Since the 1940s, baseball players have been spreading a special kind of "magic mud" on new baseballs to reduce the slick, glossy shine and give pitchers a firmer grip. Now, scientists at the University of Pennsylvania have identified just what gives that magic mud its special properties, according to a new paper published in the Proceedings of the National Academy of Sciences.

Before magic mud came along, baseballs were treated with a mix of water and soil from the infield or, alternatively, tobacco juice or shoe polish. But these substances stained and scratched up the ball's leather surface. Lena Blackburne was a third-base coach for the Philadelphia Athletics in the 1930s when an umpire complained about that, so he hunted for a better mud. Blackburne found that mud in a still-secret location purportedly near Palmyra, New Jersey, and a baseball dynasty was born: Lena Blackburne Baseball Rubbing Mud. Once harvested, the mud is strained, skimmed of excess water, rinsed with tap water, and then subjected to a secret "proprietary treatment" before being allowed to settle.

Yet there hasn't been much scientific research on the magic mud apart from one 2022 study. We do know quite a bit about the complex behavior of soil in general, including mud. Per the authors, mud is essentially "a dense suspension of predominantly clay and silt particles in water," sometimes with a bit of sand in the mix, although this has little effect on how mud behaves under shearing forces (rheology). Technically, it falls into the non-Newtonian fluid category, in which the viscosity changes (either thickening or thinning) in response to an applied strain or shearing force, thereby straddling the boundary between liquid and solid behavior.

Mud's rheological properties have been studied in the context of natural hazards like mud slides and debris flows, as well as for dredging and navigation. All those particles clump into aggregates. In shear-thinning, a shearing force will break up those aggregates and align the particles with the flow; the viscosity will decrease as the shear rate increases.