office carpet. When we did a remodel at work I grabbed the leftover pieces.



Bob

[Bluesssman]

Bob, thanks so much for the pictures and info. I am going to build a "BOB WHEEL"


Gary

[Freeform]

great exampling Bob, thanks for sharing it. Love them old 100sil belts.

[bobby1]

Here are some photos of a slab of Obsidian that I did a while back. The slab that I started with.
 
I'm actually using a very well worn 80 grit belt.

Here I've started the sanding.

Here you can see some of the scratches that often appear when sanding Obsidian. These scratches are usually from very small chips of Obsidian that have come loose and embedded in the belt. This phenomenon is common with Obsidian but rare when working with Agate. Sometimes I run a scrap piece of Obsidian on the belt to dislodge the chips.

Here I have finally gotten the scratches out.

Here I have started the polishing step.

Here I have finished the polishing.

A view of the slab backlit. It is Midnight Lace Obsidian from Glass Buttes.

Another view.

Bob

[idrockman]

Bob,

I tried this on my Covington 8 x 3" expando wheel with a 60 grit belt, it worked like a charm I was able to sand and polish about 30 thunder-eggs, I told my dad and he couldn't believe me that this method does work... Thanks again for the hint.  Saved many hours on my genie and saved my diamond wheels for finer things.

The best to you,

Brad

[bobby1]

Brad,
I have trouble sometimes convincing some people that using just one coarse belt and going directly to the polish actually works. I can tell from the look on their faces that they don't believe me. OH, Well! To each his own.
Bob

[Rockoteer]

Wow, this is exactly what I was looking for.  I have the mask, the arbor, the expandable drum, the belt/belts, the slabs.

I am going to work.....   Well I need to make myself a domed polisher.  Wished I handn't sold my lathe...drats.

Rockoteer (the other Gary)