Baseball has a muddy secret. For over 80 years, a mysterious mud sourced from an undisclosed New Jersey river location has become an essential part of every Major League Baseball game.

Known as “Rubbing Mud,” this substance is applied to every new baseball, giving it a subtle, gritty texture that pitchers find crucial for grip. Now, scientists have explored exactly what makes this mud so special.

The history of Rubbing Mud dates back to 1938, when Lena Blackburne, a coach with the Philadelphia Athletics, discovered the mud’s “de-glossing” properties.

If you’ve ever played with a new, pristine baseball, you’ve probably noticed it too: somehow, it just doesn’t seem to have enough grip. It’s silky and doesn’t feel grippy.

Even back in the early 1900s, players would treat the balls with things ranging from shoe polish and tobacco juice to water and soil from the infield to make them grippier. But when the rubbing mud came along, it became ubiquitous. Nowadays, it’s even a requirement for Major League games, with each team applying the mud to baseballs to remove the factory gloss and create roughness that improves grip.

A new study by scientists at the University of Pennsylvania analyzed the mud’s exact composition and mechanical qualities, shedding light on a century-old baseball ritual and confirming what players have been saying all along: the mud really makes a difference.

A soft matter mystery

Rubbing Mud’s effectiveness had been anecdotal, sustained by baseball superstition and tradition. But it’s backed so strongly that every time the league tried different substitutes, players kept coming back to the New Jersey mud. So, what makes this mud so special?

Shravan Pradeep and colleagues use a series of mechanical tests and advanced imaging techniques to understand how the mud interacts with baseballs. Their findings reveal that Rubbing Mud alters the baseball’s surface through a unique combination of properties that yields an excellent grip for pitches.

At a microscopic level, the mud contains a balanced mixture of fine clay and silt particles, cohesive organic materials, and a touch of angular sand grains, all suspended in water. This unique composition is key to its effectiveness.

It features just the right combination of cohesion and adhesion that it’s spreadable and uniform. And it has just the right friction to add grip without feeling too rough. Essentially, small angular grains are suspended within the mud. When the mud dries out, these grains create a gritty texture. The clay particles also fill in the small leather pores that can create the “silky” effect that pitchers dislike.

“It spreads like a skin cream and grips like sandpaper,” says Shravan Pradeep, the paper’s first author

Mud science

The research team employed a range of techniques to investigate the mud’s rheological (flow) and tribological (friction) behaviors. These properties define how a material spreads, adheres, and interacts with surfaces.

Using shear rheometry, which measures how materials flow under stress, the team discovered that Rubbing Mud has what scientists call “shear-thinning” qualities. This means that when it’s applied with force, the mud’s viscosity decreases, allowing it to spread smoothly over the baseball surface. Once spread and left to dry, it returns to a more solid, slightly gritty texture that gives pitchers a reliable grip.

To understand how Rubbing Mud enhances friction, researchers tested the mud under conditions that replicate pitching, including varied pressure and motion. Their results showed that its friction increases with speed, peaking when a pitcher throws fast. Beyond a certain speed, the mud begins to wear away, returning the baseball’s surface to its clean, factory-smooth state.

This dual action of increased friction and adhesion is rare among natural materials, making Rubbing Mud an exceptional example of a sustainable, high-performance soft material.

Could we make synthetic mud?

At the moment, it’s not clear how much of this mud there’s left or if it’s harvested sustainably. However, baseball fans and players alike have long wondered if MLB’s Rubbing Mud could be replaced by a synthetic equivalent. Judging by what this study found, the mud’s unique properties are difficult to reproduce synthetically. You could, at least in theory, create a mud with similar properties, but it’s going to be challenging and costly — definitely more costly than simply harvesting the mud.

In recent years, Major League Baseball has tried alternatives like “pre-tacked” baseballs, which aim to provide grip without the need for mud. However, none have matched the natural characteristics of Rubbing Mud, and players have been reluctant to embrace them. This study supports the players’ preference, showing that Rubbing Mud’s grip-enhancing qualities are challenging to replicate.

So, it’s unlikely that we’ll have synthetic mud in the very near future. However, having an entire league rely on an undisclosed mud source is bound to pose challenges in the long run. After all, a single source that’s kept secret is a precarious foundation for a billion-dollar sports league. If something were to disrupt the supply chain — environmental changes, contamination, or simply running out of the material — the sport would face an unexpected crisis.

But the pressure to innovate remains. As science continues to advance, it’s possible that researchers may eventually unlock a formula that mimics the mud’s unique combination of properties.

Until that day, however, the ritual of mudding baseballs will continue.

The study was published in the Proceedings of the National Academy of Sciences (PNAS).