Experimental Rock Tumbling: Skipping Fine Grit
We are sometimes skipping the fine grit step completely and getting nice results. Fine grit is expensive and messy. But, the main reason we are skipping that step is to save labor hours. Labor hours are much more costly than the extra electricity and equipment wear.
A variety of tumbled stones that we made while skipping the fine grit step completely.
Confession: Sometimes We Skip Fine Grit
For decades we religiously followed the four-step process for rotary tumbling. That means tumbling in coarse grit (60/90), then medium (150/220), then fine (500 or 600), and then a polish such as TXP aluminum oxide. Lately we are skipping the fine grit step completely on some of our tumbling. The rocks go from medium grit in a rotary tumbler to polish in a vibratory tumbler (we do all of our polishing in vibratory). Or, in vibratory we go straight from medium grit to TXP polish.
Before you decide to try this yourself, you need to know three more things: 1) we allow the rocks to tumble in medium grit for an extra two weeks in rotary, or an extra two days in vibratory; 2) we are doing this with rocks that have a Mohs hardness of seven; and, 3) about half of the time we allow the polishing step to go an extra 12 to 24 hours.
We have been very happy with the results. The quality of the polish on our tumbled stones is very close or equal to what we get when using a fine grit step. This method does add one extra week of tumbling time in a rotary tumbler, and up to 24 hours extra in a vibratory tumbler. Why are we doing this? To cut costs. Labor hours are much more costly than the electricity and equipment wear.
Another sample of stones we finished skipping the fine grit step.
The idea behind this is that extra time in medium grit causes the grit particles to break down into smaller and smaller pieces. So, we are essentially manufacturing fine grit in the tumbler. That can occur if you are tumbling relatively hard materials (Mohs hardness of seven or greater) but will probably not occur if you are tumbling materials that are so soft that the silicon carbide grit is not abraded.
In theory, you could start a batch of rocks in coarse grit, never open the tumbling barrel and if you run them long enough they should emerge as brightly-polished stones. We are not ready to try that yet, but maybe we should. :-)
|Hobart M. King has decades of rock tumbling experience and writes most of the articles on RockTumbler.com. He has a PhD in geology and is a GIA graduate gemologist. He also writes the articles about rocks, minerals and gems on Geology.com.|