employee was removing end cap bolts on a kiln trunnion w/ a 2-1/4"" box end wrench. As EE tugged on wrench, it came off the bolt head, pinching employee's R hand between the wrench and a pipe. The wound was cared for onsite, employee taken to ER & received 10 stitches. RTW on restricted duty. IE was wearing heavy leather gloves, wrench & bolt head were in good condition.