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.
Rheology is relevant to how magic mud can smear and coat a baseball. Another factor is how friction and adhesion come into play on the surface of the baseball once the magic mud has dried—its tribology. Most prior research has focused on friction in granular media like sand, although there has also been some work on how dried mud increases adhesion and friction on glass surfaces—relevant to how dust deposits on solar panels in humid environments.
Mud-slinging in the lab
So when a sportswriter approached Shravan Pradeep of the University of Pennsylvania about analyzing the magic mud for insight into its composition and flow behavior, Pradeep decided to conduct a set of experiments. Based on prior research on mud, he and his co-authors thought magic mud would exhibit shear-thinning behavior when spread into a thin, uniform coating on a baseball. And as the mud dried, the particles would adhere to the surface, increasing friction.
To test that hypothesis, Pradeep et al. conducted three sets of experiments, each designed to look at a different complementary factor. One looked at the mud's spreading behavior (rheology); one measured how sticky it is and why; and the third investigated what effect the mud has on the friction between the baseball and a pitcher's fingertips. They used a single container of Lena Blackburne Baseball Rubbing Mud purchased in 2022 and tested the mud using Major League Baseball-licensed baseballs.
In the first experiment, the authors smeared the mud between two plates and then rotated them, measuring the changes in viscosity with a rheometer. In the second, they used an atomic force microscope to peer at the atomic structure of the material to learn more about what makes it sticky. The third experiment required a bit of ingenuity in terms of building the apparatus. They mounted pieces of mudded baseball leather on acrylic base plates and then lowered a ball to contact the surface. At first, they used a steel ball, but it didn't have the same elastic properties as human skin. So they made their own ball out of PDMS, carefully tuned to the same elasticity, and coated it with synthetic squalene to mimic the secretion of sebum on the fingers by human oil glands.
Pradeep et al. found that magic mud's particles are primarily silt and clay, with a bit of sand and organic material. The stickiness comes from the clay, silt, and organic matter, while the sand makes it gritty. So the mud "has the properties of skin cream," they wrote. "This allows it to be held in the hand like a solid but also spread easily to penetrate pores and make a very thin coating on the baseball."
When the mud dries on the baseball, however, the residue left behind is not like skin cream. That's due to the angular sand particles bonded to the baseball by the clay, which can increase surface friction by as much as a factor of two. Meanwhile, the finer particles double the adhesion. "The relative proportions of cohesive particulates, frictional sand, and water conspire to make a material that flows like skin cream but grips like sandpaper," they wrote.
Despite its relatively mundane components, the magic mud nonetheless shows remarkable mechanical behaviors that the authors think would make it useful in other practical applications. For instance, it might replace synthetic materials as an effective lubricant, provided the gritty sand particles are removed. Or it could be used as a friction agent to improve traction on slippery surfaces, provided one could define the optimal fraction of sand content that wouldn't diminish its spreadability. Or it might be used as a binding agent in locally sourced geomaterials for construction.
"As for the future of Rubbing Mud in Major League Baseball, unraveling the mystery of its behavior does not and should not necessarily lead to a synthetic replacement," the authors concluded. "We rather believe the opposite; Rubbing Mud is a nature-based material that is replenished by the tides, and only small quantities are needed for great effect. In a world that is turning toward green solutions, this seemingly antiquated baseball tradition provides a glimpse of a future of Earth-inspired materials science."
DOI: PNAS, 2024. 10.1073/pnas.241351412 (About DOIs).
