Glass Running - How does it actually work?

Nail-boards, glass running and fire walking - these are effects that are associated with a fakir. Glass running  is beyond our imagination, as the effect seems to be completely impossible from a physical point of view without preparation. But glass running is actually possible with completely unprepared glass shards. All that is needed is good faith in the laws of physics. 

During glass walking, the artist walks barefoot over broken glass.

Physical explanation

It is and remains astonishing: Why does glass running work without prepared glass fragments?  

In the cabaret scene there are some artists who prepare the pieces before use. This is done, for example, by boiling the shards in a pot filled with water and fine quartz sand for a long time. During cooking, the sharp edges are ground off due to the strong friction against each other and on the quartz sand. However, this preparation does not make much sense, as shards break away under the weight of the feet and new sharp edges are formed. 

Before attempting the explanation, a small but important physical mode of action must be explained. Sharp edges (blades, shards, etc.) do not cut by pressure alone. Much more important is the so-called cutting movement of this cutting edge. The best success in cutting with a knife is achieved by moving the knife blade in the direction of the longitudinal axis of the knife - and not by pressing down the knife.

The same applies to broken glass: Pressing a shard edge onto the fingertip or the sole of the foot hardly leads to injuries. Only tiny or larger cuts cause wounds.

Several physical phenomena now interact when glass running: 

• First, the foot is usually set straight and from above on the sherd carpet. No cutting movement is required.
• Secondly, the shards, which form a thick carpet, sink down and sometimes break. The sinking of the fragments ensures that the weight force exerted by the artist is broken down into components that are not all strictly vertical.
• Thirdly, the load distribution for glass running is similar to that for the nail-board effect. The body weight is not concentrated on a single shard edge, but is distributed over many edges.  
• Fourthly, not the entire sherd carpet and above all the concrete contact surface consists of sherd edges. Many shards also lie flat on the carpet, so that they can offer the feet a good and safe hold. 

It is precisely the interplay of these individual aspects that provides an initial basis for explanation. People who believe in physics like to hear simple explanations in which a phenomenon can be explained by a single reason. This is difficult when running glass. But the four phenomena mentioned above certainly offer a plausible explanatory grid.