EE was welding a 12""X1""X3/4"" piece of metal onto a mill gang cart. He was holding the piece in his R hand and welding with his L hand. He stated he heard it pop and then the welding wire bounced up and penetrated his welding glove and R thumb. He applied 1st aid and continued to work. On 4/3/11 it was swollen/sore, went to ER, did 1st aid. On 4/4/11 got worse so he was admitted.