Science of Rock Tumbling


Mechanics of Rock Tumbling Process

The purpose of this article is to precisely define the rock tumbling process so that you will understand the dynamics of what conditions create better finished product. Therefore we must explore what occurs in the four basic stages of the rock tumbling process in a rotary tumbler

The mechanism behind the rock tumbling process is one of repetition and precision. When you consider how stones become smooth, such as river rock, you are looking at the constant flowing of granulated materials transported by water over a collection of rocks. The most critical factor in making the process work is repetition. The numeric count of how many times grains of abrasive material pass over each rock is incredibly large.  It would be hard to conceive of water wearing down stone or smoothing and shaping it, but in fact multiplied over many years it has this effect.

Refining the Process with Grit

The above river rock smoothing process references repetition. However, this is not the entire story how we shape and polish rocks in our garages and workshops.  Of course we do not have the time or patience to wait years to make things like a river does.  So in addition using repetition we employ another tool to obtain results faster. This is the use of precision by adding grit. Grit is the material (Silicon Carbide) that multiplies the abrasive action far beyond the grains in the river. It is harder, it is larger and it is shaped in a more precise way to erode jagged edges of rocks.

Stages of the Tumbling Process

We further refine the process by introducing different shapes and sizes of grit which at each stage match the grinding action needed based on the current state of the rock surface.  And then of course, as the rocks have begun to shape in the way we desire, we begin the finer process of polishing the surfaces. It is interesting to note that we cannot continue the tumbling process and wait for the coarse grit to keep working its way to shaping the rocks. 

Instead we must use more precision by changing the grade of the grit to a finer set of angles that will more directly engage with the smaller rock edges.  The best analogy would be in sculpting where you start with a large wide blade or chisel to remove large chunks of stone and then use smaller tools. If you continued with the large tool you would essential grind the piece into nothing.

Visual Observation

What is the signal indicating when to switch grit sizes you may ask. The answer to this question is arrived at by your understanding of the goal of each processing step and visual assessment of what the rocks look like.  Your goal in each step is fixed.

  • Step one - rough rock shaping, most of the edges and angles of your pieces must be substantially removed
  • Step two - the edges and facets of your rock must be almost completely removed.
  • Step three - your rocks now shaped the way you want them must begin to be surface smoothed. You goal a matted semi-smooth surface.
  • Step four - you are now, having a relatively smooth surface, attempting to create a smoother surface and a polished glossy look.

Each of the processes should be engaged in until the goal of that process is achieved. If this goal is not reached you should not proceed. You should see progress as you tumble. If you are not seeing any progress, or the results are sketchy you may not have the right rocks in terms of their overall appropriateness.

Results Expected

Please note the important lesson here is that if you do not get goal results in a process it is not a fruitful exercise to continue with the next process. This is because going on to the next step will not complete the tasks that were needed to be completed in the last step. If you are building a house and you don;t get the roof done, it is not a solution to start working on the kitchen installation. You have got to complete each step before you move on. This is a huge pill to swallow, but believe it. If you get to polish stages, you will at best be looking at some good polished edges, but there will also be dull jagged edges and your finished rock will really be an overall Fail.

Successful Experimentation

Admittedly there is a random and non-scientific art to working with rocks and grit in a tumbler. There is great success to be had with continued experimentation and continued observation. A critical element in the acceleration of learning and true mastery of the process is journal writing or good note-taking. How long did the process take, put down the times for each step. Log the type of stone you used and keep track of its progress. We always like to take before and after pictures of a batch of rocks. The fun match experiment is to match a before rock with the same rock after in the process.

The Future of Rock Tumbling

The current state of the art is okay. But it is not enough there must be a revolution in tumbling. There must be a paradigm shift in the approach to this process but for hobbyists and industrial professionals. We ask the bigger questions and talk about the progress of the community in bringing about a change through the sharing of knowledge and metrics. Gone are the days of the one size fits all approach to tumbling.