On 7/1 employee welded a pin keeper on a truck when a piece of slag went down his shirt sleeve and burned his elbow in area about the size of a pencil end. The burn became infected. Notified management on 7/11. Missed work as of 7/9; returned 7/15. Use of short-cuff leather gloves instead of long welding gloves in a overhead position allowed slag to enter the shirt.