Tool
How much grit to use in a rock tumbler
The short rule, a chart by barrel size, and a calculator that turns your barrel and your rock into amounts and a schedule.
Short answer
Use about 2 level tablespoons of grit for each pound of rock in a rotary tumbler, the same amount in steps 1, 2 and 3 and again for polish. A 3 lb barrel holds about 2 lb of rock, so it takes about 4 tablespoons per step. Add water until it sits just below the top layer of rocks.
Grit amounts by barrel size
| Barrel rating | Rock to load | Grit, steps 1 to 3 | Polish, step 4 | Soap to burnish |
|---|---|---|---|---|
| 1 lb | 0.6 to 0.8 lb | 1 to 1.5 tbsp | 1 to 1.5 tbsp | about 0.5 tbsp |
| 1.5 lb | 0.9 to 1.1 lb | 2 to 2.5 tbsp | 2 to 2.5 tbsp | about 0.5 tbsp |
| 2 lb | 1.2 to 1.5 lb | 2.5 to 3 tbsp | 2.5 to 3 tbsp | about 0.5 tbsp |
| 3 lb | 1.8 to 2.3 lb | 3.5 to 4.5 tbsp | 3.5 to 4.5 tbsp | about 1 tbsp |
| 4 lb | 2.4 to 3 lb | 5 to 6 tbsp | 5 to 6 tbsp | about 1.5 tbsp |
| 6 lb | 3.6 to 4.5 lb | 7 to 9 tbsp | 7 to 9 tbsp | about 2 tbsp |
| 12 lb | 7.2 to 9 lb | 14.5 to 18 tbsp | 14.5 to 18 tbsp | about 4 tbsp |
| 15 lb | 9 to 11.3 lb | 18 to 22.5 tbsp | 18 to 22.5 tbsp | about 5 tbsp |
Grit calculator with run times
Pick your tumbler, barrel and rock. Add a start date to get a dated schedule you can print.
Your batch
35 to 53 daysstart to shine
Load 1.8 to 2.3 lb of rock (agate, jasper, quartz, petrified wood), barrel 1/2 to 3/4 full, water just below the top layer.
- 1
60/90 silicon carbide
about 4 tablespoons · 7 days per run; often 2 to 4 runs
Move on when: Every stone is rounded with no pits, cracks or flat broken faces left.
- 2
150/220 silicon carbide
about 4 tablespoons · about 7 days
Move on when: Surfaces are smooth and evenly matte, with no coarse scratches under a bright light.
- 3
500 or 600 silicon carbide
about 4 tablespoons · about 7 days
Move on when: Dry stones look satin and feel silky. A dull or scratched stone goes back a stage.
- 4
aluminum oxide or cerium oxide polish
about 4 tablespoons · 7 to 10 days
Move on when: Dry stones shine. If they only shine when wet, read the dull stones note.
- 5
plain soap, no grit
about 1 tablespoon of grated bar soap · 1 hour to 1 day
Move on when: Rinse water runs clear and the film of polish is gone.
Amounts follow the common rule of two level tablespoons of grit per pound of rock in a rotary barrel. Your tumbler's manual wins where it differs. Times are typical ranges: the stones decide when a stage is done, not the calendar.
How the numbers are worked out
A barrel's rating counts everything inside it: rock, water and grit. Loaded between half and three quarters full, a barrel carries roughly 60 to 75 percent of its rating as rock, so a 3 lb barrel holds about 2 lb of stones.
For a rotary tumbler the calculator uses the common rule of two level tablespoons of grit for each pound of rock, the same amount in every grit step. Kit tumblers that come with pre-measured packets, such as the National Geographic models, are set up for one packet per step: use the packet and ignore the spoon.
Vibratory bowls use far less grit and their makers publish their own amounts, so the calculator shows a small starting figure and points you to the bowl's chart.
How much water goes in a rock tumbler
Add water until you can see it between the stones, just below the top layer. Too much water and the grit floats away from the rock and the stones barely grind. Too little and the slurry turns to thick mud that cushions the stones and can make the barrel leak.
What happens with too much or too little grit
Too little grit and it breaks down before the week is over, so the last days do nothing. Too much does not grind faster: the extra settles as sludge, wastes money and thickens the slurry. If the barrel is under half full of rock, add ceramic media to bring the load up, and count the media as rock when you measure the grit.
Run times are typical ranges for hard material such as agate and jasper. The real test is always the stones: a step is finished when they look the way the next step needs, not when a week has passed.
Tumbling log sheet
One line per run. Write the date in and the date out, and what the stones looked like dry.
| Step | Grit | Amount | Date in | Date out | How the stones look |
|---|---|---|---|---|---|
| 1 Coarse | 60/90 | ||||
| 1 Coarse, repeat | 60/90 | ||||
| 2 Medium | 150/220 | ||||
| 3 Pre-polish | 500 | ||||
| 4 Polish | polish | ||||
| 5 Burnish | soap |