The science behind Major League Baseball’s ‘magic mud’

As the Phillies gear up for the home opener, behind the scenes, another tradition takes place: each baseball is rubbed with the “magic mud,” a little-known ritual performed across Major League Baseball (MLB). But while the players of the sport have sworn by its effects for decades, researchers at the University of Pennsylvania have shown that not only does it work but also how.

In a study published last fall in the Proceedings of the National Academy of Sciences, researchers from the School of Engineering and Applied Science and the School of Arts & Sciences teamed up to quantify the mud’s legendary grip-enhancing properties. The study confirmed the mud improves grip through material properties and their interactions with a pitcher’s fingers.

The mud, which has been harvested by the Bintliff family at an undisclosed location along the Delaware River for nearly 90 years, “spreads like a skin cream but grips like sandpaper,” says Shravan Pradeep, a postdoctoral researcher in Penn’s School of Engineering and Applied Science.

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Douglas Jerolmack of Penn’s School of Arts & Sciences and Penn Engineering notes that the material has the right mixture to make three things happen: “spreading, gripping, and stickiness.”

Using an apparatus to mimic how human skin elasticity and oils rub mud-treated baseball leather, closely mirroring the friction a pitcher experiences, the researchers found that this mud isn’t just a lucky charm; it alters the ball’s surface in three crucial ways.

First, it fills in the microscopic pores of new leather, eliminating slick spots by laying down a thin, cohesive film. Then, the clay-rich film makes the surface slightly tacky, roughly doubling the grip (adhesion) between finger and ball. And finally, a sparse little sprinkle of tiny sand grains in the mud gets glued to the ball by the clay, creating a studded, abrasive texture that boosts friction like miniature cleats on the ball’s hide.

“This unexpected behavior of the mud, especially given its sandy constitution, intrigued us,” says Penn Engineering professor Paulo Arratia. He adds that the consistency and composition of the mud could very well vary from batch to batch each year, which has raised concerns within MLB. “They want the game to be as predictable and consistent as possible, so understanding this mud at a microscopic level can help achieve that,” he says.

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The researcher’s fundamental finding, being able to separate and tune the frictional versus attractive forces in a soil, can open the door to designing new materials with similarly “programmable” properties.

That broader vision is at the heart of what Pradeep calls “materials geomimicry,” an emerging approach to material science inspired by geology. Pradeep notes that the study underscores an emerging research area called Earth-inspired materials science, which explores how naturally occurring materials can outperform synthetic ones.

“We’re looking at soil as a material, and it has amazing, gripping properties which people have been using for years, but they never thought of it that way,” says Pradeep, noting how unique it is to treat humble dirt with the same respect as high-tech materials. And unlike synthetic alternatives, nature’s recipe has sustainability built in “because it comes from nature,” he says.

Jerolmack notes how “inherently green” and challenging to synthetically replicate the practice of using the mud is. “Nature perfected this formulation long before we began studying it,” he says.

The curious properties of the mud note the researchers could herald a new wave of materials science innovations.

Additional co-authors include Xiangyu Chen and Ali Seiphoori of the University of Pennsylvania and the Norwegian Geotechnical Institute, and David Vann of the University of Pennsylvania.

This study was conducted at the University of Pennsylvania School of Engineering and Applied Science and School of Arts & Sciences and supported in part by the National Science Foundation (NSF) Major Research Instrumentation Award (NSF-MRI-1920156), NSF Penn MRSEC (NSF-DMR-1720530), NSF Engineering Research Center for the Internet of Things for Precision Agriculture (NSF-EEC-1941529), NASA Planetary Science and Technology Through Analog Research Program (PSTAR Grant 80NSSC22K1313), Army Research Office (ARO Grant W911NF2010113), Penn Center for Soft and Living Matter Postdoctoral Fellowship, and the University of Pennsylvania’s Singh Center for Nanotechnology, a National Nanotechnology Coordinated Infrastructure (NNCI) member supported by NSF Grant ECCS-1542153.