Supt.was helping with positioning of roll mount for No.1 kiln/No.5 Pier when a hydraulic jack started leaking. Pressure on jack was released allowing jack/shims to lower, remaining jack/shims had all pressure on it & being to move. EE had his left hand on handrail and this hand was struck by a piece of shim stock 11 1/4x3x1"" weighing 11 1/2lbs. laceration 4 sitches.