Leslie and I are in the process of landscaping and because we effectively live on a rock pile we decided to feature our own rocks in our landscape designs; rock gardens, edging, dry creek bed, etc. The only problem is the rocks that have been excavated during various construction activities were stained a nasty rust color, kind of the natural color of the soil here.

We tried power washing and scrubbing the stones to no avail. So my wife came up with the idea that we need to tumble them just like you would do with small stones in the lapidary craft. "How could we possibly do that?" I asked. "With a cement mixer!" she replied! And thus began the summer our neighbors will never forget - so far we have tumbled over 200 loads of rocks!

Step 1: The Bumble Tumble

My friend John loaned us an old cement mixer for the project, however it had been badly damaged when a truck backed into it so before the tumbling could begin we had to repair the mixer. The fixup included a new yellow and black paint job - hence 'Bumble Tumble'. This Instructable is not about repairing the cement mixer but as a result I have some tips on what to do to prepare a cement mixer for the task.

1. Remove the veins from inside the drum; they are necessary for making concrete but not for tumbling rocks. In fact I think they would be a huge detriment in that they would break or bend out of shape and also cause the rocks to spill from the top of the drum - this would surely shorten the life of the mixer.

2. Change the drive pulley ratio so the mixer turns slowly (about 12 RPM).

3. Grease the drive mechanism regularly.

4. Don't put the rocks in the mixer then start it, have it running first then dump a bucket of rocks in. The starting torque may burn out the motor prematurely.

Step 2: The Operation

SAFETY NOTE: A cement mixer is a powerful piece of machinery, be very careful to stay clear of the drive belt and pulleys and exercise extreme caution when working near the moving parts. Be sure to use a GFCI receptacle to power the mixer. Do not let children come near the mixer when it is operating. Also wear appropriate eye and ear protection at all times!

We tended to sort our rocks into 5 sizes; 'peas' <1 inch, 'eggs' 1 - 2 inch, 'potatoes' 2 - 4 inch, 'grapefruits' 4 - 6 inch and LARGE - anything bigger than 6 inch we did not put in the tumbler. We'd often run batches by size, however it was easier on the machine to put 2-3 'grapefruits' in with 'peas' or 'eggs', otherwise a bucket load of 'grapefruits' got pretty violent in the tumbler.

With an empty tumbler drum, add 1 - 2 gallons of water, 1/2 a shovel of crushed gravel (or 1-2 shovels of 'pea' size gravel, a 1/2 shovel of coarse sand as well as a modest squirt of Dawn dish soap (or whatever is handy). Depending on your particular tumbler you might need to adjust these quantities.
Start the mixer.
While the tumbler is running dump in 2 - 4 gallons of stones from a 5 gallon bucket. Once again, depending on your particular tumbler you will have to gauge the quantity that best suits your machine.
Start a timer for the tumble cycle. We found that 'peas' and 'eggs' took less time - about 10 - 15 minutes, 'potatoes' about 20 - 30 minutes, 'grapefruit' about 30 - 40 minutes.
At the end of the cycle stop the tumbler and remove the rocks by hand into a wheelbarrow or wagon, etc (leave the sand and crushed gravel in the drum) . We used a garden wagon made of expanded metal; you want to rinse the stones off after each load and the wagon worked well for that.
Rinse and repeat! We usually did 10+ loads per day and would completely drain and rinse the drum at the end of each day. The crushed gravel can be reused again but you'll likely lose the sand when changing the water.

Tips:

Rinsing the tumbled stones right away prevents the slurry from the tumbler re-solidifying on your rocks, plus it is very reassuring to see the stones all clean and glistening! Yes, we picked out the gems for special placement.
Keep a tally of your loads; this becomes an interesting challenge to try and improve efficiencies and squeeze in an extra load or two each day. It can also create a healthy competition between participants. Further it provides the sense of accomplishment in your achievements.
We didn't discard the LARGE rocks, some of them were reasonably cleaned with the power washer. They found their place in the landscape design.

Step 3: Closing

We are extremely happy with the results! Cleaning our own rocks and using them in our own landscape creations has been very rewarding. Is it hard work? Well yeah! But we've never been shy about applying a little elbow grease to make our home look and feel better. It's cheaper than buying washed landscape rocks and a lot less work than foraging the beach and river beds for them.

We hope you get some inspiration from this and next time you kick a stone across your yard, ask yourself 'could this stone be put to better use?'.

Thanks for checking out this Instructable.

Kent and Leslie at the FrontierShed