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My prior research on the slippery ball problem: Baseball’s Last Mile Problem

The TL;DR is that mudding adds moisture to the surface of the ball.  Under normal conditions (i.e. stored with free airflow where it was stored before mudding), that moisture evaporates off in a few hours and leaves a good ball.  If that evaporation is stopped, the ball goes to complete hell and becomes more slippery than a new ball.  This is not fixed by time in free airflow afterwards.

My hypothesis is that the balls were sometimes getting stored in environments with sufficiently restricted airflow (the nylon ball bag) too soon after mudding, and that stopped the evaporation.  This only became a problem this season with the change to mudding all balls on gameday and storing them in a zipped nylon bag before the game.

MLB released a new memo yesterday that attempts to standardize the mudding and storage procedure.  Of the five bullet points, one (AFAIK) is not a change.  Balls were already supposed to sit in the humidor for at least 14 days.  Attempting to standardize the application procedure and providing a poster with allowable darkness/lightness levels are obviously good things.  It may be relevant here if the only problem balls were the muddiest (aka wettest) which shouldn’t happen anymore, but from anecdotal reports, there were problem balls where players didn’t think the balls were even mudded at all, and unless they’re blind, that seems hard to reconcile with also being too dark/too heavily mudded.  So this may help some balls, but probably not all of them.

Gameday Mudding

The other points are more interesting.  Requiring all balls to be mudded within 3 hours of each other could be good or bad.  If it eliminates stragglers getting mudded late, this is good.  If it pushes all mudding closer to gametime, this is bad.  Either way, unless MLB knows something I don’t (which is certainly possible- they’re a business worth billions and I’m one guy doing science in my kitchen), the whole gameday mudding thing makes *absolutely no sense* to me at all in any way.

Pre-mudding, all balls everywhere** are all equilibrated in the humidor the same way.  Post-mudding, the surface is disrupted with transient excess moisture.  If you want the balls restandardized for the game, then YOU MAKE SURE YOU GIVE THE BALL SURFACE TIME AFTER MUDDING TO REEQUILIBRATE TO A STANDARD ENVIRONMENT BEFORE DOING ANYTHING ELSE WITH THE BALL. And that takes hours.

In a world without universal humidors, gameday mudding might make sense since later storage could be widely divergent.  Now, it’s exactly the same everywhere**.  Unless MLB has evidence that a mudded ball sitting overnight in the humidor goes to hell (and I tested and found no evidence for that at all, but obviously my testing at home isn’t world-class- also, if it’s a problem, it should have shown up frequently in humidor parks before this season), I have no idea why you would mud on gameday instead of the day before like it was done last season.  The evaporation time between mudding and going in  the nylon bag for the game might not be long enough if mudding is done on gameday, but mudding the day before means it definitely is.

Ball Bag Changes

Cleaning the ball bag seems like it can’t hurt anything, but I’m also not sure it helps anything. I’m guessing that ball bag hygiene over all levels of the sport and prior seasons of MLB was generally pretty bad, yet somehow it was never a problem.  They’ve seen the bottom of the bags though.  I haven’t. If there’s something going on there, I’d expect it to be a symptom of something else and not a primary problem.

Limiting to 96 balls per bag is also kind of strange.  If there is something real about the bottom of the bag effect, I’d expect it to be *the bottom of the bag effect*.  As long as the number of balls is sufficient to require a tall stack in the bag (and 96 still is), and since compression at these number ranges doesn’t seem relevant (prior research post), I don’t have a physical model of what could be going on that would make much difference for being ball 120 of 120 vs. ball 96 of 96.  Also, if the bottom of the bag effect really is a primary problem this year, why wasn’t it a problem in the past?  Unless they’re using entirely new types of bags this season, which I haven’t seen mentioned, we should have seen it before.  But I’m theorizing and they may have been testing, so treat that paragraph with an appropriate level of skepticism.

Also, since MLB uses more than 96 balls on average in a game, this means that balls will need to come from multiple batches.  This seemed like it had the potential to be significantly bad (late-inning balls being stored in a different bag for much longer), but according to an AP report on the memo

This seems generally like a step in the smart direction, like they’d identified being zipped up in the bag as a potential problem (or gotten the idea from reading my previous post from 30 days ago :)).  I don’t know if it’s a sufficient mitigation because I don’t know exactly how long it takes for the balls to go to hell (60 minutes in near airtight made them complete garbage, so damage certainly appears in less time, but I don’t know how fast and can’t quickly test that).  And again, repeating the mantra from before, time spent in the ball bag *is only an issue if the balls haven’t evaporated off after mudding*.  And that problem is slam-dunk guaranteed solvable by mudding the day before, and then this whole section would be irrelevant.

Box Storage

The final point, “all balls should be placed back in the Rawlings boxes with dividers, and the boxes should then be placed in the humidor. In the past, balls were allowed to go directly into the humidor.” could be either extremely important or absolutely nothing.  This doesn’t say whether the boxes should be open or closed (have the box top on) in the humidor.  I tweeted to the ESPN writer and didn’t get an answer.

The boxes can be seen in the two images in https://www.mlb.com/news/rockies-humidor-stories.  If they’re open (and not stacked or otherwise covered to restrict airflow), this is fine and at least as good as whatever was done before today.  If the boxes are closed, it could be a real problem.  Like the nylon ball bag, this is also a restricted-flow environment, and unlike the nylon ball bag, some balls will *definitely* get in the box before they’ve had time to evaporate off (since they go in shortly after mudding)

I have one Rawlings box without all the dividers.  The box isn’t airtight, but it’s hugely restricted airflow.  I put 3 moistened balls in the box along with a hygrometer and the RH increased 5% and the balls lost moisture about half as fast as they did in free air.  The box itself absorbed no relevant amount.  With 6 moistened balls in the box, the RH increased 7% (the maximum moistened balls in a confined space will do per prior research) and they lost moisture between 1/3 and 1/4 as fast as in free air.

Unlike the experiments in the previous post where the balls were literally sealed, there is still some moisture flux off the surface here.  I don’t know if it’s enough to stop the balls from going to hell.  It would take me weeks to get unmudded equilibrated balls to actually do mudding test runs in a closed box, and I only found out about this change yesterday with everybody else.  Even if the flux is still sufficient to avoid the balls going to hell directly, the evaporation time appears to be lengthened significantly, and that means that balls are more likely to make it into the closed nylon bag before they’ve evaporated off, which could also cause problems at that point (if there’s still enough time for problems there- see previous section).

The 3 and 6 ball experiments are one run each, in my ball box, which may have a better or worse seal than the average Rawlings box, and the dividers may matter (although they don’t seem to absorb very much moisture from the air, prior post), etc.  Error bars are fairly wide on the relative rates of evaporation, but hygrometer don’t lie.  There doesn’t seem to be any way a closed box isn’t measurably restricting airflow and increasing humidity inside unless the box design changed a lot in the last 3 years.  Maybe that humidity increase/restricted airflow isn’t enough to matter directly or indirectly, but it’s a complete negative freeroll.  Nothing good can come of it.  Bad things might.  If there are reports somewhere this week that tons of balls were garbage, closed-box storage after mudding is the likely culprit.  Or the instructions will actually be uncovered open box (and obeyed) and the last 5 paragraphs will be completely irrelevant.  That would be good.

Conclusion: A few of the changes are obviously common-sense good.  Gameday mudding continues to make no sense to me and looks like it’s just asking for trouble.  Box storage in the humidor after mudding, if the boxes are closed, may be introducing a new problem. It’s unclear to me if the new ball-bag procedures reduce time sufficiently to prevent restricted-airflow problems from arising there, although it’s at least clearly a considered attempt to mitigate a potential 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.

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 https://tht.fangraphs.com/the-physics-of-cheating-baseballs-humidors/, 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.

Conclusion

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)

Notes

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.

As of this writing on 4/25/2022, HRs are down, damage and distance on barrels are down, and both Alan Nathan and Rob Arthur have observed that the drag coefficient of baseballs this year is substantially increased.  This has led to speculation about what has changed with the 2022 balls and even what production batch of balls or mixture of batches may be in use this year.  Given the kerfluffle last year that resulted in MLB finally confirming that a mix of 2020 and 2021 balls were used during the season, that speculation is certainly reasonable.

It may well also turn out to be correct, and changes in the 2022 ball manufacture could certainly explain the current stats, but I think it’s worth noting that everything we’ve seen so far is ALSO consistent with “absolutely nothing changed with regard to ball manufacture/end product between 2021 and 2022” and “all or a vast majority of balls being used are from 2021 or 2022”.  

How is that possible?  Well, the 2021 baseball production was changed on purpose.  The new baseball was lighter, less dense, and less bouncy by design, or in more scientific terms, “dead”.  What if all we’re seeing now is the 2021 baseball specifications in their true glory, now untainted by the 2020 live balls that were mixed in last year?

Even without any change to the surface of the baseball, a lighter, less dense ball won’t carry as far.  The drag force is independent of the mass (for a given size, which changed less if at all), and F=MA, so a constant force and a lower mass means a higher drag deceleration and less carry.

The aforementioned measurements of the drag coefficient from Statcast data also *don’t measure the drag coefficient*.  They measure the drag *acceleration* and use an average baseball mass value to convert to the drag force (which is then used to get the drag coefficient).  If they’re using the same average mass for a now-lighter ball, they’re overestimating the drag force and the drag coefficient, and the drag coefficient may literally not have changed at all (while the drag acceleration did go up, per the previous paragraph).

Furthermore, I looked at pitchers who threw at least 50 four-seam fastballs last year after July 1, 2021 (after the sticky stuff crackdown) and have also thrown at least 50 FFs in 2022.  This group is, on average, -0.35 MPH and +0.175 RPM on their pitches.  These stats usually move in the same direction, and a 1 MPH increase “should” increase spin by about 20 RPM.  So the group should have lost around 7 RPM from decreased velocity and actually wound up slightly positive instead.  It’s possible that the current baseball is just easier to spin based on surface characteristics, but it’s also possible that it’s easier to spin because it’s lighter and has less rotational inertia.  None of this is proof, and until we have results from experiments on actual game balls in the wild, we won’t have a great idea of the what or the why behind the drag acceleration being up. 

(It’s not (just) the humidor- drag acceleration is up even in parks that already had a humidor, and in places where a new humidor would add some mass, making the ball heavier is the exact opposite of what’s needed to match drag observations, although being in the humidor could have other effects as well)