“‘I am,’ said the slab artist, blowing on his benumbed digits to warm them, ‘but I ain’t an ice pitcher, blame ye!’” Frederick News (Frederick, KS), November 18, 1895, p2
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1902. “The first game found Taylor and Mathewson as rival slab artists” (William F.H. Koelsch, The Sporting Life, Aug. 16; Peter Morris).