Mar 14, 2005, 12:49 AM
Post #1 of 1
Recently, we had someone come in looking for plastic pellets. On further questioning, they were using them to try to polish agate, and to get stubborn rocks to polish.
I don't think that is going to work.
Polishing requires the rocks to be in contact with each other, and what plastic pellets do is cushion them, and slow them down. That's great for soft materials, or even delicate materials, like obsidian, or garnet, but for agate, it's going to prevent polishing!
The plastic pellets will help carry grit into the nooks and crannies if you don't have enough small rocks, but it's still better to use small agate, jasper, or petrified wood, (I personally like tiny LSA (Lake Superior Agates) since they last a long time, and look great in fish tanks, or flower vases when done).
When tumbling, it seems to be best to start with about 2x more rock than you think you need.
That means about 10 pounds of rock for a 3-4 pound tumbler, 12-15 pounds for a 6lb tumbler.
Sort the rocks in to piles, mixing up large and small rocks. the "left over" rocks are the extras you add.
So, if you have a 4 pound barrel, you make two 4 pound piles, and 2 pound pile.
You tumble the first round or two in 60/90 grit for rotary tumblers, or 220 for vibrating tumblers. Then set it aside. Then you tumble the next round of rocks the same way. You now have about 1-2/3rds barrels of rocks (remember, you are grinding off a lot of rock).
Now, pick out the best of the rocks, the ones that seem smoother, and starting to take on a better shape. Make sure you have a good mix of large/small rocks. 1:2 is a good ratio, but no less than 2:1 or your grinding/polishing will suffer.
Each time you change out the grit, your rocks are getting smaller, the weight is getting less, and your barrel is emptier. You MUST ADD rock to keep it full. Otherwise, a rotary tumbler will start banging/bruising the rocks as they slide too fast around the barrel, and a vibrating tumbler will not work properly, giving less-than-satisfactory grinding.
This is where those extra rocks come in. Start with the left over partially ground rocks, but if they are not good, add in rocks from the left over pile that are "rounder" and less cracked, if possible.
With a vibrating tumbler, you will remove rocks at each grit change that are "done" and add in rocks that are partially done, or from the extra pile, and keep grinding with 220 grit.
When you end up with about 4-5 pounds of rocks you've pulled out, that you like, you are ready to move to the next step.
Most likely, you'll have gone through all 10 pounds of rough to get that 4-5 pounds you like. And, the rest of the rough, the 3-4 pounds remaining, will be in various states of grind. Some will only be used as "filler" to help grind other rocks, some you'll add in to your next batch of rough grind.
In the 600 grit grind, you don't lose much rock, so adding in rock is not as important.
I often keep a bit of clear quartz, slightly damaged crystals, or such to add, since they only need one or two runs in 600 grit to get where they need to be.
THen, you go to polish.
Rotary tumblers are a bit more difficult to take past that first few rough grind stages, since you can't watch them grind the same way you can with a vibrating tumbler. With the Lot-O-Tumbler, you can really fine-tune the grind as you gain experience, by swapping out rock, taking out rocks, or adding rocks as you go along. It's a great tumbler, and makes rock tumbling a whole heck of a lot more fun!
But no matter what tumbler you use, the key here, is that as you grind your rocks, you MUST ADD new rough. The best way is to start two batches, then combine them, and work from that. For each new batch you start, get an average of 6 pounds of rough rough for the 4-5 pound barrel, and about 5 pounds for the 3-4 pound barrel. Do another double batch whenever you start to run out of the partially ground rocks to add.
The tumblers are set to keep a constant load (weight/size) as they are rated (3pound, 4 pound, 6 pound, etc). As you grind, you lose as much as 40% of the mass of the rock you started out with. You need to keep adding that back in, or your tumbler quits working as it should.