In this study crystals were typically deformed to compressive engineering strains greater than 50 percent in order to maximize substructural refinement, strength and finished billet size. Fractures induced in CsI by these large deformations have not been addressed in the previous studies. Fracture, and its prevention, is a central issue. Stress-strain characteristics for three less than 111 greater than oriented CsI crystals forged at different temperatures are presented. These typical characteristics indicate that little apparent work hardening occurs before compressive strains greater than 20 percent are reached. Beyond 20 percent strain, forging stresses increase. The authors speculate that the increase in stress results from an increase in friction at the platen/billet interfaces, rather than an increase in the work hardening rate.