The magic mud is applied to every ball used in Major League Baseball, including in this year’s World Series. (Credit: Mark Griffey)
In a nutshell
- Scientists at the University of Pennsylvania found that a unique mud’s blend of clay, silt, sand, and water creates a surface that’s both grippy and smooth, enhancing a pitcher’s ability to control the ball.
- The mud transforms the ball’s surface at multiple scales. It fills in pores to make the ball more uniform, leaves behind sticky clay particles that double adhesion, and embeds sand grains that increase friction—making the ball easier to grip and throw.
- This quirky tradition could inspire greener materials. The mud’s properties, spreading like skin cream and gripping like sandpaper, could be harnessed in eco-friendly alternatives for lubricants, traction aids, or construction materials, showing that old-school solutions can still lead to future innovation.
PHILADELPHIA — Professional baseball has a strange pre-game ritual that’s been happening for almost 100 years: rubbing each pristine white baseball with special mud harvested from a secret spot on a Delaware River tributary in New Jersey. This substance, known as Lena Blackburne Baseball Rubbing Mud, has been used to “de-gloss” new baseballs since 1938 when Philadelphia Athletics third-base coach Lena Blackburne found it near his home.
No one really knew why this specific mud worked so well for all these years. In a sport full of superstitions, was this mud actually helping pitchers grip the ball better, or was it just another baseball tradition that stuck around?
Scientists from the University of Pennsylvania recently tackled this muddy mystery, studying it through material science. Their findings, published in the Proceedings of the National Academy of Sciences, show that this ordinary-looking riverside mud has extraordinary properties that make it perfect for baseball.
What’s Actually in Baseball’s Secret Mud?
The research team put the mud through extensive testing to determine what makes it special. They discovered it was mostly silt and clay, with some sand and organic material mixed in. About 57% of the mud is solid particles, with the rest being water. The particles range from nanometer to millimeter sizes, with noticeably fewer particles larger than 169 micrometers, suggesting the mud is likely strained through a specific mesh during processing.
The clay and fine particles make the mud sticky, while the sand grains, which are angular and rough, create friction. Together, these components create a material with unique properties.
One of the mud’s most remarkable features is how it flows. When tested, the researchers found the mud exhibits extreme “shear-thinning” behavior, it gets less viscous when force is applied. Without pressure, it’s thicker than peanut butter, but when spread, it thins out to the consistency of cooking oil. This lets someone hold it like a solid but spread it into a thin, even coating on a baseball.
The mud creates a more uniform baseball surface by filling in pores in the leather. Using powerful microscopes, they saw that the mud fills in natural imperfections in the leather baseball surface. Ironically, this makes the surface smoother at large scales but rougher at small scales because of the tiny particles left behind.
How Mud Makes Baseballs Grippier
The mud boosts grip in three ways: First, it evens out the surface by filling pores. Second, the clay particles that remain after drying double the surface stickiness compared to clean baseballs. Third, sand grains become bonded to the surface, creating a texture that increases friction.
To measure these effects accurately, the researchers built a custom testing apparatus that mimics how human fingers interact with the baseball. They created artificial fingers using soft polymer material similar to human skin, coated them with squalene (an oil found in human skin), and tested how they interacted with clean and mudded baseballs.
The tests showed mudded baseballs have up to twice as much friction, especially at speeds relevant to pitching. At extremely high speeds and pressures, the particles could be knocked off, returning the ball to something closer to its clean state.
Why This Mud Matters
Felipe Macera)
Baseball has tried finding alternatives to this mud for years. The MLB has attempted to create “pre-tacked” baseballs several times, but none have worked well enough, forcing the league to stick with detailed instructions for proper mud application instead.
The Penn researchers think the mud’s effectiveness comes from the perfect balance of ingredients rather than anything exotic in its composition. The right proportions of clay, sand, and water create a unique material that would be hard to recreate or replace.
Similar natural materials could replace synthetic products for lubrication, improve traction on slippery surfaces, or act as binding agents in construction. The research also helps scientists better understand muddy materials, which could improve predictions for natural hazards like mudslides or help with moving through muddy environments.
Baseball players knew this mud worked well through experience, and now science confirms why. What seemed like just a quirky baseball tradition turns out to be a sophisticated material with properties that modern science is only beginning to fully understand.
Paper Summary
Methodology
The researchers analyzed the mud’s composition using laser diffraction for particle sizes, X-ray diffraction for minerals, and energy dispersive X-ray spectroscopy for elements. They used scanning electron microscopy and confocal laser scanning microscopy to create detailed images of baseball surfaces. Flow properties were measured with a rheometer that tested how the mud behaved under different forces. For friction tests, they created a custom apparatus with a soft polymer ball coated with skin oil to mimic a human finger gripping a baseball. Adhesion forces were measured using atomic force microscopy at the nanoscale level.
Results
The mud exhibited remarkable shear-thinning behavior, with viscosity inversely proportional to force across several orders of magnitude—from thicker than peanut butter to as thin as cooking oil when spread. Friction tests showed mudded baseballs had twice the friction coefficient of clean ones at pitching-relevant speeds. Adhesion measurements revealed mudded surfaces were twice as sticky as clean ones. Under microscopy, the mud created a more uniform surface by filling leather pores while simultaneously adding roughness at smaller scales through particle attachment.
Limitations
The study used a single container of Rubbing Mud purchased in 2022, which may not represent all batches. Their friction tests used synthetic materials that approximate but don’t exactly match real pitchers’ hands. The researchers didn’t compare this mud to samples from other locations, and their estimate of solid content relied on assumptions about particle density that might not fully account for the material’s mixed composition. The study focused on physical properties but didn’t investigate potential biological components that might affect performance.
Discussion and Takeaways
The mud’s effectiveness comes from the perfect balance of ingredients rather than exotic components. The right mix of clay, sand, and water creates a material that’s easy to spread but provides excellent grip once dried. This explains why baseball has stuck with natural mud despite attempts to create synthetic replacements. Beyond sports, these findings could inform development of sustainable materials for lubrication, traction improvement, or construction. Similar creek environments might produce suitable materials if properly processed by removing excess coarse material and adjusting water content.
Funding and Disclosures
The research received support from NSF, NASA, and Army Research Office grants, plus funding from the Penn Center for Soft and Living Matter. The researchers acknowledged sports journalist Matthew Gutierrez for bringing Rubbing Mud to their attention and consultations with baseball professionals from Driveline and the Philadelphia Phillies. No competing interests were declared.
Publication Information
The study, “Soft matter mechanics of baseball’s Rubbing Mud,” was authored by Shravan Pradeep, Xiangyu Chen, Ali Seiphoori, David R. Vann, Paulo E. Arratia, and Douglas J. Jerolmack from the University of Pennsylvania. The paper appeared in the Proceedings of the National Academy of Sciences on November 4, 2024 (Vol. 121, No. 47, e2413514121) and is available as an open-access article under Creative Commons Attribution License 4.0.
