HIGHLIGHTS
• A study was initiated to better understand how Rubbing Mud used by Major League Baseball assists pitchers with throwing a baseball.
• Application of rubbing mud to a baseball produces a smooth, uniform surface that leads to an increase in friction.
• Tribology studies found that significantly higher friction was observed with mudded balls compared to a clean ball.

As those of you who read this column know, we have dealt with a variety of topics with relevance to tribology because this subject we are all involved in has a wide influence on our lives both personally and professionally. This covers a myriad of automotive and industrial applications.

Throwing a baseball professionally continues to be an enormous challenge for the pitcher. As speeds have increased and orientations continue to change, pitchers are trying to figure out how they can best control the baseball and place it where they want in relation to the batter. After all, the objective is to fool the batter into swinging at a pitch that will be very difficult to hit.

One of the unknown materials that plays a role (or does it?) in this process is rubbing mud. For the past 70 years (since the 1950s), pitchers in professional baseball in the U.S. have been using a material known as Lena Blackburne Baseball Rubbing Mud to better grip the baseball. 

Images of a clean baseball (on the left) and a mudded baseball (on the right) are found in Figure 1. 


Figure 1. Rubbing mud on a clean baseball (on the left) produces mudded ball (on the right) that provides pitchers with a greater ability to grip the ball. Figure courtesy of the University of Pennsylvania.

Dr. Sharavan Pradeep, postdoctoral researcher in the department of earth and environmental science at the University of Pennsylvania in Philadelphia, Penn., says, “The Rubbing Mud used by professional baseball is sourced from a specific location near Philadelphia. The mud is obtained from an estuary tributary of the Delaware River in a secret location in the U.S. state of New Jersey.”

Once sourced, the mud is then processed in a proprietary fashion to generate the Rubbing Mud used. Major League Baseball, the organization running professional baseball in the U.S., has tried to figure out an approach for making a synthetic version of Rubbing Mud. Pradeep indicated that his colleagues were originally asked by a sportswriter to investigate what is so special about this mud. 

Unfortunately, none of the alternatives have proven to be as effective as the Rubbing Mud. Pradeep says, “As we started to better understand the unique properties of Rubbing Mud, our first thought was whether its effectiveness is due to superstition. Baseball is a game that is steeped in many traditions and players tend to have specific routines that can border on superstition.”

To gain a better understanding of Rubbing Mud, the first step is to gain information about its composition. Pradeep says, “Mud is a mixture of clay, sand and silt particles in water in combination with minor amounts of organic matter, and adequate moisture. The two main components of interest are clay and sand.”

The researchers focused on studying the rheology and tribology of Rubbing Mud on a baseball. Pradeep explains, “Rheology is important because studying yield and flow behavior under shear should determine the effectiveness of the Rubbing Mud in spreading and coating a baseball. After drying, tribology needs to be examined to find out how the presence of mud changes the surface texture and modifies the ability of a human hand to better grip the baseball due to an increase in friction.” 

Soft tribology apparatus
Prior to conducting an in-depth analysis, the researchers speculated that Rubbing Mud has shear thinning behavior which leads to forming a thin and uniform coating. This enables the clay and organic content to enhance adhesion, once dried. Increased friction which would allow for a better grip to the baseball is due to the bonding of sand-sized particles by clay-sized particles as the mud dries. 

The study compared a clean ball to a muddied ball. 

Pradeep says, “Initial analysis by scanning electron microscopy (SEM) found that the clean ball displays valleys making the surface rough. Application of the Rubbing Mud smooths the ball surface as the clay fills the valleys leading to a smooth, uniform surface. Further work with confocal laser scanning microscopy (CLSM) identified the small number of angular sand grains bonded to the surface of the ball that leads to an increase in friction. The reason for this observation is that only at high sliding speeds could the sand be removed.”

Surface friction measurements were conducted by designing and attaching a custom ball-on-plate soft tribology apparatus to an advanced strain and stress control rheometer (see Figure 2). A constant normal force was applied to the sample which was then sheared at a specific sliding speed to measure the shear force. Effective friction was then calculated as the ratio of shear force to normal force. 


Figure 2. Attaching a custom ball-on-plant soft tribology apparatus to an advanced strain and stress control rheometer was used to evaluate the difference in friction between a mudded ball and a clean ball. Figure courtesy of the University of Pennsylvania.

To evaluate friction and come close to how an actual human hand interacts with the baseball, the researchers used a ball made of polydimethylsiloxane, which mimics the elastic properties of skin. The ball is then lowered to contact the baseball surface. Initial testing produced abrasion of the ball surface so the researchers added a squalene coating to minimize wear. Squalene was selected because it has the same viscosity, mimicking sebum secreted by human oil glands. 

Significantly higher friction was observed with the mudded balls with a peak value of 0.6 achieved at a sliding speed of 10 millimeters per second. In contrast, friction for the clean ball was 0.33 over a wide range of sliding speeds. 

Pradeep says, “The added friction is due to the larger particles in the clay matrix of the Rubbing Mud applied to the baseball. Use of atomic force microscopy showed that clay imposed more than twice the adhesive force than was found with a clean baseball.”

Pradeep adds, “Our conclusion from this study is that Rubbing Mud did not have any magic chemical constituent. Its perfect composition of clay and sand produced the higher friction and adhesive forces that allow the pitcher to better grip the baseball.”

The researchers hypothesize that Rubbing Mud could be a very effective lubricant if the gritty sand is removed. Pradeep says, “The fact that Rubbing Mud is shear thinning and produces a smooth surface means there is potential for use as a lubricant. Future work will involve studying Rubbing Mud’s lubricant properties using different clay particle levels and working with mixed clays.”

Additional information on this work can be found in a recent article¹ or by contacting Douglas Jerolmack, Edmund J. and Louise W. Kahn Endowed Term Professor of Earth and Environmental Science at the University of Pennsylvania at sediment@seas.upenn.edu or Paulo Arratia, Eduardo D. Ghandi, Distinguished Scholar and Professor of Mechanical Engineering and Applied Mechanics at the University of Pennsylvania at parratia@seas.upenn.edu.

REFERENCES
1. Pradeep, S., Chen, X., Seiphoori, A., Vann, D., Arratia, P. and Jerolmack, D. (2024), “Soft matter mechanics of baseball’s Rubbing Mud,” Proceedings of the National Academy of Sciences, 121 (47), e2413514121.