Polishing slabs is one area that I hear people have problems.
Here is what I have found out: Slabs have a tendency to stick together in the 12 pound barrel tumblers I use. For best results they have to be kept apart in the tumbling action to get a good polish.
To keep the slabs apart there has to be a good quantity of real small sized rocks added to each barrel of rocks that are tumbled. These are from BB sized up to bean sized. Most people just throw these away if they crack up their own rock. I keep all of the “floor sweepings” -the stuff that is left over after I crush up my rocks. I throw all of these little pieces into the barrel with my other small broken rocks that I want to tumble.
For each load I can have as much as 1/2 slabs, 1/4 regular sized rocks and about 1/4 the real small rocks that I call “scrubbers”. They are actually the grinders as they carry the grit in between the slabs to grind off any saw marks and rough spots. They also grind in between the crevices and curves of the other broken rocks. They also carry the polishing powder so that these areas are polished. I like the way my slabs come out.
One of the reasons people don’t usually put the real small chips of rock in the tumbler is that they don’t think they will have a need for them. They might just be something that would be thrown away or discarded after the load was finished. Since I set up and sell stuff at rock shows what I do though is to put these real small rocks in bags or plastic boxes to sell to kids. Save them for club shows.
The other thing that I would mention to people is the time I let my loads run. I usually let my loads run on average 10 days. For the rough grind, I find that for many of the rocks one time through is not enough time. When I take my rocks out of the rough grind, after washing, I will spread them out to dry. All the rocks that are not smooth as I want them go back into another rough grind. When the rough grid is really good the rocks then go into the second (220 grit) stage of grinding. This is also for at least 10 days. It never hurts them to go longer. The next stage is a pre polish. I am using an aluminum oxide for this. This again is for 10 days. The next step is the cerium oxide polish. 10 days is good for this also. I give the rocks an hour or two in a laundry soap wash (burnishing) to finish it off. The rocks are then dried and ready for sorting and separating. If there are some rocks (and there usually is only a few) that I don’t really like the finish, I either throw them into a kids box or if I think they would be good if re- tumbled, I will put them in the next batch of rough.
My washing process for the coarse grit of a 12 pound barrel after 10 days is as follows. I work outside on a bench. I use plastic tubs that I can put a plastic strainer in. I dump the rocks into the strainer so the mud drains through. All this mud is disposed of except any small rocks that go through the holes in the strainer. I then wash the rocks in two or three of these plastic tubs. The water gets cleaner by the second or third wash.
Now I want to save all the rocks but no coarse grit if there is any left. Something like gold panning classifiers or strainers: ï¿½ inch, 1/4 inch, 1/8 inch, and then screen wire size. ( I made my own with metal mesh material.) For the next ï¿½ 220 grit ï¿½ I want all of the rocks except anything that goes through the screen wire size strainer. If it goes through the screen wire it may contain some coarse grit. All this rock material is saved for the next coarse grit. Remember: I need all the small rocks I can get to scrub between the slabs.
I use a coarse grit, a 220 grit, a pre-polish, polish and then a 2 hour laundry soap burnishing. (Rinse the soap off these rocks and they should still appear wet even when dry.)
MORE ABOUT MY TUMBLING FROM A LETTER WRITTEN TO A FELLOW LAPIDARY
On my web page about tumbling I didn’t say anything about the grit I used as I assumed most people had the directions that came with their tumblers or had done research for that. Now I don’t really measure the grit I use but in the 12 pound barrels I think I use about 1/2 to 3/4 cup of coarse grit. I am sure it is never the same because I just have a small scoop or cup that is in the grit container and I just put in what looks OK to me. When I get through running the coarse grit for the two weeks, I never want to see any or at least hardly any black grit left in the bottom of my wash pan. If I see a lot of grit left over as I clean out the rocks I feel that I have put too much grit into the barrel to begin with but since I don’t measure I can’t tell myself to cut back a tablespoon or so. If I do see some black grit in the bottom of the wash water, I try to pour off the water and then catch and save the black grit in a plastic butter dish/container and throw it into the next coarse barrel I set up. In each new load I try to add a couple of hands full of the real small BB sized rock to help with the tumbling action. Even two or three hands full of this small stuff just fills in between the rock so it doesn’t really add anything more to the load. It just takes a little less water.
One thing I do with my pre-polish is to save it and reuse it for the next load. I also do this for the polish from a load. I keep the milk jugs separate and don’t mix the materials. I just add a little more of the pre-polish or polish to the next load. I may add a 1/4 to 1/2 the usual amount when I have some from a previous load. The jugs will usually settled some so the top is just water and I will pour off some of the water and just add the more solid (not a good word) slush (liquid polish) to the next load. My pre-polish loads will have a lot of “foam” in the barrel while the Polish does not have the “foam”. These milk jugs will freeze in the winter time and then the stuff will run out on the floor. This happened in my “dirty” shop so it didn’t do anything more than make a mess for me when they thawed out.
There have been a few times (2 or 3) that I did not get a good polish on a load of stones and I found out it was my fault. I was tumbling some of the material I received when my brother passed away. He had some stones that would not polish. (The last big one I gave to some kid at the bazaar.) These were all harder than agate, jasper and wood and all through the polishing step instead of polishing it would just scratch so the whole batch of stones just came out all cloudy. I knew it had to be something in the batch of rocks do I sorted through each stone to see if I could see something different about it. What I found were a few “real heavy” stones that had to be the culprit. I left these out and put the load of rocks through the 220, pre polish and polish again and they came out perfect.