On December 23, 1902 — John A. Hillerich is issued a patent which devises a method of hardening the surface of a bat, improving the wood’s ability to drive a baseball and to preserve the material from checking, chipping, or splintering on its surface from exposure to the sun or the weather. The name “Louisville Slugger” had become the Kentuckian’s business registered trademark in 1894.

Sources:
National Pastime

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