Rotary Tumbling: Coarse Grit Cycle
The first stage of tumbling your rocks is called the coarse grit stage. This stage typically takes 7 - 10 days. During this initial stage the sharp corners, rough edges and larger pits are removed from the rocks. If you have stones that have multiple sharp points, or are tumbling crushed material, you might need to repeat this stage to adequately ensure the sharp edges are rounded. Stones that are already round (like from beaches or creek beds) still need to go through the coarse grit cycle because they might have pits in them that need to be smoothed out.
When filling your barrel(s) with rock to tumble, it is important to make sure you are filling it 2/3 - 3/4 full. Les than that the action can be too loose or fast, causing chips. Too full and the action will be too slow, and the rocks wont be able to move and will not be able to accomplish much rounding and smoothing out. Also, for maximum tumbling results using a filler is a good idea. A filler can be ceramic pellets or small stones. If using smaller stones as filler, it is best if you use stones that are of the same type and hardness of the larger stones you are tumbling as a harder filler can scratch softer stones. These filler rocks should be small, between ¼ and ½ inches. They help carry the grit, and rub it against the larger stones. We recommend 1/3 to 1/2 of your batch be filler. Your filler can be used over and over. Just keep adding more to keep your barrel ¾ full throughout the different cycles.
Once you have your rocks in the barrel it is time to add the coarse grit. We use 1 tbsp (tablespoon) per pound of rock. So 3 pounds equals 3 tbsp. of grit, for example. When you are changing grits and cleaning your barrel, if you find a lot of grit at the bottom you have added too much.
After adding the grit you can add water to the barrel. Fill the barrel with water until the water level just covers the top of the rocks. After adding the water, put the top back on, making sure not to forget the washer and nut on top! Then place it on the tumbler, and let 'er go! Congrats! You are now tumbling your first group of rocks.
The grit on the left is 60-90 (coarse) grit. The grit on the right is 150-220 (medium) grit. The number or "range" in the grit is determined by the number of granules per inch if they were placed end to end.
After 7 - 10 days: When looking at your stones after the first coarse grit stage, if there are some that seem to be a lot smaller, they are probably softer than the rest of the batch and should be removed and tumbled in a separate batch of like stones. Look at the shapes to see if you want to put them in another coarse grit cycle or move them on to the medium grit cycle.
The image on the left is rough crushed material. The image on the right has been tumbled with 60-90 grit, and is now ready for the 150-220 grit stage.