Spitball

Stan Covaleski

"A spitball is an illegal baseball pitch in which the ball has been altered by the application of saliva, petroleum jelly, or some other foreign substance. Such a pitch presents an additional challenge to the hitter because it causes the ball to move atypically during its approach due to the altered wind-resistance and weight on one side of the ball. There is also thought that a spitball works by allowing the ball to 'slip' off of the pitcher's fingers withouth the usual spin that accompanies a pitcher's throwing off of the threads. In this sense, a spitball can be thought of as a fastball with knuckleball action. (See Bob Adair's analysis for example). Alternative names for the spitball are spitter, mud ball, shine ball, supersinker, vaseline ball (because originally, Vaseline was used to give the ball a little more break), and emery ball, although technically, an emery ball is one where the ball has been abraded in much the same way that the original cut ball had been physically cut (an emery ball is also known as a scuff ball)."
Wikipedia

"A spitball (aka spitter, wet one, or unsanitary pitch) is a pitch in which the pitcher applies saliva to the baseball, either to change its aerodynamic properties or to reduce friction between his fingers and the ball. The term is sometimes applied loosely to pitches in which the ball is treated with other foreign substance, such as vaseline, and occasionally to any type of pitch that involves doctoring the baseball. The spitball rose to prominence in the early 1900s and was widely used into the 1910s. It, and all other pitches involving doctoring the ball, was banned before the 1920 season, though some 'bona fide' spitball pitchers were allowed to continue throwing the pitch for the remainder of their careers. Many pitchers since have been accused of throwing spitballs illegally, and a few were either caught or admitted to doing so after retiring."
Baseball Reference

Ed Walsh

The Last Legal Spitball Pitchers
"I just finished reading Spitballers by Charles F. and Richard B. Faber (McFarland & Co. 2006), which I purchased at a Salvation Army thrift store earlier this summer for a buck. It was well worth my modest investment. The book consists of short biographies of the 17 spitball pitchers who were grand-fathered by their major league teams when the pitch was banned prior to the 1920 season: Red Faber, Burleigh Grimes, Jack Quinn, Urban Shocker, Stan Coveleskie, Bill Doak, Ray Caldwell, Clarence Mitchell, Dutch Leonard, Ray Fisher, Dick Rudolph, Allen Sothoron, Phil Douglas, Allan Russell, Doc Ayers, Dana Fillingim and Marvin Goodwin."
Burly's Baseball Musings

Spitballs, Splitballs, Dry Spitters, and Physics
"In 1920 the spitball was outlawed in baseball. Arguably this was in response to the death of Ray Chapman of the then Cleveland Indians, who was hit with a ball and killed while at the plate. Witnesses state that he never attempted to avoid the ball, which led to speculation that he never saw it coming. This was the end of the era known as the dead-ball era in baseball, which I wrote about previously in 'Into the Swing of Things'. The dead-ball era was characterized by the use of balls that literally wore out and died during the game, because they were practically never replaced. Knowing this, every pitcher considered it his responsibility to hurry along the death of the ball with the application of any and every substance possible: spit, tobacco juice, grease, licorice, sand paper, nail files, nails, blades, spikes, you name it. Dead balls were hard to see clearly."
Think Science

[PDF] The Evolution of the Spitball - A Page from Baseball's Past