Baseball’s Last Mile Problem

2022 has brought a constant barrage of players criticizing the baseballs as hard to grip and wildly inconsistent from inning to inning, and probably not coincidentally, a spike in throwing error rates to boot.  “Can’t get a grip” and “throwing error” do seem like they might go together.  MLB has denied any change in the manufacturing process, however there have been changes this season in how balls are handled in the stadium, and I believe that is likely to be the culprit.

I have a plausible explanation for how the new ball-handling protocol can cause even identical balls from identical humidors to turn out wildly different on the field, and it’s backed up by experiments and measurements I’ve done on several balls I have, but until those experiments can be repeated at an actual MLB facility (hint, hint), this is still just a hypothesis, albeit a pretty good one IMO.

Throwing Errors

First, to quantify the throwing errors, I used Throwing Errors + Assists as a proxy for attempted throws (it doesn’t count throws that are accurate but late, etc), and broke down TE/(TE+A) by infield position.

TE/(TE+A) 2022 2021 2011-20 max 21-22 Increase 2022 By Chance
C 9.70% 7.10% 9.19% 36.5% 1.9%
3B 3.61% 2.72% 3.16% 32.7% 0.8%
SS 2.20% 2.17% 2.21% 1.5% 46.9%
2B 1.40% 1.20% 1.36% 15.9% 20.1%

By Chance is the binomial odds of getting the 2022 rate or worse using 2021 as the true odds.  Not only are throwing errors per “opportunity” up over 2021, but they’re higher than every single season in the 10 years before that as well, and way higher for C and 3B.   C and 3B have the least time on average to establish a grip before throwing.  This would be interesting even without players complaining left and right about the grip.

The Last Mile

To explain what I suspect is causing this, I need to break down the baseball supply chain.  Baseballs are manufactured in a Rawlings factory, stored in conditions that, to the best of my knowledge, have never been made public, shipped to teams, sometimes stored again in unknown conditions outside a humidor, stored in a humidor for at least 2 weeks, and then prepared and used in a game.  Borrowing the term from telecommunications and delivery logistics, we’ll call everything after the 2+ weeks in the humidor the last mile.

Humidors were in use in 9 parks last year, and Meredith Wills has found that many of the balls this year are from the same batches as balls in 2021.  So we have literally some of the same balls in literally the same humidors, and there were no widespread grip complaints (or equivalent throwing error rates) in 2021.  This makes it rather likely that the difference, assuming there really is one, is occurring somewhere in the last mile.

The last mile starts with a baseball that has just spent 2+ weeks in the humidor.  That is long enough to equilibrate, per, other prior published research, and my own past experiments.  Getting atmospheric humidity changes to meaningfully affect the core of a baseball takes on the order of days to weeks.  That means that nothing humidity-wise in the last mile has any meaningful impact on the ball’s core because there’s not enough time for that to happen.

This article from the San Francisco Chronicle details how balls are prepared for a game after being removed from the humidor, and since that’s paywalled, a rough outline is:

  1. Removed from humidor at some point on gameday
  2. Rubbed with mud/water to remove gloss
  3. Reviewed by umpires
  4. Not kept out of the humidor for more than 2 hours
  5. Put in a security-sealed bag that’s only opened in the dugout when needed

While I don’t have 2022 game balls or official mud, I do have some 2019* balls, water, and dirt, so I decided to do some science at home.  Again, while I have confidence in my experiments done with my balls and my dirt, these aren’t exactly the same things being used in MLB, so it’s possible that what I found isn’t relevant to the 2022 questions.

Update: Dr. Wills informed that that 2019, and only 2019, had a production issue that resulted in squashed leather and could have affected the mudding results.  She checked my batch code, and it looks like my balls were made late enough in 2019 that they were actually used in 2020 with the non-problematic production method.  Yay.

Experiments With Water

When small amounts of water are rubbed on the surface of a ball, it absorbs pretty readily (the leather and laces love water), and once the external source of water is removed, that creates a situation where the outer edge of the ball is more moisture-rich than what’s slightly further inside and more moisture-rich than the atmosphere.  The water isn’t going to just stay there- it’s either going to evaporate off or start going slightly deeper into the leather as well.

As it turns out, if the baseball is rubbed with water and then stored with unrestricted air access (and no powered airflow) in the environment it was equilibrated with, the water entirely evaporates off fairly quickly with an excess-water half-life of a little over an hour (and this would likely be lower with powered air circulation) and goes right back to its pre-rub weight down to 0.01g precision.  So after a few hours, assuming you only added a reasonable amount of water to the surface (I was approaching 0.75 grams added at the most) and didn’t submerge the ball in a toilet or something ridiculous, you’d never know anything had happened.  These surface moisture changes are MUCH faster than the days-to-weeks timescales of core moisture changes.

Things get much more interesting if the ball is then kept in a higher-humidity environment.  I rubbed a ball down, wiped it with a paper towel, let it sit for a couple of minutes to deal with any surface droplets I missed, and then sealed the ball in a sandwich bag for 2 hours along with a battery-powered portable hygrometer.  I expected the ball to completely saturate the air while losing less mass than I could measure (<0.01g) in the process, but that’s not what actually happened.  The relative humidity in the bag only went up 7%, and as expected, the ball lost no measurable amount of mass.  After taking it out, it started losing mass with a slightly longer half-life than before and lost all the excess water in a few hours.

I repeated the experiment except this time I sealed the ball and the hygrometer in an otherwise empty 5-gallon pail.  Again, the relative humidity only went up 7%, and the ball lost 0.04 grams of mass.  I calculated that 0.02g of evaporation should have been sufficient to cause that humidity change, so I’m not exactly sure what happened- maybe 0.01 was measurement error (the scale I was using goes to 0.01g), maybe my seal wasn’t perfectly airtight, maybe the crud on the lid I couldn’t clean off or the pail itself absorbed a little moisture).  But the ball had 0.5g of excess water to lose (which it did completely lose after removal from the pail, as expected) and only lost 0.04g in the pail, so the basic idea is still the same.

This means that if the wet ball has restricted airflow, it’s going to take for freaking ever to reequilibrate (because it only takes a trivial amount of moisture loss to “saturate” a good-sized storage space), and that if it’s in a sealed environment or in a free-airflow environment more than 7% RH above what it’s equilibrated to, the excess moisture will travel inward to more of the leather instead of evaporating off (and eventually the entire ball would equilibrate to the higher-RH environment, but we’re only concerned with the high-RH environment as a temporary last-mile storage condition here, so that won’t happen on our timescales).

I also ran the experiment sealing the rubbed ball and the hygrometer in a sandwich bag overnight for 8 hours.  The half-life for losing moisture after that was around 2.5 hours, up from the 70 minutes when it was never sealed.  This confirms that the excess moisture doesn’t just sit around at the surface waiting if it can’t immediately evaporate, but that evaporation dominates when possible.

I also ran the experiment with a ball sealed in a sandwich bag for 2 hours along with an equilibrated cardboard divider that came with the box of balls I have.  That didn’t make much difference. The cardboard only absorbed 0.04g of the ~0.5g excess moisture in that time period, and that’s with a higher cardboard:ball ratio than a box actually comes with.  Equilibrated cardboard can’t substitute for free airflow on the timescale of a couple of hours.

Experiments With Mud

I mixed dirt and water to make my own mud and rubbed it in doing my best imitation of videos I could find, rubbing until the surface of the ball felt dry again.  Since I don’t have any kind of instrument to measure slickness, these are my perceptions plus those of my significant other.  We were in almost full agreement on every ball, and the one disagreement converged on the next measurement 30 minutes later.

If stored with unrestricted airflow in the environment it was equilibrated to, this led to roughly the following timeline:

  1. t=0, mudded, ball surface feels dry
  2. t= 30 minutes, ball surface feels moist and is worse than when it was first mudded.
  3. t=60 minutes, ball surface is drier and is similar in grip to when first mudded.
  4. t=90 minutes, ball is significantly better than when first mudded
  5. t=120 minutes, no noticeable change from t=90 minutes.
  6. T=12 hours, no noticeable change from t=120 minutes

I tested a couple of other things as well

  1. I took a 12-hour ball, put it in a 75% RH environment for an hour and then a 100% RH environment for 30 minutes, and it didn’t matter.  The ball surface was still fine.  The ball would certainly go to hell eventually under those conditions, but it doesn’t seem likely to be a concern with anything resembling current protocols.  I also stuck one in a bag for awhile and it didn’t affect the surface or change the RH at all, as expected since all of the excess moisture was already gone.
  2. I mudded a ball, let it sit out for 15 minutes, and then sealed it in a sandwich bag.  This ball was slippery at every time interval, 1 hour, 2 hours, 12 hours. (repeated twice).  Interestingly, putting the ball back in its normal environment for over 24 hours didn’t help much and it was still quite slippery.  Even with all the excess moisture gone, whatever had happened to the surface while bagged had ruined the ball.
  3. I mudded a ball, let it sit out for 2 hours, at which point the surface was quite good per the timeline above, and then sealed it in a bag.  THE RH WENT UP AND THE BALL TURNED SLIPPERY, WORSE THAN WHEN IT WAS FIRST MUDDED. (repeated 3x).  Like #2, time in the normal environment afterwards didn’t help.  Keeping the ball in its proper environment for 2 hours, sealing it for an hour, and then letting it out again was enough to ruin the ball.

That’s really important IMO.  We know from the water experiments that it takes more than 2 hours to lose the excess moisture under my storage conditions, and it looks like the combination of fresh(ish) mud plus excess surface moisture that can’t evaporate off is a really bad combo and a recipe for slippery balls.  Ball surfaces can feel perfectly good and game-ready while they still have some excess moisture left and then go to complete shit, apparently permanently, in under an hour if the evaporation isn’t allowed to finish.

Could this be the cause of the throwing errors and reported grip problems? Well…

2022 Last-Mile Protocol Changes

The first change for 2022 is that balls must be rubbed with mud on gameday, meaning they’re always taking on that surface moisture on gameday.  In 2021, balls had to be mudded at least 24 hours in advance of the game, and while 2021 changed the window to 1-2 days in advance, the window used to be up to 5 days in advance of the game.  I don’t know how far in advance they were regularly mudded before 2021, but even early afternoon for a night game would be fine assuming the afternoon storage had reasonable airflow.

The second change is that they’re put back in the humidor fairly quickly after being mudded and allowed a maximum of 2 hours out of the humidor.  While I don’t think there’s anything inherently wrong with putting the balls back in the humidor after mudding (unless it’s something specific to 2022 balls), humidors look something like this.  If the balls are kept in a closed box, or an open box with another box right on top of them, there’s little chance that they reequilibrate in time.  If they’re kept in an open box on a middle shelf without much room above, unless the air is really whipping around in there, the excess moisture half-life should increase.

There’s also a chance that something could go wrong if the balls are taken out of the humidor, kept in a wildly different environment for an hour, then mudded and put back in the humidor, but I haven’t investigated that, and there are many possible combinations of both humidity and temperature that would need to be checked for problems.

The third change (at least I think it’s a change) is that the balls are kept in a sealed bag- at least highly restricted flow, possibly almost airtight- until opened in the dugout.  Even if it’s not a change, it’s still extremely relevant- sealing balls that have evaporated their excess moisture off doesn’t affect anything, while sealing balls that haven’t finished evaporating off seems to be a disaster.


Mudding adds excess moisture to the surface of the ball, and if its evaporation is prevented for very long- either through restricted airflow or storage in too humid an environment- the surface of the ball becomes much more slippery and stays that way even if evaporation continues later.  It takes hours- dependent on various parameters- for that moisture to evaporate off, and 2022 protocol changes make it much more likely that the balls don’t get enough time to evaporate off, causing them to fall victim to that slipperiness.  In particular, balls can feel perfectly good and ready while they still have some excess surface moisture and then quickly go to hell if the remaining evaporation is prevented inside the security-sealed bag.

It looks to me like MLB had a potential problem- substantial latent excess surface moisture being unable to evaporate and causing slipperiness- that prior to 2022 it was avoiding completely by chance or by following old procedures derived from lost knowledge.   In an attempt to standardize procedures, MLB accidentally made the excess surface moisture problem a reality, and not only that, did it in a way where the amount of excess surface moisture was highly variable.

The excess surface moisture when a ball gets to a pitcher depends on the amount of moisture initially absorbed, the airflow and humidity of the last-mile storage environment, and the amount of time spent in those environments and in the sealed bag.  None of those are standardized parts of the protocol, and it’s easy to see how there would be wide variability ball-to-ball and game-to-game.

Assuming this is actually what’s happening, the fix is fairly easy.  Balls need to be mudded far enough in advance and stored afterwards in a way that they get sufficient airflow for long enough to reequilibrate (the exact minimum times depending on measurements done in real MLB facilities), but as an easy interim fix, going back to mudding the day before the game and leaving those balls in an open uncovered box in the humidor overnight should be more than sufficient.  (and again, checking that on-site is pretty easy)


I found (or didn’t find) some other things that I may as well list here as well along with some comments.

  1. These surface moisture changes don’t change the circumference of the baseball at all, down to 0.5mm precision, even after 8 hours.
  2. I took a ball that had stayed moisturized for 2 hours and put a 5-pound weight on it for an hour.  There was no visible distortion and the circumference was exactly the same as before along both seam axes (I oriented the pressure along one seam axis and perpendicular to the other).  To whatever extent flat-spotting is happening or happening more this season, I don’t see how it can be a last-mile cause, at least with my balls.  Dr. Wills has mentioned that the new balls seem uniquely bad at flat-spotting, so it’s not completely impossible that a moist new ball at the bottom of a bucket could deform under the weight, but I’d still be pretty surprised.
  3. The ball feels squishier to me after being/staying moisturized, and free pieces of leather from dissected balls are indisputably much squishier when equilibrated to higher humidity, but “feels squishier” isn’t a quantified measurement or an assessment of in-game impact.  The squishy-ball complaints may also be another symptom of unfinished evaporation.
  4. I have no idea if the surface squishiness in 3 affects the COR of the ball to a measurable degree.
  5. I have no idea if the excess moisture results in an increased drag coefficient.  We’re talking about changes to the surface, and my prior dissected-ball experiments showed that the laces love water and expand from it, so it’s at least in the realm of possibility.
  6. For the third time, this is a hypothesis.  I think it’s definitely one worth investigating since it’s supported by physical evidence, lines up with the protocol changes this year, and is easy enough to check with access to actual MLB facilities.  I’m confident in my findings as reported, but since I’m not using current balls or official mud, this mechanism could also turn out to have absolutely nothing to do with the 2022 game.

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