The strongly rising R-curve behavior, typical of coarse grain sized spinels, has been ascribed to crack face tractions in the wake zone which reduce the crack tip stresses. A novel experimental approach is presented which isolates these tractions for direct characterization in tension. With this method, a detailed study of the crack face separation events has identified the toughening contributions from the active microstructural constituents. Critical wake stresses, varying from 3.75 MPa for a small grain size to 13 MPa for a large grain size, are estimated for this high purity spinel containing only minor quantities of a crystalline grain boundary phase. Grain boundary interactions in the process zone are characterized through optical techniques and evidence the load-carrying role of bridging ligaments. The post fracture tensile test results of a medium grain size spinel are validated through a simple model to reconstruct the experimentally determined R-curve.